The Supply of Small Sewage Discharges (SSDs) to ground risk mapping for England

Diffuse Water Pollution (DWP) can come from a variety of source including, agriculture, small un-consented point discharges and other diffuse sources such as runoff from roads or urban areas.

DWP is a major cause for SSSI and Natura 2000 sites not meeting favourable condition and also not meeting objectives under the Water Framework Directive. Therefore failure to effectively tackle diffuse water pollution presents a significant risk to the delivery of the Biodiversity 2020 and Water Framework Directive requirements.

There is now a body of evidence (May et al, 2015a - NECR170; May et al, 2015b - NECR171; May & Woods, 2015 – NECR179) which suggests that small domestic discharges may pose an environmental risk to freshwater habitats under certain situations.

Currently 166 SSSIs have a DWP plan remedy assigned to the whole or part of the site, reflecting concern about the adverse impact of DWP on protected site condition. Many of these plans flag up small domestic discharges such as septic tanks and package treatment plants as a potential source of nutrients within the catchment. They also highlight the need to have a greater understanding of the risk they pose to be able to understand their significance and where action might be needed to reduce any impacts.

It has also often been difficult to confidently identify where new SSDs can be located without causing unacceptable risks to freshwater SSSIs. Natural England considers that it is important to address this evidence gap to improve the future management of SSDs in the catchments of vulnerable SSSIs.

May et al (in press) developed and applied a risk assessment framework and mapped the risk from SSDs to ground for 20 SSSIs. This project mapped the risk based on 3 key environmental factors:

• (i)Proximity to watercourse
• (ii)Depth to high water table
• (iii)Slope

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