Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2025
Publication Date: 4/8/2025
Citation: Arboleda-Zapata, M., Osterman, G.K., Li, X., Sasidharan, S., Dahlke, H.E., Bradford, S.A. 2025. Time-lapse ensemble-based electrical resistivity tomography to monitor water flow from a managed aquifer recharge site. Journal of Hydrology. 659; 133282. https://doi.org/10.1016/j.jhydrol.2025.133282.
DOI: https://doi.org/10.1016/j.jhydrol.2025.133282

Interpretive Summary: Drywell technology can be used to inject recharge water into the unsaturated zone, bypassing shallow impermeable layers and more likely reaching the underlying aquifer. However, there is little research to date regarding the efficacy of drywells. We employ a geoelectric array to image the changing electrical resistivity of the subsurface during recharge operations, where the changing water content and solutes in the subsurface will appear as changes in our resistivity images. There is inherent uncertainty in any image of subsurface resistivity, so we apply a novel scheme to derive an ensemble of possible resistivity models at each time step. These solutions illuminate features that were not visible using standard image recovery schemes, such as perched water layers forming in the vadose zone. Additionally, our results indicate that the drywell operations likely flushed solutes stored in the vadose zone, and the movement of water in the subsurface slowed considerably after the cessation of water injection into the drywells. These results have implications for modeling recharge efficacy and efficiency using drywell technology.

Technical Abstract: Depleted groundwater resources can be replenished using a variety of managed aquifer recharge (MAR) strategies such as drywells. An understanding of water flow in the vadose zone is crucial for assessing the performance of drywells. To obtain reliable estimations of water flow, point-scale observations from monitoring wells can be complemented with geophysical monitoring tools such as time-lapse electrical resistivity tomography (TL-ERT), which is sensitive to water and salinity changes. In this study, we demonstrate the applicability of TL-ERT for monitoring the performance of five drywells in the Central Valley of California over one year (March 2023 – February 2024). This MAR operation aims to alleviate stress on the regional aquifer caused by excessive over-pumping and changing climate conditions, which caused a water table decline of over 75 meters in 100 years. Drywell technology allows for bypassing impermeable layers and possible contaminated soils near the land surface. To invert our TL-ERT data sets, we use geostatistical constraints that favor layered models as expected due to the alluvial deposits in our study area. By considering different correlation lengths, we obtain an ensemble of resistivity model solutions per time-step instead of a single model solution (as typically performed). Model differences between the mean model of the baseline data set and the models from the subsequent time steps allowed us to image the wetting front development until reaching the regional aquifer, a perched water table, and flush of salts that were otherwise not visible when using single model solutions from standard deterministic TL-ERT inversion approaches.