EESS Spring Seminar Series, Thomas Harter, Professor, Department of Land, Air and Water Resources, University of California, Davis
EESS Spring Seminar Series, Thomas Harter, Professor, Department of Land, Air and Water Resources, University of California, Davis, "N Cycling Gone Underground: Addressing nitrate in Groundwater", Abstract: A significant fraction of U.S. agricultural regions suffers from poor drinking water quality due to elevated nitrate, particularly in private domestic and local small system wells, often affecting economically disadvantaged communities. Understanding and addressing this issue requires an integrated, multidisciplinary approach and includes development of new methods to identify sources and reduction/prevention options for nitrate in groundwater used as drinking water, monitoring and modeling historic, current, and potential future nitrate contamination of groundwater through developing novel modeling tools, identifying methods and costs associated with treatment of or alternative supply to nitrate contaminated drinking water, and developing funding options and policies. We selected one of the country’s most productive agricultural regions as a pilot area: the Tulare Lake Basin and Salinas Valley, home to 2.6 million people and to nearly half of California’s 3.5 million hectares of irrigated agricultural land, with more than 80 different crops and housing more than half of California’s nearly 2 million head dairy herd. Shallow groundwater nitrate contamination is widespread and increasing with over 40% of domestic wells in some counties exceeding nitrate maximum contaminant levels. We developed a high-resolution basin scale groundwater nitrate loading model for over 2 million hectare of the Central Valley and Salinas Valley, identifying groundwater nitrate loading at the parcel (field) scale (0.25 ha resolution). Agriculture is by far the largest source of nitrate, loading an estimated 160 Gg N/yr to groundwater from 1.3 million ha of non-leguminous crop farmland. Loading reduction options in agriculture largely depend on improved nutrient and water use efficiency. Some improvements in groundwater nitrate loading can be achieved at relatively low cost, but compliance with water quality standards will come at high cost in some cases. A novel groundwater modeling framework is used to assess and evaluate the dynamic, spatio-temporally distributed linkages between non-point sources above a groundwater basin and groundwater discharges to wells and streams. We show that groundwater nitrate contamination is a long-term, multi-decadal issue, even under progressive future scenarios. In the immediate future, therefore, treatment and alternative water supply options for safe drinking water must be developed and depend on current service areas of public water supply systems, while a new framework for NPS regulation of groundwater is emerging for California.