Use of Isotopic Tracers to Assess Nitrate in Deep Aquifers

In many of California's groundwater basins, shallow groundwater is affected by nitrate contamination or solvent plumes, and as drinking water standards are exceeded, wells are simply drilled to greater depths to avoid shallow contamination. In contrast to near-surface alluvial aquifers, deep aquifers are often chemically reducing, providing a potential mechanism to attenuate contaminants such as volatile organic compounds (VOCs; through reductive dehalogenation) and nitrate (through autotrophic denitrification). Isotopic tracers can be used to examine whether changes in concentration with depth are the result of dilution of contaminated water with ‘pristine' water (recharged under pre-development conditions), oxidation-reduction reactions, or long term physical isolation of deep groundwater beneath continuous confining units. A large data set that includes results for tritium, dissolved noble gases, nitrate, VOCs, isotopic composition of nitrate, and measures of redox-sensitive constituents such as dissolved oxygen, methane, Mn and Fe, collected under the State of California's Groundwater Ambient Monitoring and Assessment (GAMA) program, is examined for patterns in contaminant concentration. Two measures of groundwater residence time, tritium-helium, and radiogenic helium-4 are used to assess the degree of isolation of deep groundwater in several of the heavily exploited California coastal and Central Valley groundwater basins. Comparing the isotopic composition of nitrate in groundwater that contains detectable tritium versus ‘tritium-dead' groundwater suggests that the observed low nitrate concentrations in deep groundwater are more often due to isolation beneath effective confining units or dilution with low nitrate water rather than a denitrification reaction. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. DE-AC52-07NA27344.