A critical component of the State Water Resource Control Board's Groundwater Ambient Monitoring and Assessment (GAMA) Program is to assess the major threats to groundwater resources that supply drinking water to Californians (Belitz et al., 2004). Nitrate is the most pervasive and intractable contaminant in California groundwater and is the focus of special studies under the GAMA program. This report presents results of a study of nitrate contamination in the aquifer beneath the cities of Morgan Hill and Gilroy, CA, in the Llagas Subbasin of Santa Clara County, where high nitrate levels affect several hundred private domestic wells. The main objectives of the study are: (1) to identify the main source(s) of nitrate that issue a flux to the shallow regional aquifer (2) to determine whether denitrification plays a role in the fate of nitrate in the subbasin and (3) to assess the impact that a nitrate management plan implemented by the local water agency has had on the flux of nitrate to the regional aquifer. Analyses of 56 well water samples for major anions and cations, nitrogen and oxygen isotopes of nitrate, dissolved excess nitrogen, tritium and groundwater age, and trace organic compounds, show that synthetic fertilizer is the most likely source of nitrate in highly contaminated wells, and that denitrification is not a significant process in the fate of nitrate in the subbasin except in the area of recycled water application. In addition to identifying contaminant sources, these methods offer a deeper understanding of how the severity and extent of contamination are affected by hydrogeology and groundwater management practices. In the Llagas subbasin, the nitrate problem is amplified in the shallow aquifer because it is highly vulnerable with high vertical recharge rates and rapid lateral transport, but the deeper aquifers are relatively more protected by laterally extensive aquitards. Artificial recharge delivers low-nitrate water and provides a means of long-term remediation. Examination of nitrate concentration in relation to groundwater age indicates that the nitrate management plan has not yet resulted in a decrease in the flux of nitrate to the shallow aquifer in the areas tested.

Nitrate is the number one drinking water contaminant in the United States. It is pervasive in surface and groundwater systems, and its principal anthropogenic sources have increased dramatically in the last 50 years. In California alone, one third of the public drinking-water wells has been lost since 1988 and nitrate contamination is the most common reason for abandonment. Effective nitrate management in groundwater is complicated by uncertainties related to multiple point and non-point sources, hydrogeologic complexity, geochemical reactivity, and quantification of denitrification processes. In this paper, we review an integrated experimental and simulation-based framework being developed to study the fate of nitrate in a 25 km-long groundwater subbasin south of San Jose, California, a historically agricultural area now undergoing rapid urbanization with increasing demands for groundwater. The modeling approach is driven by a need to integrate new and archival data that support the hypothesis that nitrate fate and transport at the basin scale is intricately related to hydrostratigraphic complexity, variability of flow paths and groundwater residence times, microbial activity, and multiple geochemical reaction mechanisms. This study synthesizes these disparate and multi-scale data into a three-dimensional and highly resolved reactive transport modeling framework.

A critical component of the State Water Resource Control Board's Groundwater Ambient Monitoring and Assessment (GAMA) Program is to assess the major threats to groundwater resources that supply drinking water to Californians (Belitz et al., 2004). Nitrate concentrations approaching and greater than the maximum contaminant level (MCL) are impairing the viability of many groundwater basins as drinking water sources Source attribution and nitrate fate and transport are therefore the focus of special studies under the GAMA program. This report presents results of a study of nitrate contamination in the aquifer beneath the City of Livermore, where high nitrate levels affect both public supply and private domestic wells. Nitrate isotope data are effective in determining contaminant sources, especially when combined with other isotopic tracers such as stable isotopes of water and tritium-helium ages to give insight into the routes and timing of nitrate inputs to the flow system. This combination of techniques is demonstrated in Livermore, where it is determined that low nitrate reclaimed wastewater predominates in the northwest, while two flowpaths with distinct nitrate sources originate in the southeast. Along the eastern flowpath, {delta}{sup 15}N values greater than 10{per_thousand} indicate that animal waste is the primary source. Diminishing concentrations over time suggest that contamination results from historical land use practices. The other flowpath begins in an area where rapid recharge, primarily of low nitrate imported water (identified by stable isotopes of water and a tritium-helium residence time of less than 1 year), mobilizes a significant local nitrate source, bringing groundwater concentrations above the MCL of 45 mg NO{sub 3} L{sup -1}. In this area, artificial recharge of imported water via local arroyos induces flux of the contaminant to the regional aquifer. The low {delta}{sup 15}N value (3.1{per_thousand}) in this location implicates synthetic fertilizer. Geochemical modeling supports the hypothesis of separate sources, one including organic carbon, as from animal waste, and one not. In addition to these anthropogenic sources, natural nitrate background levels between 15 and 20 mg NO{sub 3} L{sup -1} are found in deep wells with residence times greater than 50 years.