Development of a risk assessment tool to assess the significance of septic tanks around freshwater SSSIs: Phase 1 - Understanding better the retention of phosphorus in the drainage field.

Published online
02 Feb 2021
Published by
Natural England
Content type

May, L. & Withers, P. J. & Stratford, C. & Bowes, M. & Robinson, D. & Gozzard, E.

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This project was undertaken to investigate some of the processes to protect SSSIs and provide information to underpin the development a 'family' of buffer zones to help protect SSSIs from P enrichment associated with rural development. The project was divided into two parts: (1) Factors affecting the discharge of P from the septic tanks to the drainage field. (2) The movement of P in septic tank effluent plumes through the aerated zone of the drainage field, i.e. within the soil that is above the water table. It should be noted that the results and conclusions from this study do not apply to situations where STS discharge directly to water, or where there is enhanced connectivity between STS discharges and waterbodies via pipes and field drains or through groundwater movement. The results apply to the part of the soakaway that comprises aerated soil that sits above the water table, only. The average P concentration in effluent discharged from the 11 treatment tanks studied was about 11 mg P/litre of soluble reactive P (SRP) and 15 mg/litre of total phosphorus (TP). Although a package treatment plant (PTP) had been installed at one of the sites to improve the quality of the effluent discharged, there was no evidence that this had been effective in reducing P concentrations. Discharges from this tank were still high, ie 10.7 mg P/l of SRP and 12.9 mg P/litre of TP, and were very similar to those measured in effluent from the older and more traditional tanks. Although no sound conclusions can be drawn from this one example, it is recommended that this issue is investigated further. This is because the current perception that discharges from PTPs are much lower in P content than those from standard septic tanks underpins guidance that allows PTPs to discharge directly into watercourses if installed correctly. The borehole data showed significant P enrichment of the soils to a depth of just over 1 m below the effluent distribution pipes. The observed reduction in soil P enrichment with depth suggests that much of the P in the effluent that moves vertically through the aerated part of the soil profile is retained by those soils. Although based on a small number of samples from only 2 sampling points with the same soil type, this strongly suggests that the soils can provide an important function in terms of P retention within these systems. This function is likely to be compromised if the soil becomes waterlogged, as may occur during periods when the water table is high. The results suggested that it may be very important that soil soakaways should be situated in areas that provide a significant depth of aerated soil below the effluent distribution pipes. However, this requires further investigation.

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