Stream quality in small urbanised catchment.

River-length patterns in the chemistry and biology of the Charlton Brook, an unclassified watercourse in northwest Sheffield have been examined. Sampling sites for macroinvertebrates and pollutant analysis were used, in conjunction with Environment Agency General Quality Assessment (GQA) methodologies and hydraulic analysis of the catchment. Sites were strategically located to account for the tributaries and the brook downstream of their confluence, to assess the potential impact from surface water outfalls (SWOs).Variations in GQA parameters indicate a significant drop in quality downstream of the SWOs that discharge to the study watercourse, with a marked drop in biological diversity noted at the onset of urbanisation. The decline in biological quality however, is greater than that suggested by physico-chemical analysis alone. There was a significant inverse relationship between impermeable area and biological diversity.Analysis of polycyclic aromatic hydrocarbons (PAHs) and trace metals in sediment from the watercourse showed significant, yet irregular between site variations. PAHs in conjunction with metals as a function of the PEL-quotient method employed, suggest that all the sites sampled for macroinvertebrates have the potential for being adversely affected by the pollutants contained within the brook's sediment. Mean PEL-quotients suggest that sediment contamination within the brook is indicated at all sites.The potential toxicity of instream metal concentrations was determined using cumulative criterion unit (CCU) scores. CCU analysis highlighted cadmium, copper and lead as the major sources of potential instream toxicity with all sites exceeding the threshold for likely harm to aquatic life.In the absence of different physical characteristics, comparisons of the chemical and biological data indicate that the benthic macroinvertebrate population of such watercourses are adversely affected by the stormwater inputs.