UC Davis

Dependence on groundwater for irrigation and consumptive use has resulted in the widespread depletion of groundwater aquifers across the world. In most of the semi-arid Southwest of the United States, groundwater is increasingly being regulated in efforts to sustainably manage this limited resource. Various Managed Aquifer Recharge (MAR) techniques are used to increase the sustainable management of groundwater resources. Agricultural Managed Aquifer Recharge (Ag-MAR) is a promising form of managed aquifer recharge, where farmland is flooded during the winter using excess surface water resources from runoff in order to recharge the underlying groundwater aquifer. In addition to increasing the security of groundwater resources and improving general drought resilience, Ag-MAR may have additional beneficial outcomes including flood risk reduction, drought preparedness, maintenance of environmental flows in aquatic ecosystems, prevention of saltwater intrusion in coastal aquifer systems, mitigating land subsidence as well as water quality benefits such as flushing salts from the shallow vadose zone. One main concern surrounding Ag-MAR implementation is the potential for increased nitrate (NO3-) leaching from historically cultivated agricultural land. When ingested in high concentrations, NO3- has been linked to methaemoglobinaemia (“blue baby syndrome”), miscarriages, and non-Hodgkin’s lymphoma. In order to evaluate the general viability of Ag-MAR as an appropriate MAR technique that presents minimal risk to agricultural production systems and groundwater contamination, a thorough understanding of nitrogen cycle dynamics under Ag-MAR must be developed. This dissertation focuses on evaluating the hydrologic and biogeochemical processes driving NO3- leaching and nitrogen cycling in the shallow vadose zone, under varying soil textures and Ag-MAR best management practices. First, I present field and laboratory experiments, which examine how different recharge practices affect NO3- leaching, mineralization and denitrification processes in different soil textures. Results show that short-lived, pulsed Ag-MAR flooding events cause NO3- leaching and organic N mineralization, whereby the dominant soil texture of an Ag-MAR site impacts both the timing of NO3- leaching, and the conditions for biogeochemical processes under Ag-MAR. Specifically, reducing time between flooding events for Ag-MAR reduces mineralization/nitrification, which in turn decreases mass of NO3- leached. This has implications for the development of Ag-MAR best management practices (BMPs), suggesting that in a N mineralization dominated system, short flooding frequencies may decrease mineralization/nitrification and NO3- leaching potential of Ag-MAR. Next, I developed a dual-porosity, mobile-immobile zone (MIM), reactive transport model using HP1 (HYDRUS-1D and PHREEQC), simulating NO3- leaching and biogeochemical processes under large water application events using observed datasets from the previous field and laboratory experiments. When comparing this HP1-MIM model to traditional NO3- leaching models, I find that the incorporation of environmental factors, and physical non-equilibrium dynamics improve model performance when estimating cumulative NO3- leached from the shallow vadose zone, and the amount of NO3- in the residual soil profile following water application events. Finally, I use this reactive transport model to perform a multi-scenario analysis, examining NO3- leaching potential, residual NO3- in the soil profile after water application events, and biogeochemical rates during water application events, over a variety of Ag-MAR best management practices (flooding frequency, flooding magnitude) and climate scenarios (dry year, wet year) to determine what Ag-MAR practices minimize NO3- leaching to the groundwater. For this analysis, I estimate the range of NO3- leached under Ag-MAR for two soil textures. Results of this analysis indicate that soil texture has a large influence on the moisture and oxygen regime of the soil, which in turn defines whether the soil system is dominated by denitrification or mineralization during and after Ag-MAR events. This has implications regarding Ag-MAR best management practices, and the general viability of Ag-MAR implementation. Across all soil types, we see the ability of high-magnitude water applications to dilute the mass of NO3- being leached below the shallow vadose zone to low concentrations in the bulk recharge. This indicates that under proper management, NO3- leaching during high magnitude water applications can be minimized.