A detailed desk study, intrusive ground investigation and hydrogeological modelling allowed this project to become viable.
Our client was looking to purchase the existing, operational, petrol filling station with a view to converting the existing garage and workshop buildings to residential use and to build further residential properties at the rear of the site. Initially Ashdown Site Investigation was commissioned to carry out a preliminary ground contamination risk assessment for the proposed development.
Our assessment identified a number of potential sources of contamination, including an existing underground fuel tank, fuel pumps and above ground storage tanks within the vehicle repair workshop. The site is underlain by a sensitive aquifer and is situated within a source protection zone, with a potable water abstraction being located some 770m to the north. The site is also located adjacent to a SSSI to the northwest.
A targeted ground investigation was undertaken using shallow dynamic sampler boreholes. Standpipes were installed around the existing building to assess the potential risk that may have been posed by fuel vapours. Initial monitoring indicated that fuel vapours were present, but the shallow soil testing did not identify any significant widespread contamination. The most likely explanation was that contamination from the existing below ground fuel storage tanks had migrated downwards through the underlying sands and gravels to reach the groundwater which was expected to be present at between 8 and 10 metres depth below ground level.
To assess the risk to end users as well as the nearby sensitive sites, including the water abstraction, a series of deeper boreholes were drilled using a cable percussion rig. The exact location of the underground tank was determined by use of a ground probing radar survey and one of the boreholes was excavated immediately adjacent to the edge of the tank. Standpipes were installed in each of the deeper wells to intercept the groundwater. Samples of the groundwater and gas samples were collected from the deep and shallow standpipes, respectively.
Both the in-situ observations and the laboratory testing confirmed that fuel had leaked from the underground tank and had moved directly downwards to contaminate the underlying groundwater. However, by carrying a detailed risk assessment, using the ConSim model, demonstrated that the fuel contamination did not represent a significant risk to the groundwater abstraction. The detailed gas analysis also confirmed that the fuel contamination did not pose an unacceptable risk to end users of the site.