WRc consulted the Environment Agency regarding Combined Storm Overflow (CSO) impacts on inland compared to coastal bathing waters.
How WRc model catchment water qualitydedicated investigations involving tailored simulation of the client's case using expert modelling software
the client with the required answers, along with detailed explanations of the observed consequences
legislative decisions regarding human health were thoroughly investigated and not based on assumptions
There is increased interest in designating more riverine waters as safe to bathe in under the legislative framework that ensures bacterial monitoring and classification of both inland and coastal bathing waters. Bathing Water status is undertaken during the May to September bathing season by comparing the weekly monitoring data to 95%ile (90%ile for Sufficient) standards for intestinal enterococci and E. coli. These bacteria are introduced to rivers from point and diffuse sources, including: continuous sewage discharges, intermittent overflows (such as Combined Sewer Overflows (CSOs)), agricultural runoff, and wildlife.
Loads introduced to rivers vary considerably. In extreme wet weather, contributions from intermittent and rainfall-generated runoff can increase relative to continuous discharges operating in both dry and normal wet weather conditions. In recognition of this, spill frequency standards have been developed to protect the status of estuarine and coastal designated bathing waters; an average of either 2 or 3 spills per bathing season is permitted for either Excellent or Sufficient/ Good waters.
The primary question to be addressed for the EA was whether the 3 (or 2) spill frequency design standard applied to estuarine and coastal bathing waters is also appropriate to riverine bathing waters, given the different factors that arise relating to intermittent discharges.
A SIMPOL modelling exercise was undertaken on an idealised river catchment that simulated representative inputs of E. coli from point and diffuse sources to a river comprising a bathing water.
The investigation concluded that it was likely that background concentrations of E.coli would cause failure of the Sufficient bathing water classification standard; adopting the same approach as used in the coastal situation is not appropriate for the following reasons:
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