Managed aquifer recharge site assessment with electromagnetic imaging: Identification of recharge flow paths

Abstract

Surface spreading recharge, the intentional flooding of the ground surface to replenish a groundwater system, is one approach used to mitigate groundwater overdraft in California's Central Valley (CV). Choosing appropriate sites for surface spreading recharge, in regard to the sites' ability to convey water from the ground surface to the desired recharge depth, can be challenging because of limited knowledge of the properties of the subsurface. In this study, we present an approach for using a towed time-domain electromagnetic (tTEM) imaging method to develop three-dimensional (3D) models of sediment type, map potential flow paths through the subsurface, and evaluate sites for surface spreading. We began with tTEM data from seven sites in the CV along with an existing resistivity-to-sediment type transform. We leveraged geostatistical methods to generate multiple 3D models of binary (flow and no-flow) sediment type from the tTEM data. We then developed two metrics to assess the quality of sites for recharge: (a) the depth to the shallowest no-flow unit beneath each point at the surface and (b) preferential flow paths lengths measured by finding the shortest distance through connected flow units between surface points and the desired depth. We explored how these metrics can be used to identify optimal areas within a site, then developed a way to compare and assess the relative suitability of each site using the decay in the number of vertical flow paths as a function of depth. Our methods can be used to rapidly identify potential sites for surface spreading recharge.

Controls on flood managed aquifer recharge through a heterogeneous vadose zone: hydrologic modeling at a site characterized with surface geophysics

“…Despite their growing popularity, however, geophysical methods have historically only been used to qualitatively interpret subsurface lithology. In one of the first quantitative assessments of recharge sites with geophysical data, Pepin et al (2022) evaluated recharge pathways using a ray-tracing approach at sites previously surveyed using a towed transient electromagnetic (tTEM) system. At each site, the authors developed 3D realizations of sediment type and measured the shortest path between the surface and the water table along coarse-grained facies.…”

Mapping Hydrogeological Structures Using Transient Electromagnetic Method: A Case Study of the Choushui River Alluvial Fan in Yunlin, Taiwan

“…The heterogeneous distribution of groundwater in space and time is linked to those structural features. It is essential to establish rules based on those features for sustainable groundwater resource management; to use the resource appropriately for the present and long term in the future [1][2][3][4][5][6]. Improper groundwater management leads to inappropriate groundwater utilization [7].…”