Electrical Resistivity Mapping of Lower Rio Grande River-Groundwater Interactions

Published Date:
July 2023

Autumn Joy Person, Kenneth C. Carroll, Dale F. Rucker, Chia-Hsing Tsai, Erek H. Fuchs

In the current era of rapid environmental changes, more rivers are projected to dry up and transition to disconnected systems in unprecedented duration and frequency. However, surface-groundwater interactions including connectivity remain a challenge to characterize, especially for managed-ephemeral rivers such as the lower Rio Grande in southern New Mexico where conjunctive use for irrigated agriculture is prevalent. 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, which has been validated using groundwater table and conductivity monitoring well data. Time-lapse monitoring of electrical resistivity (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. The relationships between electrical conductivity from the geophysical analysis versus the large array of ancillary data helped to narrow down the driver of observed temporal changes in connectivity after the transition from disconnection to connection. Results have shown resistivity impacts due primarily to variations in water saturation during Rio Grande water arrival, and some influence after water arrival from water temperature and aqueous electrical conductivity (i.e., salinity) differences between resident groundwater and the infiltrating surface water. The relationships between bulk conductivity versus aqueous salinity and water saturation have been developed using laboratory experiments and correlation coefficient analysis to evaluate the time-lapse resistivity data, the characterization of surface-groundwater connectivity, and the transition from losing to gaining stream. The surface water temperature showed a direct and strong correlation to the electrical conductivity of shallow sediments, which was similar to the river stage/groundwater elevation drivers of infiltration. 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 non-perennial systems.

Technical Report 405

Rio Grande; electrical resistivity; ephemeral; non-perennial; surface water disconnection