An assessment of the impact of motorway runoff on a pond, wetland and stream

The impact of soil filtered runoff from a section of the M25 outer London m otorway (constructed in 1981) on a pond. wetland and stream in a nature res erve was investigated by monitoring water, sediment. The tissues of the eme rgent plants Typha latifolia and Glyceria maxima collected from the pond we re analysed for the heavy metals, Cd, Pb, Cu and Zn. Macroinvertebrates wer e monitored in the stream and biotic indices applied to the data. The plant tissue concentrations for Typha and Glyceria show decreasing metal concent rations from root to rhizome to leaf. This trend has previously been report ed for Typha exposed to runoff although the tissue concentrations are lower in this study with the exception of Cd in root tissue. The Biological Moni toring Working Party (BMWP) score and Average Score Per Taxon (ASPT) for th e stream at sites above and below the pond outlet are lower than the scores recorded by the Environment Agency for England and Wales at an upstream si te above the Pond/Wetland. The sites have an Overall Quality Index of 'mode rate water quality', and there is no evidence of a deterioration of biologi cally assessed water quality between them. The results of the study show th e long-term impact on sediment of filtered road runoff discharges to a natu ral wetland and pond located in a nature reserve. The use of natural wetlan ds for the discharge of road runoff is inadvisable. Constructed wetlands in combination with other structures including settlement trenches and ponds should be considered as an alternative treatment option. (C) 2001 Elsevier Science Ltd. All rights reserved.