Risk Assessments for Treated Effluent Soakaways

As part of an application for an Environmental Permit for a soakaway discharge,  a risk assessment needs to be completed. For small discharges in non-vulnerable environmental settings, the Environment Agency applies a screening level assessment to identify whether they need further information before issuing the permit. For larger discharges or those in sensitive environments (for instance on principal aquifers, within catchments of public supply boreholes etc.) a more detailed, qualitative, risk assessment is needed.

The procedure for undertaking a qualitative risk assessment is given in the agency’s Horizontal Guidance note H1 Annex J4. It requires a firm conceptual understanding of the local groundwater flow system and physico-chemical properties of the contaminants that are being disposed of.  The hydrogeological system is modelled using a spreadsheet model to predict the impact of the discharge on a receiving water. Uncertainty analysis is then used to make sure that the model predictions are robust.

Dr Stephen Buss has been undertaking risk assessments for protection of the water environment for more than 15 years. He has extensively used the Environment Agency’s risk assessment model and taught numerous courses on its use. He also co-wrote Environment Agency guidance on risk assessment of ammonium in groundwater – the contaminant of most concern in discharges of treated sewage to ground.

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