SOURCES AND TRANSPORT OF NITRATE IN THE LIVERMORE VALLEY GROUNDWATER BASIN

Nitrate concentrations approaching and exceeding the maximum contaminant level (MCL) in several San Francisco Bay region groundwater basins impair their viability as drinking water sources. This study identifies a typically unrecognized nitrate contamination risk of enhanced recharge of surface water through groundwater management activities. Augmented recharge can mobilize nitrate sources that would not affect the regional aquifer under natural, low recharge-rate conditions. An investigation of a nitrate contaminated groundwater basin in Livermore, California, demonstrates that nitrate isotope data are effective in determining contaminant sources, and that other isotopic tracers such as oxygen isotopes and tritium-helium ages give insight into the routes and timing of nitrate inputs to the flow system.

Low nitrate reclaimed wastewater predominates in the northwest, while two flowpaths with distinct nitrate sources originate in the southeast. Along the eastern flowpath &delta¹⁵N values greater than 10‰ indicate that animal waste is the primary source. Diminishing concentrations over time suggest that nitrate 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 up to 53 mg/L. In this area, artificial recharge of State Project water via local arroyos induces a flux of the contaminant to the regional aquifer. The low &delta¹⁵N (3.1‰) value in this location implicates synthetic fertilizer, although nitrification in the root zone of natural vegetation is another likely contributor. 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/L are found in deep wells with residence times greater than 50 years. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.