UC Davis

Groundwater overdraft in the state of California has resulted in many undesirable results, including land subsidence, water quality degradation, loss of interconnected surface water/ groundwater locations, seawater intrusion, and overall reduction in groundwater storage. These consequences were exacerbated by the 2012-2016 drought period, resulting in the passage of the California Sustainable Groundwater Management Act, the first legislation that explicitly required sustainable use of groundwater resources in the state. This legislation also acknowledged the importance of conjunctive use of surface water and groundwater resources, in which excesses of one resource can supplement deficiencies of the other. This conjunctive use of resources is the main motivation for relevant parties incorporating managed aquifer recharge projects into their groundwater sustainability portfolios.Managed aquifer recharge projects have the potential to allow for increased surface water resources in the wet season to be transferred to aquifers for future use. Many of these projects are in use in the state and range from injection wells to large scale flooding of agricultural fields. These projects can be costly to implement and are limited in locations due to the need to create infrastructure for diversions, find willing landowners to allow the project to occur, and receive the proper permits. Because of these costs, there is an importance in understanding what specific parameters are most important to understand and quantify when determining whether these recharge projects will be successful. This dissertation focuses on the creation of models for managed aquifer recharge sites utilizing large amounts of publicly available data and understanding how the uncertainty of that data impacts recharge results. The first body chapter focuses on the creation and utilization of a data-dense fine resolution geologic model of the recharge site and surrounding area using previously proprietary geologic data. Many realizations were developed to quantify the uncertainty of this geology. Results of this chapter acknowledge the importance of geologic characterization of recharge study sites because there can be great uncertainty between realizations of geologic results, specifically in the location of high conductivity connected geologic units. In the second body chapter, these geologic realizations are incorporated into groundwater models, with publicly available information for pumping and recharge. This geology was then simplified using a vertical upscaling process, to conclude if computationally intensive geologic models could be simplified and still produce similar groundwater flux and head results. Geologic upscaling resulted in similar groundwater head results at low levels of upscaling, but as upscaling increased, the impact of the pumping and recharge boundary conditions increased, resulting in increasing unrealistic model results. Finally, artificial recharge scenarios were applied at varying magnitudes and times to the recharge sites in a transient groundwater model in the third body chapter to quantify any changes in large scale or local scale model results. Large amounts of recharge applied at once resulted in increased gradients at the recharge locations, which drove more flow out of the model domain than in a no recharge case, but overall, recharge provided a benefit to river dynamics and cumulative storage rates. This work emphasizes the importance of subsurface characterization and understanding the impact of the boundary conditions that are applied to groundwater model results and provides scenario results that can be presented to relevant parties to discuss how the timing and magnitude of recharge at the study sites can affect the underlying groundwater table. Future work could include the incorporation of more data, such as isotopes for groundwater dating and recharge pathways and modeling of the unsaturated zone to visualize how recharge flows from the surface to the water table.