PDFDataDocumentPDFLLNLSWRCBGAMADSTgroundwaterThe white paper to accompany the Well Vulnerability Assessment Tool (WVAT) web app.thumbnail/thumbnail1760039696810.pngThis document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.<div><br /></div><div>This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.</div><div><br /></div><div>This report was prepared in cooperation with the California State Water Resource Control Board GAMA Program.</div><div><br /></div><div>Report, and all accompanying data to create this tool, is available to the public and can be attained through this publication or by contacting GAMA@waterboards.ca.gov or Dori Bellan (dori.bellan@waterboards.ca.gov).</div><div><i>The term &quot;Vulnerability Assessment Tool (VAT)&quot; has ben updated to &quot;Well </i><i>Vulnerability Assessment Tool </i><i>(WVAT)&quot; Please consider this when accessing these documents. </i></div><div><br /></div><div>The purpose of the Well Vulnerability Assessment Tool (WVAT) (described herein) is to provide possible reasons why water supply wells in California’s Central Valley are adversely affected by contaminants. The WVAT compliments the Groundwater Ambient Monitoring Assessment (GAMA) Program’s online tools, which were developed to help users understand groundwater quality in California (California State Water Resources Control Board, n.d.-a). Groundwater is a major component of California’s water supply; 16,000 public water supply wells and more than 600,000 private wells are used as drinking water sources. Public water systems, including public supply wells, are tightly regulated for contaminants with state and federal maximum contaminant limits (MCLs). Domestic and small system wells (serving 1-14 connections) are often outside of regulatory reach though, and nearly one million Californians, mostly in rural, disadvantaged communities, may access groundwater that does not meet regulatory guidelines. Access to a reliable drinking water source may also be compromised by drought and over-pumping by nearby wells. Assessing the vulnerability of wells to contaminant and water supply reliability challenges is an important step to ensuring every California resident’s human right to clean, safe, and affordable drinking water.</div><div><br /></div><div>Contaminant concentrations are measured precisely and frequently in public supply wells and requirements for remedial actions are straightforward in relation to MCLs. These wells are monitored for regulated contaminants, while private domestic wells may be monitored infrequently, or not at all. In all cases, however, a lack of understanding of why a contaminant is present in a well hinders management and remedial planning. While large amounts of concentration data have been collected for some 90 chemical constituents with primary MCLs at public supply wells, concentration data alone cannot typically be interpreted to determine the reasons for the presence or absence of contaminants. The WVAT aims to assess the hydrogeologic and geochemical drivers of contaminant occurrence, and to indirectly assess the source and fate of contaminants.</div><div><br /></div><div>For more than 20 years, the California State Water Resources Control Board’s (State Water Board) GAMA program has included the analysis of environmental tracers in wells tested under the program. When combined with contaminant concentrations, these tracers can be used to understand hydrogeologic and geochemical drivers of contaminant occurrence and transport. The tracers include stable isotopes of the water molecule which allow identification of the source(s) of recharge to wells, whether local precipitation or river water generated from snow melt. Groundwater age, or residence time, as determined using tritium or the tritium-helium system, can indicate a well’s susceptibility to contaminants from the land surface when it produces recent recharge, or, on the other end of the time scale, to geological contaminants that often are sourced from long term water-rock interaction (Belitz et al., 2022). When contaminants are not present because sources are not present, these tracers still provide useful information about groundwater sustainability, including susceptibility of wells to going dry during droughts, vulnerability to unsustainable extraction, and the likelihood that recharge sources to the well will be affected.</div><div><br /></div><div>The WVAT is based on statistical analyses of tracer-contaminant pairs and on hydrogeological conceptual models that delineate groundwater flowpaths from recharge to discharge. For example, Visser et al., (2018) show that river water from Sierra Nevada runoff can be identified in wells in the Central Valley, while Castaldo et al., (2021) show that river water brings lower nitrate. Thus, to follow the example, the WVAT would indicate that the reasons for (surface-sourced) nitrate occurrence at problematic concentrations in a well may be because nitrate correlates positively with young groundwater and negatively with (low-nitrate) river water contribution.</div><div><br /></div><div>The WVAT is not intended to provide a spatial or temporal predictive capability for contaminant occurrence or concentration trends. Groundwater ages and groundwater age distributions, calculated using the same data set, are interpolated and predicted in other recent studies (Jurgens, 2016; Visser et al., 2016; Faulkner et al., 2023; Azhar et al., 2024). Furthermore, only a handful of contaminants, particularly those from AB1249 (Assembly Bill No. 1249, 2014) (arsenic, nitrate, perchlorate, hexavalent chromium), and uranium (some of the most frequently occurring in the Central Valley), are included in the VAT. Emerging contaminants, contaminants with advisory levels, and volatile organic compounds are not included in the WVAT.</div><div><br /></div><div>The WVAT focuses on contaminant occurrence at approximately 3,000 individual wells where all or most of the tracers and contaminant concentrations have been measured. The goal is to provide well owners and water managers with a hydrogeologic context, based on evidence from tracers, for contaminant occurrence.</div><div><br /></div><div>Functionally, the WVAT is envisioned as a Geographic Information System (GIS) enabled, web-based tool that uses ‘stoplights’ (red, yellow, green) to depict contaminant concentrations relative to benchmarks, and to represent vulnerability to contamination using explanatory factors based on tracer results, at individual wells. Based on stoplight outcomes, when vulnerabilities are identified, the tool offers general recommendations for possible remedial actions by well owners and water managers. In particular, the tool can be used by Groundwater Sustainability Agencies (GSAs) engaged in activities that boost water supply while guarding against water quality degradation under California’s Sustainable Groundwater Management Act (SGMA).</div><div><br /></div><div>This report describes the data set upon which the WVAT is based, how the tracers are applied as explanatory indicators for contaminants and sustainability, the statistics used to test the relationships between tracers and contaminants, criteria for stoplight cutoffs, and example results for wells. Limitations posed by the data set and by the methods are also outlined.</div>LLNL and CA State University East Bay, in cooperation with the State Water Resources Control Board GAMA Program.