Performance of Storm Overflows Impacting on Shellfish Waters in England

Storm overflow (SO) discharges to shellfish growing waters are a concern for shellfish growers and may pose a health risk to consumers. We investigated the performance of permitted SOs with a shellfish water spill monitoring requirement against the design criterion of 10 spills per year (averaged over 10 years) used in England. Performance against this criterion over the period 2019–2021 differed between the five water companies whose data was analysed. Across all companies, over half of SOs spilled more frequently than this criterion (percentage greater than 10 spills: 52%, 55% and 57% for 2019, 2020 and 2021, respectively). The number of SOs with the monitoring requirement also differed considerably between the water companies and consequently some companies deal with a significantly higher ‘burden’ than others. The number of SOs spilling more than 100 times in a year also differed between water companies, from 0% (Wessex Water) to 16% (United Utilities). Discharges from SOs can lead to short-term reductions in water quality that may be missed by routine monitoring programmes using faecal indicator bacteria such as E. coli . Such discharges can lead to a higher incidence of norovirus in shellfish and thus potential illness in consumers. We conclude that site-specific impact assessments, supported by spill event-based monitoring, are required given the increased demands on sewerage networks from urban growth and climate change and the need to improve shellfish production area classifications.