Federal Register of Legislation - Australian Government

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Measures as amended, taking into account amendments up to National Environment Protection (Assessment of Site Contamination) Amendment Measure 2013 (No. 1)
Administered by: Agriculture, Water and the Environment
Registered 03 Jun 2013
Start Date 16 May 2013

Commonwealth Coat of Arms

National Environment Protection (Assessment of Site Contamination) Measure 1999

as amended

made under section 14(1) of the

National Environment Protection Council Act 1994 (Cwlth), the National Environment Protection Council (New South Wales) Act 1995 (NSW), the National Environment Protection Council (Victoria) Act 1995 (Vic), the National Environment Protection Council (Queensland) Act 1994 (Qld), the National Environment Protection Council (Western Australia) Act 1996 (WA), the National Environment Protection Council (South Australia) Act 1995 (SA), the National Environment Protection Council (Tasmania) Act 1995 (Tas), the National Environment Protection Council Act 1994 (ACT) and the National Environment Protection Council (Northern Territory) Act 1994 (NT)

Compilation start date:                     16 May 2013

Includes amendments up to:            National Environment Protection (Assessment of Site Contamination) Amendment Measure 2013 (No. 1)

This compilation has been split into 22 volumes

Volume 1:       sections 1-6, Schedules A and B

Volume 2:       Schedule B1

Volume 3:       Schedule B2

Volume 4:       Schedule B3

Volume 5:       Schedule B4

Volume 6:       Schedule B5a

Volume 7:       Schedule B5b

Volume 8:       Schedule B5c

Volume 9:       Schedule B6

Volume 10:     Schedule B7 - Appendix 1

Volume 11:     Schedule B7 - Appendix 2

Volume 12:     Schedule B7 - Appendix 3

Volume 13:     Schedule B7 - Appendix 4

Volume 14:     Schedule B7 - Appendix 5

Volume 15:     Schedule B7 - Appendix 6

Volume 16:     Schedule B7 - Appendix B

Volume 17:     Schedule B7 - Appendix C

Volume 18:     Schedule B7 - Appendix D

Volume 19:     Schedule B7

Volume 20:     Schedule B8

Volume 21:     Schedule B9

Volume 22:     Endnotes

 

Each volume has its own contents

 

 

 

About this compilation

The compiled instrument

This is a compilation of the National Environment Protection (Assessment of Site Contamination) Measure 1999 as amended and in force on 16 May 2013. It includes any amendment affecting the compiled instrument to that date.

This compilation was prepared on 22 May 2013.

The notes at the end of this compilation (the endnotes) include information about amending Acts and instruments and the amendment history of each amended provision.

Uncommenced provisions and amendments

If a provision of the compiled instrument is affected by an uncommenced amendment, the text of the uncommenced amendment is set out in the endnotes.

Application, saving and transitional provisions for amendments

If the operation of an amendment is affected by an application, saving or transitional provision, the provision is identified in the endnotes.

Modifications

If a provision of the compiled instrument is affected by a textual modification that is in force, the text of the modifying provision is set out in the endnotes.

Provisions ceasing to have effect

If a provision of the compiled instrument has expired or otherwise ceased to have effect in accordance with a provision of the instrument, details of the provision are set out in the endnotes.

 

 

  

  

  


 

National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of Site Contamination) Measure 1999 National Environment Protection (Assessment of

Schedule B4





 

 

 

 

 

 

 

 

 

 

 

 


                                                          

 

 

 

 

 

 

 

 

 

 

 

 

GUIDELINE ON

Site-Specific Health
Risk Assessment Methodology


 

 


 

Explanatory note
The following guideline provides general guidance in relation to investigation levels for soil, soil vapour and groundwater in the assessment of site contamination.

This Schedule forms part of the National Environment Protection (Assessment of Site Contamination) Measure 1999 and should be read in conjunction with that document, which includes a policy framework and assessment of site contamination flowchart.

The original Schedule B4 to the National Environment Protection (Assessment of Site Contamination) Measure 1999 has been repealed and replaced by this document.
The National Environment Protection Council (NEPC) acknowledges the contribution of the National Health and Medical Research Council (NHMRC), enHealth, ERM and enRiskS to the development of this Schedule.



Contents
Site-specific health risk assessment methodology
            Page

1                             Introduction                                                          1

1.1             Objectives                                                                                           1

1.2             Overview of Schedule B4                                                                  1

1.3             Introduction to quantitative health risk assessment in contaminated land decision-making                                                                                                1

1.4             Site assessment process and terminology                                         2

1.4.1                Health investigation levels                                                      2

1.4.2                Conceptual site model                                                             2

1.4.3                The tiered approach                                                               3

2                             The Australian risk assessment framework         4

2.1             The enHealth framework                                                                 4

2.2             Risk assessment framework for contaminated sites                       5

2.3             Fundamentals of the risk assessment approach                              8

2.3.1                Issues identification                                                                 8

2.3.1.1                 Planning and scoping                                                                            8

2.3.1.2                 Problem formulation                                                                            9

2.3.2                Exposure pathways                                                                 9

2.3.3                Conceptual site model                                                             9

2.4             The tiered approach                                                                        13

2.4.1                Fundamentals of the tiered approach                                   13

2.4.1.1                 Tier 1                                                                                                      13

2.4.1.2                 Tier 2                                                                                                      13

2.4.1.3                 Tier 3                                                                                                      13

2.4.2                General risk assessment assumptions                                   14

2.4.3                Risk assessment endpoints                                                    15

2.4.4                Deterministic versus probabilistic estimates                        15

3                             Data collection and data evaluation                  16

3.1             Data collection                                                                                  16

3.2             Source variables                                                                               16

3.2.1                Organic speciation                                                                16

3.2.2                Metals speciation                                                                  16

3.2.3                Background concentrations                                                  17

3.2.4                Vapour and particulate (dust) sampling                               17

3.3             Exposure pathway variables                                                          17

3.3.1                Organic carbon content                                                        17

3.3.2                Other key pathway parameters                                           18

3.4             Data evaluation                                                                                18

3.4.1                Data quality assessment and data quality objectives           18

3.4.1.1                 Analytical methods                                                                             19

3.4.1.2                 Data quality objectives                                                                       19

3.4.1.3                 Limits of detection                                                                              19

3.4.1.4                 Density and distribution of samples                                                19

3.4.2                Three dimensional source definition                                     19

3.4.3                Refining the exposure pathways                                           20

3.4.4                Tier 1 screening                                                                    20

4                             Exposure assessment                                           22

4.1             Introduction                                                                                     22

4.2             Exposure settings                                                                             22

4.2.1                Defining model inputs to represent the contaminant source 22

4.2.2                Exposure pathway input values                                            23

4.2.3                Exposed population input values                                          24

4.3             Exposure point concentrations                                                       25

4.4             Exposure point concentrations - volatiles                                     26

4.4.1                Introduction                                                                          26

4.4.2                Indoor air concentrations:                                                   27

4.4.3                Outdoor air concentrations                                                  28

4.4.4                Finite and infinite sources                                                     28

4.4.5                Biodegradation                                                                     28

4.4.6                Vapours from non-aqueous phase liquids                            29

4.5             Exposure point concentrations - particulates                               30

4.6             Exposure point concentrations - food consumption                    31

4.6.1                Fruit and vegetable consumption                                         31

4.6.2                Poultry, meat and fish consumption                                     32

4.7             Estimation of contaminant intake                                                  33

4.7.1                Introduction                                                                          33

4.7.2                Ingestion intakes                                                                   33

4.7.3                Dermal intakes                                                                      34

4.7.4                Inhalation intakes                                                                 35

4.8             Specific considerations in exposure modelling                              37

4.8.1                Blood lead modelling                                                             37

4.8.2                Bioavailability and bioaccessibility                                      37

4.8.3                The approach for petroleum hydrocarbons                         39

5                             Toxicity assessment                                             41

5.1             Introduction                                                                                     41

5.1.1                Sources of toxicity information                                            41

5.1.2                Sources of physical and chemical data                                 43

5.2             Hazard identification                                                                       43

5.2.1                Acute effects                                                                          44

5.2.2                Chronic threshold effects                                                      44

5.2.3                Chronic cancer effects                                                          45

5.3             Dose-response assessment                                                              47

5.3.1                Overview                                                                               47

5.3.2                Threshold toxicity reference values                                     47

5.3.3                Cancer toxicity reference values                                          47

5.4             Other considerations in toxicity assessment                                   50

5.4.1                Absence of information                                                         50

5.4.2                Early-life susceptibility                                                         50

5.4.3                Metal speciation                                                                    50

6                             Risk Characterisation                                         53

6.1             Overview                                                                                          53

6.2             General risk characterisation principles                                        53

6.3             Risk estimation                                                                                 54

6.3.1                Threshold risk estimation                                                     54

6.3.2                Non-threshold risk estimation                                              54

6.4             Risk evaluation                                                                                55

6.4.1                Threshold risk evaluation                                                     55

6.4.2                Non-threshold risk evaluation - acceptable level of cancer
risk                                                                                       
56

6.5             Risk evaluation of mixtures                                                            58

6.6             Uncertainty and sensitivity analysis                                               59

6.6.1                Uncertainty analysis                                                             59

6.6.2                Sensitivity analysis                                                                61

7                             Risk communication and management             63

7.1             Risk communication                                                                        63

7.2             Risk management                                                                            63

8                             Bibliography                                                       65

9                             Appendix 1: Structure of a risk assessment
report                                                                 
73

9.1             Introduction                                                                                     73

9.2             General                                                                                             73

9.3             Key principles                                                                                  73

9.3.1                Overview                                                                               73

9.3.2                Issues identification                                                               74

9.3.3                Data collection and evaluation (development of a conceptual site model) 74

9.3.4                Exposure assessment                                                            75

9.3.5                Toxicity assessment                                                              75

9.3.6                Risk characterisation                                                            75

9.3.6.1                 Overview                                                                                               75

9.3.6.2                 Uncertainty                                                                                           76

9.3.6.3                 Sensitivity analysis                                                                              76

10                          Glossary                                                              77

11                          Shortened forms                                                 81


1                  Introduction

1.1              Objectives

The objectives of this revised Schedule B4 guideline to site-specific health risk assessment methodology of the National Environment Protection (Assessment of Site Contamination) Measure 1999 (the NEPM) are to:

·         establish the fundamental principles of risk assessment as they relate to contaminated land decision-making in Australia to ensure protection of human health

·         provide a framework for policy making and undertaking risk assessments that is transparent, logical and compatible with current scientific principles and practice

·         provide a basis for deriving the health investigation levels (HILs) presented in Schedule B7 guideline on derivation of health-based investigation levels

·         provide a guide for deriving site-specific criteria, which may act as clean-up levels.

The intended audience for this Schedule includes policy makers, risk practitioners and regulators.

 

This guidance is not intended to be used to assess the risks from occupational exposure to substances that may occur in an occupational setting or workplace. These risks are dealt with under work health and safety legislation and associated guidelines.

1.2              Overview of Schedule B4

This document provides an approach to conducting site-specific health risk assessments and the development of response levels at contaminated sites. Due to the complexity of the risk assessment approach, a standard approach to all sites is not practicable and site-specific considerations will often need to be accounted for. This document is intended to provide a guide to assist the decision-making process and, where possible, risk assessment procedures are recommended. The principles and guidelines in this Schedule are intended to assist in determining whether or not remediation is required at a site given the proposed land use, and to aid in decision-making with regard to the end-point for remedial works

1.3              Introduction to quantitative health risk assessment in contaminated land decision-making

The enHealth Committee (enHealth) of the Australian Health Protection Committee (enHealth 2012a) defines risk assessment as ‘the process of estimating the potential impact of a chemical, physical, microbiological or psychosocial hazard on a specified human population or ecological system under a specific set of conditions and for a certain timeframe’.

 

Quantitative (health) risk assessment is a step-wise process used to inform and assist the contaminated land decision-making process by modelling the dose or exposure of humans to observed site contamination. It is used to estimate, in a way that is adequately protective of health, the potential for site contamination to have an adverse effect on the health of those potentially exposed to it (referred to as exposed populations). This is achieved by modelling the dose that an individual may receive through incidental exposure to contaminated soil and/or water as a result of everyday activities. This estimated dose can then be compared against doses that are considered to result in no observable adverse impact to health, as published by authoritative bodies and health protection agencies.

 

The quantitative risk assessment process, in the context of this guidance, is primarily designed to evaluate the long-term or chronic risks to exposed populations from contamination in the environment. Short-term or acute risks can be dealt with using similar means though normally this is not necessary, because contamination that is severe enough to pose acute risks is sufficiently obvious that a complex assessment is not required.

 

There are two different approaches to risk assessment, which can be referred to as ‘forward’ and ‘backward’ assessments. In a forward assessment, the risks associated with a measured contaminant concentration are estimated by comparing an estimated dose that a receptor (i.e. a person) may receive with a dose considered to result in no significant observable impact to health. It is possible to undertake the modelling in reverse (a backward assessment), starting with the dose considered to result in no significant observable impact to health, and back-calculating ‘tolerable’ contaminant concentrations at the site. These concentrations can then be used to define management requirements or as remediation targets or site-specific clean-up criteria.

1.4              Site assessment process and terminology

The general site assessment process for contaminated land is described in Schedule A of this NEPM. Once the need for an assessment is triggered, a preliminary site investigation should be conducted using guidance outlined in Schedule B2. The objectives of the preliminary investigation are to identify the potential contaminants of concern, the areas of potential contamination and the potentially affected media (i.e. soil, water, sediment and air). The preliminary investigation report should clearly identify any significant data gaps and include an assessment of the accuracy of the information collected.

 

Where there is potential for soil and/or groundwater contamination to be present, then a detailed site investigation should be conducted. This involves the collection of soil, groundwater and soil vapour samples for field and laboratory analysis, and should be conducted using appropriate guidance such as that outlined in Schedule B2 and Schedule B3.

 

The sample analysis results are then compared against appropriate guidance values, such as HILs (after appropriate checking of the reliability of the data).

1.4.1        Health investigation levels

HILs and health screening levels (HSLs) are defined as ‘the concentration of a contaminant above which further appropriate investigation and evaluation will be required’. They are defined for specific land uses and will not necessarily be protective for other land uses. There are a number of assumptions (for example, relating to site geology or the nature of exposure) built into the HILs and HSLs, and careful review should be undertaken to determine whether these assumptions are applicable on a given site. The absence of an HIL for a specific contaminant does not indicate that no potential risk is posed; rather that a site-specific risk assessment may be required.

 

The HILs are presented in Schedule B7. HSLs have been developed separately by CRC CARE (Friebel & Nadebaum 2011) and may be adopted where applicable (for further information refer to Schedule B1). Levels marginally in excess of the HILs and HSLs (where applicable) do not imply unacceptability or that a health risk is likely to be present. Similarly, levels less than the HILs may not imply acceptability or that a health risk does not exist for a sensitive sub-population (for example, people with pre-existing illness and people with pica (relatively common in some groups with severe or profound intellectual disability)). These issues would need to be addressed in site-specific assessment. Subject to an appropriate investigation and assessment process, a decision not to take further action or to take further action may be justifiable.

 

HILs and HSLs (where applicable) are not intended to be clean-up levels. The decision on whether clean-up is required and if so, to what extent, should be based on site-specific assessment. Health risk assessment is one aspect of making such a decision though other considerations such as practicality, timescale, effectiveness, cost and durability are also important.

1.4.2        Conceptual site model

A conceptual site model (CSM) is a representation of site-related information regarding contaminant sources, receptors and the exposure pathways between those sources and receptors. The development of a CSM is an essential part of all site assessments and provides the framework for identifying how the site became contaminated and how potential receptors may be exposed to contamination either in the present or in the future.

 

In order to commence a risk assessment, it is necessary to develop a preliminary CSM. Factors to be considered include:

·         the typical and maximum concentrations of contaminants on-site

·         the vertical and horizontal distribution of the contaminants

·         the physical and chemical properties of the contaminants and their likely mobility in the environment

·         the physical properties associated with the geology and hydrogeology underlying the site

·         the potential presence of subsurface geology or structures that may act as preferential pathways for vapour migration on and off the site

·         the people who may be exposed to the contaminants

·         the means by which exposure could occur and the frequency of exposure.

Additional information is presented in Schedule B2.

1.4.3        The tiered approach

The human health risk assessment process for assessment of site contamination is generally undertaken in stages or ‘tiers’ involving progressively more detailed levels of data collection and analysis. In this guidance, the tiers are referred to as Tier 1, Tier 2 and Tier 3. The approach provides for assessment at a level of complexity that is appropriate for the problem under consideration; the degree of health protection achieved is equal at each tier. As the amount of data and assessment detail increases and the conceptual understanding of site conditions (that is, the CSM) is refined, the level of uncertainty decreases.

 

A risk assessment progresses from Tier 1 to Tier 2 when uncertainty and risks at Tier 1 may be unacceptable and further assessment is needed. Progression from Tier 2 to Tier 3 is similarly driven. Tier 3 provides more detailed and specific focus on risk-driving factors. It should be noted that the activities within the tiers may vary depending on the scale and complexity of the project.

2                  The Australian risk assessment framework

2.1              The enHealth framework

The Environmental Health Committee (enHealth) provides national leadership on human health issues from environmental hazards, coordinates national policies and programs, and provides a pivotal link between international and environmental health stakeholders in Australia. It is also responsible for the implementation of the National Environmental Health Strategy. Additional information on enHealth, including access to its publications, is available via the Department of Health and Ageing website:

 http://www.health.gov.au/internet/main/publishing.nsf/Content/ohp-environ-enhealth-committee.htm.

In 2004, enHealth developed Environmental Health Risk Assessment - Guidelines for assessing human health risks from environmental hazards (enHealth 2004) to provide a national approach to undertaking environmental health risk assessments  and updated these guidelines in 2012 (enHealth 2012a). The guidelines present a general environmental health risk assessment methodology that has been adopted nationally to evaluate risks and establish standards for the protection of human health and the environment.

 

With respect to assessment of risks from contaminated land, the guidelines draw on other documents as follows:

·         Assessment and management of contaminated sites (ANZECC & NHMRC 1992)

·         SA Health Commission contaminated sites monograph series 1991, 1993, 1996 and 1998 (El Saadi & Langley 1991; Langley & van Alphen 1993, Langley et al. 1996a, 1996b)

·         Environment Protection and Heritage Council, Proceedings of the 5th national workshop on the health and environmental assessment of site contamination (Langley et al. 2003)

·         National Environment Protection (Ambient Air Quality) Measure, Report of the risk assessment task force (NEPC 2000)

·         National Research Council, Science and decisions: advancing risk assessment (NRC 2008)

·         US EPA, Risk assessment guidance for Superfund, volume I, Human health evaluation manual, Part A (US EPA 1989)

·         US EPA, Risk assessment guidance for Superfund, volume I, Human health evaluation manual, Part B, Development of risk-based preliminary remediation goals (US EPA 1991a)

·         US EPA, Risk assessment guidance for Superfund, volume I, Human health evaluation manual, Part C, Risk evaluation of remedial alternatives (US EPA 1991b)

·         US EPA, Risk assessment guidance for Superfund, volume I, Human health evaluation Manual, Part D, Standardized planning, reporting and review of Superfund risk assessments (US EPA 1998)

·         US EPA, Risk assessment guidance for Superfund, volume I, Human health evaluation manual Part E, Supplemental guidance for dermal risk assessment (US EPA 2004b)

·         US EPA, Risk assessment guidance for Superfund, volume I, Human health evaluation manual Part F, Supplemental guidance for inhalation risk assessment (US EPA 2009)

·         US EPA, Risk assessment guidance for Superfund, volume I, Human health evaluation manual, Supplement to Part A: Community involvement in Superfund risk assessments (US EPA 1999)

·         World Health Organization, IPCS Risk assessment terminology, Harmonisation project, document no. 1 (WHO 2004)

·         World Health Organization, Principles of characterising and applying human exposure models, Harmonisation Project, Document no. 3 (WHO 2005)

·         World Health Organization, Part 1: Guidance document on characterising and communicating uncertainty in exposure assessment, and Part 2: Hallmarks of data quality in chemical exposure assessment, Harmonisation project, document no. 6 (WHO 2008).

The enHealth guidelines adopt a framework for evaluating risks that was developed by and for environmental health agencies, including the World Health Organization (WHO) and the United States Environmental Protection Agency (US EPA). As discussed in Section 2.2, the framework presented in enHealth forms the basis for the risk assessment framework adopted in this schedule.

 

The framework comprises the following components:

·         Issues identification

·         Hazard assessment (often called toxicity assessment)

·         Exposure assessment

·         Risk characterisation

·         Risk communication and management.

2.2              Risk assessment framework for contaminated sites

The risk assessment process for contaminated land is intended to achieve the following objectives:

·         to provide a consistent methodology for appraising and recording public health risks at contaminated sites

·         to establish the baseline risks and determine whether site remediation is required

·         to determine tolerable levels of contaminants in soil and groundwater that are protective of public health and ecosystems (as required)

·         to enable the comparison of potential health benefits and impacts (where relevant) of various remedial technologies.

The assessment framework was originally outlined by NEPC in the Assessment of Site Contamination National Environmental Protection Measure (NEPC 1999). The major difference between the framework originally outlined by NEPC and that of enHealth is the first step in the assessment process, which in enHealth (2012a) is referred to as ‘issues identification’. The term ’issues identification’ is intended to establish the context for the risk assessment by a process of identifying the concerns that need to be addressed, such as ‘what is causing the identified concern?’, and ‘why is the concern an issue?’.  Inclusion of the ‘issues identification’ as an explicit need is consistent with US conclusions that increased attention to scoping and planning of risk assessment is necessary (NRC 2008).

 

This revised schedule has adapted the enHealth framework with additions intended to provide guidance specific to a contaminated land context. Consistent with guidance provided in enHealth (Section 16.1 of enHealth (2012a)), in the assessment of contaminated sites, this Schedule takes precedence over the enHealth framework, and documents referenced therein, where there are contradictions. It is to be noted that the enHealth framework has a wider remit than the assessment of contaminated sites only, and some elements of the guidance are not relevant in a contaminated sites context.

 

The framework followed in this Schedule is illustrated in Figure 1. Detailed guidance for each stage of the process is provided in the body of this Schedule.

 

All risk assessments should be fully documented in order to ensure transparency, consistency in decision-making and ease of understanding by interested parties.

 

This means it should be supported with references to policy, scientific literature and other sources, including expert opinion. Jurisdictions may also require that a risk assessment is subject to review and revision should significant new information (that has the potential to change the objectives or outcome of the risk assessment) become available.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 1. Risk assessment framework for contaminated sites


2.3              Fundamentals of the risk assessment approach

2.3.1        Issues identification

The issues identification stage of a contaminated land risk assessment is fundamental to the production of a useful output. Issues identification is a process of communication between stakeholders in the project, and its scope and complexity depends upon the scale of the project and the issues being dealt with. Issues identification covers both a planning and scoping phase and a problem formulation stage.

2.3.1.1       Planning and scoping

In the planning and scoping phase, a team of decision-makers, stakeholders and risk assessors identifies the issue (or concern, problem or objective) to be assessed and establishes the goals, breadth, depth and focus of the assessment. The primary product of planning and scoping is a statement, with an explanation of why the assessment is being performed and what it will include and exclude, that is, how comprehensive it will be (NRC 2008).

 

Stakeholders in a contaminated land risk assessment are likely to be a subset of the following groups:

·         regulators (environment protection agencies)

·         local government (potentially several departments, e.g. planning, development control, road engineering, drainage, traffic, ecological issues)

·         state and territory departments of health

·         landowners

·         land developers

·         tenants, land management companies

·         local residents

·         occupants of neighbouring properties

·         water, sewerage, electricity, gas and telecommunication utilities

·         other interested parties (e.g. non-governmental organisations, local interest groups)

·         local politicians

·         consultants.

The planning and scoping phase should be undertaken before work begins on the risk assessment. The steps recommended are:

·         identify the stakeholders

·         decide which of them it is appropriate to consult at this stage of the project.

Frame the answers to the following questions and discuss them with the stakeholders:

·         Is a risk assessment the right type of decision-making tool?

·         What is the issue that the proposed risk assessment is considering?

·         Why is a risk assessment necessary?

·         What do we want to find out?

·         What are the hazards posing a risk?

·         The source of the risks?

·         What exposure pathways should be investigated?

·         Who might be exposed and who will not be exposed?

·         What decisions need to be made?

·         Timing of the assessment (urgency of answers)?

·         Level of complexity/detail of risk assessment needed?

With the outcome of the consultation known, determine the objectives for the risk assessment. Where not all the stakeholders have been consulted, consideration of their likely objectives should be included.

2.3.1.2       Problem formulation

The problem formulation stage is where discussions are conducted on how to conduct a risk assessment that covers the matters identified in the planning and scoping stage. Two critical products of the problem formulation stage are a conceptual model that explicitly identifies the stressors, sources, receptors, exposure pathways and potential adverse human health effects that the risk assessment will evaluate, and an analysis plan (or work plan) that outlines the analytic and interpretive approaches that will be used in the risk assessment (NRC 2008).

2.3.2        Exposure pathways

The fundamental concept of risk assessment is that there should be an exposure pathway linking the source of contamination and the exposed population. Where this linkage exists, an assessment of the nature and significance of the exposure pathway is required to determine the level of risk.

2.3.3        Conceptual site model

A key concept behind all risk assessments is the definition of a suitable CSM specific to each site. The CSM describes the source(s) of contamination, the pathway(s) by which contaminants may migrate through the various environmental media, and the populations (human or ecological) that may potentially be exposed.

 

A detailed CSM should include information on the following (on- and off-site as relevant):

·         the contaminants: nature of the contaminants identified, concentration, fate and transport, distribution and media in which they occur (soil, water, vapour, sediment or air)

·         physical characteristics of the environment:  soil type, porosity, vadose zone thickness, groundwater gradient and velocity and hydraulic conductivity of the saturated zone and the potential presence of preferential migration pathways

·         physical characteristics above-ground: sizes and locations and structures of current or future buildings (if known); potential presence of preferential vapour pathways; nature, size and location of outdoor spaces

·         characteristics of the exposed populations: exposed populations may be people residing or working at the site or off-site areas, future occupiers of the site after redevelopment, or environmental populations such as ecosystems in receiving environments e.g. natural surface waters.

Consideration of preferential migration pathways is an essential part of the development of the CSM where volatile and/or dissolved contaminants are present in the subsurface (refer to Schedule B2 and US EPA (2012a) for further information).

A CSM is generally a written description of the site that is accompanied by a schematic, graphical interpretation that depicts what is known or has been inferred about the site. It can also be presented as a flow diagram, as shown by the example in Figure 2. It can be simple or complex, with a more complex example shown in Figure 3.


Primary
Sources

Secondary
Sources

Transport
Mechanisms

Exposure
Pathways

Exposed Population
Characterisation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 2. Example Conceptual Site Model flow diagram (modified from ASTM, 1995)

A conceptual site model can inform the development of and be incorporated into the detailed scope for a human health risk assessment such as is shown in Figure 3. This example deals with the scoping of a risk assessment for hazardous air pollutants (HAP) and persistent, bioaccumulative and toxic pollutants (PBT) 

The schema identifies the sources, contaminants of concern (stressors), exposure pathways, potential receptors, and adverse human health effects that the risk assessment will address. The pathways presented are for illustrative purposes only and are not relevant to any specific scenario.

 

Figure 3: Example of scoping a risk assessment for air pollutants, indicating pathways considered (bold lines) and pathways not considered. (Adapted from NRC 2008) 


Sources

 

 

 

Stressors

 

 

 

Pathways/

Media

 

 

Routes

 

 

 

 

 

Populations

 

 

 

Endpoints

(Specific non-cancer target
organ endpoints shown;
for example purposes)

 

 

Metrics

(HAP-specific and cumulative (e.g. by cancer type, weight of evidence, by target organ-specific hazard index)  by

State


2.4              The tiered approach

2.4.1        Fundamentals of the tiered approach

2.4.1.1       Tier 1

The Tier 1 (or screening level) assessment is the first stage of assessment at the site. It includes a comparison of known site data with published risk-based guidance levels, such as the HILs. The assessment provides an initial screening of the data to determine whether further assessment is required. HILs are generic Tier 1 guidance values that are designed to be protective of most exposed populations under a variety of circumstances. The assumptions on which the HILs are based (including site conditions and the exposure scenarios) should be understood in order to determine whether the HILs are applicable for a given site. HSLs are similar to Tier 1 values and can be applied in the same manner provided that the associated guidance on suitability and application of the values is considered.

 

Exceedence of Tier 1 criteria is generally used to define the contaminants that require more detailed assessment at Tier 2. However, chemicals that act via the same toxicological mechanism should be considered carefully before being excluded from Tier 2 assessment. An assessment of the significance of exceedences may be necessary where they are marginal or present over a limited area. Under some circumstances further assessment of contaminants exceeding Tier 1 criteria may not be conducted (e.g. where the extent of the exceedence and cost of remediation is small and further assessment is not cost-effective). Where no further assessment is proposed, a clear and transparent explanation should be provided.

 

A Tier 1, screening level, assessment is often bypassed where there are no appropriate risk-based guidance levels (including HILs and HSLs), for example, if:

·         there are no risk-based guidance levels for a particular contaminant identified at the site or

·         the land use applicable to the site is not covered by the risk-based guidance levels or

·         the physical characteristics (e.g. geology, hydrogeology) of the site are such that the risk-based guidance levels may not be appropriate, for example, the presence of preferential migration pathways where there is a potential vapour risk.

2.4.1.2       Tier 2

A Tier 2 assessment is typically required when one or more contaminants are present at the site at levels that exceed Tier 1 guidance criteria, if there are no appropriate Tier 1 criteria, or if there are unresolved and significant uncertainties (limiting the reliability of the assessment conducted) identified in the Tier 1 assessment. Tier 2 assessment includes a site-specific risk assessment and the development of site-specific risk-based criteria for comparison with site data. Site-specific risk-based criteria are derived to be adequately protective of human health, but also to take into account site-specific conditions such as relevant exposure pathway linkages to avoid being unnecessarily conservative. Exceedence of Tier 2 criteria may result in a need for a Tier 3 assessment. As with Tier 1 exceedences, an assessment of the significance of exceedences may be necessary where they are marginal or present over a limited area. If Tier 2 criteria are exceeded, but further assessment (or action) is not proposed, the information and logic used to inform the decision should be documented clearly and transparently.

2.4.1.3       Tier 3

A Tier 3 assessment may be required where exceedence of Tier 2 site-specific risk-based criteria is judged to represent a potentially unacceptable risk to human health. The Tier 3 assessment typically focuses on the risk-driving contaminants in more detail, although studies aimed at reducing the uncertainties inherent in the modelling of exposure pathways are also common at Tier 3. This level of assessment may include statistical methods and mathematical modelling to assess the significance of the site contamination. The collection of additional data, such as soil vapour sampling, ambient air sampling, analysis of dust, biological monitoring and additional site investigations may be needed to support Tier 3 assessments in order to reduce uncertainties.

 

The tiered approach should provide a process for addressing site contamination methodically, with the level of complexity and cost proportional to the significance of the risk. Increased levels of site-specific data reduce uncertainties inherent in the assessment. It is important to note that for a given site the level of protection of exposed populations should remain the same regardless of the tier of assessment conducted.

2.4.2        General risk assessment assumptions

Risk assessment is a tool to help risk managers make decisions about contaminated sites. Risk assessment should incorporate an appropriate level of health protection such that sensitive exposed populations are adequately protected. Because of the many uncertainties inherent in risk assessment, it is desirable that quantitative risk assessment methods should overestimate risk to some extent.

 

Risk assessors should transparently describe the assumptions and uncertainties involved in a risk assessment.

 

Risk assessors should select exposure model inputs carefully, and consider the reasonableness of the exposure settings when taken together. Some examples of how assumptions may be applied to a model are:

·         using an average (where there is sufficient data and it is relevant to the exposure being assessed) and high-end estimate (e.g. upper 95th percentile or maximum where relevant) of the contaminant concentrations to represent the source

·         choosing an appropriate sensitive exposed population for the CSM, such as young children or pregnant women

·         assuming that the behaviour of the sensitive population results in plausible high-end exposures (e.g. long exposure periods, exposure of large areas of the body, high activity rates)

·         assuming that exposure to the population may occur by several pathways (e.g. a child who plays in contaminated soil also eats vegetables grown in this soil)

·         using fate and transport models (e.g. for vapour intrusion pathways) which provide estimates of the amount of contamination that reaches the exposed populations.

Models should only be used where they are suitable for the exposure scenario being evaluated, and the limitations of the model are adequately addressed and discussed in the assessment report. An overview of contaminant fate and transport modelling is presented in Schedule B2. Specific expertise and experience are required to carry out this type of modelling because of the highly complex nature of most contaminant fate and transport problems.

 

Some exposure settings and assumptions (as in Tier 1 assessments) may not be realistic for the site under consideration as they are based on generic assumptions and parameters that are not going to be realistic for all sites. A Tier 2 assessment may be used to derive more site-specific values by amendment of the assumptions to reflect actual site conditions.

 

Where available, data on biodegradation of contaminants and bioavailability of chemicals should be considered (by an appropriately qualified toxicologist), and exposure factors (and assumptions) should reflect the scenarios under consideration. It is not necessary to assume that the HIL assumptions detailed in Schedule B7 prevail under all circumstances; both sites and the exposed populations may be very different from the HIL scenarios and this can and should be accounted for in a Tier 2 or Tier 3 assessment.

2.4.3        Risk assessment endpoints

There are two different approaches to risk assessment which can be referred to as ‘forward’ and ‘backward’ assessments.

 

In a forward assessment, site data is assessed to estimate whether the observed contaminant concentrations potentially pose a health risk to exposed populations. Risks are expressed as hazard indices or as increased lifetime cancer risks and these risk estimates are used to support a decision regarding the acceptability of the risk.

 

In the backward assessment, the starting point of the assessment is the level of risk or exposure that is deemed to be acceptable for the site. The endpoint is an HIL or site-specific risk-based level, which may be used for further assessment or to provide a basis for clean-up.

2.4.4        Deterministic versus probabilistic estimates

A deterministic approach means that variables input to an exposure model are expressed as single values or point estimates, which are considered by the assessor to represent the best estimate of the value of the variable. The advantage of this approach is that it is simple and easily understood. Its potential disadvantage is that selection of many point estimates at the upper end of their likely ranges leads to compounding of the uncertainty. Sensitivity and uncertainty analysis are used to overcome this disadvantage and provide increased understanding and clarity on which values are risk-driving; this is itself a useful part of the risk assessment.

 

Probabilistic techniques can be used to overcome the potential for compounding conservatism and may provide increased understanding of the uncertainty inherent in the assessment results. Monte Carlo analysis and other probabilistic statistical techniques rely on the use of probability distribution functions instead of point estimates to represent the values of variables. A probabilistic exposure model is run over several thousand iterations, and values for each parameter are selected randomly from each distribution at each iteration. The output is also expressed as a probability distribution function.

 

The advantage of probabilistic methods is that it becomes possible to express the range of potential exposure and risk outcomes. This can aid decision-making by increasing the level of understanding around how likely different risk outcomes may be. The disadvantage is that in order to generate meaningful output, a high level of data and information on the shape of the probability distribution inputs is required. Often this information is not available, and in this case incorrect selection of a probability distribution can introduce error. Some variables are linked (for example, body weight and skin surface area) and if not carefully constructed, probabilistic models may be capable of generating some results that are physically impossible. Another disadvantage of probabilistic risk assessments is the difficulty in explaining this complex approach to the stakeholders who are involved in a site.

 

In summary, probabilistic techniques are generally not practicable for the majority of assessments at Tier 2, on the basis that there is often insufficient data to support the method. When probabilistic techniques are used, the justification for the probability distribution functions selected as inputs should be clearly given, and dependent variables should be identified and linked.

3                  Data collection and data evaluation

3.1              Data collection

Data collection entails the acquisition and analysis of information about contaminants at a site that may affect human health and which will be the focus of the risk assessment. The purpose of data collection is to gather data that will improve the CSM and hence enable a more site-specific assessment of risk to be made. Data will relate to not only contaminants (that is, the source) but the physical environment in which they are present (that is, the pathways of exposure). In some instances, it may also be appropriate to gather additional information about the potential exposed populations.

 

It is recommended that a systematic planning process is used for defining the objectives of a site risk assessment and to develop a sampling plan for the collection and evaluation of representative data to achieve those objectives. The elements of the data collection stage are described in more detail in Schedule B2, including the data quality objective process and the preparation of a sampling and analysis quality plan. The general requirements for data collection in site investigation apply equally to data collected for the purpose of risk assessment. There are a number of issues in data collection that are specific to health risk assessment, which are discussed below.

3.2              Source variables

There are a number of variables commonly required for health risk modelling, which can be measured on a site-specific basis. Site-specific data is preferable to the use of literature or model default values, because environmental variables may have large ranges. Choosing a worst-case value to account for uncertainty adds conservatism to the risk assessment. Consistent selection of worst-case values results in compounding of the conservatism and can result in unrealistic modelled outcomes. Getting site-specific data permits use of realistic values and provides more confidence in the risk assessment outcome.

3.2.1        Organic speciation

Several organic compounds are commonly analysed as groups of substances, for example, petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and phenols. Such tests may be useful for providing cost-effective estimates of the quantity of contaminant present, but are problematic in risk assessment because toxicity data is generally only available for specific substances. For certain complex mixtures commonly found on contaminated sites (e.g. petroleum hydrocarbons (as discussed in Section 4.8.3), PAHs and PCBs as discussed in Schedule B7, Appendix A2) there are established ways of mitigating this problem, which provide the preferred way of assessing these mixtures. On sites where the contamination can be fully defined by reference to individual specific substances (for which toxicity data is available), it is preferable to assess these specific substances.

 

Chemical analyses providing detailed breakdowns for groups of organic compounds into individual substances (e.g. PAH), or a limited range of individual indicator compounds plus fractions containing groups of compounds with similar physicochemical properties (e.g. petroleum hydrocarbon fractions) are commercially available. It is commonly necessary to undertake such analyses of mixtures in order to understand the mixture sufficiently to assess risk, because individual compounds within these groups have very different physicochemical and toxicological properties.

3.2.2        Metals speciation

The toxicity of metals and metalloids can be highly dependent on the form in which they occur in the environment. A common example is chromium (Cr), where the environmentally stable Cr (III) oxidation state is relatively harmless to humans, but the more oxidised Cr (VI) state is more hazardous to humans. For most metals and metalloids, the HIL assumes that the most toxic form is 100% of the contaminant present in soil. This is very unlikely to be true in most cases, and knowledge of the actual or probable form can be very useful at Tier 2.

 

Routine metal speciation analysis is commercially available in Australia for species and compounds of arsenic (As), selenium (Se), mercury (Hg), tin (Sn) and lead (Pb), while metal speciation of some other elements is routinely conducted internationally. At sites where potentially toxic metals are present, consideration should be given to whether speciated metal analysis would be relevant to the assessment. Speciated metals data can be useful in refining the risk assessment process. Further discussion on the toxicity of metal species is provided in this Schedule in Section 5.4.3.

3.2.3        Background concentrations

It is helpful to understand the prevailing background concentrations of chemical substances, since in some circumstances background concentrations may already exceed screening criteria.

 

For metal contaminants, if reliable ambient background concentrations cannot be measured, then either the estimation method of Hamon et al. (2004) or collations of ambient background data such as Olszowy et al. (1995) for four Australian cities could be used. To use the Hamon et al. method, it is necessary to establish whether the iron and manganese concentrations of the soil at the site in question are elevated by co-contamination, as the Hamon et al. relationships are based on soils from sites with no known history of contamination other than farming. Further information on these two methods can be found in Section 2.4.9 of Schedule B5b.

 

Most organic contaminants of interest at contaminated sites have no natural background concentration, though there are notable exceptions, including polycyclic aromatic hydrocarbons. Therefore, ambient background concentrations should either be measured at a suitable reference site, or a default assumption of zero be made for organic contaminants. There are no equivalent models to that of Hamon et al. (2004) available for organic compounds.

 

Further information on the establishment of background concentrations in soil and groundwater may be found in SA EPA (2008), ANZECC & ARMCANZ (2000) and US EPA (2002b, 2002c).

3.2.4        Vapour and particulate (dust) sampling

Direct sampling of ambient air, soil vapour and/or dust can be useful in the risk assessment process, as it provides actual data for inhalation exposure points as opposed to results obtained from fate and transport modelling applied to soil or groundwater data.

 

Air sampling may be useful in scenarios where human populations are potentially exposed to airborne contaminants over prolonged periods (for example, occupational or residential scenarios).

 

Australian standards for ambient air quality are provided by the NEPC (2003). Standard methods for sampling and analysis of ambient air and dust are available in various Australian standards (AS 3580 Series; AS 2800-1985; AS 2985-2004; AS 2986-1987; and AS 3640-2009) and US EPA TO Methods (including TO-15 and TO-17, US EPA 1999). Australian guidance on air sampling, analysis, assessment and modelling is available from New South Wales Department of Environment, Climate Change &Water (NSW DECCW, 2010).

 

Soil vapour sampling guidance is included in Schedule B2 and references therein. Specific guidance on dust sampling for lead is available from US EPA (1995a, 1995b) and ASTM (E1792-03 Standard specification for wipe sampling materials for lead in surface dust, 2011).

3.3              Exposure pathway variables

3.3.1        Organic carbon content

The potential for a contaminant to volatilise from soil is strongly influenced by the compound’s soil partition coefficient (Kd), which is a parameter that estimates the potential for the contaminant to adsorb to soil particles. For organic compounds, which sorb most readily to organic carbon within soil, the partition coefficient is a function of the compound’s organic carbon partition coefficient (Koc) and the fraction of organic carbon (Foc) in the soil. The organic carbon partition coefficient represents the chemical partitioning between organic carbon and water in soil. For organic compounds, the relationship between Kd, Koc and Foc is expressed by the following equation:

                        Kd = Koc x Foc

Fraction of organic carbon is a soil property that can be measured, and which often has a significant effect on the risk assessment outcome for volatile and persistent contaminants. Samples of uncontaminated soil should be tested for Foc such that the concentration of carbon measured is not influenced by the potential presence of organic contaminants. The number of samples required depends upon the inherent variability of the soil being characterised. Sufficient (based on site-specific factors) samples should be taken to represent each soil type being characterised.

3.3.2        Other key pathway parameters

Other key physical and chemical parameters that may be required for the risk assessment process include the following:

·         site topography and drainage direction

·         depth to groundwater

·         groundwater flow direction and velocity

·         groundwater geochemical parameters including pH, redox potential, dissolved oxygen and major element chemistry

·         presence of oxygen in the vadose zone

·         soil type classification

·         presence of physical barriers or conduits (that may act as preferential pathways)

·         moisture content

·         depth of contamination.

Data collection for the establishment of these variables is part of the site investigation process. Guidance on site investigation is provided in Schedule B2.

3.4              Data evaluation

The data evaluation stage is the first stage of the development of the CSM. The level of effort expended and detail of reporting necessary should be proportionate to the amount of data available.

The data evaluation steps include:

·         assessing data quality

·         rejecting unreliable data or those that fail to meet data quality objectives (DQOs)

·         defining the source in three dimensions

·         refining the understanding of pathways

·         undertaking Tier 1 screening of chemical data against HILs or relevant criteria for those chemicals without HILs (checked with auditor/regulatory authority)

·         identifying contaminants of concern.

3.4.1        Data quality assessment and data quality objectives

Guidance on data quality assessment is presented in Schedule B2 and enHealth guidance (enHealth 2012a) and should be followed when collecting data for risk assessment. Data quality and precision should be such that uncertainty in the risk assessment can be determined and minimised; data quality uncertainties should be explicitly discussed in the uncertainty analysis. Where a number of studies are being combined to provide definition of the source term, particular attention should be paid to the following:

3.4.1.1       Analytical methods

Use of different analytical and sample preparation methods can cause significant differences in results. For example, use of a ‘clean-up’ step (silica gel) in petroleum hydrocarbon analysis removes polar hydrocarbons such as soil humic acids, resulting in a much lower result in many cases. Further information is provided in Section 2.4.4 of Schedule B1. Guidance on analytical methods and their selection is given in Schedule B3.

3.4.1.2       Data quality objectives

Data collection is a vital and integral part of the risk assessment process. All data should be collected to meet pre-determined data quality objectives. In many instances, data from a preliminary or detailed site investigation may be available prior to commencing the risk assessment. In such cases, the assessor should determine whether the data quality objectives of any previous investigations are compatible with the objectives of the risk assessment and whether the original data quality objectives have been satisfactorily met. Guidance on the development of data quality objectives is given in Schedule B2.

3.4.1.3       Limits of detection

The detection limit of the analytical method used must be lower than the level at which the contaminant might become a concern (that is, lower than the HIL or Tier 1 screening level).

3.4.1.4       Density and distribution of samples

Under most circumstances, the data should adequately characterise the contamination at the location where the population is likely to be exposed to it. Where sampling density guidelines cannot be followed, the effects of the lack of data on the risk assessment should be considered. Guidance on the development of sampling programs is given in Schedule B2.

3.4.2        Three dimensional source definition

Understanding the source is a critical part of risk assessment. There are many factors that control the risk to health—the contaminant concentration is only one of them. The CSM should include a detailed description of the source, bringing together information from the site history, soil and geological information, the depth and extent of the source, and the chemical data. Guidance on developing CSMs can be found in Schedule B2.

 

Site history should provide information on how the contaminants were released to the soil, and the form in which they were likely to have been released. It may also provide information on the length of time the contaminants have been on the site. This information permits judgements to be made on the likely form and mobility of the contaminants and, together with geological and hydrogeological information, on whether groundwater may be impacted. Site history should also allow judgements to be made on where the contamination is likely to be located.

 

Contaminants may not be uniformly distributed through the soil profile; they may be associated with a particular soil stratum, such as a layer of imported fill material, or a layer of clay that preferentially adsorbs a contaminant. If the distribution or depth of the contamination is not characterised correctly, the risk can be overestimated or underestimated. The risk of exposure to contaminant vapours derived from soil or groundwater contamination is often driven by the soil type and porosity, the depth of the contamination and the presence of organic carbon in the soil profile. Therefore, by understanding which soil stratum is impacted, key parameters such as soil type, soil depth and Foc can be obtained from the appropriate zone, and a more accurate assessment of the risk can be made.

3.4.3        Refining the exposure pathways

Site investigation data may either introduce or rule out exposure pathways in the CSM. For example, establishing the groundwater flow direction with improved certainty might show that a pathway to a potentially exposed population does not exist because that exposed population proves to be located upgradient of the source. Detailed guidance is available in Schedule B2.

 

Guidance on physical hazards such as inhalation of asbestos fibres, risk of fire or explosion from flammable gases and risk of exposure to asphyxiating atmospheres (for example, methane, carbon dioxide, carbon monoxide) is beyond the scope of this Schedule. Guidance on the assessment of asbestos is provided in Schedules B1 and B2. Appropriate guidance should be followed for the assessment of physical hazards.

 

Exposure pathway assessment should lead to a clear conclusion on which pathways are considered viable and which are not, with reasoning and appropriate evidence. Where viable pathways cannot be assessed in the risk assessment process, appropriate controls for mitigating the risk should be provided and documented.

3.4.4        Tier 1 screening

Tier 1 screening involves comparison of site analytical results with appropriate screening criteria. In Australia, appropriate HILs (including interim HILs for vapour and, where applicable, HSLs for petroleum hydrocarbons and assessment criteria for asbestos) and GILs are used for Tier 1 screening to provide a rapid assessment of whether the site contamination may be of concern with respect to human health. Should contaminant concentrations at a site occur at levels that are below the Tier 1 levels, this implies that for the majority of the people in the population there is no significant health risk from contamination and that remedial action may not be required to protect human health.

 

For contaminants where HILs are not available, the methodology set out in Schedule B7 could be adopted to derive HIL-equivalent screening criteria. Sources of toxicity reference values should be considered in the same way as has been done for those chemicals with HILs (same hierarchy and other considerations). Exposure scenarios should be used as laid out in Schedule B7. This should only be undertaken by suitably qualified professionals. The resulting values may be used in the same way as HILs. All assumptions and calculations should be transparent, clearly referenced and justified.

 

Tier 1 values may be adopted from external peer reviewed sources only if the assumptions used to generate the values are equivalent or more conservative than those used to develop the HILs and are suitable for use. In these cases, the relevance of the values and assumptions included in the development of the proposed Tier 1 values should be clearly justified and referenced.

 

Exceedences of the HILs should be identified and considered. HIL exceedences do not imply that a risk is necessarily present but that further assessment may be justified. HILs are not intended to indicate a clear demarcation between acceptable and unacceptable. Marginal exceedences may not require quantitative Tier 2 risk assessment to conclude that further assessment is not necessary. The magnitude of the exceedence should be considered in the context of the CSM (that is, whether the exposure pathways are plausible and whether exposure will result in harm).

 

Background concentrations may also be an important consideration at the Tier 1 screening stage. If it can be clearly demonstrated that site concentrations are consistent with natural regional background levels (natural or anthropogenic), this can provide evidence that site contamination has not significantly increased the background level and further assessment is not justified. However, it is important to note that background concentrations in some cases may present a risk: for example emissions from motor vehicles can result in higher exposure to residents in a transport corridor. Residences adjacent to refuelling stations may be exposed to high hydrocarbon background. Naturally elevated background can result from highly mineralised geologic environments, resulting in enrichments of potentially toxic trace metals or from anthropogenic inputs such as atmospheric deposition in highly industrialised areas that are often found in major Australian cities. In such cases, consultation with local environment regulators and health protection agencies would be appropriate.

 

Tier 1 screening criteria (including HILs and HSLs) should only be used where there has been adequate characterisation of a site (that is, appropriate representative sampling has been carried out). At the very least, the maximum and the 95% UCL should be compared to relevant Tier 1 screening criteria. Where sufficient data is available it may be appropriate for the arithmetic mean (AM) to be compared to the relevant Tier 1 criteria. However, the implications of localised elevated values should also be considered. The results should also meet the following criteria:

·         the standard deviation (SD) of the results should be less than 50% of the Tier 1 screening criteria

·         no single value exceeds 250% of the relevant Tier 1 screening criteria.

No single summary statistic will fully characterise a site and a range of relevant statistical measurements should be considered in the data evaluation process and iterative development of the CSM (refer to Schedule B2, Section 4). It is preferable to examine a range of summary statistics including the contaminant range, median, geometric mean and geometric SD and 95% UCL as well as the AM and AM SD. Further information is provided in Schedule B2.

 

The end-point of the Tier 1 screening is the selection of the contaminants of potential concern that require further assessment. If the Tier 1 screening assessment concludes that there are no contaminants with plausible pathways to exposed populations, then the assessment is complete.

 

Further assessment after Tier 1 screening may comprise either additional assessment at Tier 2 (that is, risk assessment as described by the remainder of this guidance) or, alternatively, additional data collection may be required.

 

A weight-of-evidence approach, whereby the consistency of data from more than one line of evidence is considered, is recommended. For example, soil contamination at a site should also be evaluated by examining data from other media such as air, groundwater and surface water. This data should also be analysed in order for it to be used in the appropriate exposure scenario at a site. There is less guidance available on the characteristics of such data and indicators of its sufficiency. An initial review of these other types of data is required to determine if the sampling and analysis methods used were appropriate and if the detection limits achieved were appropriate given the risk-based screening criteria.

 

Groundwater data being used to assess human exposure may consider a reasonable maximum and relevant average at the site or off-site (as appropriate from the CSM). Where groundwater is to be extracted and used for any purpose, the suitability of groundwater data for the assessment of average and maximum exposures should be on a site-specific basis.

 

If air data or soil vapour data is available for the site, then the use of that data needs to be considered within the context of the CSM and the activities at the site that may affect the presence of the chemicals in the air. Consideration of both a reasonable maximum and a relevant average case should be developed where possible.

 

Data generated from chemical analyses may fall below the limit of detection (LOD) or limit of reporting (LOR) for the analytical procedure. Although substitution methods (such as replacing with the LOD/2) are reported widely in the literature for analysing data with non-detects, these approaches result in bias of summary statistics calculated from the adjusted data set. Where less than about 15% of the relevant data set comprises non-detects, substituting half the detection limit may be satisfactory (US EPA 2006). More detailed adjustments may be appropriate where greater than 15% of results are below detection. Further information can be found in Schedule B2 and US EPA (2006 and 2007a).

4                  Exposure assessment

4.1              Introduction

Exposure assessment involves the estimation of the magnitude, frequency, extent and duration of exposures to contaminants. In the data evaluation stage, the CSM is refined to produce an understanding of the source(s), pathway(s) and exposed population(s) that require assessment at Tier 2. In the exposure assessment stage, the CSM is used to generate a numerical representation of the exposure pathways that can be modelled quantitatively; these are the model input values. The input values are then used to model exposure point concentrations and estimate intakes of contaminants by the exposed populations.

 

To promote consistency and transparency in making exposure assessment assumptions, enHealth has established a framework for exposure assessment, set out in enHealth (2012a). This section follows the framework, adding details specific to contaminated land assessment.

 

Exposure assessment modelling methods have largely been developed for use in contaminated land risk assessment by the US EPA, and in the following section there are many references to US EPA guidance documents. These documents are extensive, well referenced and provide model algorithms and guidance on their use. Adoption of the modelling methodology in this Schedule does not imply endorsement of the US guidance; rather the US methods should be adapted to meet Australian policy objectives and environmental circumstances.

 

In the following sections, the derivation of model input values generally using point estimates (that is, single value estimates) is discussed. A more complete discussion of the use of probability distributions as input values (for example, the Monte Carlo method) is provided in Section 2.4.4. There may be circumstances where the use of probabilistic methods is appropriate in Tier 2 assessments, and it is not the intention of this Schedule to discourage the use of probabilistic models where their use can be beneficial to the outcome of the assessment.

 

The key stages of exposure assessment, as applied to contaminated land risk assessment, are to:

1.       determine input values for contaminant concentrations

2.       determine physicochemical properties of contaminants

3.       determine physical input values for pathways

4.       determine input values for exposed populations

5.       estimate exposure concentrations

6.       estimate chemical intake

7.       collect data to test predictions in stages 4 and 5 (where relevant).

Stages 1 to 4 comprise the translation of the CSM into modelling terms as described. Stages 5 and 6 are achieved using a quantitative health risk assessment model. Stage 7 is a test of the model.

4.2              Exposure settings

4.2.1        Defining model inputs to represent the contaminant source

The data evaluation stage should provide a good understanding of the source. Consideration needs to be given as to how the contaminant concentration will be applied in a Tier 2 human health risk assessment; that is, what values will be used as the input concentrations representative of site conditions. Note that, depending on the complexity of the site, there may be more than one ‘source’ requiring further assessment. Note also that the source may be in soil, water, non-aqueous phase liquid or vapour. Sources in different physical forms generally should be assessed separately and require separate input values. The depth of different sources also requires consideration. For example, contamination may be present in soil at depth during site investigations, but be moved to the surface of the site during construction.

There are a number of options for choosing the value to use as an input concentration. The most appropriate method will depend on the data set, and different methods may be required for different source areas or contaminants, since these may show very different distributions. If a series of data over time is available, consideration of trends will be needed; this is particularly important for groundwater sources. Some commonly used approaches are described herein; however, more sophisticated statistical methods may be used if the data set is suitable. Whatever approach is used to define the source input value, it should be clearly explained and justified.

 

Maximum observed contaminant concentration. This generally provides a conservative assessment because if estimated risks from the maximum concentrations are not of concern, then the site should be suitable for use under the CSM considered. Maximum concentration is often suitable for groundwater sources where trends are poorly defined. However, a maximum concentration may not be representative of the source as a whole and may result in an overestimation or underestimation of risk if the data is extremely limited.

 

Mean concentration. The mean contaminant concentration can be a suitable input concentration provided that it can be shown that it adequately represents the source being considered. It is important that small areas of high concentrations or hotspots are not ignored by averaging with lower values from other parts of the site. The mean value may be more representative of the source as a whole than the maximum, and may provide a better estimation of the actual concentration that a population would be exposed to over a period of time.

 

95% upper confidence limit (UCL) of the arithmetic mean contaminant concentration. This provides a 95% confidence level that the true population mean will be less than, or equal to, this value. The 95% UCL is a useful mechanism to account for uncertainty in whether the data set is large enough for the mean to provide a reliable measure of central tendency. Note that small data sets result in higher 95% UCLs. Further guidance on the use of 95% UCLs can be found in NSW DEC (1995), US EPA (2006) and US EPA (2007a).

 

Monte Carlo (or other probabilistic) techniques. Refer to Section 2.4.4.

 

Source input values will normally be soil, groundwater or soil vapour data. In more detailed assessments, more specific sources may be defined, such as dust, bore water or ambient air.

 

Considerations in this section are also relevant to contaminant source input values derived using fate and transport or other exposure point estimation methods. These are described elsewhere in this Schedule.

4.2.2        Exposure pathway input values

This stage involves describing the physical environment in terms of the input values that will be used to represent exposure pathways in the model. The scenario being modelled should clearly relate to the existing or proposed land use for which decisions on contamination are required. Depending on the pathways modelled, a number of variables will need to be defined, for example, soil type, soil properties, depth to groundwater, soil and vapour sources, climactic variables and building characteristics and dimensions. Schedule B7 provides a complete list of the exposure pathway variables used to derive the HILs, together with justification of the parameter values selected.

 

Where the CSM is similar to that used to derive the HILs, exposure pathway input parameters used to conduct a site-specific risk assessment should consider those adopted in the derivation of the HILs, with further consideration of site-specific factors (where relevant). Where the CSM is different from the HIL scenarios, further consideration of site-specific input parameters and variables may be warranted.

 

Each variable used in a model should be clearly referenced and justified. Some commercially available models do not permit amendment of all the variables listed and the user has to rely on the default values supplied with the software. Where this is the case, it should be demonstrated that the model defaults are applicable to the site. It should be appreciated that models developed for the use in other countries may incorporate assumptions that are not justified in the Australian environment (science policy and physical environment). Reference should be made to the physical setting assumptions outlined in Schedule B7 for guidance on values likely to be suitable for Australian sites. Site-specific data from site investigations should be used wherever possible.

4.2.3        Exposed population input values

The purpose of this part of the risk assessment is to determine the characteristics and behaviour of the critical exposed populations. Exposed populations may relate to the current or proposed future use of the site, and it should be made clear which land use assumptions are being made. Exposed populations may be located at some distance from the site, with pathways involving transport via groundwater, surface water or wind. Exposed populations may also be linked to the site via the food chain, for example, consumers of fish, meat or agricultural products that may be affected by site contaminants.

 

The physical characteristics and behaviour patterns representative of the exposed population should be selected for modelling. This is essentially a common sense exercise, which does not normally require any specific assessment methodology or data.

 

The main considerations are described below.

 

Physical characteristics - selection of representative values for physical aspects such as age, life expectancy, body weight and respiration rate need to be applied. It is not expected that risk assessors will be generally required to generate these assumptions (refer to those listed in B7 and/or enHealth (2012a and 2012b) where relevant)—considerable uncertainty is involved and variations in assumptions can have a significant impact on the risk assessment outcome. Exposed population physical characteristics should be sourced from applicable Australian guidance on exposure assessment (i.e. enHealth 2012b). Schedule B7 provides the values that have been selected to derive the HILs, which are primarily sourced from enHealth (2012a and 2012b).

 

Exposed population behaviour - exposure assessment requires the development of a model behaviour pattern that is judged to represent an exposed population. Some data on Australian behaviour patterns is available (for example, see EPHC 2004, enHealth 2012b). Important considerations in contaminated land risk assessment include factors such as the distribution of hours spent indoors and outdoors, amount of time spent in the location where exposure is predicted, level of physical activity, the nature of work or leisure activities, and the exposure duration. In selecting values to represent exposed population behaviour, it is important to consider the following:

·         Plausible high-end exposure — is the recommended approach to judging receptor population behaviour. The likelihood of the modelled scenario should be considered, and behaviours which might reasonably apply to real people should be selected for modelling.

·         Consistency— Schedule B7 provides behavioural and exposure duration assumptions for four standard exposure scenarios. Where site-specific assessments are essentially considering the same exposed populations in similar circumstances to the standard scenarios, the Schedule B7 behavioural assumptions should be adopted. This promotes consistency between site-specific risk assessments. Where the exposed population differs from the standard scenarios, amendments to behavioural assumptions can be made and should be clearly justified. Note that amending the commercial/industrial and public open space scenarios may be a common requirement, since activities within these land uses are prone to be variable.

·         Exposure via food and drink—the standard scenarios described in Schedule B7 provide limited consideration of exposure via food and drink because most Australians do not source a significant proportion of their food or water from their own property. However these sources may need to be considered in a site-specific assessment.

·         Allocation of background exposure (where it may contribute towards exposure to threshold contaminants)—related to the above point is the extent to which the exposed population is exposed to the contaminants under consideration as part of their daily lives. As well as presence in food and drink, contaminants may be present in the air, in consumer products, in household goods and in building materials. It is therefore necessary to consider the extent to which the risk assessment will allow for this exposure. At a screening level, simple (but appropriately justified) assumptions may be adopted. In more detail, background exposure can be considered on a site-specific basis depending on the site location and land use, as well as considering the contaminants individually.

It is recommended that in designing the exposure assessment, worst-case scenarios (particularly those where many ‘high-end‘ assumptions are compounded) should generally be avoided. The sensitivity and uncertainty of the assumptions adopted in the exposure assessment should be considered.

4.3              Exposure point concentrations

An exposure point concentration is the estimate of the concentration of the source contaminant in the medium that the population is exposed to, at the location where exposure is predicted to occur.

 

It is preferred that, where possible, exposure concentrations are derived from direct measurements in the relevant media (soil, bore water, indoor air, fruit and vegetables and eggs).

 

However, under some circumstances it is not practical to measure concentrations directly, and in these cases exposure point concentrations are typically estimated using computer models.

 

The most commonly used exposure point estimation methods are:

·         vapour intrusion modelling used to estimate vapour concentrations in ambient air from measured soil vapour, soil, groundwater and non-aqueous phase product data

·         particulate modelling used to estimate the concentrations of dust generated from surface soil

·         groundwater fate and transport modelling, used to estimate groundwater concentrations at an exposed population location where direct measurement is not possible, or to predict future groundwater concentrations at an exposed population location

·         plant uptake modelling, used to estimate concentrations of contaminants in crops and vegetables from soil and groundwater data

·         animal uptake modelling, used to estimate concentrations of contaminants in eggs, fish and meat.

These methods are described herein, with the exception of groundwater fate and transport modelling for which guidance is presented in Schedule B2. It should be noted that the level of uncertainty associated with the use of any model for the purpose of estimating exposure concentrations should be carefully considered and discussed.

4.4              Exposure point concentrations - volatiles

4.4.1        Introduction

Volatile substances are those that are capable of changing from liquid to vapour phase (that is, volatilising) under ambient conditions. While there are a few definitions of what may be considered of significance with respect to volatilisation, a volatile substance can be defined as having a Henry’s law constant of greater than or equal to 10-5 atm/m3/mol and its vapour pressure greater than 1 mm Hg at room temperature (NJDEP 2005). In addition to these measures, a substance should be assessed as volatile if its saturated vapour concentration results in exposure concentrations that are a risk to the exposed population. Some chemicals with low Henry’s law constants, or low vapour pressures, are so toxic that even a small amount that moves into the vapour phase could be enough to contribute to a risk. Hence both measures of volatility and toxicity need to be considered.

 

Vapours may arise primarily from three processes (refer to US EPA (2004a) for additional detail and equations):

·         by desorption from soil organic matter, described by a function of soil concentration, Henry’s law partition coefficient, soil-water partition coefficient (a function of organic carbon content and soil organic carbon partition coefficient) and soil properties (bulk density, volumetric air content and volumetric water content)

·         from groundwater plumes, described by the Henry’s law partition coefficient between water and air

·         from non-aqueous phase liquids, depending on the vapour pressure of the volatile compounds in the non-aqueous phase and the mole fraction of the compound of interest (US EPA 2000).

To assess exposure to volatiles, it is necessary to estimate the concentration of the vapour in the air that the exposed population breathes. The most direct approach to the quantification of these exposures is to use direct measurements of indoor or ambient air. Vapours in ambient air are relatively easy to sample; however, the collection and interpretation of this data can be difficult.

 

An indoor air sampling program may be expensive if many samples over a reasonably long period are needed to get representative results. In homes and workplaces, gaining access can be difficult and may lead to unnecessary concern on the part of the occupants.

 

Depending on the volatile compounds considered, ambient air results may be difficult to interpret since many other sources in addition to the site soil and groundwater can be present. For these reasons, ambient air measurements are not generally available. When they are available they may be unsuitable as a means to assess risks associated with soil and groundwater source alone, because of the inability to distinguish whether a soil- or groundwater-derived component is present or not. Where affected by background sources, the collection of indoor or ambient air measurements may not be considered the most appropriate approach.

 

Where direct measurements are not available, indoor and outdoor ambient air contaminant concentrations can also be predicted by modelling from measured soil vapour concentrations. Soil vapour measurement is the preferred route in most situations where a vapour issue (from a subsurface source) is considered likely to exist. However where soil vapour measurements are used in risk assessments, the relevance of the data with respect to the CSM and the data quality need to be evaluated prior to use. Guidance on the development and use of vapour CSMs is provided in Schedule B2 and references there-in.

 

In the absence of measured soil vapour concentrations, it is also possible to model the generation of vapour from soil, groundwater and non-aqueous phase liquids. This procedure adds another level of uncertainty to the process, and may lead to inaccurate results.

 

The uncertainties associated with the use of a model for these purposes should be well understood and discussed in relation to the nature of the volatile contaminants assessed.

 

Where unresolved uncertainties or unacceptable risks are predicted by modelling vapour concentrations, direct measurement of soil vapour and/or exposure concentrations indoors and outdoors should be obtained.

 

Additional information on developing a multiple-lines-of-evidence approach to assessing vapour risk can be found in Schedule B2.

4.4.2        Indoor air concentrations:

The direct measurement of indoor air concentrations may be appropriate where practical and where the data can be adequately interpreted. When assessing impacts from contaminated land it is important that background influences on indoor air concentrations are identified and characterised.

 

There are many circumstances where the measurement of indoor air concentrations is not the preferred method of assessing exposure from a subsurface contamination source. The information should be used in conjunction with other measurements such as sub-slab soil vapour, soil vapour at depth and outdoor air.

 

If direct measurements of indoor air (soil vapour and outdoor air) are conducted, sufficient numbers of samples to address temporal and spatial variability need to be collected.

 

Alternatively, indoor air concentrations can be modelled (estimated) using an attenuation factor (refer to US EPA 2012b for a range of values that could be considered), a model such as the Johnson and Ettinger (1991) model, or another appropriate (justified) model. The Johnson and Ettinger (1991) model is a one-dimensional ‘heuristic’ analytical solution to model advective and diffusive vapour transport into indoor spaces. It provides an estimated attenuation coefficient that relates the vapour concentration in the indoor space to the soil vapour concentration at the source of contamination (US EPA 2004a). A vapour attenuation factor, ‘’, is calculated, which is the ratio of the concentration of a chemical vapour in an indoor scenario relative to that measured in the soil. This model has been updated and modified since 1991 (Abreu & Johnson 2005, 2006) and is also described in Davis et al. (2004, 2009a). Inputs to the model include chemical properties of the contaminant, saturated and unsaturated zone soil properties, and structural properties of the building (US EPA 2004a).

 

The Johnson and Ettinger model as described by US EPA (2004a) is the most commonly used model for estimating vapour concentrations in indoor air and has been used in the derivation of the Health Screening Levels (HSLs) for petroleum hydrocarbons. The US EPA model provides additional functionality permitting the estimation of soil vapour concentrations from soil, groundwater and phase separated liquid. It is provided at: http://epa.gov/oswer/riskassessment/airmodel/johnson_ettinger.htm

 

There are a number of ways in which this vapour model can be manipulated to improve the confidence in the outcomes from the model; however, confidence in any model output without corresponding data for the purpose of validation is not high. Additional guidance is provided in Davis et al. (2004, 2009a).

 

It is noted that the Johnson and Ettinger model (as described by US EPA (2004a) and other similar vapour intrusion models do not adequately address vapour risk issues where there are preferential vapour migration pathways connecting a vapour source with the building, where the building structure extends into a saturated contaminated zone (that is, into the groundwater table) or where biodegradation is of significance (see Section 4.4.5). These issues should be addressed on a site-specific basis using more suitable (justified) techniques.

4.4.3        Outdoor air concentrations

The direct measurement of outdoor air concentrations is appropriate where practical, taking care with any data collected to minimise background influences.

 

Alternatively, outdoor air concentrations can be estimated using models such as the Jury et al. (1983) model. The Jury et al. model calculates the maximum flux of contaminant vapours from an infinite soil contaminant source via vapour phase diffusion. Chemical movement to the atmosphere is modelled via volatilisation loss through a stagnant air boundary layer at the soil surface, making it appropriate for use in an outdoor exposure setting. The Jury et al. (1983) model is widely accepted as an appropriate methodology for vapour modelling into outdoor air and has been applied by regulatory agencies in the United Kingdom (EA 2009) and United States (US EPA 1996) in the development of Tier 1 soil investigation levels. The Jury et al.  model is also recommended for estimating outdoor vapour concentrations in the Standard guide for risk-based corrective action (ASTM E1739–95 (2010)).

 

The modelling of outdoor air exposures also needs to account for vapour dispersion, between the soil surface and breathing zone of potentially exposed populations. This can be done using an outdoor box model which may predict ambient vapour concentrations on the downwind edge of the area source at the breathing zone height, as described by ASTM (E1739–95 (2010)). Alternatively, vapour dispersion in a well-mixed box may be estimated using calculated air dispersion factors, as described in US EPA (1996). Either method is considered appropriate.

4.4.4        Finite and infinite sources

The Johnson and Ettinger model is constructed as both a steady-state solution to vapour transport (infinite or non-diminishing source) and as a quasi-steady-state solution (finite or diminishing source) for soil contamination. A finite source model was not provided for groundwater in US EPA (2004) since groundwater migration reduces the certainty of concentration attenuation.

 

In situations where the soil or non-aqueous phase liquid source of dissolved phase volatile groundwater contamination is no longer present, dissolved phase concentrations should diminish or attenuate as the contaminants volatilise (or biodegrade). Dissolved phase contamination therefore becomes ‘finite’.

 

In circumstances where relatively small amounts of contaminants are present in the source zone, the infinite source assumption can give rise to a physically impossible output when a long period of time is modelled. Assessments should consider whether sufficient source (considered on a site-specific basis and may address aspects that include mass, extent and potential for biodegradation) exists to support the volatilisation modelled for the time period under consideration.

 

The finite source model can be used for site-specific risk assessment, provided that field evidence for the finite nature of the source is presented.

4.4.5        Biodegradation

In common with experience in the United States (US EPA 2012c), there are two classes of volatile substances that together account for a large percentage of sites affected by soil and groundwater contamination in Australia:

·         petroleum hydrocarbons compounds such as from diesel and petrol

·         chlorinated hydrocarbons such as dry cleaning solvents (tetrachloroethene or PCE) and degreasing agents (trichloroethene (TCE), 1,1,1-trichloroethane (TCA) and PCE).

The principal behaviour difference between petroleum hydrocarbons and chlorinated solvent vapours in the subsurface is that petroleum hydrocarbons readily biodegrade under aerobic (oxygenated) conditions whereas chlorinated solvents typically biodegrade much more slowly and under anaerobic conditions (Davis et al. 2009b, US EPA 2012a, 2012b and 2012c). 

 

Important factors that influence aerobic biodegradation in the unsaturated zone include source concentration, total oxygen demand (the oxygen required to biodegrade the organic contaminants as well as any soil organic matter), the distance between the source and the building and soil type (US EPA 2012a). For some petroleum vapour concentrations there may be sufficient separation distance between a persistent vapour source and a building foundation, referred to in some published literature as ‘exclusion distance’, that attenuation of biodegrading petroleum chemicals is essentially complete, as the supply of oxygen is non-limiting. Consideration of exclusion distances may be useful in the assessment of the significance of the vapour pathway for petroleum hydrocarbons. Further information is provided in Schedule B2 (Section 9).

 

However, aerobic biodegradation may be limited by the supply of oxygen such as may occur under large areas of hardstand or where soil organic matter consumes the available oxygen. Aerobic biodegradation is also not applicable to chlorinated solvents (except vinyl chloride where evidence suggests good biodegradation in the presence of oxygen) and other volatile contaminants. Chlorinated hydrocarbons, however, are known to biodegrade in highly reducing environments. Incomplete degradation of chlorinated solvents can produce toxic degradation products such as vinyl chloride and this should be considered where relevant.

 

Davis et al. (2009b) recommend a process for incorporating biodegradation into vapour assessment of petroleum hydrocarbons where there is sufficient evidence to justify its use (and where site-specific biodegradation data is not available). This approach may be applied in the context of the HSLs for petroleum hydrocarbons (see Schedule B1 Section 2.4.9). It is noted that the measurement of oxygen in the soil profile can be difficult and care should be taken when using this data to support biodegradation.

 

Approaches to assessing and modelling biodegradation of petroleum hydrocarbon vapours are developing rapidly. Useful methods are likely to be developed and will come into general use. For example the American Petroleum Institute (API) in 2009 released Biovapor, a model permitting simulation of biodegradation (including the consideration of the presence of a slab) on a site-specific basis. It can be accessed and downloaded at www.api.org/ehs/groundwater/vapor/index.cfm.

 

While currently it is not possible to recommend Biovapor as an appropriate method for use in Australia, it is not the intent of this guidance to restrict development of new approaches and techniques for improved assessment. Therefore, where new methods can be justifiably applied, their use may be considered.

4.4.6         Vapours from non-aqueous phase liquids

Non-aqueous phase occurs when the sorbed phase, aqueous phase, and vapour phase of a chemical have reached saturation in soil. Concentrations above this saturation limit (Csat) for all of the specified chemicals of a mixture result in a non-aqueous phase liquid or solid (US EPA 2000).

 

At contaminant concentration less than Csat, the equilibrium vapour concentration at the contaminant source is proportional to the soil concentration, according to the vapour modelling equation presented by Johnson and Ettinger (US EPA 2004a). When a non-aqueous phase is present however, the vapour concentration at the contaminant source is independent of the soil concentration but proportional to the mole fraction of the individual component of the non-aqueous phase mixture, according to Raoult’s Law (US EPA 2000). It is noted that the calculation of Csat relies on laboratory derived parameters and hence there may be some variability in calculated Csat values and site-specific values associated with the observed presence of non-aqueous phase liquids.

 

Raoult’s Law states that ‘the vapour pressure of each chemical component in an ideal solution is dependent on the vapour pressure of the individual component and the mole fraction of the component present in the solution’.

 

Therefore, as the number of components in a solution increases, the individual vapour pressures decrease as the mole fraction of each component decreases with each additional component.

 

In order to calculate the mole fraction for mixtures, or a solution of compounds, it is necessary to know the concentrations of the individual components comprising the non-aqueous phase liquid. This cannot be achieved by estimating the proportion of components in non-aqueous phase liquid from dissolved phase results. A sample of the free phase liquid should be collected and analysed if practicable.

Mole fraction  =          Number of moles of compound

                                    Total number of moles

Number of moles       =         Concentration of compound in solution

                                                Compound molecular weight

The saturation vapour concentration can therefore be calculated by:

Csat

=

X l ρ l MW

 

 

  R l T

where:

            Csat      =          Saturation vapour concentration (g/cm3)

            X                   =          Mole fraction of chemical in product

            r                    =          Vapour pressure of the chemical (mm Hg)

            MW          =          Molecular weight of compound (g/mole)

            R                   =          Molar gas constant (62, 361 mm Hg - cm3/mole - k)

            T          =          Absolute temperature (293 K for ambient conditions)

Where non-aqueous phase liquid is present, exposure point concentrations may be calculated using models such as NAPL-SCREEN and NAPL-ADV (US EPA (2004a).

4.5              Exposure point concentrations - particulates

Where contamination is present in surface soils or there is potential for near-surface contamination to be brought to surface during construction activities, exposure to particulates (dust) requires consideration.

 

The concentration of particulates relevant for different exposure scenarios can be determined using either a dust concentration factor or a soil particulate emission factor (PEF). The PEF approach is presented in the soil screening guidance (US EPA 1996) and supplemental soil screening guidance (US EPA 2002a). The methodology uses a fixed conservative soil particulate release rate combined with a box model to determine dust concentrations in air. The PEF assumes loosely packed surface soil so that a relatively large concentration of dust is entrained in air. The entrained soil particles are considered to mix in the ambient air breathing zone directly above the soil source. The calculation of a PEF using the US EPA equations may not be applicable to Australian conditions and hence suitability of the equation should be considered prior to use. In addition, the  model does not address exposures associated with dust that is generated from dry, exposed soil, generated during active use of a site (for example, during use of dry sporting fields) or  mechanically generated (such as during vehicle movements). Dust generated during these scenarios may be better estimated using a dust concentration (loading) relevant to the area and nature of activity assessed or alternatively, exposure may be assessed by conducting air quality monitoring.

4.6              Exposure point concentrations - food consumption

Ingestion of contaminated food is a potential pathway for humans to be exposed to hazardous chemicals and, in some site assessments, may be a significant pathway requiring quantitative risk assessment. Guidance on how to calculate the mean daily (contaminant) intake from ingestion of contaminated food is provided in the following hierarchy of sources:

·         Environmental health risk assessment (enHealth 2012a) and Australian exposure factor guidance (enHealth 2012b)

·         Australian total diet study (formerly the Australian market basket survey), Food Standards Australia and New Zealand. Reports are accessible at
www.foodstandards.gov.au/scienceandeducation/monitoringandsurveillance/australiantotaldiets1914.cfm  

·         Exposure factors handbook (US EPA 2011)

·         UK contaminated land exposure assessment model: technical basis and algorithms (DEFRA and EA 2002) and updated technical background (EA 2009).

Patterns in food consumption vary between individuals and groups of individuals and therefore the likelihood exists that some groups of people will have a different diet from the population as a whole (Cross & Taylor 1996). Australian national food consumption data, including percentage of each food type or group consumed on a daily basis is available in enHealth (2012b) and has been compiled from a number of dietary surveys.

 

EnHealth (2012a) strongly recommends that Australian dietary survey data is used in exposure assessments, as overseas diets are likely to be of less relevance, though the algorithms from overseas guidance are applicable to Australia. When using the dietary survey data, it is important to consider the limitations of the survey conducted and the site-specific situation being addressed (particularly in relation to food consumption patterns) within a multiple-lines-of-evidence approach. Additional guidance, including contaminant intake algorithms for different produce that are also applicable to the Australian context, is available in the US EPA exposure factors handbook (US EPA 2011) and EA (2006).

 

When assessing contaminant intake from food consumption, it is important to consider not only the percentage of a particular food group in the diet, but also the percentage of the food group that could potentially have been grown/reared in a contaminated environment.

4.6.1        Fruit and vegetable consumption

In circumstances where home-grown fruit and vegetable consumption is likely to be significant (for example, more than 10% of the diet), the consumption of garden vegetables grown in soil on a contaminated site is likely to represent the main potential transfer of soil contamination to adults and children. An assessment of exposure from this pathway depends on three critical factors: how much contamination is likely to be accumulated by garden vegetables from the surrounding soil, how much home-grown produce is likely to be consumed by those in the household, and how much contamination in food is absorbed by the human body (Paustenbach 2000).

 

Limited published information is available on the percentage of Australian households having domestic fruit and vegetable gardens, and on the percentage of home-grown fruit and vegetables consumed in comparison to those purchased. However, health risk assessments are site-specific and therefore this information may be obtained through discussion with the site owner/occupier at the CSM development stage. In the absence of site-specific information, Cross and Taylor (1996) provide a summary of information currently available.

 

Contaminant uptake behaviour varies markedly between plant species and for different contaminants. Ideally, the concentration of contaminants in home-grown vegetables and fruits should be measured directly on a site-specific basis, but this is often impractical in contaminated land assessments. In the absence of site-specific contaminant uptake data, the chemical concentrations in the edible portions of fruits and vegetables can be predicted from the soil-to-plant concentration factors (CFs), which describe the relationships between the concentrations of contaminants in the soil and plant contaminant concentrations, on a fresh weight basis.

 

A methodology to estimate contaminant intake from fruit and vegetables for home-grown vegetable intakes of up to 10% of the diet is provided in Schedule B7.

 

It is recommended that caution be employed in the evaluation of potential human health risks associated with ingestion of fruits and vegetables using generic predictions of plant uptake. Data is limited, and the methods for predicting plant uptake of contaminants from soil and groundwater data are not well validated, particularly for many organic substances. Further site-specific investigations are likely to be justified in situations where the consumption of home-grown produce is likely to constitute a significant portion (>10%) of the diet of a household or where commercial production of vegetables and/or fruit may occur.

4.6.2        Poultry, meat and fish consumption

The calculation of intake concentrations from the consumption of poultry, meat and fish is complicated and suffers from a number of uncertainties that limit the reliability of the assessment. In order to estimate contaminant concentrations in the potential food source using soil, groundwater and surface water concentrations, the following information is required as a minimum:

·         an understanding of the diet of the exposed population

·         an estimation of contaminant concentrations in the food source consumed by the livestock or fish </