Development of a risk assessment tool to evaluate the significance of septic tanks around freshwater SSSIs.

Published online
21 Jan 2021
Published by
Natural England
Content type

May, L. & Dudley, B. J. & Woods, H. & Miles, S.

Publication language
UK & England & Scotland


There is growing evidence suggesting that small sewage discharges (SSDs), in practice mainly septic tank systems but also including package treatment plants, may pose a significant environmental risk to freshwater habitats in certain situations and under certain conditions. However, the extent of this risk and its potential impact across the freshwater site of Special Scientific Interest (SSSI) series are not well understood. For this reason, there is limited information with which to make evidence based decisions on where such systems can be located safely in rural areas. The overall aim of the project was to develop a general methodology that could be used to estimate the number and location of SSDs within the catchment of freshwater SSSIs and assess their relative likelihood (low, moderate, high) of causing phosphorus (P) pollution to those waterbodies. The assessment was a desk study focusing on the use of nationally available datasets and did not involve sampling trips or site visits. It focused on three main factors: proximity to a surface waterbody forming part of the catchment drainage system, slope of the terrain and depth to high water table. The project focused on 20 freshwater SSSI demonstration sites, as agreed with Natural England in consultation with local site officers. The risk assessment methodology was developed and applied across a wide range of freshwater SSSI sites. These varied greatly in terms of catchment area, the smallest being Hawes Water (2.5 km2) and the largest being the River Avon above Southampton (1,669 km2). They also varied in terms of geographical location and waterbody type: ie river, lake or wetlands. The main output from the project is a series of three ArcGIS map layers for each SSSI and its catchment. These comprise: (1) surface water catchment boundary; (2) map of low, moderate and high risk zones for SDD locations within the catchment; (3) map showing the likely locations of SSDs within the SSSI and its catchment and the relative level of risk that they pose in terms of causing eutrophication problems in a SSSI waterbody through P pollution. Following testing and validation at the 20 sites included in this study, consideration should be given to addressing any issues raised and rolling this risk assessment process out across all of the freshwater SSSIs that have P pollution issues. If testing and validation are successful, rolling out the risk assessment process at the national scale would also enable Natural England to identify and prioritise SSSIs where P pollution from SSDs is likely to pose the highest risk to water quality. This would also enable limited resources to be targeted effectively at the national scale. This risk based approach reflects the general aspirations of the new general binding rules approach to the management and control of SDD discharges, which was introduced in England in January 2015. Although developed for use in England, this approach may also be of interest to conservation agencies and regulatory authorities in other countries with similar problems, especially Wales and Scotland.

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