Electrical Resistivity Mapping of Rio Grande River-Groundwater Interactions

Abstract:

Surface-groundwater interactions including connectivity remain a challenge to characterize, especially for ephemeral rivers such as the lower Rio Grande. This investigation used a noninvasive and spatially distributed geophysical method (i.e., time-lapse electrical resistivity) for mapping the water table below and adjacent to the Rio Grande River, which has been validated using groundwater table monitoring well data. Time-lapse monitoring of electrical resistivity (i.e., inversely related to electrical conductivity and also water saturation) before, during, and after the irrigation season has been used to characterize the transient and spatial connectivity of the water table with the base of the Rio Grande from disconnection, to connection, and back to disconnection. Results have shown resistivity impacts due to a coupling of both variations in water saturation and an aqueous electrical conductivity (i.e., salinity) difference between resident groundwater from the previous irrigation season and the infiltrating surface water. The relationships between bulk resistivity versus aqueous salinity and bulk resistivity versus water saturation have been developed using laboratory experiments to evaluate the time-lapse resistivity data and characterization of surface-groundwater connectivity. This type of spatiotemporal groundwater level assessment advances our disconnection process characterization capabilities, and will support the sustainable conjunctive use of surface water and groundwater especially for ephemeral systems.