Abstract

The remediation of a former manufactured gas plant in the north of England is the subject of a multi-disciplinary research programme into the hydrobiological controls on the transport and remediation of organic pollutants. Production of town gas at the 0.7 ha site started in the 1850s and continued for about one hundred years. The aquifer beneath the site is unconfined fine-grained sand, with depth to groundwater of approximately 1m. Following an initial remedial phase, which involved the removal of major contaminant sources (tar tanks and gasholder bases), groundwater and residual phase contamination remained. Target contaminants include phenols, BTEX and light fraction polynuclear aromatic hydrocarbons.

A full-scale air sparging treatment curtain was installed at the site, and was operated between August 1999 and May 2003, ultimately achieving successful closure with the regulator. In addition to monitoring dissolved phase organic contaminant concentrations, detailed monitoring of the physical behaviour of the system during pulsed air injection cycles was undertaken, which provides detail of air flow patterns at field scale. Results provide evidence that air channels under a pulsed injection are ephemeral features, and air coverage of the subsurface is considerably more uniform than antic-ipated. This has major implications for modelling mass removal for such systems.

Monitoring of dissolved phase inorganic parameters implied the use of sulphate as an electron acceptor in the biodegradation of BTEX contaminants (principally benzene) - although oxygen was being supplied to the subsurface, monitoring showed that the demand for oxygen exceeded supply, hence other electron acceptors may also have been utilised. This hypothesis is consolidated by the observation that sulphate concentrations began to rise again once the benzene contaminant concentrations were reduced to below detection limits. Similar variations were also seen in iron^II concentrations, which suggest that it was being utilised in a similar way.

Key words:air sparging, BTEX, groundwater mounding, sulphate