Accurate mapping of the electrical conductivity and redox potential of groundwater is important in delineating the shape of a contaminant plume. A map of redox potential in an aquifer is indicative of biodegradation of organic matter and of concentrations of redox-active components; a map of electrical conductivity provides information on the mineralisation of the groundwater. Both maps can be used to optimise the position of pumping wells for remediation. The self-potential method (SP) and electrical resistivity tomography (ERT) have been applied to the contaminant plume associated with the Entressen landfill in south-east 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. There is good linear correlation (R²=0.91) 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.

[1] In situ measurements of redox potential are rather difficult to perform and provide only sparse information on its spatial distribution. To delineate redox fronts in a contaminant plume, the self-potential (SP) method can be a helpful complement to geochemical measurements. Here, we apply the SP method to the Entressen municipal waste landfill (south-eastern France) over a 20 km 2 area. The results show a large negative SP-anomaly of $À400 mV with respect to a reference station taken outside the contaminant plume. Once removed the electrokinetic component associated with groundwater flow, the residual self-potential signals are linearly correlated with in situ measurements of redox potential. We propose a quantitative relationship between self-potential and redox potential, which would be used to invert self-potential measurements in terms of in situ redox potential values in contaminant plumes.

[1] Two sets of experiments were designed to understand the change in induced polarization associated with the sorption of copper and sodium, exhibiting distinct sorption behavior on a silica sand. A sand column experiment was first performed to see the change in the complex conductivity during the advective transport of a copper sulfate solution. A second set of experiments was done with the sand at equilibrium with various solutions of NaCl and CuSO 4 . In the first experiment, the copper sulfate solution replaced a sodium chloride solution, keeping the electrical conductivity of the solution nearly constant. During the passage of the copper sulfate solution, the apparent phase angle decreased from 3 6 0.2 to 0.5 6 0.2 mrad, while the magnitude of the conductivity of the sand remained nearly constant. A quantitative model is proposed to explain the change in the complex conductivity as a function of the chemistry assuming a polarization mechanism associated with the Stern layer (the inner part of the electrical double layer coating the water-mineral interface). The Stern layer polarization is combined with a complexation model describing the competitive sorption of copper and sodium at the pore water interface. The change of the phase lag is directly associated with the ion exchange between sodium and copper at the surface of the silica grains. The explanation of the observed phase differences between Na and Cu relies on their different complexation behaviors, with Na being loosely absorbed, while Cu forms relatively strong complexation with both inner (monodentate) and outer sphere (bidentate) complexes. The replacement of Cu 2þ by Na þ is less favorable; therefore, the kinetics of such a replacement is much slower than for the opposite replacement (Na þ by Cu 2þ ). We were able to reproduce the changes in the phase lags at thermodynamic equilibrium near the relaxation frequency and in the frequency domain. These measurements and modeling results open the door to the quantitative interpretation of spectral induced polarization data in the field in terms of quantification of the sorption processes.Citation: Vaudelet, P., A. Revil, M. Schmutz, M. Franceschi, and P. Bégassat (2011), Induced polarization signatures of cations exhibiting differential sorption behaviors in saturated sands, Water Resour. Res., 47, W02526,

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