Combined use of 15N and 18O of nitrate and 11B to evaluate nitrate contamination in groundwater

isotopic composition of NO3 (delta(15) N-NO3 and delta(18)O(NO3)) and B (delta(11)B) were used to evaluate NO3 contamination and identify geochemical processes occurring in a hydrologically complex Basin and Range valley in northern Nevada with multiple potential sources of NO3. Combined use of these isotopes may be a useful tool in identifying NO3 sources because NO3 and B co-migrate in many environmental settings, their isotopes are fractionated by different environmental processes, and because wastewater and fertilizers may have distinct isotopic signatures for N and B. The principal cause of elevated NO3 concentrations in residential parts of the study area is wastewater and not natural NO3 or fertilizers. This is indicated by some samples with elevated NO3 concentrations plotting along delta(15)N(NO3) and NO3 mixing lines between natural NO3 from the study area and theoretical septic-system effluent. This conclusion is supported by the presence of caffeine in one sample and the absence of samples with elevated NO3 concentrations that fall along mixing lines between natural NO3 and theoretical percolate below fertilized lawns. Nitrogen isotopes alone could not be used to determine NO3 sources in several wells because denitrification blurred the original isotopic signatures. The range of delta(11)B values in native ground water in the study area (-8.2 parts per thousand. to +21.2 parts per thousand) is large. The samples with the low delta(11)B values have a geochemical signature characteristic of hydrothermal systems. Physical and chemical data suggest B is not being strongly fractionated by adsorption onto clays. 61113 values from local STP effluent (-2.7 parts per thousand.) and wash water from a domestic washing machine (-5.7 parts per thousand.) were used to plot mixing lines between wastewater and native ground water. In general, wells with elevated NO3 concentrations fell along mixing lines between wastewater and background water on plots of delta(11)B against 1/B and Cl/B. Combined use of delta(15)N and delta(11)B in the study area was generally successful in identifying contaminant sources and processes that are occurring, however, it is likely to be more successful in simpler settings with a well-characterized delta(11)B value for background wells. Published by Elsevier Ltd.