Robust Conceptualisation and Detailed Quantitative Risk Assessment at a Former Clay Pit infilled with Chemical Waste
Shortlisted for Brownfield Awards Category 7: Best Project Closure/Verification
Atkins were awarded a site in 2019 to assess the controlled waters risks, associated with the site's Black County industrial heritage, following four phases of previous works that had identified potential contamination, but no clear conclusions or solutions.
The site was known to have been occupied by clay pits, a colliery and a brickworks since the 1880s, and during the site’s redeveloped in the late 1960s, the historical land uses were reportedly infilled with chemical waste.
Atkins approach was outcome focussed from the outset. Through a diligent review of historical information and literature review, a targeted site investigation was designed, focussed on data gaps and uncertainties in the conceptual model.
Following the data collection and identification of source-pathway-receptor linkages, a holistic, lines of evidence based interpretation of the data sources gathered (historical and current investigation) was used to undertake a robust and appropriate Detailed Quantitative Risk Assessment (DQRA). The cornerstone of this project was strong conceptualisation, robust risk assessments and alignment with SuRF UK principles. The project outcome exceeded the client’s expectations, as it was able to a close out the risks with a high standard of reporting, achieving a reduction in the sites’ risk provision.
Source: Blue Billy - But Not That One!
Our first goal was to understand the potential contaminants that were associated with the chemical waste.
The historical site investigation reports documented that the two on-site clay pits were dewatered and then filled with chemical fill, brick and ash from a nearby chemical tip. This material was also used in the landscaping mounds. An earlier site investigation reported bluish-green material with high sulphate concentrations and a reference to ‘blue billy’ within the landscaping mounds.
We undertook a review of chemical tips within the area. In depth literature and online research established that wastes from two chemical works that produced white and blue-grey ash of calcium sulphate (‘The Blue Billies’) from production of ‘salt cake’. Documentary evidence indicates that these waste stockpiles produced ‘noxious liquids’ and ‘flooding the clay pits’ (1).
The stockpiles were moved from their locations approximately 500 m to 1 km away from their original locations in the 1960s, and transported along a road, adjacent to the site, for disposal. Given that the site was redeveloped in the late 1960s, it is inferred that the on-site clay pits were filled with the chemical waste.
Atkins conceptualisation, based on thorough data review and a lines of evidence approach, that ‘The Blue Billies’ were the source of the chemical waste used to fill the clay pits.
Pathway: Geology & Hydrogeology Model Uncertainties
Historical reports state that the clay pits were filled with chemical waste from a level of 138 m above ordnance datum (M AOD), approximately 10 m bgl from the site compound level. Landscaping mounds up to 152 m AOD, 6 m above the site level were also constructed. The Etruria Formation comprising mudstone, sandstone and conglomerate underlies the site.
A small stream is present in the south of the site that feeds the main river, located 820 m to the north east.
Our site investigations targeted the two infilled clay pits (see Figure 2) and the landscaping mounds to understand the Made Ground depths, the composition of the underlying Etruria Formation and groundwater quality and levels within the pits and the bedrock.
The investigation of the clay pits confirmed they were deeper than the historical evidence suggested.
At its deepest in the north east of the site, the infilling was up to 30 m bgl, comprising chemical waste as described in the ‘Blue Billies’ article, with elevated concentrations of cyanides, sulphates and metals including mercury and arsenic. There were similar JAGDAG hazardous substances reported in the groundwater, which varied dramatically in colour across the infill material: vivid yellow in the centre of the site; yellow-green in the north; and blue-green in the north east (see Fig 2).
Results from the groundwater monitoring programme showed that the chemical waste was affecting the groundwater quality within the Etruria Formation, with the same potential contaminants that were being reported within the infilled clay pits.
Figure 2: Aerial image of the site from 1945 showing two clay pits (left), variability of groundwater colouration from the groundwater wells installed within the infilled clay pits (right)
Boreholes drilled into the Etruria Formation predominately encountered mudstones with occasional sandstone espleys between 141 m and 126 m AOD. The sandstone espleys varied in thickness from 0.05 m to 3.0 m.
Published sources indicate that groundwater flow within the Etruria Formation is dominated by fracture flow and flow within the higher permeability sandstone espleys.
Yields within the horizon can therefore be highly variable and largely dependent on the location and extent of the espleys within the mudstone dominant rock (2). This was reflected in our findings to the west of the clay pits along a North-South transect, which records a 2 m sandstone espley at 141 m to 139 m AOD in the north, no sandstone in the centre, and a 3 m sandstone espley in the south (see Figure 3).
In theory where sandstone was encountered at depths that could intersect the clay pits (higher than the base at ~120 m AOD), then the chemical waste contaminants would be reported in the bedrock, as groundwater would use the more permeable sandstone as a preferential pathways.
Our extensive groundwater conceptualisation showed the distribution of the potential contaminants appeared to be related to bedrock in close proximity to the clay pits rather than migrating through sandstone espleys.
Receptors: Who, What and Where?
With a strong understanding of the source area, definition of the controlled water receptors that could be impacted by the site was crucial.
The groundwater aquifer on-site are limited to the Etruria Formation bedrock which is classified as a Secondary A Aquifer.
In-situ permeability testing was undertaken in four bedrock boreholes with varying thicknesses of sandstone. The indicated hydraulic conductivity ranging from 0.42 m/d to 0.004 m/d. The difference in conductivity relates to the proportion of sandstone logged within the screened section of each location, with lower conductivity in the mudstone. Given the demonstrated discontinuous nature of the sandstone espleys, off-site migration beyond 20-30 m could not be via discrete high permeability sandstone espleys. A bulk permeability of the aquifer was utilised to represent off-site migration.
Figure 3 - Site Conceptual Model showing the sandstone espleys (orange), thickness of infilling and the contaminant exceedances
The closest surface water features are a small pond located within the central western portion of the site and a stream which, is at a lower elevation than the rest of the site and, flows westward along the southern site boundary.
The on-site steam joins to a culverted surface watercourse to the east of the site.
The closest un-culverted section of this watercourse is 820 m north east of the site. The Birmingham Canal Network is located approximately 160 m northeast of the site, and is fully engineered.
Using the network of 12 borehole installed across the site, we were able to determine that the groundwater flow direction was towards the north east (see Figure 4), away from the on-site stream in the south. Therefore the stream was not a receptor for sources within the infilled clay pits.
Chemical sampling from the up-stream and down-stream across six monitoring visits reported no evidence that the potential contaminants identified in the on-site soils and groundwater were contributing any contaminant load to the stream.
Figure 4 - Groundwater Potentiometric Elevation Contour Plot.
Our conceptualisation, based on a robust potentiometric model, discounted the on-site stream as a receptor and determined that the closest surface water receptor was the un-culverted section of the receiving watercourse, located 820 m north east, and down gradient, of the site.
The DQRA showed that impacts present in present in soils and groundwater would not pose an unacceptable risk to either groundwater resources or identified surface water receptors. Sensitivity analysis was undertaken on critical input parameters through the evaluation process to ensure the modelling process was robust.
An evaluation was also undertaken with reference to SuRF UK sustainable management practices (SMP), which supported the outcomes of the DQRA and the long-term strategy for the site.
Challenges & Solutions
To summarise the close out of the site, the consideration of multiple challenges were required to produce a robust and defensible model, that would pass peer review and achieve the client’s requirements.
The table below provides a summary of this decision-making process.
Critical review of historical site information and online resources to determine the contamination profile of the material we should expect to find. Use that information to provide cost effectiveness in our analysis and investigations.
Targeted site investigation and use of best practice guidance for logging and soil and groundwater sample recovery to profile the potential contaminants and the ground conditions, to identify preferential pathways and produce a defensible ground model
Review of technical papers on the regional hydrogeological model to infer site groundwater characteristics alongside our own data collection. Allowing for appropriate conservatism in our modelling of the aquifer’s conductivity and potential preferential pathways.
Our DQRA modelled two scenarios, one for the protection of groundwater resources and one for the receiving un-culverted surface water receptor. To ensure appropriate conservatism in our the model, the surface water receptors were kept at 50 m for hazardous and 250 m for non-hazardous substances for our Potential Contaminants of Concern using current Environment Agency’s approach to groundwater protection and Remedial Targets Methodology (RTM) V.3.2.
Site wide exceedances of soil SSAC (from both models) for ammonium were identified. However, through rigours process and risk evaluation, accounting for site-specific leachability results to calibrate model outputs, it was determined that the identified exceedances did not represent a realistic risk to controlled waters.
Further to the results, we considered the sustainability of our assessment with respect to SuRF UK SMPs (3), reviewing our environmental indicators and determined that our assessment was robust in its conceptualisation for:
1. the source’s leaching potential through available pathways is low,
2. reduced natural resource use is low considering the regions industrial heritage and
3. reducing waste production required if a ‘hard’ solution was recommended
Throughout the project, Atkins worked closely with the client to achieve a cost effective and technically robust solution to assess the risks. The assessment was peer-reviewed and the sustainable, robust and cost-effective outcome meant there was no requirement to remediate the infilled clay pits. The project outcome exceeded the client’s expectations due to our approach and diligence in developing a detailed and robust assessment that achieved a substantial reduction in the sites’ risk provision.
Source identification and contaminants of concern
Understanding the ground model, developing an understanding of pathways and connectivity.
Determining the receptors and their attributes
Producing a fit for purpose model representative of our conceptualisation
Interpretating the model outputs
Potential environmental impact being caused by the site to the wider area
Current site use and client drivers
1. Made in Oldbury, Various. 2021. The Blue Billies. Available at: https:// madeinoldbury.co.uk/articles/the-blue-billies/ (Accessed: 18/05/2021).
2. R. A. Downing, D. H. Land, R. Allendar, P E R Lovelock and a. L. R. Bridge. “The hydrogeology of the Trent River Basin,”. 1970.
3. Sustainable Remediation Forum UK. 2014. Sustainable Management Practices for Management of Land Contamination, March 2014