Abstract

Historic soil and groundwater contamination has been observed at a former gas works site subject to redevelopment in Dartford, Kent, UK. Site characterisation has confirmed low-level dissolved phase PAH contamination in groundwater. Tar residues have also been observed in the matrix of the sand and gravels on site. These tars represent highly weathered coal tars that include high molecular weight, low mobility compounds. These tar deposits have given rise to elevated concentrations of dissolved phase compounds that are attenuating rapidly. An assessment of fate and transport has indicated that the concentrations of all analytes modelled are projected to be below adopted guideline standards at a maximum distance of 5 m horizontal from the individual source areas. The sources have been identified to be associated with a former benzol plant and liquor plant, together with tar storage tank area. Dartford Borough Council subsequently granted planning approval for the site, subject to the implementation of a monitoring and assessment strategy to establish the stability of the dissolved and free-phase source areas and to demonstrate that natural processes of attenuation are effectively mitigating any risks. The process of monitored natural attenuation (MNA) as an appropriate remedial option has been agreed with the Environment Agency (EA) based on a modified version of R&D 95, and has been implemented at the site. Abbott's approach to this site was one of establishing an effective relationship with the regulators early in the process, and combining site mitigation with development. This facilitated efforts required in the long term, and allowed all parties’ goals to be achieved.

Key words: natural attenuation, regulatory discussion, site development