ESI Ltd. was appointed to develop a groundwater flow and contaminant transport model of the low level radioactive waste repository (LLWR) site in Cumbria. The findings of the model were used to update the hydrogeological conceptual model of the site and to assist in parameterising the risk assessment model for the site.

The hydrogeological system at the LLWR is complex and is defined as having two groundwater systems. Within the upper groundwater system, there are downwards hydraulic gradients as well as lateral gradients. Underlying the upper groundwater is the regional groundwater system where the vertical gradients are much smaller. Historical waste disposal trenches are located within the upper groundwater, with protection of groundwater afforded by cut-off walls. It was not clear as to whether potential source, pathway, receptor linkages exist between the trenches and nearby surface water bodies, or whether the contaminant transport direction is downwards to the regional groundwater, with discharge to the sea.

ESI developed a FEFLOW finite element model of the groundwater system. The model was calibrated to observed groundwater levels, hydraulic gradients and estimates of base flow to the site streams, by adjusting the model parameters.

It was found that a good match to the site data could be achieved for most of the groundwater monitoring wells and the model reproduced the downwards hydraulic gradients between the upper and regional groundwater systems across most of the model domain.

The model was then used to undertake scoping contaminant transport calculations which again largely agreed with the site data.

The study concluded that the majority of potential contamination from the trenches would migrate downwards to the regional groundwater and thence discharge to sea rather than to nearby surface water receptors. This information was taken forward in the post-closure risk assessment model.

Please contact ESI (01743 276100 / email) for more information on 'Groundwater and contaminant transport modelling'.