Water utilities and environmental regulators in the UK are investigating surface water run-off to determine the degree to which surface water sewers are contaminated by bacterial indicator species. This contamination may occur because of sewer abuse; for example, from misconnections of domestic foul waste water to the surface water sewer system instead of the foul sewer system. WRc has carried out research to investigate this potential risk to bathing water quality. The work included a literature review of relevant UK published sources, plus an analysis of WRc's own ''back catalogue'' of environmental evidence.
The research provided evidence to show that under dry weather conditions, flow in surface water sewers is derived from ground water infiltration or culverted streams and the concentrations of faecal coliforms will be expected to be low. However, the level of bacterial pollution will fluctuate considerably between storm events in urban areas and often by several orders of magnitude. A geometric mean concentration of approximately 6000 faecal coliform forming units per 100 ml for urban runoff in the UK may be expected. However, there can be a wide range of faecal coliforms of between 100 - 50,000 coliform forming units per 100 ml, with the higher numbers occurring during storm events following periods of dry weather.
This work illustrates the significance of rainfall run-off from urban areas as an important source of pollution.
Further details on this study can be obtained from Robert Moore at robert.moore@wrcplc.co.uk or telephone +44 (0) 1793 865054.