Nitrate concentrations approaching and greater than the maximum contaminant level (MCL) 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 tritium-helium ages to give insight into the routes and timing of nitrate inputs to the flow system. This combination of techniques is demonstrated in Livermore, CA, where it is determined that low nitrate reclaimed wastewater predominates in the northwest, while two flowpaths with distinct nitrate sources originate in the southeast. Along the eastern flowpath, δ This dual isotope technique is sometimes limited by the overlap of source isotope values 3 and by the variety of potential processes that affect nitrate (Aravena et al., 1993;Mengis et al., 2001). Successful studies of nitrate behavior and distribution must take into account the many environmental and historical factors that affect nitrate fate and transport (Böhlke and Denver, 1995).Our purpose is to improve upon traditional nitrate investigation methods that often yield ambiguous interpretations. We apply an integrated analytical approach using multiple lines of evidence to resolve the manifold origins and pathways of nitratecontamination. This approach is demonstrated in Livermore, CA, a city that relies on groundwater for a significant portion of its drinking water, but where MCL exceedances have occurred at 6 of the 13 public supply wells in the contaminated portion of the basin.As in many regions where this approach may be beneficially applied, Livermore has a decades long history of varied nitrate inputs in a complex groundwater system.Previous studies of Livermore's nitrate problem have estimated nitrate loading from various sources using literature ranges of potential nitrate inputs from these sources (Steinbergs and Wong, 1980;Raines, Melton, and Carella, Inc., 2002 Both layers are water bearing with higher yields observed in the alluvial deposits (State of California, 1967; Sorenson et al., 1984;Moran et al., 2002). Vertical transport (Burow, et al., 1998), while another demonstrated that, even with closely controlled drip irrigation, 67% to 79% of fertilizer nitrogen applied to vineyards in the spring remained in the soil at harvest time, available for leaching with winter rains (Hajrasuliha, 1998).Another possible nitrate source in the valley is that formed from the oxidation of organic nitrogen, naturally found in soil as a result of plant decomposition and microbial activity (Kendall and Aravena, 2000 METHODS Sample CollectionThe thirty-three well sampling locations from 2003 are shown in Figure 2 and described in Table 1 Denitrification produces signatures both in the nitrate isotopes and dissolved gas ratios. As denitrification occurs, nitrogen and oxygen isotopes in nitrate from freshwater are typically enriched in a characteristic pattern with a ratio close to 2:1 (Kendall and Aravena, 2000). Saturated zon...

Introduction and BackgroundNitrate contamination is a significant threat to groundwater in California and other states. Successful groundwater management requires science-based simulation of future impacts in affected aquifers and science-based assessment of management practices designed to reduce nitrate levels. An important component of developing science-based approaches is the need to develop new and better tools for the characterization and quantification of biogeochemical reactions in the subsurface that control the fate and transport of nitrate in groundwater. When coupled with our ability to characterize groundwater flow and to model reactive transport, these new tools will allow an accurate assessment of the future distribution of nitrate in California aquifers and of the impact of different management practices on nitrate input to groundwater. Such assessment is vital to making cost-effective management and policy decisions regarding land use and groundwater remediation. Research ActivitiesOur research investigated the fate and transport of nitrate in groundwater at two scales, with an emphasis on microbial denitrification :• Field scale: denitrification in the shallow saturated zone at a dairy farm in the Central Valley, and • Basin scale: nitrate transport in two impacted basins.The research focused on developing and applying rapid methods to quantify denitrification in order to assess the role of denitrification in the shallow and deep saturated zone, and to allow quantitative assessment of the kinetics of microbiallymediated denitrification in groundwater systems.The research supported a number of students, including two LLNL postdocs.• Tracy LeTain, SEP, LLNL postdoc (Harry Beller, advisor) • Michael Singleton, CMS Directorate Postdoc (Bradley K. Esser advisor) -4-• Keara Moore, University of Arizona (Brenda Ekwurzel, advisor): Keara's work on nitrate source attribution and transport in the Livermore-Amador Basin formed the core of her M.S. thesis.• Brad Cey, University of Texas at Austin (Bridget Scanlon, advisor): Brad is using the dairy site that was developed with LDRD funding in his doctoral research.• Glenn Shaw, UC-Merced (Martha Conklin, advisor): Glenn participated in sampling at the dairy site, and spent a summer working at the lab. Technical accomplishments• We developed a quantitative polymerase chain reaction method for functional nitrite reductase genes to assay denitrifier populations in soil and in water; • We determined denitrification rate constants for an autotroph with Fe monosulfide; • We installed several multilevel wells and performed a synoptic water and soil survey at a Central Valley dairy that allowed us to demonstrate localized denitrification in stratified groundwater; • We used real-time qPCR to determine denitrifier populations in soil cores taken from the dairy wells, and demonstrated that denitrification was strongly localized at the oxic-anoxic interface at this site; • We developed a new tracer for manure lagoon seepage that allows us to distinguish wastewater ...