June On behalf of: Harrow Estates Plc

Transcription

1 Further Quantitative Risk Assessment for Controlled Waters and Preliminary Post-Remediation Validation Model Former Bayer Crop Science Site Hauxton Cambridgeshire June 2011 On behalf of: Harrow Estates Plc Vertase F.L.I. Limited 3000 Aviator Way Manchester Business Park Manchester M22 5TG Tel +44 (0) Fax +44 (0)

2

3 Executive Summary The report has been prepared to reassess the risk assessment with respect to controlled waters at the former BayerScience site, Hauxton following the collection of further data during the ongoing remediation. This report describes the site setting and outlines previous work undertaken to date with respect to the controlled waters risk assessment. It takes on board information gathered during the current remedial works and builds upon previous investigation data to provide a revised conceptual site model for the post remediation conditions that has significantly more certainty with regard to volumes and types of soil material and thus the geometry of restored soil horizons compared to previous models. This assessment follows the previous risk assessment (Further Quantitative Risk Assessment for Risks to Groundwater and Surface Water, Former Bayer Crop Science Site, Hauxton, Cambridgeshire February 2011) and has been adapted to the new post remediation conceptual site model for and expanded to cover all contaminants of concern. This is now a major step towards the final post remediation risk assessment model that was discussed in the remediation method statement. This assessment will now be updated on a continual and ongoing basis with site data from the ongoing validation. An appropriate level of conservatism and variation within parameters has been built in to allow for any future variability in the treated soils at site. Geotechnical and hydrogeolgical parameters for the treated soils have been assessed through laboratory and in-situ testing. These site specific parameters have subsequently been used in the risk assessment model to enable the potential risks from any residual contaminants in the reinstated remediated soils to be assessed. The output from the model represents the maximum concentrations in soils and leachate that would not result in an exceedance of the selected compliance target at the receptor, the Riddy Brook (i.e. the Environment Agency Environmental Quality Standards or equivalent.) These maximum concentrations will be used as such and in most cases material will be treated much below these concentrations to ensure that the risks are minimised further. The actual achieved concentrations post treatment so far can be as much as several orders of magnitude lower than these maximum concentrations. In order to be protective of human health also a set of working targets has been prepared to ensure remediated soils are both acceptable within the controlled waters risk assessment and that of human health. These are presented in separate documents. We will now continue to build on this model and will add further data to the model as we continue to progress with validation and restoration of material. Once material is restored at the site we wil have final residual contaminants of concern concentrations in the reinstated soil materials, updated geotechnical and hydrogeological parameters and fraction organic carbon which will be entered into the models as probability density functions (or PDF s) to represent actual material at the site. The model will be re-run periodically to ensure that no risk to controlled waters remain resulting ultimately in a post remediation model accurately reflecting ground conditions at the site.

4 Table of Contents 1.0 Introduction Limitations Remediation Strategy and Approach to Risk Assessment Environmental Setting Site Description Pre Remediation Conditions Pre-Remediation Ground Conditions Existing Remedial Measures Hydrogeology Hydrology Observed Conditions During Remedial Works Ground Conditions Additional Contaminant Source Hydrogeology Review of Previous Reports Enviros Consulting Part IIA Site Investigation Report for Bayer Crop Science, January Atkins Environment, Remediation of Former Bayer Site, Hauxton, Preliminary Conceptual Model Report August Atkins Environment, Former Bayer Site, Hauxton, Groundwater Modelling Report, June MODFLOW QRA Vertase FLI, Further Quantitative Risk Assessment for Risks to Groundwater and Surface Waters, January Summary of Previous Works Conceptual Site Model Original CSM Post Remediation Site Conditions Ground Conditions Groundwater Flow Through the Site Aquifer Parameters Post Remediation CSM Contaminant Source Pathways Receptor Selection of Risk Assessment Software and Modelling Approach Environment Agency Remedial Targets Methodology Assessment of Risks from Soils Assessment of Risks from Contaminated Groundwater Conceptual Model Comparison of EA RTM and Site Conceptual Site Model Risk Assessment Approach Applicability of EA RTM Risk Assessment Levels Selection of Modelling Software Assessing Risks from Type B and C Soil Material Assessing Risks from Type A Soil Material i

5 7.0 Model Inputs Contaminant Sources Receptors Selection of Screening Criteria Selection of Aquifer Pathway Parameters Level 3a Assessment Aquifer Thickness Dry Bulk Density Mixing Zone Thickness Hydraulic Conductivity Effective Porosity Hydraulic Gradient Groundwater Flow Direction Dispersivity Fraction of Organic Carbon Summary Model Settings Model Correlations Soil Source Parameters Level 1 Assessment Contaminant Parameters Selection of Parameters Model Outputs Leachate/Groundwater Maximum CoC Threshold Values Soil Maximum CoC Threshold Values Comparison of Leachate and Soil CoC concentrations ConSim Level 1 Assessment Comparison of Soil Threshold Values Summary Further Work ii

6 Appendices Appendix A Appendix B Vertase FLI Drawings Drawing D907_87 Surveyed Geological Section Drawing D907_154 Post Remediation Conceptual Site Model Drawing D907_134 - ConSim Graphical Input of Source Zones and Receptor, Type B and C model Drawing D907_169 ConSim Graphical Input of Source Zones and Receptor, Type A model Atkins Drawings Drawing /002, Conceptual Site Model for Proposed Site Development, July 2006 Drawing /Figure 5, Groundwater Contours (maod) April 29 th 2006 Drawing /Figure 6, Groundwater Contours (maod) May 11 to 16 th 2006 Drawing /Figure 7, Groundwater Contours (maod) 16 th December 2006 Appendix C Appendix D Appendix E Water Pumping Volumes Geotechnical Results Soil Source Parameter Supporting Data Fraction of Organic Carbon Moisture Content Appendix F Appendix G Appendix H Appendix I Contaminant of Concern Input Value Justifications Contaminants of Concern Literature Values ConSim Model Inputs Type A Material ConSim Model Inputs Type B and C Material Appendix J Relationship Between Soil and Leachate CoC iii

7 Concentrations Appendix K Data CD ConSim Model Outputs iv

8 1.0 Introduction VertaseFLI have been appointed by Harrow Estates Plc to undertake remedial works at the former Bayer Crop Science agrochemicals works in Hauxton, Cambridgeshire (the site). The site has been determined as a Special Site under Part IIa of the Environmental Protection Act (EPA) 1990 due to identified significant pollutant linkages being present with respect to groundwater and surface water resulting from the former use in the production and storage of agrochemicals. The majority of the buildings and structures at the site have been demolished, and at the time of writing, remedial works comprising the excavation of contaminated soil material, the formation of biopiles (including the addition of organic matter) and turning of the contaminated soil material were being undertaken. On completion the remediated soils will be reinstated at the site and the site will be developed for primarily residential use. In order for the remediated soil material to be reinstated, it must be demonstrated through the development of an appropriately detailed risk assessment, that the reinstated soils do not present a significant risk to local receptors such as groundwater and surface water. The initial risk assessment and remedial targets were based on site investigation data collected prior to remediation. As the remedial works have progressed, more data has been obtained on the volumes and properties of the different soil material present at the site so that a more detailed and representative post remedial conceptual site model can be developed. Therefore, in accordance with the remediation method statement (VertaseFLI, Apr 2010) this report reassesses and where necessary revises the risk assessment for the site in light of the additional data collected during the remedial works. 1.1 Limitations It is important to note that this document refers to the CSM with respect to the controlled waters risk assessment only. A human health risk assessment has been undertaken and presented in the report, Derivation of Pesticide Soil Screening Values & Evaluation of the Spatial Extent of Contamination (Atkins, July 2007). It is also discussed in the VertaseFLI Remediation Strategy Report (VertaseFLI, Nov 2008). 1

9 2.0 Remediation Strategy and Approach to Risk Assessment The main strategy for the remediation at the site was to excavate all materials at the site to ensure that all uncertainty regarding contaminants and geological conditions are removed. All excavated soil material was to be segregated, classified and treated as appropriate before being reinstated and validated. Any contaminated groundwater was to be separated, treated and disposed from the site under discharge consent. Following the remediation and reinstatement of soils, a clean cover system will be imported from off-site above the finished levels. Paragraphs 6.3 and 6.4 of the Remediation Strategy set out the approach to be used in developing the risk assessment with respect to environmental receptors as follows: 6.3 An important part of the approach of our remedial strategy will be to collect further information on the geology, hydrogeology, contamination, material parameters and characteristics during the remedial works. It is our intention that this information will be used to further develop the site model to reevaluate the remediation targets. This will be continually re-assessed as the remediation is continued and may ultimately result in the preparation of a numerical model that represents exact site conditions with a high degree of certainty to prove that materials present on site post remediation do not represent a significant risk of significant harm to the environment and that adequate remediation works have been completed to satisfy the requirements under Part IIa of the Environmental Protection Act This modelling approach will be calibrated by site based monitoring and the mode calibrated appropriately. It does mean that some material will be replaced at the site that does not meet the present generic criteria but through the remediation we will have detailed data and knowledge of this material which will allow clearer understanding of the site. This knowledge and understanding will be used to present a new conceptual model and appropriate risk assessment. Therefore, an iterative approach has been used to allow the groundwater risk assessment to be further developed as more data becomes available during the risk assessment, so that the post remediation site conditions and conceptual model, and therefore the site specific remedial targets, can be developed with as much accuracy as possible. The level 2

10 of data now available has enabled a robust model for all contaminants of concern to be developed with revised risk based remediation targets to work towards. As the remedial work continues and soil material is reinstated, the validation data for the reinstated soils will be incorporated into the risk assessment model as part of the validation process so that on completion of the work the final model will accurately represent the remediated conditions at the site. 3

11 3.0 Environmental Setting 3.1 Site Description The site is situated approximately 200 m northwest of the village of Hauxton (National Grid Ref TL ), and covers an area of approximately 9 hectares. It was previously occupied by Bayer CropScience and used for the production and storage of agrochemicals including pesticides, insecticides and herbicides. The site is bounded to the west by the A10 trunk road beyond which is agricultural land and the Waste Water Treatment Plant (WWTP) for the Site. The northern and eastern site boundaries are formed by the Riddy Brook, with Church Road forming the southern site boundary and the southeast of the site bounded by agricultural land. The site is generally level with a ground elevation of between 12 and 13 m AOD. 3.2 Pre Remediation Conditions Pre-Remediation Ground Conditions Based on the information provided by Enviros (2005) and Atkins (2006), the preremediation ground conditions at the site are presented in Table 1. Table 1: Ground Conditions Description Made Ground (consisting of reworked sand and gravel, chalk marl, alluvium, brick rubble and clinker), foundations, drainage features and voids Superficial Deposits Alluvium and River Terrace Gravels West Melbury Marly Chalk Formation (WMMCF) Marly chalk with thin limestone bands, typically described in available logs as a stiff light grey clay Gault Clay Thickness Typically up to 2 m bgl, with a maximum thickness of 5 m Generally < 3 m thick where present. Completely replaced by Made Ground in parts of the site Present in the south and northwest of the Site only. Typically less than 3m thick with a maximum thickness of 7m in some areas. Typically present at a depth of 5 m bgl underlying Made Ground/Superficial Deposits/WMMCF, the thickness is understood to be up to 50 m (based on historic borehole data presented in Atkins (2006) 4

12 Lower Greensand m Existing Remedial Measures Prior to the current remedial works at the site, the following measures had been undertaken at the site to control groundwater: A bentonite and cement cut-off wall was constructed along the Riddy Brook. It is understood to have been installed in 1974; A groundwater abstraction system was in use in the north of the site to prevent the migration of contaminated groundwater to the north of the bentonite wall. The system comprised several sumps which were pumped to the waste water treatment works (WWTW) to the north of the site; and A dewatering system was present in the south of the site associated with the warehousing in this area. The construction of the warehouse and associated loading areas is understood to have included the excavation of soil material such that they were constructed below the natural water level on the site. As a result, a dewatering system comprising a sump was constructed to artificially lower the groundwater level in this area with all waste water pumped to the adjacent WWTW Hydrogeology Geological Units Made Ground and Drift Deposits The natural drift deposits at the site comprise River Terrace Gravels and Alluvium and are classified by the Environment Agency as a Secondary A Aquifer which are described as: permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. These are generally aquifers formerly classified as minor aquifers. The granular Made Ground was considered by Atkins to be part of the same hydraulic units for modelling purposes. The presence of cohesive Made Ground was considered to be associated with the presence of perched groundwater. 5

13 West Melbury Marly Chalk Formation The Lower Chalk (which includes the WMMCF) is classified by the Environment Agency as a Primary Aquifer. A Primary Aquifer is described as: These are layers of rock or drift deposits that have high intergranular and/or fracture permeability - meaning they usually provide a high level of water storage. They may support water supply and/or river base flow on a strategic scale. In most cases, principal aquifers are aquifers previously designated as major aquifer. However, the investigations undertaken by Enviros and Atkins indicated that the WMMCF was largely absent from the centre and north of the main site. The WMMCF had a high content (typically described in available borehole logs as a stiff light grey clay) and was considered by Atkins to have poor aquifer properties Gault Clay The underlying Gault Clay is considered to act as an aquiclude, preventing continuity between any shallow groundwater present in on the site and the Lower Greensand which underlies the Gault Groundwater Levels and Flow Direction Groundwater was typically present at depths between 0.69 and 2.42 m below ground level (bgl) with an average depth on the site of 1.3 m bgl. Based on the available site investigation data, pre remediation, groundwater flow was assumed to occur within the granular Made Ground and drift deposits, site infrastructure, and within the dis-continuous sand and gravel lenses within the underlying WMMCF. The local groundwater flow direction was generally from the south and west towards the Riddy Brook and the River Cam. However, groundwater flows on site were significantly altered due to the presence of a bentonite cut off wall along the northeast site boundary and the ongoing abstraction of groundwater in both the north and the south of the site. As can be seen from the 2006 Atkins groundwater contours presented in Appendix B, a groundwater low was present in the south of the site as a result of the dewatering/groundwater abstraction in this area which resulted in groundwater flow towards this area and a steeper hydraulic gradient over much of the site compared to off-site. Following remediation including the removal of the bentonite wall and cessation of groundwater abstraction, it is anticipated that groundwater flow across the site will generally be to the northeast, towards the Riddy Brook. 6

14 3.2.4 Hydrology The Riddy Brook and Hauxton Mill Race are the closest water bodies to the site, forming much of the northern and eastern site boundary. The Riddy Brook and Hauxton Mill race meet immediately to the north of the site where they enter the River Cam. Shallow groundwater at the site is considered to be in direct continuity with the Riddy Brook. Due to the existing bentonite wall and groundwater abstraction system, the site does not currently contribute significantly to the Riddy Brook base flow. 3.3 Observed Conditions During Remedial Works As the remediation has progressed, the excavation of soil material across the site has allowed a far more detailed understanding of the pre-remediation ground conditions including the extent of contamination, and the hydrogeological properties of the soil material to be developed. The following sections describe the additional observations made during the remediation of the site so far Ground Conditions The excavation of soil material as part of the remedial works has generally confirmed the distribution and thickness of the Made Ground, WMMCF and Gault Clay described by Enviros and Atkins. The presence of natural superficial deposits typically comprising sand and gravel was also confirmed. The excavations showed the deposits to have been discontinuous across the site, as described in the previous site investigations, and comprised several large shallow lenses of sand and gravel with a maximum thickness of 3m. Thin discontinuous lenses of sand and gravel were also identified in the WMMCF. Drawing 907_85 in Appendix 1 shows a surveyed section of the excavation with exposed isolated sand and gravel lenses present within the surrounding stiff grey clay material. It is important to note, that post remediation, the ground conditions and properties may be significantly different to the pre-remediation conditions due to the homogenisation of soil material during the remediation and the compaction on reinstatement Additional Contaminant Source The excavations generally confirmed the contaminant distribution identified in the previous reports with contaminant levels generally highest in the former process areas. However, during the excavations a discreet lens of sand and gravel in the top of the WMMCF was identified to the northeast of the site (surveyed on Drawing 907_85), adjacent to the Riddy 7

15 Brook which also contained between 20 and 30 corroded steel drums. The sand and gravel was associated with significantly elevated contaminant concentrations. The sand and gravel lens extended to the Riddy Brook and may have acted as a direct contaminant pathway until the bentonite wall was constructed, On discovery, the drums were appropriately disposed of and the associate sand and gravel moved to a treatment bed Hydrogeology Prior to remediation, groundwater flow to the Riddy Brook through the site was assumed to be through granular Made Ground and granular drift deposits with the majority of any flow through the discontinuous lenses of sand and gravel within the WMMCF as shown in the original Atkins CSM (Appendix B). However, as described in and 3.3.1, the WMMCF predominantly comprises stiff clay with thin isolated discontinuous lenses of sand and gravel. The full thickness of WMMCF has been exposed in the sides of the remediation excavations. Based on the exposed sections, groundwater flow within the in-situ WMMCF surrounding the site is very low with any flow generally occurring as seepages through the discontinuous sand and gravel lenses. These seepages through the exposed lenses typically decrease with time suggesting negligible recharge through natural strata as would be expected given the discontinuous nature of the sand and gravel lenses and the surrounding stiff clay/marl. As a result, the excavations which are to depths significantly below the natural water table over much of the site, have largely remained dry (with the exception of rainfall) for the duration of the remediation so far. In areas where excavations have not commenced groundwater flow appears to be largely associated with the presence of drainage features and other site infrastructure, and the presence of granular Made Ground following periods of rainfall. The pumping system in the north of the site is no longer in use. The remaining pumping system in the south of the site receives all water from the site drainage system. The sump for the pumping system is located adjacent to a lens of sand and gravel and this is likely to contribute to the total volume of water removed from the site. 8

16 4.0 Review of Previous Reports A number of investigations and risk assessments have been undertaken at the site since For clarity, a summary is provided in the following sections of some of the previous work carried out. The following reports relevant to the assessment of risks to groundwater and surface waters were made available to VertaseFLI for review: Enviros Consulting Ltd, Part IIA Site Investigation Report, Report for Bayer Crop Science, January 2005, Reference SH A; Atkins Environment, Remediation of Former Bayer Site, Hauxton, Preliminary Conceptual Model Report Prepared for Bridgemere UK Limited and Harrow Estates Plc, August 2006; Atkins Environment, Former Bayer Site, Hauxton, Groundwater Modelling Report, Prepared for Harrow Estates Plc, June 2007; and Vertase FLI, Former Bayer Crop Science Site, Hauxton Further Quantitative Risk Assessment for Risks to Groundwater and Surface Waters, February 2011,. In accordance with the Remediation Strategy, a review of the previous site investigations and risk assessments was undertaken. The principal findings of previous investigations and risk assessments at the site are detailed in the following sections. 4.1 Enviros Consulting Part IIA Site Investigation Report for Bayer Crop Science, January 2005 The report details the findings of the site investigation undertaken to satisfy the determination of the site as a Special Site under Part IIa of the Environmental Protection Act The report built on previous work at the site from 1991 onwards which had included installation and monitoring of 59 No. piezometers and 19 No. monitoring boreholes. Additional intrusive work comprised the drilling of 10 No. new boreholes and 41 window sample holes. The key findings of the report were: Identified soil contamination was largely restricted to localised hot spots within the site, typically in the immediate vicinity of the former production areas. Identified contaminants included a wide range of pesticides and VOCs; 9

17 Soil contamination was considered to make a small and localised contribution to the contaminant source with only small volumes of contaminated soil identified in the unsaturated zone. It was considered that greatest potential impact on controlled water receptors would be from the presence of contaminants within groundwater; The greatest concentration of contaminants in groundwater was identified in the centre of the site; Contaminant concentrations in groundwater were significantly lower outside of the identified contaminant source areas. The decrease in concentrations was much greater than would be anticipated through contaminant migration alone and it was therefore considered that biodegradation of contaminants was occurring. Additionally, one of the main solvent contaminants in groundwater at the site was trichloroethene (TCE) and the presence in groundwater of breakdown products which increased as TCE decreased away from the source was taken as further evidence of biodegradation occurring; The WMMCF was proven to be absent under much of the site and less than 4m thick downstream of the contaminant sources. The WMMCF was also of low permeability. Therefore, the WMMCF was not considered to be a significant receptor and the potential pollutant linkage between the site and the WMMCF was considered invalid; and A significant pollutant linkage was confirmed between contaminated shallow groundwater and the Riddy Brook and River Cam. 4.2 Atkins Environment, Remediation of Former Bayer Site, Hauxton, Preliminary Conceptual Model Report August 2006 Atkins Environmental (Atkins) prepared the report on behalf of Harrow Estates to further develop and refine the conceptual site model (CSM) for the site and to address uncertainties identified in the Enviros report. An additional 12 boreholes and 17 window sample holes were drilled over the site to further assess the ground conditions overlying the Gault Clay and to install new groundwater monitoring boreholes. Samples of soil and groundwater were analysed for contaminants including individual pesticides and herbicides, some geotechnical testing was also undertaken and, falling head tests were conducted between 1.5 m and 3.6 m bgl during drilling. The principal findings of the work were: 10

18 The investigations confirmed the highest contaminant concentrations were in the former production and storage areas of the site; Leachate analysis indicated that there was the potential for contaminants, including pesticides and herbicides to leach into groundwater at the site; Groundwater analysis identified high levels of pesticides and organic substances although there did not appear to be a consistent pattern of groundwater contamination, transport or migration across the site, with the presence of cohesive soil material appearing to limit migration; Concentrations of light non aqueous phase liquids (LNAPL) and dense non-aqueous phase liquids (DNAPL) were identified in concentrations that may have been indicative of free phase contaminants to be present; The report confirmed the Enviros findings that biodegradation of pesticides, herbicides and organic compounds was occurring; The report considered that both the WMMCF and surface water courses (Riddy Brook and the River Cam) should be considered as receptors; and The report recommended the use of the use of MODFLOW and the Environment Agency Remedial Targets Worksheet to derive suitable remedial targets for the site. 4.3 Atkins Environment, Former Bayer Site, Hauxton, Groundwater Modelling Report, June 2007 Based on the findings of the Preliminary CSM report (discussed in Section 4.2), a quantitative risk assessment (QRA) was conducted using MODFLOW and the Environment Agency Remedial Targets Worksheet (EA RTW). Additionally, further assessment of the presence of DNAPL was carried out and pumping tests were also undertaken to provide more information on the hydraulic conductivity in the soil material overlying the Gault Clay. The approach taken for the groundwater modelling for the QRA was as follows: Development of a detailed water balance for the site and surrounding catchment area; 11

19 The development of the MODFLOW model based on the local geography, geology and hydrogeology which was calibrated based on observed groundwater levels and hydraulic conductivities from the site investigation data; The calibrated MODFLOW model was used to plot travel pathways and the relative unretarded travel times between known contaminant sources and the controlled water receptor (the Riddy Brook) using MODPATH; Qualitative risk screening was conducted based on contaminant concentrations, distance and travel time to receptor and the retardation characteristics of individual contaminants to select a representative list of priority contaminants at the site; and Quantitative Risk Assessment of the priority contaminants was using the EA Remedial Targets Worksheet (EA RTW) which set the preliminary remedial targets MODFLOW The modelled catchment which included the site covered an area of ha. A water balance calculation was undertaken for the catchment area based upon the estimated groundwater base flow to the River Cam and predicted infiltration rates for grassland. This gave an infiltration rate of 1.45 x 10-4 m/day which was used to derive a total water balance for the catchment of 310 m 3 /day. A groundwater flow model for the site was developed using MODFLOW. An iterative approach was taken in the development of the model with input parameters being refined to reflect observed site conditions and data obtained from the site investigation such as the hydraulic conductivities obtained from the pumping tests. A total of 20 models were run to develop the final model of the site which included the modelling of the bentonite wall and groundwater abstraction which was ongoing at the time of writing. Two additional models were also run for the proposed redevelopment of the site with the groundwater abstraction and bentonite wall removed from the model although it was not clear if any other groundwater parameters were altered to reflect the likely changed in groundwater flow. Uniform infiltration rates of m/d for residential properties and m/d for commercial properties were assumed, the modelling conducted with a uniform distribution between the two values. It was considered that the models gave a reasonable representation of the likely future groundwater flux rates and therefore the hydraulic conductivity and hydraulic gradients developed for the models were considered appropriate to use in the QRA. 12

20 4.3.2 QRA The QRA used the EA RTW to model the potential risks. UK Drinking Water Standards were used as appropriate target criteria for the risk assessment due to the limited number of Environment Agency Fresh Water Environmental Quality Standards (EQS) available for organic compounds. The site was divided into two zones which were modelled separately. The first zone comprised the area within 20 m of the Riddy Brook (the receptor) with the second zone comprising the remainder of the site. Twenty three contaminants of concern (CoC) were identified and remedial targets were developed for all CoCs in both zones. However, there were recognised issues with assessing the cumulative impact on the Riddy Brook from two separate sources. As a result for the outer zone, remedial targets were derived based on 90% of the modelled remedial target assuming a distance of 60 m to the receptor. For the inner zone, the distance to receptor was assumed to be 1 m Aquifer Parameters The modelled aquifer was the Made Ground, and the inputs for bulk density and porosity were based on assumed values for cohesive and granular soils. The hydraulic conductivity and hydraulic gradient were determined through the MODFLOW analysis. Typically, for each parameter a range of values was obtained and a representative distribution (Probability Distribution Function (PDF)) selected so that the natural variability of the aquifer could be represented. The aquifer parameters used in the model are presented in Table 2. Table 2 Aquifer Properties Used in Atkins QRA Parameter Units PDF Minimum Most Likely Maximum Water filled soil porosity % Triangular Effective Aquifer Porosity % Triangular Air Filled Porosity % Uniform 1-2 Bulk Density of soil gcm -3 Uniform material in aquifer 2 Infiltration rate m/day Uniform 1x x 10-4 Hydraulic Conductivity m/day Log Triangular Hydraulic Gradient m/m Triangular

21 Contaminant Parameters Contaminant parameters (K d, Henrys Law Constant and half-life in groundwater) were obtained from literature values, primarily obtained from the following references: Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals Second Edition, Mackay et al; ToxNet: The United States Natural Library of Medicine, Hazardous Substances Data Bank EEC active substance reports and Relevant Environment Agency publications. A representative range of values and PDF was selected for each contaminant parameter. Values of partition coefficient K d were calculated based on literature values of the soil organic carbon partition Coefficient K oc and averaged site specific values for the fraction of organic carbon (FOC) using the equation K d = K oc x FOC. As an additional factor of safety, 10% of the calculated value of K d for each contaminant was used to develop the remedial targets. For several pesticides and herbicides, there were no published values for contaminant half life in water. In these cases, a range of half lives considered by Atkins to be conservative (relative to modern pesticides) were used in the model, typically 100 to 730 days Selection of Input Values The EA RTW requires single inputs for each aquifer and contaminant parameter. However, as detailed above, the aquifer and contaminant properties comprised a range of values and representative PDFs. Therefore, the predictive modelling software Crystal Ball TM was used with the parameter ranges and PDFs to derive a single value based on the 95 th percentile value for each parameter Remedial Targets Remedial targets were developed for soil and groundwater in both the inner and outer zones of the site. A number of the calculated remedial targets were below the commercially available limits of detection, and for these contaminants the remedial targets were set at the detection limit. 14

22 4.4 Vertase FLI, Further Quantitative Risk Assessment for Risks to Groundwater and Surface Waters, January 2011 The report reassessed the remedial targets for five priority contaminants of concern (dicamba, schradan, Bis(2-chloroethyl)ether, ethofumesate and 1,2-dichloroethane) using ConSim. ConSim was selected as a suitable risk assessment tool as it can assess the combined effects of different contaminant sources at a receptor and it allowed a range of input values (in the form of probability functions) for all parameters using Monte-Carlo analysis so that natural variations in inputs can be modelled to produce 95 th percentile probability. Due to the geometry of the site, for the modelling, the site was divided into four zones, Zone 1 (the 20 m buffer zone), Zone 2S and 2N (in the centre of the site) and Zone 3 in the southwest. For consistency, and where appropriate, inputs from the Atkins RTW model were used. Soil material was divided into three types, Type A (Granular Made Ground and sand and gravel), Type B (WMMCF) and Type C (Gault Clay). Environment Agency EQS values for freshwater were used for screening criteria at the Riddy Brook, or if not available UK drinking water standards. If neither was available either detection limits or screening values from other regulatory frameworks (such as United States Environmental Protection Agency) were used. Remedial targets were derived for each soil type in each zone with respect to the Riddy Brook for soil, leachate and groundwater. 4.5 Summary of Previous Works A CSM and initial remedial targets were developed by Atkins using EA RTW before remediation commenced for twenty three contaminants of concern. The initial CSM and derivation of remedial targets was largely based on the existing pre-remediation conditions, available Enviros and Atkins site investigation data and modelling of the groundwater regime at the site using MODFLOW. Subsequently five CoCs (dicamba, schradan, Bis(2-chloroethyl)ether, ethofumesate and 1,2-dichloroethane) were identified as priority CoCs by VertaseFLI and remedial targets for these were reassessed using ConSim to provide a more detailed probabilistic risk assessment. 15

23 5.0 Conceptual Site Model 5.1 Original CSM The original CSM for the site was developed by Atkins in 2006 to 2007 and is presented in Appendix B. The CSM presents a detailed model of the sub-surface contaminant pathways prior to remediation. The contaminant source is shown as granular Made Ground and cohesive Made Ground, much of which was located below the water table. The main environmental receptors were groundwater and surface water (Riddy Brook). The key groundwater contaminant pathways in the model are: Vertical migration through the unsaturated zone; Migration of contaminants through sand and gravel lenses within the cohesive Made Ground and WMMCF; and Migration through existing site infrastructure such as sumps and utility trenches. The original CSM and therefore the initial risk assessment and remedial targets were based on the available data at the time which comprised the findings of the site investigations and therefore reflected the pre-remediation site conditions (See sections 4.2 and 4.3) and it was assumed that the majority of groundwater flow was through the sand and gravel lenses associated with the WMMCF. Although significant amount of data had been collected, the investigations were still limited by the presence of buildings and site infrastructure. As described in Section 3.3.2, additional contaminant sources have been located and treated during the remediation process that were not identified during the site investigations. The presence of the bentonite cut off wall and the on-going groundwater abstraction are also likely to have affected groundwater levels and flow directions on the site and the presence of site infrastructure such as sumps, and trenches may also have impacted on the results of pumping tests. 5.2 Post Remediation Site Conditions Ground Conditions Post remediation, ground conditions at the site will generally comprise the following three remediated soil types (see Drawing D907_153, Appendix A): 16

24 Type A Sand and gravels, including granular Made Ground. The total volume is approximately 10,000 m 3 at present; Type B Cohesive soils largely comprising the West Melbury Marly Chalk Formation (WMMCF) including previously reworked material. The total volume is approximately 62,000 m 3 at present; and Type C Gault Clay. The total volume is approximately 12,000 m 3 at present. The soil material will be reinstated in layers replicating the distribution of undisturbed soils prior to development of the site, so that the Type A overlies Type B which will overlie Type C. As described in Section 2.1, prior to remediation the distribution of the natural granular deposits at the site were limited to several shallow discontinuous lenses of sand and gravel. As a result of this and the limited presence of granular Made Ground there is only a relatively small volume (10,000 m 3 at present) of the Type A material present which is insufficient to provide cover over the entire site. Given a thickness of 0.3 m the Type A material would cover approximately 3.5 ha, less than half the area of the site. Therefore, no Type A material will be placed in Zone 1 and the Type A material is also unlikely to be continuous over Zones 2S, 2N and 3 replicating the pre remediation distribution of sands and gravel. The anticipated shortfall in reinstated Type A material in will be made up with Type B material. Given the volumes of remediated material and the area of the site, typical thicknesses of the reinstated units are likely to be as follows: Type A Between 0.3 and 0.6 m (where present); Type B Between 3 and 5 m; and Type C Approximately 0.5 m. The Type C material (comprising remediated Gault Clay) will be placed directly over a significant thickness of undisturbed Gault Clay. It should also be noted the development proposals include the placement of imported low permeability cover material on top of the reinstated soil material. The cover material will be imported from an off-site source by the developer following the completion of the remedial works. 17

25 5.2.2 Groundwater Flow Through the Site Pre Remediation and Observed Groundwater Conditions The original CSM for the site assumed groundwater flow at the site was predominantly through the WMMCF and the associated lenses of sand and gravel. However, as discussed in Section 3, the lenses of sand and gravel within the WMMCF are relatively small and discontinuous with the WMMCF typically comprising a stiff clay. Site observations during the ongoing remediation have indicated that the majority of groundwater flow on site was through the granular Made Ground, site drainage system and site services result from periods of rainfall. The excavations undertaken for the remediation provide a complete cross section through the top and base of the in-situ WMMCF and there has been negligible flow observed in the excavation walls other than very slight seepages through the discontinuous lenses of sand and gravel. The seepages have generally decreased with time following exposure suggesting very limited recharge from the surrounding marl material as would be expected from a stiff clay. As a result, the excavations including those below the water table have also remained dry (with the exception of rainfall) for the duration of the remedial works so far. The groundwater contours presented in Appendix B represent the pre-remediation site conditions. The impact of the dewatering system in the south of the site can be seen with the presence of a groundwater low and the surrounding groundwater contours indicating groundwater flow towards the dewatering system. It is likely that the artificial lowering of groundwater in this area was driving groundwater flow through the sand and gravel lenses within the WMMCF on site. It is important to note that the sump for the dewatering system is situated adjacent to a sand and gravel lens so that the volume of groundwater removed is higher than may be anticipated if it had been installed in the stiff light grey clay marl material. Therefore, as a result of the groundwater flow induced by the dewatering, the observed flow in the WMMCF prior to remediation is not considered representative of the post remediation conditions. Without the dewatering, groundwater levels in the south of the site would return to natural levels (in the order of 1 m bgl) and the likely groundwater flow rate would be significantly reduced to rates more comparable with the observed low hydraulic conductivity and hydraulic gradient in the surrounding natural WMMCF. 18

26 Comparison of Waste Water Pumping and Rainfall Data Drainage at the site is designed to collect all surface water, including any groundwater that may flow into the excavations. Given the existing ground conditions and ground cover it is anticipated that the majority of rainfall will be collected by the drainage system. Over an 8 month period between September 2010 and April 2011, a total volume of 31,963 m 3 was pumped from the site to the waste water treatment plant (see Appendix C). Based on the average annual rainfall for the area of 586 mm/year and an approximate site area of 8.5 ha, over an 8 month period a total rainfall volume of 33,207 m 3 would be anticipated which is very similar to the volume of water pumped from the site. Recorded rainfall data for Cambridge gave a total rainfall for the same 8 month period of mm, equivalent to m 3 over the site. A significant proportion of the difference between pumped groundwater volume and total rainfall volume (approximately 10,000 m 3 ) is likely to result from the very low rainfall in March and April 2011 (a total of 4.7 mm over both months) and the subsequent use of large volumes of water on site for dust suppression. During this period, approximately 180 m 3 of water a day was used for dust suppression, equivalent to total of approximately 8,100 m 3. Based on the recorded pumped water volumes relative to local rainfall, and making allowance for loss of rainfall as evaporation etc there appears to be only a very low contribution to the pumped water volumes from groundwater ingress into the excavations. Considering the depth of dewatering achieved (approximately 1 m) this suggests that the volume of water present in the natural WMMCF including the sand and gravel lenses and the hydraulic conductivity are generally very low Reinstated Soil Material and Likely Impact on Local Groundwater Regime The reinstated Type B and Type C soil material will have been homogenised during the remediation and compacted. As a result of the sorting and homogenisation of the Type B material, the discontinuous sand and gravel lenses will no longer be present. Additionally, all historic site infrastructure and drainage will have been removed and the limited volume of granular material will be replaced in isolated lenses above the water table. These activities will have removed the majority of previously identified contaminant pathways and reduced the hydraulic conductivity/permeability of the reinstated material relative to the surrounding natural strata. Without the impact of the current dewatering system in the south of the site and based on the observed negligible flow through sides of excavations, groundwater flow through the 19

27 natural strata surrounding the site and therefore through the reinstated soil material is likely to be very low. Although the reinstatement of reduced permeability soil material may locally alter the groundwater regime, given the relatively low hydraulic gradient observed in the surrounding area, and the lack of significant groundwater flow through the natural WMMCF and associated sand and gravel lenses, any changes to groundwater levels in and surrounding the site are not likely to be significant Aquifer Parameters Groundwater Levels Monitoring of groundwater levels before and during remediation indicates that groundwater levels at the site are shallow, with groundwater typically being encountered between 0.8 and 2 m below ground level Hydraulic Gradient The presence of the bentonite cut-off wall and groundwater abstraction on site make accurately determining the post remediation hydraulic gradient and groundwater flow direction across the site very difficult. It should also be noted that the MODFLOW models previously developed by Atkins were based on the original CSM (discussed in Section 5.1) and site investigation data and reflect the ground conditions at the time including the presence of granular Made Ground, drainage channels and assumed flow through the WMMCF. Therefore the modelled groundwater parameters used in previous reports are unlikely to be fully representative of the remediated site conditions. Available Atkins groundwater monitoring data from the adjacent wastewater treatment plant (immediately to the northwest of the site) appears to be outside of the influence of both the bentonite wall and groundwater abstraction. It is therefore considered most representative of the local groundwater conditions and the likely post remediation groundwater conditions on site. The local groundwater flow is in a northeastern direction towards the Riddy Brook and the local hydraulic gradient is typically between and Hydraulic Conductivity and Permeability Testing As the soil material has been largely homogenised as part of the remediation process, the variability within the reinstated compacted soil material and therefore the variability in hydraulic conductivity is likely to be significantly reduced relative to ground conditions prior to remediation. The reinstated soil materials will also be compacted which will further reduce variability. 20