Assessing Aquifer Recharge with 4D Resistivity Tomography at Urban SCMs

Abstract Managed aquifer recharge in urban basins increasingly relies on small-footprint stormwater control measures (SCMs), yet their actual infiltration performance and contribution to groundwater recharge remain poorly constrained after installation. This study evaluated two contrasting SCMs in the Los Angeles Basin — a deep drywell beneath a paved parking lot and a shallow bioswale within a narrow right of-way — using controlled recharge experiments monitored with borehole-based, time-lapse electrical resistivity tomography (ERT). Instrumented boreholes were installed around each SCM to enable 3D (drywell) and 2D (bioswale) imaging, and ERT data were collected autonomously over several days, during which known volumes of water were applied to each system for calibration. Soil moisture was independently measured with neutron probes and in situ sensors to validate resistivity-derived wetting patterns. Results showed that the drywell rapidly conveyed water from shallow pretreatment features and deeper screened intervals into laterally extensive and stratigraphically controlled pathways, with preferential f low observed along higher-permeability layers. By contrast, infiltration in the bioswale was limited to the upper 2 m, even under elevated water levels exceeding typical storm conditions, indicating shallow storage and limited deep percolation. In both settings, changes in resistivity closely matched changes in measured soil moisture, confirming the ability of bore hole ERT to resolve hydrologic processes in constrained built environments. The findings demonstrated that SCM infiltration performance is highly site specific, influenced by engineering design and subsurface heterogeneity, and that geophysical monitoring provides critical information for accurate recharge estimation, SCM efficacy and siting, maintenance planning, and long-term water-resource management in urban basins.