Combined self-potential and electromagnetic imaging provides insights into the spatial variation of wetland soil hydro-biogeochemical properties

Abstract

This study assesses the innovative use of self-potential (SP) combined with electromagnetic imaging and direct soil properties measurements for characterizing the distribution of wetland soil organic matter (SOM), soil moisture content (SMC), and redox potential (E_H). SP distributions were measured using a high-impedance multimeter, Cu-CuSO4 non-polarizing electrodes, and a fixed reference electrode approach at two wetlands in northwest Ohio. The SP data were compared with laboratory measurements of SMC and SOM on 16 soil samples and 95 direct field measurements of E_H and SMC. Soil apparent electrical conductivity (ECa) and magnetic susceptibility (MSa) were also acquired along co-located transects with the SP to assess the source strength of the SP and to aid the interpretation of the SP signals. Results of this study show variation in SP magnitude of up to 45 mV over 400 m in the wetlands. Negative SP anomalies were observed around areas with drainage tiles with preferential infiltration. The measured SP correlates with SOM, SMC, and E_H, with R² values of 0.67, 0.51, and 0.74. A combination of SP signals with soil electrical conductivity produced current density cross-sections, which provided information on the depth and intensity of the source charges generating the SP signals. Regions with high ECa and low SP signals show corresponding low MSa interpreted as anaerobic reducing zones. These results show the existence of natural potential gradients within wetland soils related to electrokinetic and electrochemical effects and validate SP as a useful complementary technique for characterizing wetland soils and monitoring their biogeochemical functioning.