A modelling assessment of contaminant distributions in the Severn Estuary

The regulatory requirements imposed by the Habitats Directive (EU 93/43/EEC) require the Environment Agency for England and Wales (EA) to review consented discharges and determine whether they are compliant with Environmental Quality Standards (EQS). Since the EQS are annual averages, model predictions, and sample comparisons, should be made on an annual average basis. Advection and dispersion of metal contaminants in the Severn Estuary were computed using a coupled 1-D and 2-D hydrodynamic-water quality model. The external inputs of dissolved copper, arsenic, mercury and chromium to the model were from 66 industrial discharges and sewage treatment works and 30 rivers. The annual average predicted concentrations were compared with the annual average dissolved metal concentrations from the 2004 and 2005 monitoring programme, and any discrepancy used to identify the role of additional processes, mainly involving the sediments. This ability to separate anthropogenic inputs from internal estuarine processes contributes to a better understanding of the functioning of the estuary and hence an improved management capability. The paper discusses the approach in designing scenarios and characterising uncertainty, when decision-making in the regulatory context.