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Title: Nitrogen Cycle Dynamics Revealed Through δ18O-NO3- Analysis in California Groundwater

Abstract

Nitrate is a significant water-quality issue in California, the United States as a whole, and the world. Critical to addressing nitrate contamination is understanding the presence and extent of denitrification, and further refining the techniques used to identify nitrate sources. The use and understanding of nitrate isotopic signatures to identify nitrate sources have advanced tremendously; however, knowledge gaps remain concerning specific fractionation pathways and the role of denitrification in altering source values. Using a large unique database of California groundwater nitrate isotopic compositions, we explored the utility of nitrate–oxygen isotope ratios in determining specific nitrate origins. Lawrence Livermore National Lab (LLNL) samples were supplemented by United States Geological Society (USGS) data to create a dataset of over 1200 dual-isotope results. Methods used at LLNL allowed for the determination of δ15N-NO3-, δ18O-NO3-, δ18O-H2O, δ2H-H2O, excess air, major dissolved gases, and excess N2. Results were examined for the degree to which δ18O-NO3- conforms to the model of nitrification in which two atoms of oxygen are sourced from ambient water and one from the atmosphere. Almost 80% of the results fall within one standard deviation of predicted values. However, 19% of samples had significantly higher values, suggesting the preservation of a synthetic nitratemore » source signature, mixing of sources, or widespread denitrification. Results were examined with respect to general land-use classifications and, while nitrate concentrations followed the expected pattern of being higher in agricultural settings, δ18O-NO3-patterns are complicated by application of N-fertilizer in various forms, and subsequent N cycling in the soil zone. We found that the current understanding of oxygen isotope-fractionation mechanisms cannot yet explain the prevalence of oxygen-isotope compositions with higher than predicted δ18O values, but when paired with related data such as land use and indicators of denitrification, oxygen-isotope compositions of nitrate can help to assess nitrogen cycle dynamics.« less

Authors:
 [1];  [2]; ORCiD logo [2];  [2];  [1]
  1. California State Univ. East Bay, Hayward, CA (United States). Dept. of Earth and Environmental Sciences
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1529179
Report Number(s):
LLNL-JRNL-776814; LLNL-JRNL-791486
Journal ID: ISSN 2076-3263; GBSEDA; 970055
Grant/Contract Number:  
AC52-07NA27344
Resource Type:
Accepted Manuscript
Journal Name:
Geosciences
Additional Journal Information:
Journal Volume: 9; Journal Issue: 2; Journal ID: ISSN 2076-3263
Publisher:
MDPI
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; 54 ENVIRONMENTAL SCIENCES; δ15N-NO3−; δ18O-NO3−; oxygen isotope; nitrate; denitrification; fractionation; California groundwater

Citation Formats

Veale, Nate, Visser, Ate, Esser, Bradley, Singleton, Michael, and Moran, Jean E. Nitrogen Cycle Dynamics Revealed Through δ18O-NO3- Analysis in California Groundwater. United States: N. p., 2019. Web. doi:10.3390/geosciences9020095.
Veale, Nate, Visser, Ate, Esser, Bradley, Singleton, Michael, & Moran, Jean E. Nitrogen Cycle Dynamics Revealed Through δ18O-NO3- Analysis in California Groundwater. United States. doi:10.3390/geosciences9020095.
Veale, Nate, Visser, Ate, Esser, Bradley, Singleton, Michael, and Moran, Jean E. Mon . "Nitrogen Cycle Dynamics Revealed Through δ18O-NO3- Analysis in California Groundwater". United States. doi:10.3390/geosciences9020095. https://www.osti.gov/servlets/purl/1529179.
@article{osti_1529179,
title = {Nitrogen Cycle Dynamics Revealed Through δ18O-NO3- Analysis in California Groundwater},
author = {Veale, Nate and Visser, Ate and Esser, Bradley and Singleton, Michael and Moran, Jean E.},
abstractNote = {Nitrate is a significant water-quality issue in California, the United States as a whole, and the world. Critical to addressing nitrate contamination is understanding the presence and extent of denitrification, and further refining the techniques used to identify nitrate sources. The use and understanding of nitrate isotopic signatures to identify nitrate sources have advanced tremendously; however, knowledge gaps remain concerning specific fractionation pathways and the role of denitrification in altering source values. Using a large unique database of California groundwater nitrate isotopic compositions, we explored the utility of nitrate–oxygen isotope ratios in determining specific nitrate origins. Lawrence Livermore National Lab (LLNL) samples were supplemented by United States Geological Society (USGS) data to create a dataset of over 1200 dual-isotope results. Methods used at LLNL allowed for the determination of δ15N-NO3-, δ18O-NO3-, δ18O-H2O, δ2H-H2O, excess air, major dissolved gases, and excess N2. Results were examined for the degree to which δ18O-NO3- conforms to the model of nitrification in which two atoms of oxygen are sourced from ambient water and one from the atmosphere. Almost 80% of the results fall within one standard deviation of predicted values. However, 19% of samples had significantly higher values, suggesting the preservation of a synthetic nitrate source signature, mixing of sources, or widespread denitrification. Results were examined with respect to general land-use classifications and, while nitrate concentrations followed the expected pattern of being higher in agricultural settings, δ18O-NO3-patterns are complicated by application of N-fertilizer in various forms, and subsequent N cycling in the soil zone. We found that the current understanding of oxygen isotope-fractionation mechanisms cannot yet explain the prevalence of oxygen-isotope compositions with higher than predicted δ18O values, but when paired with related data such as land use and indicators of denitrification, oxygen-isotope compositions of nitrate can help to assess nitrogen cycle dynamics.},
doi = {10.3390/geosciences9020095},
journal = {Geosciences},
number = 2,
volume = 9,
place = {United States},
year = {2019},
month = {2}
}

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