Former Petrol Filling Station, West Midlands

CALA Homes

Geo Environmental Group were commissioned by CALA Homes to undertake a Phase III geo-environmental assessment, remediation design, supervision and verification of a former petrol filling station site in the West Midlands in order to satisfy the planning requirements for a proposed residential development.

The site has had a relatively long industrial history. Initially existing within a large sand pit excavation in the early 1900s, the site was backfilled and developed as a garage in the mid-1960s.

In environmental terms, the site was relatively sensitve as it was directly underlain by a Major Aquifer with a groundwater abstraction approximately 500m to the SW.

At the time of the investigation, the site contained a number of former underground fuel storage tanks (USTs) and associated pipes to the former petrol pumps together with oil interceptors and the derelict forecourt structures associated with its former use.

GEG proposed a phased approach to the remediation, intially reviewing and updating a Phase II investigation (including the Groundwater Detailed Quantitative Risk Assessment - DQRA) and subsequently providing a Remediation Strategy. The aim of the Strategy was to remove the contamination sources in accordance with the soil remedial targets created by the DQRA. This was followed by periodic monitoring of the groundwater to determine the impact the removal of the contamination source would have on the groundwater, which was identified as contaminated and requiring remediation.

Due to the historical sand extraction from the site and its subsequent infilling, there was some potential for ground gas generation. However, due the age of the fill it was considered likely that any ground gas would be residual rather than generated from existing degrading organic matter. GEG undertook further gas monitoring and confirmed that only residual gas was present. Utilising guidance from CIRIA C659, the low gas flow values indicated a ‘green’ condition. However, due to the maximum concentrations of gases falling within a general ‘amber 1’ condition together with the localised potential for residual concentrations of hydrocarbons, it was considered prudent to adopt an ‘amber 1’ condition in terms of protection with a hydrocarbon resistant gas membrane within all properties.

GEG supervised the removal of the tanks, fuel pipes, petrol pump bases, interceptors and associated fuel impacted spoil by the remediation contractor to prevent cross contamination. The selectively excavated contaminated spoil was removed under the supervision of an experienced geo-environmental engineer from GEG and placed in separate stockpiles (depending on its hydrocarbon content) on an impermeable base and covered to prevent leaching by precipitation. Waste Acceptance Criteria (WAC) testing was undertaken on completed stockpiles prior to sending them for disposal to a licenced landfill facility, as there was insuffient space and quantity for on-site treatment such as bioremediation.

Although the contamination sources were removed, as stated previously it was considered prudent to incorporate hydrocarbon resistant gas membranes within all properties together with placing a clean cover layer over all garden and landscaped areas to further reduce the human health risk to a negligible level. This was fully approved by the Local Authority and NHBC.

Post-contaminant source removal groundwater sampling and analysis was undertaken by GEG over a period of 2 months. This indicated an initial increase of contamination in the groundwater which was anticipated and associated with disturbance of the solid strata during removal creating a plume. However, there was subsequently a rapid decline in contamination, and GEG successfully demonstrated to the Environment Agency, that, by removing the source contamination, the groundwater quality was markedly and consistently improving to below target concentrations. Therefore, no groundwater treatment was considered necessary and the client saved in excess of £100,000 that had been allocated for treatment from specialist groundwater remediation contractor tenders.