Advances in Water Treatment and Enviromental Management

204 WATER TREATMENTThe investigations confirmed the conceptual model whereby chlorinated solvents, spilled atthe ground surface, accumulate at the top of the drift, in small depressions on the irregularsurface of the low permeability putty chalk. The solvents then seep slowly through this layerinto the fissured chalk aquifer below, where flow rates are rapid. This is shown schematicallyin Fig. 4.The Phase II shallow drilling programme identified severe contamination by organic solventsat 4 of the 5 locations investigated. The profiles of contamination with depth at one of thesites are shown in Fig 3. All four boreholes at this site were within 100 m of Phase I Borehole6 that had detected high concentrations of TCE and, to a lesser extent, PCE and TCA at thewater table in the chalk aquifer. One of the boreholes (2/3), sited adjacent to a building inwhich solvents had been used, detected very similar concentrations of TCE (18100 µg/1),PCE (1720 µg/1) and TCA (1740 µg/1), at least 2m above the water table; CTC was alsopresent at higher concentrations than those found at any site during Phase 1. The threeremaining boreholes failed to detect high concentrations of contaminants.Similarly varied profiles of solvent contamination were obtained from closely-spaced boreholesat the other sites.Fig 4 Sectional View through Chalk Aquifer—Case Study ‘A’Fig 5 Site Illustration—Case Study ‘B’ (Schematic)