Managed Groundwater Recharge on Agricultural Lands: Implications for Nitrate Contamination of Groundwater.

California is in the midst of a historically unprecedented four-year drought, with similarly unprecedented reductions of surface water allocations to farmers, especially in the San Joaquin Valley. In response to the current drought, farmers, out of necessity, have increasingly turned to groundwater to offset surface water reductions and meet their irrigation needs. Climate change forecasts predict increased precipitation variance with expected increases in flood frequency, as well as droughts. Agricultural groundwater recharge presents an innovative climate change adaptation tool for farmers to secure a long-term water supply and buffer against surface water allotment reductions during future droughts, while mitigating potential damage from flood events. The potential benefits of groundwater recharge, in addition to a reliable future basin-wide water supply, include decreasing downstream flood risks by removing excess water from near flood stage rivers, reducing pumping costs by increasing groundwater levels, flushing salts from the rooting zone, increasing water storage in the root zone, and mitigating land subsidence. However, of particular concern is the potential for groundwater recharge to exacerbate nitrate contamination of already at risk aquifers. Nitrate, when ingested, has been linked to methaemoglobinaemia, or “blue baby syndrome”, miscarriages, and non-Hodgkin’s lymphoma. In this study, using a water flow and solute transport model, HYDRUS 1D, combined with physiochemical data taken from nine meter boreholes, we examine the effects of varying groundwater banking management strategies on time and concentration of nitrate to groundwater and the factors controlling transport in the deep alluvial vadose zone of California’s Central Valley.