4D ERT Monitoring of Simulated Leaks from a Nuclear Storage Silo at Sellafield, UK

Results are presented from a 2-year full-scale field trial of Electrical Resistivity Tomography (ERT) monitoring of simulated leaks from a waste storage silo at Sellafield, UK. The trial included the monitoring of a series of injections of conductive tracer fluids to simulate leaks, as well as an extended monitoring period to assess the fate of the resulting saline plume. The trial constituted the first application of ERT subsurface monitoring at a UK nuclear licensed site. The work was undertaken to evaluate the Technology Readiness of ERT in terms of detecting, spatially characterising and monitoring potential leaks to the subsurface of silo liquor from a storage structure. Despite the weak conductivity contrast of the tracer fluids compared to the groundwater, and the loss of one of the ERT boreholes during the trial, full 4D inversion proved sufficiently sensitive to detect and image the passage of the tracers. Images of resistivity changes relative to a reference baseline have revealed likely pathways of tracer flow in the vadose zone and upper groundwater system. These pathways were found to be in close agreement with the locations of historic contamination detected in sediment cores retrieved from the ERT boreholes.