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

Authors: PERZAN, ZACH - Stanford University; Osterman, Gordon; MAHER, KATE - Stanford University

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2023
Publication Date: 3/6/2023
Citation: Perzan, Z., Osterman, G.K., Maher, K. 2023. Controls on flood managed aquifer recharge through a heterogeneous vadose zone: Hydrologic modeling at a site characterized with surface geophysics. Hydrology and Earth System Sciences. 27(5):969-990. https://doi.org/10.5194/hess-27-969-2023.
DOI: https://doi.org/10.5194/hess-27-969-2023

Interpretive Summary: Agricultural managed aquifer recharge (AgMAR) is a method of flooding agricultural fields and orchards and allowing the water to seep into the ground and eventually recharge the underlying aquifer. While this technique has received considerable attention as a means of mitigating groundwater depletion in California's Central Valley, little work has been done to explore how conditions in the shallow, unsaturated part of the subsurface will impact the efficacy of recharge efforts. Poor recharge rates can impact crop and tree health, while failing to meaningfully impact the total water storage in the underlying aquifer. In order to study the effect of the unsaturated zone, we use a dense, 3D model of electrical resistivity underlying an almond orchard to a depth of roughly 70 meters. We translate this resistivity model to a suite of estimated sediment texture models as a basis for parameterizing groundwater models where we simulate a range of recharge scenarios. This allows us to evaluate three outcomes: surface infiltration rate, recharge water residence time in the root zone, and saturated zone recharge efficiency. The results show that recharge efforts should target areas with high amounts of coarse materials, as fine-grained sediments such as clays can retain water long enough to harm almond trees. Additionally, recharge efficiency can be limited when the fine-grained sediments are unsaturated, as they will draw recharge water into them, limiting the downward movement of the recharge water. Characterizing MAR efficiency at future locations should focus on characterizing the fine-grained sediments.

Technical Abstract: In water-stressed regions of the world, managed aquifer recharge (MAR), the process of intentionally recharging depleted aquifers, is an essential tool for combating groundwater depletion. Many groundwater-dependent regions, including the Central Valley in California, USA, are underlain by thick unsaturated zones (ca. 10 to 40'm thick), nested within complex valley-fill deposits that can hinder or facilitate recharge. Within the saturated zone, interconnected deposits of coarse-grained material (sands and gravel) can act as preferential recharge pathways, while fine-textured facies (silts and clays) accommodate the majority of the long-term increase in aquifer storage. However, this relationship is more complex within the vadose zone. Coarse facies can act as capillary barriers that restrict flow, and contrasts in matric potential can draw water from coarse-grained flow paths into fine-grained, low-permeability zones. To determine the impact of unsaturated-zone stratigraphic heterogeneity on MAR effectiveness, we simulate recharge at a Central Valley almond orchard surveyed with a towed transient electromagnetic system. First, we identified three outcomes of interest for MAR sites: infiltration rate at the surface, residence time of water in the root zone and saturated-zone recharge efficiency, which is defined as the increase in saturated-zone storage induced by MAR. Next, we developed a geostatistical approach for parameterizing a 3D variably saturated groundwater flow model using geophysical data. We use the resulting workflow to evaluate the three outcomes of interest and perform Monte Carlo simulations to quantify their uncertainty as a function of model input parameters and spatial uncertainty. Model results show that coarse-grained facies accommodate rapid infiltration rates and that contiguous blocks of fine-grained sediments within the root zone are >20'% likely to remain saturated longer than almond trees can tolerate. Simulations also reveal that capillary-driven flow draws recharge water into unsaturated, fine-grained sediments, limiting saturated-zone recharge efficiency. Two years after inundation, fine-grained facies within the vadose zone retain an average of 37'% of recharge water across all simulations, where it is inaccessible to either plants or pumping wells. Global sensitivity analyses demonstrate that each outcome of interest is most sensitive to parameters that describe the fine facies, implying that future work to reduce MAR uncertainty should focus on characterizing fine-grained sediments.