REVIEW OF RISK ASSESSMENT PRACTICE IN THE UK AND ITS APPLICABILITY IN CHINA Mengfang Chen and Liz Waterfall Capita Symonds, Wood Street, East Grinstead, West Sussex RH9 UU Abstract Quantitative risk assessments have increasingly played a major role in regeneration of contaminated land and brownfield sites in the UK. The UK technical guidance for assessing risks to human health and controlled waters is divided into two parts, both issued by the Environment Agency. The potential risks to human health can be assessed using the Contaminated Land Exposure Assessment (CLEA) model, while a separate tiered methodology known as P20 model has been produced for assessment of controlled waters. The UK has experienced an unprecedented change in risk assessment guidance following the release of the CLEA- UK model in November 2005 and DEFRA Way Forward Document (CLAN6/06) in November 2006. CLAN 6/06 outlines a major revision of the CLEA-UK model, presenting changes that are to be made with an immediate effect and other potential changes which are still being reviewed. While anticipating the revision of the CLEA-UK model, an alternative risk modelling approach, that is consistent to the current UK practice for both human health and controlled water risk assessment, is required. At present, the RBCA model for chemical release sites is considered to be the most appropriate model for evaluating health risks from both soil and groundwater contamination when fully integrated with exposure and other relevant parameters defined for UK standard land use scenarios under the CLEA model guidance. This paper firstly provides an overview of current status of risk assessment practice in the UK, secondly describe how the RBCA model can be used to implement CLAN6/06 strategies for human health and P20 guidance for the controlled waters. Finally risk assessment practice in China is briefly reviewed and applicability issues of CLEA-UK and RBCA model in China are examined. Key Words: Contaminated Land Exposure Assessment (CLEA), Risk Based Corrective Action (RBCA), P20 Remedial Target Sheets, Soil Guideline Values (SGV), Generic Assessment Criteria (), Site Specific Assessment Criteria (SSAC), State Environmental Protection Administration (SEPA). Introduction Recent changes in contaminated land and other relevant legislation have created statutory regimes for the identification and remediation of contaminated land in the UK. The legislation includes Part IIA of the Environmental Protection Act 990, Landfill Directive 2002, the PPC Regulations 2002 and Water Framework Directive. The principal feature of these legislations is that the hazards associated with contaminated land should be evaluated in the context of a sitespecific risk based framework. The purpose of the risk-based process is to apply site-specific conditions to site assessment and remediation, and to minimise expenditure while protecting human health and the environment. The UK has experienced an unprecedented change in human health risk assessment guidance following the beta release of UK Contaminated Land Exposure Assessment (CLEA-UK) model in November 2005 and DEFRA Way Forward in November 2006. CLAN 6/06 outlines a major revision of the CLEA-UK model, presenting changes that are to be made with an immediate effect and other potential changes which are still being reviewed []. While anticipating the 5
revision of the CLEA-UK model, an alternative risk modelling approach, that is consistent to the current UK practice for both human health and the controlled water risk assessment is required. This paper primarily reviews technical guidance related to human health risk assessment as no technical issues have been identified regarding the controlled water guidance in the UK. The paper aims to: ) Provide an overview of the current risk assessment practice in the UK including nonstatutory model guidance and risk assessment protocol; 2) Review alternative modelling tools that are required in response to CLAN 6/06 publication; 3) Undertake a limited model validation exercise to illustrate how the RBCA model fully complies with the UK regulatory guidance; and 4) Discuss how the risk assessment guidance in the UK may be applied in China s contaminated land sector. Review of the UK Risk Assessment Guidance The UK technical guidance for assessing risks to human health and the controlled waters is divided into two parts, both issued by the Environment Agency and DEFRA. The potential risks to human health can be assessed using the CLEA-UK model [2], while a separate tiered methodology has been produced by the UK Environment Agency for assessment of controlled waters, known as the P20 model [3] (Table ). In addition to the guidance documents summarised in Table, the Environment Agency also published the Contaminated Land Research (CLR) series of reports and technical materials aiming to provide regulators, developers and other interested parties with authoritative and researched advice and information to assist in identifying, assessing and managing the problems land contamination can pose. The CLR papers relevant to the CLEA are summarised below: CLR 7 Assessment of risks to human health from land contamination: an overview of the development of Soil Guidance Values (SGV) and related research [7]. CLR 8 Potential contaminants for the assessment of land [8]. CLR 9 Contaminants in soil: collation of toxicological data and input values for humans [9] CLR Model procedure for the management of land contamination [0] Human Health Controlled Waters The following guidance documents define a general The following guidance documents define technical architecture for CLEA-UK model originally designed approaches to derive soil and groundwater remedial to derive human health SGV or Generic Assessment targets: Criteria (): R & D Publication P20 [3] CLR 0 The contaminated land exposure General assumptions are summarised below: assessment model: technical basis and Unsaturated zone attenuation not considered in algorithms [2] P20 model CLEA Briefing Note : Human exposure to Dilution by surface water not considered contaminants in soil via the dermal pathway [4] Biodegradation for dissolved phases only CLEA Briefing Note 2 and 3: Conceptual Biodegradation for both the solid and dissolved exposure model for indoor vapour intrusion and phases only sufficient evidence of natural update to building parameters [5] attenuation occurring. CLEA Briefing Note 4: Dealing with multiple exposure pathways [6] Table Summary of UK Risk Assessment Model Guidance 52
CLR 0 provides a basis for the release of CLEA2002 model with software interface programmed using Visual C++. However the CLEA2002 model was designed for Generic Quantitative Risk Assessment (GQRA), thus can not be used to undertake Detailed Quantitative Risk Assessment (DQRA) as parameters related to human exposure and soil properties can not be altered by the user. In recognition to apparent disadvantages inherent with CLEA2002, a beta version of the CLEA UK model was released in November 2005 following the changes in indoor vapour and dermal contact algorithms published as a series of CLEA Briefing Notes (Table ). The CLEA UK model was developed using VBA (Visual Basic Application) associated with Microsoft Excel spreadsheet. Because of the choice of the programming language selected, it has led to a significant increase in computer runtime compared with the CLEA2002 model. As part of the CLEA model development, the Environment Agency have only published 23 toxicity reports and 0 SGV (Soil Guideline Values) reports and they are predominantly for metals [, 2]. Clearly the pace of the production and numerous other technical and practical issues are unlikely to satisfy local authorities and consultants alike. SGVs are intended for local authorities to facilitate the determination of land as contaminated land under the Part IIA regime where there is a significant possibility of significant harm (SPOSH) in relation to human health effects []. SGV Task Force (SGVTF) was formed in 2004 in response to concerns expressed on the production and use of SGV. After a wider consultation and debates, CLAN6/06 Way Forward Document was published by DEFRA in November 2006 []. CLAN 6/06 represents a major revision of the CLEA-UK model and relevant guidance. Two significant recommendations are to change the probabilistic CLEA into a deterministic model and change soil ingestion rate from a probabilistic parameter with a mean value of 00 mg/d and a 95 th percentile value of 300 mg/d to a deterministic value of 00 mg/d for a critical child receptor under residential land use. These changes are likely to have an impact on the already published SGV for metals. Review of Risk Assessment Models There are a number of computer models commercially available that have been developed for the purposes of Quantitative Risk Assessment (QRA) that allow human health risk assessment to be carried in accordance with the regulatory guidance outlined above. These models vary in sets of algorithms used to implement and numerically represent exposure pathways. Also, the models do not all implement the same set of exposure pathways. These two basic differences between the various models mean that both guidance relating to the applicability of the various algorithms and the type of exposure pathways requiring consideration must be evaluated. Consideration must also be given to practical issues such as the user interface, and any limitations known to affect the practicalities of completing a QRA. The EA has formally withdrawn the CLEA2002 software because it is no longer compliant with current technical guidance and lacks the versatility of the more recent CLEA UK model. The CLEA UK model implements the Johnson Ettinger algorithm detailed in 'Briefing Notes 2 and 3' in which default parameters relating to building, air and soil properties are defined [5]. In addition the dermal algorithm within the CLEA model has been changed to the dermal absorption approach that was implemented with the RBCA model (as summarised in CLEA briefing note [4]). It is considered impractical to use the CLEA UK model due to its poor interface design, inconsistency of modelling results, and the prolonged simulation duration for 53
multiple contaminant scenarios. The CLEA UK model also excludes groundwater vapour and controlled water pathways which need evaluation as part of risk based assessment. Exposure Pathway BP Risc (V4) RBCA (V.3b) CLEA UK (beta) Soil Ingestion Identical [2, 4] Dermal Contact Identical (Dermal Absorption Approach) [4, 4] Consumption of Home grown YES (not CLEA vegetable Compliant) NO Yes Indoor Vapour Identical (Johnson & Ettinger) [5] Outdoor Vapours Identical (Subsurface Option) [2, 4] Particulates/Dusts NO GSI Model [4] USEPA (6) Soil Leaching to Groundwater Yes (including saturated soil model) Yes (P20 Compliant with limited modification) [4, 7] Not Implemented Groundwater Migration Offsite Yes Yes Not implemented GW Outdoor Vapour (Onsite and Offsite) Yes Yes Not implemented Groundwater Vapour (on Site) Yes Yes Not implemented ) RBCA V2 is due to be released in June 2007 which implements plant uptake pathway (personal communication with GSI Inc). Table 2 Comparison of RBCA, BP Risc and CLEA UK model At present, the RBCA model for chemical release sites, developed by the Groundwater Services Inc [3, 4] is considered to be the most appropriate model for evaluating health risks from both soil and groundwater contamination when it is fully integrated with exposure and other relevant parameters defined for the UK standard land use scenarios under the CLEA framework. Though the current version of RBCA model (Version.3b) excludes a plant uptake pathway, Version 2, to be released shortly, will allow flexibilities to implement the UK compliant algorithm for calculating soil to plant concentration factors. BP RISC uses many of the same algorithms as RBCA and would seem to be equally as applicable, however, practical issues such as being limited to 20 contaminants in any one model indicate the RBCA model to have practical advantages for use in relation to risk assessment. Comparison of the RBCA, BP Risc and the CLEA UK model is summarised in Table 2. Framework for Contaminated Land Risk Management A pragmatic approach to contaminated risk assessment can transform what may sometimes appear to be an extremely detailed, complex and resource-intensive process into a practical aid to decision-making [8]. Figure provides a general framework for a tiered approach to contaminated risk assessment and management. Tier GQRA in which Generic Assessment Criteria are derived under certain generic conditions mainly related to land uses and soil properties. Tier 2 DQRA in which Site Specific Assessment Criteria (SSAC) are derived by incorporating site specific parameters such as depth to the contaminated soil, fraction of organic carbon and hydraulic parameters in case of groundwater. Site specific remedial targets (SSRT) can then derived to assist in developing appropriate remedial strategies following the integration of human health and controlled water SSAC. Both the CLEA UK and P20 models are applied in Tier GQRA and Tier 2 DQRA. It should not be confused with a multi-tiered structure in relation to migration pathways employed by the P20 model. Tier 3 DQRA may be required for sites where groundwater needs treatment. Numeric models such as USGS MODFLOW and related contaminant transport 54
models such as RT3D and MT3DMS are used to evaluate remedial alternatives and conduct cost and benefit analysis. Problem Formulation Exposure Assessment Toxicity Assessment Risk Characterisation INCREASING COMPLEXITY & LEVEL OF EFFORT Tier (GQRA) Assessment under generic conditions - toxicity based - fate and transport -Generic - relevant pathways - groundwater excluded Soil: SGV CLEA UK GW: P20 Tier 2 (DQRA) Risk based site specific criteria - SSAC -adjustment of based on site specific information - more realistic - less conservative Soil: CLEA UK GW: P20 INCREASING NEED FOR REALISM AND Tier 3 (DQRA) Assessment of controlled water Risks (SSAC) - detailed fate, transport and exposure modelling -realistic - in-situ management - long term monitoring MODFLOW, RT3D MT3DMS Risk Based Remedial Strategy REDUCED CLEAN-UP COST Figure Flow chart illustrating framework for contaminated land risk management Each tier comprises four key stages in which potential human health risks can be quantified: problem formulation, exposure assessment, toxicity assessment and risk characterisation. Problem Formulation The identified soil contamination and the predicted land use are used to formulate the site conceptual model, which highlights the significant pollutant linkages (source, exposure pathways and critical receptors). Exposure Assessment The objective of the exposure assessment is to estimate the type, magnitude and duration of the exposure given media concentrations. In this assessment, average inhalation exposure concentration (AIEC) is calculated using the equation below: C VF EF ED AIEC = AT 365 Where C is the representative soil (mg/kg) or groundwater (mg/l) concentrations; EF, the exposure frequency (days/year); ED, the exposure duration (years); AT, the averaging time (years); VF, the volatilisation factor from soil to indoor vapour (kg/m 3 ) or from groundwater to indoor vapour (l/m 3 ). For exposure related to soil ingestion and dermal contact, average daily intakes (ADI) are calculated using the equation below: 55
( IR + SA M RAF) EF ED ADI = AT BW Where IR is the soil ingestion rate (mg/d); SA, the skin surface area for soil dermal contact (cm 2 ); M, the soil to skin adherence factor (mg/cm 2 /d); RAF, the relative adsorption factor for soil dermal contact (unitless); EF, the exposure frequency (days/year); ED, the exposure duration (years); AT, the averaging time (years); BW, the body weight (kg). Toxicity assessment A toxicity data review is central to quantitative risk assessment as it is chronic health effects as opposed to acute effects that are being considered. Reliable toxicological data is thus an essential component in developing a conservative model. Key toxicity parameters to be collated are Health Criteria Values (HCV): Tolerable Daily Intakes (TDI) for non-carcinogens and Index Dose (ID) for carcinogens: Tolerable Daily Intake is a threshold intake of a chemical by human, below which no adverse health effect will occur. Index Dose is an intake of a chemical representing minimum risk levels subjected to ALARP principals (i.e. as low as reasonably practicable). A target risk level of E-5 is generally considered acceptable in the UK, which has been used to derive Index Dose for benzene by DEFRA and the Environment Agency [2]. UK guidance requires that the MDI (mean daily intake) is considered when deciding upon a suitable TDI. The MDI is a measure of the background exposure of the particular contaminant that a receptor is likely to be exposed to. This is important because TDI s and MDI s are not additive as TDIs are generated based on laboratory studies. MDI should be subtracted from the TDI to produce a revised Tolerable Daily Soil Intake (TDSI). Where a MDI is not available then it is considered prudent to set the TDSI at 0.2 of the TDI [9]. This ensures that background exposure is accounted for where it is not known. Risk Characterisation The final step of the risk assessment is to compare the average exposure concentrations for the various pathways with relevant HCV to determine levels of the risk posed to the identified receptors. Soil and groundwater are derived by setting: AIEC/RfC = for inhalation pathways and ADI/TDSI = for other exposure pathways Implementation of RBCA Model within the CLEA Framework Human Health A number of RBCA manual adjustments are required to comply with the CLEA UK Model Guidance as described below: Because the RBCA model uses RfC for inhalation exposure, TDSI for noncarcinogens and ID for carcinogens need be converted using the equation below to RfC using the UK specific respiration volume and body weight under standard UK land uses. 56
( HCV BW ) RfC = RV Where RfC is the reference concentration (mg/m 3 ); HCV, health criteria values (i.e. TDSI or ID (mg/kg body weight/day); BW, the body weight; RV, the respiration volume (m 3 /d). The Henry's Law dimensionless constant characterises the equilibrium distribution of dilute concentrations of volatile, soluble chemicals between gas and liquid. The constant is temperature dependent and should been corrected for the UK ambient soil temperature of 0 C. The constants are converted using a method developed by the EPA Office of Solid Waste and Emergency Response, which is outlined in the USEPA Soil Screening Guidance [6]. RBCA calculates indoor Volatilisation Factors (VF) using Johnson and Ettinger algorithm [5] and mass balance equation [4]. However only Johnson and Ettinger algorithm is UK compliant. To eliminate the mass balance equation, the depth to the base of contaminated soil is set to 00,000 m to force RBCA to use Johnson and Ettinger model for soil indoor pathway. Similar to the calculation of VF for indoor air, the thickness of the source is set to a large number resulting in the non UK compliant mass balance equation being rejected by the model. For many contaminants significant exposure may potentially arise from simultaneous exposure of inhalation of outdoor vapours and particulates, soil ingestion and dermal contact. Although one route may be dominant, the other routes of entry may make large contributions to total exposure. In order for a combined assessment criteria to be protective of human health this additional exposure must be taken into account. The combined as result of these simultaneous exposures is calculated using the equation below [6]: For Soil soil = oral + dermal + indoor _ inhalation + outdoor _ inhalation For Groundwater gw = indoor _ inhalation + outdoor _ inhalation Controlled Water RBCA model can be made to comply with the Environment Agency P20 model guidance [3, 4, 7]. A fundamental difference between the P20 and RBCA model is the representation of soil leaching process. The P20 uses a simple partition equation from soil to pore water in the Tier soil and groundwater dilution is modelled in the Tier 2 Soil. No attenuation processes within the unsaturated zone are modelled in the P20 model for the purpose of conservatism. However, in the RBCA Tier, additional attenuation processes within the unsaturated zone are modelled by using ASTM Soil Attenuation Model (SAM). The SAM is defined as L/L2 (L, thickness of contaminated soil; L2, distance from the top of contaminated soil to water table). RBCA model also has an option to select ASTM Soil Model in which case SAM is set to and RBCA is identical to P20 model. Equations used in the P20 and RBCA model, together with parameter description are illustrated in Table 3. 57
Exposure Pathway RBCA Tier Soil Leaching (Based on Controlled Water Assessment Criteria) P20 Tier and Tier 2 Soil (Combined) Soil Leaching (Based on Groundwater Assessment Criteria) Equations to Calculate Soil Remedial Target Levels V σ GWAC gw gw ( k + θ + ρ + H θ ρ I W ws s s as)sam s GWAC + ρ s I W V σ gw gw ( θ + k ρ + H θ ) ws s s 58 as Parameter Descriptions GWAC, Groundwater Assessment Criteria (mg/l); ρ, soil bulk density (g/cm 3 ); I, infiltration (mm/year); W, width parallel to groundwater flow (m); V gw, groundwater Darcy velocity (m/d); σ gw, groundwater mixing zone thickness; k s, soil water sorption coefficient (l/kg); θ ws, volumetric water content in soil (-); θ as, volumetric air content in soil (-); H, Henry's Law constant (-); SAM = L/L2; L, Thickness of contaminated soil; L2, distance from top of contaminated soil to water table. Table 3 Comparisons of P20 and RBCA Model for Soil Leaching For groundwater migration pathway, P20 model uses the Ogata Banks equation [7] without consideration of retardation factor as the Environment Agency s preferred option to derive remedial targets for the purpose of conservatism. However RBCA uses the Domenico equation (the simplified version of Ogata Banks equation) incorporating retardation process [4]. In order to comply with P20 guidance the half lives in RBCA model are multiplied by retardation factor. The effect of this is equivalent to applying biodegradation in dissolved phase only in P20 model. Given an identical set of physical and chemical parameters, remedial targets calculated from both RBCA and P20 model are identical for contaminants with no half-life such as metals and marginally different for organic contaminants. Model Validation In order to demonstrate the compliance of RBCA model with CLEA UK and P20 models, a limited validation are undertaken under generic UK residential (without garden) for human health and a postulated scenario of groundwater migration to surface water. Impact of implementing soil ingestion rate of 00 mg/d on the published SGV for a child receptor under residential land use as recommended by CLAN6/06 is also evaluated. Input Parameters Parameter values for exposure, receptor characteristics, air and building environments and soil properties were taken from CLR0, CLEA Briefing Notes, CLEA UK model handbook and CLAN 6/06 [, 2, 3, 4, 9]. Groundwater properties are assumed for the purpose of validation (Table 4). Physio-chemical and toxicity parameters used in the derivation of were taken from SGV and TOX reports and other relevant regulatory guidance [, 2, 20]. Parameters Unit Value Plume Width m 50 Plume Thickness at Source m 3 Saturated Aquifer Thickness m 3 Bulk Density of Aquifer Materials g/cm 3.6 Effective Porosity of Aquifer - 0.3 Hydraulic Gradient - 0.005 Hydraulic Conductivity of Aquifer m/d 25 Distance to Compliance Point m 00 Fraction of Organic Carbon - 0.00 Note: ) Parameter values are assumed for the purpose of the validation. Table 4 Groundwater Properties for Groundwater Migration
Validation Results Results of the validation for human health and controlled water are summarised in Tables 5 and 6 respectively. Implication of implementing CLAN 6/06 recommendations using deterministic RBCA and soil ingestion rate 00 mg/d under residential land use has also been evaluated, results of which are summarised in Table 7. COC Soil Guideline Values (mg/kg) % SOM 2.5% SOM 5% SOM CLEA RBCA CLEA RBCA CLEA RBCA Toluene 3 3.3 8 7.74 5 5.3 Ethylbenzene 6 6.9 4 4.07 80 8.35 Note: SGV was rounded figure. Table 5 Comparisons of SGV derived using RBCA and CLEA-UK Remedial Targets for Groundwater Migration Contaminant of Concern (mg/l) P20 (Version 3.) RBCA (Version.3b) Ethylbenzene 0.059 0.060 Toluene 9.7 9.7 Arsenic 0.0734 0.0734 Cadmium 0.00734 0.00734 Chromium 0.00734 0.00734 Mercury 0.0047 0.0047 Nickel 0.0734 0.0734 Selenium 0.047 0.047 Table 6 Results of P20 Model Validation for Groundwater Migration RBCA Implementation of CLAN 6/06 Published SGV Contaminant of Concern (Residential without garden) Recommendations: Deterministic Mode with Soil Ingestion Rate of 00 mg/d mg/kg Arsenic 20 32 Cadmium 30 56 Chromium 200 240 Mercury 5 2 Nickel 75 93 Selenium 260 340 Note: ) 5 th percentile body weight of.5 kg of female children was used to evaluate the implication of CLAN 6/06. Table 7 Implementing CLAN 6/06 Using Deterministic RBCA Model It can be concluded that RBCA is fully compliant to the UK regulatory models in ) successfully reproducing the published SGV for ethylbenzene and benzene calculated from the CLEA-UK model (Table 5), and 2) groundwater remedial targets matching to those calculated from P20 model (Table 6). It is shown that implementing CLAN 6/06 recommendations would lead to a significant increase in the published SGV for metals (Table 7). 59
Applicability of UK Risk Assessment Guidance in China Current Status Risk based environmental assessment is not new in China. In 999, State Environmental Protection Administration (SEPA) published environmental quality risk assessment criteria for soil at manufacturing facilities for 89 contaminants including metals, pesticides, petroleum hydrocarbon, poly-aromatic hydrocarbon and chlorinated solvents [2]. These standards were linked with human toxicity and soil leaching potential on the groundwater, but appeared to have only addressed soil ingestion and dermal contact pathway in respect to human health. These soil criteria have apparently neglected indoor vapour pathway being a driver for remediation for volatile contaminants. Quality standards for soil were published in 996 aiming to protect allotment, forestry, crop, and orchard [22]. It is unclear how soil criteria were derived, but these were certainly not toxicity based. Environmental quality standards for 68 contaminants were also available for surface water published in 999 and revised in 2002 [23] and only inorganic contaminants for groundwater published in 993 [24]. China has developed the most comprehensive environmental law and legislation both on state and provincial levels. But there appeared to have no policy guidance documents in application of risk based approaches in China. But the situation will change dramatically in next 0 and 20 years following the World Health Organization (WHO) sponsored Forum in National Environment and Health in November 2005 [25]. As a result an action plan for national environment and health was drafted with major aims to establish relevant environmental law and guidance documents, and publish health based assessment criteria. Already, a key research project is underway in 2005 to study health effects of arsenic, lead, mercury and cadmium in soils; the training course on human health risk assessment theory was delivered in April 2006 in collaboration with US Copper Development Association, attended by 60 research scientists; and 3) Formation of national expert panel was called in January 2007. The action plan also recognised skill shortages in risk assessment and the need for international cooperation. It is with no doubt that China will benefit greatly by learning from US and UK experience in risk based practice which span over 30 years. Applicability Issues The CLEA-UK model represents one of the most sophisticated human health risk models in Europe. The recent CLAN 6/06 recommendations has brought the UK approaches in line with those implemented in the RBCA model that originated from US ASTM standard guide for Risk Based Corrective Action (RBCA). All these models employ a similar set of algorithms in estimating average daily exposures from soil contamination. There are no scientific reasons why China can not adopt these algorithms in its guidance document. However a direct application of the model in China requires review and certain modifications in the following key areas: Conceptual exposure model: CLR 0 has defined four generic land uses (residential without gardens, residential with garden, allotment and industrial/commercial) each with a set of exposure, human, building and air parameters. It is almost certain that these land uses will not be suitable to use in China. China-specific land uses and 60
exposure characteristics need be defined as a priority in the forthcoming guidance documents. Toxicity data: the US and UK have contrasting views on the use of toxicity parameter for carcinogen compounds [2, 26]. The US uses a slope factor to describe a risk level per unit intake and thus require setting acceptable target risk levels when estimating exposures from soil. However, the UK prefers the use of index dose based on the ALARP principal [9]. Both the US and UK terms are not dissimilar mathematically. China is likely to adopt the slope factor approach as it has already been used to derive soil criteria for a manufacturing site [2]. Dealing with multiple exposures: when deriving or SSAC for a multiple exposure scenario, there are two recognised approaches to choose from: the minimum in the US [3] and the harmonic mean in the UK [6], with the later being relatively conservative. Soil temperature: many countries in Europe and US use standard temperature of 25 C when the most of physio-chemical parameters are derived. However the UK has an average soil temperature of 0 C, which was used in the CLEA model. Soil temperature will impact exposure calculations for vapour contaminants. Background exposure: the UK guidance requires that the MDI (mean daily intake) is considered when deciding upon a suitable TDI for non-carcinogenic or threshold contaminants. The failure to consider background exposure would result in underestimating exposures in the risk assessment. Site assessment: the UK guidance (CLR 7) requires the application of mean and maximum tests in an averaging area to determine widespread and hotspot contamination respectively. Summaries and Conclusions The UK has experienced an unprecedented change in risk assessment guidance following the release of CLEA-UK model in November 2005 and DEFRA Way Forward Document (CLAN6/06) in November 2006. While anticipating the revision of the CLEA-UK model, an alternative risk modelling approach, that is consistent to the current UK practice for both human health and the controlled water risk assessment, is required. The RBCA methodology for chemical release sites is considered to be the most appropriate model for evaluating health risks from both soil and groundwater contamination when it is fully integrated with exposure and other relevant parameters defined for UK standard land use scenarios under the CLEA model guidance. Example case studies were provided to illustrate how the RBCA model can be used to implement CLAN6/06 strategies for human health and P20 guidance for the controlled waters. Applicability issues of CLEA-UK and RBCA model in China have been examined and key areas requiring debate and modifications have been identified. Acknowledgements The authors wish to thank Mr. Neil Greenwood and Dr. Robert Hares of Capita Symonds Contaminated Land and Regeneration Team in East Grinstead who have provided critical review of the paper. 6
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