Sources of groundwater nitrate revealed using residence time and isotope methods

Abstract Nitrate concentrations approaching and greater than the maximum contaminant level are impairing the viability of many groundwater basins as drinking water sources. Nitrate isotope data are effective in determining contaminant sources, especially when combined with other isotopic tracers such as stable isotopes of water and 3 H–He ages to give insight into the routes and timing of NO 3 inputs to the flow system. This combination of techniques is demonstrated in Livermore, CA, where it is determined that low NO 3 reclaimed wastewater predominates in the NW, while two flowpaths with distinct NO 3 sources originate in the SE. Along the eastern flowpath, δ 15 N values greater than 10‰ 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-NO 3 imported water (identified by stable isotopes of water and a 3 H–He residence time of 3 source, bringing groundwater concentrations up to 53 mg NO 3  L −1 . In this area, artificial recharge of imported water via local arroyos increases the flux of NO 3 to the regional aquifer. The low δ 15 N value (3.1‰) in this location implicates synthetic fertilizer. In addition to these anthropogenic sources, natural NO 3 background levels between 15 and 20 mg NO 3  L −1 are found in deep wells with residence times greater than 50 a.