Nitrate source history for Livermore, California using environmental isotopes, noble-gases, and major ions

Abstract

This investigation of a nitrate contaminated groundwater basin in Livermore, California, demonstrates that nitrate isotope data are especially effective in determining contaminant sources when paired with other chemical and isotopic analyses. Major ion data distinguish three water types. Low-nitrate wastewater predominates in the northwest region, while two flowpaths with distinct nitrate sources originate in the southeast. Along the eastern flowpath, δN values >10%o 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, mobilizes a significant local nitrate source, bringing groundwater concentrations to >50 mg/L. The low 615N value (3.l%) in this location implicates synthetic fertilizer. In addition to these anthropogenic sources, relatively high natural nitrate background levels (15- 20 mg/L) are found in deep wells with residence times greater than 50 years.