Saturated zone denitrification: Potential for natural attenuation of nitrate contamination in shallow groundwater under dairy operations

We present results from field studies at two central California dairies that demonstrate the prevalence of saturated-zone denitrification in shallow groundwater with H-3/ He-3 apparent ages of < 35 years. Concentrated animal feeding operations are suspected to be major contributors of nitrate to groundwater, but saturated zone denitrification could mitigate their impact to groundwater quality. Denitrification is identified and quantified using N and 0 stable isotope compositions of nitrate coupled with measurements of excess N-2 and residual NO3- concentrations. Nitrate in dairy groundwater from this study has delta N-15 values (4.3-61 parts per thousand), and delta O-18 values (-4.5-24.5 parts per thousand) that plot with delta O-18/delta N-15 slopes of 0.47-0.66, consistent with denitrification. Noble gas mass spectrometry is used to quantify recharge temperature and excess air content. Dissolved N-2 is found at concentrations well above those expected for equilibrium with air or incorporation of excess air, consistent with reduction of nitrate to N-2. Fractionation factors for nitrogen and oxygen isotopes in nitrate appear to be highly variable at a dairy site where denitrification is found in a laterally extensive anoxic zone 5 m below the water table, and at a second dairy site where denitrification occurs near the water table and is strongly influenced by localized lagoon seepage.