Abstract The long term disposal of radioactive waste in an underground repository requires the detailed geological evaluation of a potential site. Owing to their inherent sensitivity to the presence of fluids in rocks, electromagnetic (EM) methods have an important role in this assessment. Controlled-source EM techniques are especially useful in strong anthropogenic noise environments such as industrial locations. However the complexity of modeling and inversion can limit the quantitative interpretation of controlled-source EM data. A potential radioactive waste disposal site at Sellafield in Great Britain has been investigated using a variety of EM exploration techniques. Controlled-source audiofrequency magnetotelluric (CSAMT) data have given the best subsurface information in an environment that has a high level of cultural noise. One-dimensional inversions of the Sellafield CSAMT data were found to be inadequate; 2.5-D forward modeling and inversion were used to interpret the data. The resulting resistivity models show good agreement with well log data collected at the site. These resistivity models show the presence of a large zone of hypersaline groundwater extending 1 km inland towards the potential repository and indicate the effect of faults on the hydrogeology.