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Best Practice Site Conceptualisation to Optimise Remediation, Northern Ireland by AECOM

Shortlisted Brownfield Awards Category 1 - Best Project Preparatory Work

Background

AECOM was employed as the client’s advisor for remediation of an area of historical 1,2-dichloroethane (EDC) impact at a confidential site in Northern Ireland.  The EDC impact arose from legacy chemical manufacturing operations and waste disposal practices.  The focus of the remediation was to address the residual EDC source present within saturated fluvial and glacial deposits down-gradient of a former effluent treatment pond.  Through treatment of saturated soil, the objective of the remediation was to reduce the EDC discharge in shallow groundwater below a risk-based level protective of surface water quality within the adjacent water course.

Although the site is a brownfield site it has re-vegetated and is located in a very scenic location. It is bounded by ecologically sensitive rivers and coastal habitats, with designations for Atlantic salmon, otter, water birds and a diverse range of fauna.

The remediation formed part of a series of remedial measures that had previously included stabilisation of the pond and related sediments and interception of deep groundwater via the installation of a horizontal well.

The previous environmental consultant had identified a soil treatment volume of 6,500 cubic metres (m3) above the risk-based remedial target value protective of surface water quality.  This target volume had been developed through soil and groundwater sampling and a series of membrane interface probe (MIP) investigations.

The remediation technology selected by the client was permeability reduction and enhanced reductive dechlorination through in-situ soil mixing with a combination of bentonite and an organic-carbon electron donor / zero valent iron reagent (PeroxyChem’s Daramend®). The first phase of the work completed by the remediation contractor was further delineation and refinement of the source zone through the drilling of 20 additional boreholes to a maximum depth of 16 metres (m) and the collection of methanol-preserved soil samples for analysis of EDC and its degradation product vinyl chloride (VC).  The remediation strategy and budgets had been developed for a source area and volume delineated on the basis of historical soil sampling and analysis by the previous consultant that had not included methanol preservation.  Following the further delineation work the extent of the area and volume of soil requiring remediation and the concentrations of target contaminants to remediate increased significantly.  With these increases the remediation was no longer financially and practically viable.

Exemplary best practice

AECOM was commissioned to integrate the historical and new delineation data and review the site conceptualisation to develop a revised remediation strategy that was financially viable whilst still achieving the required protection of surface water quality down-gradient.

A process flow path was developed by AECOM for this site conceptualisation review that included the following steps:

  1. Review of site geology / stratigraphy and hydrogeology;
     

  2. Review of method used to define target volume for remediation prior to delineation works;
     

  3. Delineation of EDC soil remedial volume incorporating new delineation data:
     

    • Delineation of EDC soil remedial volume using methanol preserved (MP) samples;

    • Incorporation of historical soil data (non-methanol preserved):

      • Review of conversion of historical unpreserved samples to equivalent MP concentration through assessing the results from duplicate samples analysed with both methods;

      • Incorporation of historical soil data where EDC above the soil remedial target;
         

  4. Use of field data and geology to constrain vertical extent:
     

    • Correlation between headspace photo-ionisation detector (PID) (11.7 eV) and MP EDC concentrations used to constrain upper elevation of EDC impact;

    • Upper surface reviewed against historical MIP dry electrolytic conductivity detector (DELCD) data that responds to halogenated compounds and adjusted accordingly;

    • Optional lower extent defined as 1m lower than deepest transmissive strata (fluvial/glacial sands and gravels) from new delineation bores;

    • Optional lower surface reviewed against historical borehole logs and adjusted accordingly;
       

  5. Review of total mass of EDC within soil and mass contained within various plume shells/cuts;
     

  6. Review of mass distribution of EDC within the volume of soil exceeding the remedial target (uncut and basally constrained) and utilization to assess options for partial treatment; and
     

  7. Assessment of proposed optimum soil remediation solution on EDC mass discharge in groundwater to the adjacent water course.
     

AECOM utilised CTech Development Corporation’s Earth Volumetric Studio 2018 (Studio) to integrate these diverse data sets and understand their distribution in three-dimensions (3D).  Studio was then used to assess remediation options working within 10 m grid squares across the site to assess EDC concentrations, EDC mass and clean overburden thickness within each grid square to inform the final optimum remedial solution.

The initial step was to digitize the borehole records and upload into the Studio model.  This allowed the viewing of all the geological data collected through site investigation in 3D and to identify the locations and distribution of transmissive units.  To complement this step AECOM reviewed regional and local geology and hydrogeology and utilised environmental sequence stratigraphy to interpret the MIP electrical conductivity and DELCD data in the context of the depositional environment along two cross-sections through the site (Figure 1).

Figure 1: Geological cross-section West-East through target area.

Soil and groundwater laboratory data for EDC and VC was uploaded into the Studio model, together with MIP-DELCD results, headspace PID data collected during drilling and groundwater levels.  These data were used to calculate the volume of soil exceeding the remedial target.  Figure 2 illustrates the initial volume of soil identified for remediation (6,500 m3) and the volume of soil above the EDC remedial target solely based on the MP-delineation soil sampling (25,500 m3).

Figure 2: Soil remediation area: (A) prior to delineation works; (B) based solely on delineation works (excluding previous site investigation data). Grid squares 10 x 10 metres.

A statistically significant relationship between EDC concentrations in MP samples and non-MP preserved samples was not identified and hence the historical soil data were only incorporated where EDC had been detected above the remedial target.

Due to the greater abundance of headspace PID data (Figure 3A), the correlation between EDC concentrations in MP-soil samples and corresponding headspace PID results was investigated to establish a threshold PID result to constrain the upper boundary of the EDC soil volume above the remedial target.  The headspace PID data were uploaded into the Studio model together with the historical MIP-DELCD data and the PID constrained upper surface applied and then reviewed and adjusted using the MIP-DELCD data.

Figure 3: Field data and geology used to constrain upper and lower boundaries of revised remediation volume: (A) upper boundary - headspace photoionization data collected during delineation works; (B) lower boundary – 1 metre below surface of subglacial basal silt strata.

As the objective of the remedial works was to undertaken treatment of saturated soil to reduce EDC discharge in shallow groundwater below a risk-based level protective of surface water quality within the adjacent water course, the transmissivity of the EDC host strata was a key consideration.  Applying the sequence stratigraphy learnings, the surface of the basal subglacial silt strata was established within the Studio model. A cut plane 1m below this surface was then applied to the EDC soil samples exceeding the remedial target to isolate those EDC impacted soils above the remedial target within the more transmissive overlying fluvial and glacial deposits and at the interface with the lower transmissivity silt (Figure 3B).  To assess the contribution of back diffusion from the underlying silts into the overlying remediated zone the ESTCP Matrix Diffusion Toolkit v1.23 was utilized and concluded that the EDC within the silt below the 1m cut plane would not contribute to an exceedance of the remedial targets.

Figure 4: Initial revised soil volume for remediation based on existing site investigation data and new delineation data and incorporating upper and lower boundary constraints.

The integration of the above data and constraints resulted in an initial revised soil volume for remediation of 11,400 m3 (Figure 4).  This soil volume was cross-checked against EDC concentrations in groundwater that exceeded the groundwater remedial target.

To further optimize the remedial works, the area occupied by the revised soil volume was divided into 10 m grid squares.  Within each square the average EDC concentration and mass and soil volume above the soil remedial target was calculated together with the volume and thickness of clean overburden (Figure 5).  A number of scenarios were evaluated with the final solution adopted comprising grid squares containing greater than 2% of the EDC mass above the soil remedial target and considering EDC impact in groundwater.  The final revised volume of soil for treatment was 8,300 m3, a 68% reduction in the volume of soils identified to contain EDC above the soil remedial target within the Studio model and only a 28% increase in the original remediation volume identified at tender stage.

Figure 5A: EDC concentration and mass distribution per 10m x 10m grid square.

Figure 5B: Volume of EDC contaminated soil and clean overburden and thickness of clean overburden per 10m x 10m grid square.

An assessment of the impact of the proposed soil remediation on EDC concentrations in groundwater was undertaken (Figure 6) and indicated that treatment of the final revised volume of soil would result in a 45-55% reduction in EDC concentrations discharging to the water course and would achieve and surpass the remediation objective.

Figure 6: Expected EDC reduction in groundwater discharge to the river following remediation.

Real environmental/economic/social benefit

The site conceptualisation allowed the development of a targeted revised soil volume for remediation that considered mass distribution and transmissivity.  The assessment indicated that the solution was suitably protective of the adjacent sensitive aquatic ecosystem through the reduction in EDC loading and this approach was approved by the regulator.  The revised soil volume could be addressed within the financial constraints of the project to allow remedial works to progress and the return of the area to beneficial use.

Cost effectiveness

The works undertaken resulted in a 68% reduction in the volume of soil requiring treatment following the delineation works.  This reduction in soil volumes made the remediation viable.  AECOM also recommended changes to the reagents utilised for soil remediation (moving from PeroxyChem’s Daramend® to EHC® and the removal of the bentonite) to optimise the remediation and its longevity.  These changes were supported by the Northern Ireland Environment Agency (NIEA). 

Compliance with legislation, codes and guidance

The desk-based work aligned with best-practice guidance to focus on site conceptualisation to inform the development of a cost-effective remedial solution.

Effective stakeholder engagement

Stakeholder engagement was a key element of the success of this project.  With regular, transparent engagement with the client and remediation contractor and successful presentation of the revised remediation approach to the client’s international technical team and the NIEA.  To aid engagement, the Studio model was used to generate 3D pdf figures which were utilised by the stakeholders as a highly effective tool to visualise and interrogate the data.

The client praised the work completed by the AECOM project team in resolving this issue and singled out the great technical input, the drive to maintain the project momentum under very tight time pressures and ultimately coming up with a robust, sustainable and defensible solution.

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