A broad-based approach has been used to assess the impacts of discharges to rivers from 47 surface water severs, with the objective of determining whether such discharges are damaging to stream quality.

In order to study as many sites as possible, sampling, laboratory and data analysis techniques were designed to be as simple and rapid as possible. This broad approach was deliberately chosen to contrast with other UK studies in which a small number of sites have been investigated in detail.

Three parameters were studied, all of which could reflect the effects of intermittent pollution on stream quality during dry weather. These were the numbers and types of benthic nacroinvertebrates upstream and downstream of the outfalls, the concentrations of metals in algae upstream and downstream of the outfalls, and the concentrations of metals in sediments upstream and downstream of the outfalls.

Information relating to the study catchments has been collected from local authorities and by observation at the time of sampling. This information includes catchment areas, land uses and receiving stream quality.

Methods used for site selection, sampling, analysis and data interpretation are described. Results show that there is a small but significant fall in biological water quality downstreap of outfalls, but no consistent detectable impact on the concentrations of metals in sediments or algae.

The biological effects are compared with the catchment characteristics to identify the factors governing the impact, and upstream water quality is found to be an important factor. Other factors influencing the impact are sewered catchment area and land use.

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