Nitrate Biogeochemistry and Reactive Transport in California Groundwater: LDRD Final Report

Abstract

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 ...

Importance of river water recharge to the San Joaquin Valley groundwater system

“…The groundwater data for this study were collected within the framework of a number of studies funded by Lawrence Livermore National Laboratory (LLNL; n = 54; Esser et al, ) or the State Water Boards GAMA programme, including California Aquifer Susceptibility ( n = 130) assessment studies (Moran, Hudson, Eaton, & Leif, ; Wright, Belitz, & Johnson, ), GAMA Special Studies (Esser et al, ; Singleton, Roberts, & Esser, ; Visser, Moran, Singleton, & Esser, ), and GAMA Priority Basin project studies ( n = 242) (Belitz et al, ; Burton & Belitz, ; Landon & Belitz, ; Shelton, Pimentel, Fram, & Belitz, ; Shelton, Fram, & Belitz, ). Samples were collected by LLNL or United States Geological Survey (USGS) staff and analysed for dissolved noble gas concentrations at LLNL (Ekwurzel, ; Visser et al, ).…”

“…The groundwater data for this study were collected within the framework of a number of studies funded by Lawrence Livermore National Laboratory (LLNL; n = 54; Esser et al, 2006) (Vogel, 1967). Calculated 14 C ages in this study are referred to as "uncorrected" because they have not been adjusted to consider exchanges with sedimentary sources of carbon (Fontes & Garnier, 1979).…”

Groundwater residence time estimates obscured by anthropogenic carbonate

“…S8). While NO 3 − and SO 4 2− are common indicators of agricultural seepage (49,53), Sr 2+ is associated with AL dissolution due to its compatible valence and thus frequent incorporation into carbonate minerals (52). Irrigation seepage carrying particulate agricultural carbonates to the vadose zone thus provides a compelling mechanism for a modern shift toward greater open-system carbonate dissolution, leading to near atmospheric 14 C activities and higher DIC in modern tritiated groundwaters (Fig.…”