Time-lapse electrical resistivity monitoring of salt-affected soil and groundwater

In order to develop and test a methodology for incorporating time-lapse electrical resistivity imaging (ERI) into the monitoring of salt-affected soil and groundwater, a multifaceted study including time-lapse electrical resistivity imaging, push tool conductivity (PTC), and core analysis was conducted to monitor the movement of a saline contaminant plume over the span of 3 years. The survey was done on a field site containing salt-affected soils and groundwater to depths of over 7 m. The site contained a tile drain system at approximately 2 m below ground level. Temperature and saturation changes were accounted for in electrical conductivity (EC) measurements to isolate changes in electrical conductivity due to changes in salt distribution. ERI inversion parameters were selected so that the inverse models gave the best match to PTC depth profiles and the best correlation with core EC data. A strong correlation between the core data and the ERI results was observed. Time-lapse ERI difference images showed that the subsurface EC distribution was consistent with preferential solute removal above the tile drains in some locations. The ERI-delineated reduction in solute concentration is consistent with nonuniform flushing due to depression-focused recharge. The addition of time-lapse ERI to the study allowed delineation of details of solute redistribution that would not have been possible with point measurements alone.