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Ground investigation
Geotechnical consultancy
Contaminated land assessment

Former Petrol Filling Stations

There were some 40,000 petrol filling stations in the UK in the mid-1960s and this number has steadily reduced to currently around 8,500. Many of the closed sites have now been remediated and redeveloped, with the residual sites often being those of low economic value, in difficult locations or with significant soil contamination problems.

Older petrol filling stations typically stored the fuel in single skin steel underground tanks which over the years have corroded such that the vast majority of the pre-1970 tanks are found to have leaked fuel into the ground. Leaks are also commonly found in association with the elbows in petrol supply pipes and around buried petrol interceptor structures. Where former filling stations are located upon clay soils, with the tanks excavated into the cohesive deposits, soil contamination is typically found localized within the former tank backfill where it can be easily isolated and removed. However, where the tanks are located in granular soils, and particularly where the groundwater is located at a depth below the base of the tanks, a significant degree of contaminant migration through the soil can have occurred, which can necessitate a major scheme of insitu or ex-situ remediation. GEA have gained a wealth of experience in the investigation, remediation and validation of petrol station sites across the country and in a variety of soil conditions and have developed excellent working relationships with the Environment Agency, the Local Authority Environmental Health Officers and with specialist remediation contractors.

GEA have developed a system for working on both occupied and vacant petrol stations using low disturbance techniques. Following a detailed desk study, a soil vapour survey is carried out to target the areas with the greatest contamination potential. The results of this survey are presented as a contour plot to highlight the zones of soil contamination, which can then be further investigated through the use of small diameter percussive opendrive sampling. The soil cores obtained can be screened on site with a Photo-ionisation Detector (PID) to vertically profile the hydrocarbon contamination and allow targeted sub-sampling to be carried out

Once we have characterised the contamination and its extent across the site, the initial conceptual site model is updated to allow a detailed quantitative risk assessment to be carried out. This assessment is utilised to assess the scale of the risk posed to human health or to the groundwater, which are usually the most common drivers for remediation and a decision can then be made as to whether remedial action is required.

On many sites remediation may simply take the form of the removal of the buried tanks and pipework, together with the surrounding contaminated soil, whilst on other sites a full remedial options appraisal will be carried out. Being both volatile and biodegradable, hydrocarbon contamination can be remediated insitu or on the surface following its excavation, which can allow the groundworks and structural works to commence whilst the remediation proceeds. GEA will provide the client with assistance in the selection of an appropriate specialist contractor and in selecting the optimum remedial technique. During the remediation we will monitor the remediation and validate the work carried out to provide independent confirmation that the remedial objectives have been met. This confirmation, together with details of the remedial work undertaken and the results of the validation testing, are presented in a completion report for submission to the Local Authority and Environment Agency as well as to the funders of the scheme and any potential tenants who may require this information as part of their due diligence.

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