Great Bridge Marl Pit: a case study in the prevention of contaminant migration

Abstract

Abstract Great Bridge Marl Pit is a former brick pit located in the West Midlands. The pit is infilled by an assortment of materials including a wide range of toxic and aggressive substances (most significantly phenolic liquids). Phase 2 of the Black Country Spine Road (opened 1995) crosses the pit on a raised embankment. Prior to the construction of the road, the site was derelict and contaminated groundwater within the pit was thought to present only a minor risk to the adjacent River Tame and virtually no risk to local groundwater abstractions. Mobile contaminants within the pit were contained by a combination of the geometry of the pit, the location of the bulk of the contaminative materials within the base of the excavation and the low permeability of the base of the excavation. However, impact of road construction over the site, in particular the preconsolidation of the pit infill by surcharging, was considered to have the potential to mobilize significant volumes of contaminated groundwater from within the pit and therefore to present a considerable risk of pollution of the River Tame. To minimize the potential for contaminant migration, groundwater flow within the Marl Pit and in its immediate vicinity was controlled during the surcharging period by the installation of a groundwater abstraction system. Contaminated water abstracted during the surcharging was disposed of to trunk sewer and treated at the local water treatment works. The efficiency of the groundwater abstraction system was assessed by monitoring groundwater pressures and concentrations of contaminants in a number of strategically located boreholes around the margins of the site. During the surcharging period the groundwater control system successfully controlled excess groundwater pressures generated beneath the surcharge and there was no evidence of an increase in contaminant migration from the pit; in fact a considerable reduction in the volume of contaminated groundwater present within the pit is likely to have occurred.