Problems of quality designation in diffusely polluted urban streams — the case of Pymme's Brook, north London

Abstract

Downstream patterns in the biology and bacteriology of Pymme's Brook (north London) between 1985 and 1992, are compared with the local Environment Agency (EA, England and Wales) quality classification of the site, revealing a considerable discrepancy. Although downstream contaminant dispersal patterns showed that at low flow the brook was less successful in absorbing contaminants at polluted surface water outfall (PSWO) entry points than it was at high flow (supporting the low flow strategy of contemporary audit schedules), this effect was found to vary downstream in response to reoxygenation from less polluted outfalls. Additionally, temporal variations in suspended solids, Escherichia coli counts, sediment-bound and soluble pollutant concentrations at low flow, and during two sampled storms, revealed that significant pollutant transfer was concentrated in the ‘first flush’ of storm events. A downstream survey of sediment-bound lead (Pb) found that concentrations in the bed sediments, which were likely to be mobilised during a first flush, were significantly higher than in solution. So, a complex pattern of downstream contaminant dispersal emerges, which varies with differing antecedent conditions, and through storms. These results indicate that: (1) the new General Quality Assessments (GQA) audit schedule proposed by the EA for England and Wales remains inappropriate for diffusely polluted, urban environments, because it omits routine E. coli counts and sediment-bound heavy metals; and that (2) when audit is based on chemical determinants alone, the choice of site and timing of audit excessively influence quality designations. Pre-audit planning surveys and more reliable alternatives to the use of chemical audit for urban watercourses, are discussed as possible ways forward for the design of quality audit schedules. Implications for the monitoring schedules in operation in other EU countries and the USA are also considered.