Innovative Use of Mass Discharge Remedial Targets UK by AECOM

Shortlisted Brownfield Awards Category 6 - Best Project Closure/Verification



Since 2013 AECOM has been undertaking verification and risk-management works at this former petrol filling station (PFS) and associated nearby residential properties. The objective has been to manage risks to groundwater from an historical fuel release, and to achieve regulatory sign-off for the PFS site and adjacent properties.  To accomplish this, AECOM developed and agreed the use of innovative mass discharge remedial targets for the first time with the regulator, instead of the more common approach using concentration-based targets.  Mass discharge was then used to focus further remedial activities.

In 2019, through the assessment of mass discharge against regulatory-agreed targets, AECOM successfully achieved regulator-endorsed closure of remedial activities on two properties adjacent to the PFS, thereby allowing a return to residential use.  Verification works are ongoing in 2020 on the two remaining properties.


In 2002 methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) were detected during routine analysis of drinking water quality within a reservoir supplied by a public water supply (PWS) well.  Site investigations identified an active PFS located 750 metres from the PWS as the source.  Light-non-aqueous phase liquid (LNAPL) was identified beneath the PFS and extended to off-site properties.  Dissolved-phase benzene, toluene, ethylbenzene, xylenes and naphthalene (BTEXN), MTBE and TAME were associated with the LNAPL, with the BTEXN plume primarily limited to the superficial deposits and extending a maximum distance of 80 metres from the PFS due to natural attenuation, in contrast to both MTBE and TAME which extended 750 metres to the PWS.

Figure 1: Plan showing BTEXN and MTBE/TAME plumes prior to HCS operation

Between 2002 and 2014 the PFS was decommissioned and four off-site properties in the vicinity of the PFS were purchased by the PFS operator to facilitate remediation activities. A mixed remediation strategy was adopted focusing on source reduction and pathway interception to protect the PWS.  One of the abstraction wells at the PWS was used as a scavenger well to protect the other supply wells, and natural attenuation was identified as an effective remedial strategy to protect groundwater quality at the PWS from BTEXN.  However, more active remediation was required to protect the PWS from impacts from MTBE and TAME. 





Figure 2: Aerial photo of the project area, and inset plan showing the boundaries of the former PFS site and locations of four off-site properties in the vicinity of the PFS purchased by the PFS operator to facilitate remediation activities

Between 2005 and 2013 aggressive source depletion remedial techniques were applied and circa 60,000 litres of hydrocarbon were removed from the ground.  In 2010 pathway interception was implemented through the installation of a hydraulic containment system (HCS) approximately 100m down hydraulic gradient of the PFS.  Once the HCS was started, MTBE concentrations at the PWS rapidly reduced to below method detection limit and have remained so since 2012.  Since mid-2014 all water abstracted at the PWS has been returned to public supply, returning the PWS as a key resource and asset.

Figure 3: Migration pathway for MTBE & TAME to reach PWS and inferred MTBE plume above 1 mg/l following HCS operation – side view1

Agreement with the regulators that no further remedial works were required at the PFS site was achieved in 2015.  Post 2015, residual impacts present beneath adjacent properties continued to sustain the MTBE and TAME impacts to groundwater and required ongoing operation of the HCS to protect the PWS. 


Notable innovation / exemplary best practice

To assess the likely timescale for future operation of the HCS to protect the PWS from taste and odour impacts, AECOM carried out an assessment of MTBE decline curves in monitoring wells installed within the off-site properties.  This concluded that achieving the remedial target for MTBE (3.3 mg/l) across the extensive network of wells would likely require continued operation of the HCS for decades. 

Through updating the existing numerical groundwater flow model and sensitivity testing under different likely future PWS abstraction regimes AECOM determined that the HCS captured and would continue to capture all of the dissolved MTBE plume (and MTBE source area) between the PFS and HCS.  Therefore, MTBE mass discharge could be easily calculated at the HCS from the groundwater abstraction rates and MTBE concentrations in the abstracted groundwater.




Figure 4: Modelled capture zone for HCS (forward particle tracking) and inferred MTBE plume above 3 mg/l following HCS operation – plan view

Establishing the above improved stakeholder confidence in the efficacy of the HCS achieving pathway interception, allowing the development of mass discharge as a more effective metric of assessing source depletion and determining project closure than the use of the previous concentration-based remedial target.

Mass discharge remedial targets (MDRT) for assessing remedial close-out for MTBE (and latterly TAME) were developed by AECOM adopting the 10th percentile abstraction rate for the PWS (i.e. 90% of the time the PWS will be pumping at volumes greater than assumed target rate) and an agreed ceiling concentration based on taste and odour within the blended PWS. The MDRT were agreed with the regulators and stakeholders, including the water supply company, in July 2015. The Environment Agency confirmed this was the first time a MDRT has been agreed with the UK regulators. 

Following agreement of the MDRT, aquifer-testing and groundwater sampling were utilized to quantify the mass discharge of MTBE (the dominant residual risk to the PWS) across a series of transects through the source area. 

Figure 5: MTBE mass discharge transects across the source area.

The adoption of mass discharge remedial targets (rather than concentration-based metrics) allowed those areas of the source responsible for sustaining the MTBE plume to be clearly identified.  Focussed short-duration (<60 days) source depletion work using total-fluids abstraction was used to deplete MTBE within two key source areas, and this was supplemented with enhanced natural attenuation.  Quantitative analysis (census-DNA) of sulphate-reducing bacteria and MTBE-degrading bacteria provided the additional tertiary line of evidence to support the observed reduction in BTEX and MTBE groundwater concentrations.





Figure 6: Sulphate and MTBE concentrations in groundwater prior to, and post, passive injection of sulphate

Achievement of residual MTBE mass discharge below the MDRT was then utilized as one of the key metrics for successfully negotiating regulatory close-out for individual properties affected by the PFS. In 2019, through the application of the MDRT, AECOM successfully achieved regulatory sign-off for two off-site properties despite MTBE concentrations in individual wells exceeding the previous remedial target of 3.3 mg/l by up to an order of magnitude.

Real environmental/economic/social benefit

The assessment of MTBE mass discharge across transects through the source area has guided further active remediation to focus on the key source areas sustaining the exceedance of the MTBE MDRT.  The rate of MTBE mass recovery by the HCS has been utilized as a metric for evaluating the duration of further active remediation.  Active remediation has ceased when the rate of MTBE recovery fell below that achieved by the HCS. This focused, short-duration remediation in combination with the significantly earlier close-out of works on off-site properties (by over 10 years in relation to the application of the previous remedial target) has significantly reduced the economic and environmental cost of the remediation. In addition, the accelerated close-out of off-site properties has real social benefit allowing the adjacent properties to be returned to market to accelerate the regeneration of the area.

In 2019 AECOM also successfully achieved regulatory approval to align the compliance point for the other chemicals of concern with that for MTBE.  The resulting move of the compliance point from a default 50m to 750m distance at the PWS has significant implications for the duration of operation of the HCS.  Historical monitoring (prior to HCS operation) indicated periodic water target value exceedances at the default 50m compliance point that did not extend to the PWS.  A SuRF-UK Tier 1 Sustainability Assessment was completed and showed a clear sustainable benefit to aligning the compliance points rather than continuing to undertake active remedial works. This benefit was driven by electricity consumption (and associated air emissions), natural resources wastage (in terms of discharging the groundwater abstracted by the HCS to sewer) and the economic cost of continuing to operate the HCS.

Cost effectiveness

As the HCS effectively captures all groundwater discharging from the source area, the compliance point for achieving the MDRT was agreed as the discharge from the HCS itself, thus reducing the need for extensive groundwater monitoring across a wide area. Using mass discharge, it is possible that some areas with persistently higher concentrations of MTBE may remain without contributing to an exceedance of the MDRT, thus reducing the remedial burden and accelerating project closure.

Two areas which were identified as discharging enough mass to be potentially sustaining the MTBE mass discharge above the MDRT were targeted through focused total fluids abstraction in the source area. In both areas, it was demonstrated that the achievable mass recovery through groundwater abstraction was below the mass discharge from the HCS and therefore further remediation works in these areas was not required. Aquifer testing was undertaken during these works to further refine the estimates of mass discharge across the source area. Better constrained hydraulic conductivities for different strata provided a more accurate description of the mass discharge and demonstrated that the mass of MTBE being discharged from these areas was insufficient to cause exceedance of the MDRT at the HCS. This has allowed regulatory close-out for several of the properties adjacent to the former PFS despite the presence of MTBE concentrations above the previous remedial target.

Compliance with legislation, codes and guidance

All works have been undertaken in full compliance with legislation and appropriate guidance.  The Environment Agency has visited the site on a number of occasions, complementing the work and utilising the project as a model site for promoting to its staff.

Throughout all the works on-site AECOM has ensured safety as the highest priority, an approach actively supported by our client and the regulators. Testimony to this is that since work started on this project in 2013 there have been no injuries or safety incidents. In 2020 AECOM celebrated working over 2,500 days on this project without any lost time injuries.

Effective public/stakeholder engagement

The unique regulator agreement to the MDRT together with the compliance point extension was achieved through AECOM’s advocacy, supported by a robust conceptual site model and effective stakeholder engagement.

Key stakeholders included the Client team, the Environment Agency, the Local Authority and the Water Company which operated the PWS. A close, collaborative working relationship ensued between client, consultant and stakeholders from the onset of the project, building trust amongst all the stakeholders through the whole process. All stakeholders have been involved at the various stages of development of remedial strategies and targets, with regular updates, workshops, site visits and clear communication and collaboration in developing and executing the remedial strategy. 

A robust, sustainable and defensible solution

For this complex project the adoption of mass discharge as a remedial target has allowed for a simpler, more robust and defensible approach to verification and project closure. Use of the MDRT for MTBE and TAME, along with aligning the compliance points for other chemicals of concern, has resulted in significant reductions in both costs and project duration, and enhanced value by restoring property for future use.

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