The effectiveness of a sedimentation tank, a lagoon and a french drain in removing pollutants from motorway run-off has been assessed at an experimental catchment on the M1 motorway near Toddington, Bedfordshire. As expected, the drainage systems reduced the levels of suspended solids but had no effect on the concentrations of dissolved material, but there were seasonal variations in their effectiveness. The lagoon and the french drain produced substantially larger reductions in pollutants than the sedimentation tank and also have a predicted longer period of effective operation. (A)

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