The Impact of Storm Sewage Discharges on the Ecology of a Small Urban River

Abstract

Intermittent discharges of storm sewage from combined sewer overflows continue to be one of the principal causes of poor water quality in many urban rivers in the UK. Despite the persistent nature of this problem, very little attention has been given to the study of how discharges of varying magnitude, duration and frequency affect the ecological quality of receiving waters. This information is of critical importance for deriving meaningful water quality criteria for the control of intermittent pollution. This paper describes the results of a study which has been carried out on Pendle Water, a river which flows through the urban catchment of Burnley, Lancashire, UK. Both the chemical and biological quality of Pendle Water are adversely affected by storm sewage discharges during heavy rainfall events. The ecological investigation has been primarily concerned with impact of these episodic discharges on benthic invertebrate communities and physiological responses in fish. Quantitative sampling of macroinvertebrates has indicated that storm sewage discharges may have a significant impact on the structure and diversity of benthic communities in receiving waters. Physico-chemical properties of habitats appear to be altered in a way which tends to favour the proliferation of certain pollution-tolerant species and decrease the abundance of taxa intolerant of organic pollution. Insitu bioassays, including the WRc Mark III Fish Monitor, have been deployed to investigate physiological responses to storm events of different magnitude, duration and frequency. Results are discussed in relation to their application in the field validation of proposed water quality criteria for the control of intermittent pollution from combined sewer overflows.