Groundwater redox conditions and conductivity in a contaminant plume from geoelectrical investigations

Accurate mapping of the electrical conductivity and the redox potential of the groundwater is important to delineate the shape of a contaminant plume. The map of the redox potential in an aquifer indicates the presence of biodegradation of organic matter and concentrations of redox-active components. The map of the electrical conductivity provides information on the mineralization of the groundwater. Both maps can therefore be used to optimize the position of pumping wells for remediation. We applied the self-potential method (SP) and the electrical resistivity tomography (ERT) to the contaminant plume associated with the Entressen landfill located in the South East of France. The self-potential depends on groundwater flow (electrokinetic contribution) and redox conditions (“electro-redox” contribution). Using the variation of the piezometric head in the aquifer, the electrokinetic contribution is removed from the SP signals. A good linear correlation (R²=0.85) is obtained between the residual SP data and the redox potential values measured in monitoring wells. This relationship is used to draw a redox potential map of the overall contaminated site. The electrical conductivity of the subsoil is obtained from 3D-ERT analysis. A good linear correlation (R²=0.91) is observed between the electrical conductivity of the aquifer determined from the 3D-ERT image and the conductivity of the groundwater measured in boreholes. This indicates that the formation factor is nearly homogeneous in the shallow aquifer at the scale of the ERT. From this correlation, a map of the pore water conductivity of the aquifer is obtained.