Statewide cumulative human health risk assessment of inorganics-contaminated groundwater wells, Montana, USA.

Across the United States (U.S.), approximately 43 million people – about 15% of the U.S. population – lack access to public water services and rely instead on private wells (Dieter and Maupin, 2017; Dieter et al., 2018; Fox et al., 2016). Public supply drinking water contaminants are federally regulated under the Safe Drinking Water Act (SDWA) National Primary Drinking Water Regulations (U.S. Environmental Protection Agency, 2024a), but no federal laws govern private well water quality. Most states do not regulate water quality in private wells, beyond establishing standards for location and construction of new wells (National Ground Water Association, 2021). This lack of regulation is a public health concern because groundwater can contain a range of naturally occurring and anthropogenic toxics, including metals, metalloids, nitrate, fluoride, pesticides, polycyclic aromatic hydrocarbons, per- and polyfluoroalkyl substances (PFAS), and many other organic compounds (Bradley et al., 2018a; Bradley et al., 2018b; Bradley et al., 2021a; Bradley et al., 2021b; Bradley et al., 2022b; Bradley et al., 2022c; DeSimone, 2009; DeSimone et al., 2015; Focazio et al., 2006; Fox et al., 2016). Hence, people dependent on private wells are at increased risk of unrecognized exposures to unsafe groundwater (DeSimone, 2009; Nigra, 2020; Rogan and Brady, 2009).

Despite well-established potential health risks, private well water testing has multiple barriers (de Franca Doria, 2010; de França Doria et al., 2009; Eggers et al., 2018; Flanagan et al., 2015; Knobeloch et al., 2013; MacDonald Gibson and Pieper, 2017; Zheng and Ayotte, 2015; Zheng and Flanagan, 2017), and the costs of remediating home well water or even purchasing safe water can be prohibitive (Eggers et al., 2018; Fizer et al., 2018; MacDonald Gibson and Pieper, 2017; Martin et al., 2021; Seltenrich, 2017; Stillo et al., 2019). For example, 86% of home well water in a Tribal community in Montana exceeded the U.S. Environmental Protection Agency (EPA) Secondary Maximum Contaminant Level (SMCL) for total dissolved solids (TDS), yet 80% of study participants consumed their well water and only about 2% had installed water treatment systems (Eggers et al., 2018). Almost half of Tribal wells tested were found to be unsafe for lifetime consumption due to the cumulative risk from arsenic (As), uranium (U), manganese (Mn), and nitrate-nitrogen (NO₃-N) (Eggers et al., 2018). As one participant explained (Eggers et al., 2013):

“As a country, we may imagine our citizens have universal access to safe drinking water—but for millions of rural residents with poor quality well water, and who can’t afford cisterns, treatment systems, or all the bottled water they might want—this simply is not the case. In our communities, people are cooking with poor tasting, contaminated water, and living with the health consequences.”

In the absence of widespread private well water testing, federal, state, and university well water surveys have been conducted to assess potential drinking water risks. In a nationwide study of 62 major U.S. aquifers, the U.S. Geological Survey (USGS) found 22% of home wells sampled had one or more inorganic contaminants exceeding a public supply enforceable, EPA Maximum Contaminant Level (MCL) or other human health benchmark, most often strontium Sr, (7.3%), As (6.8%), Mn (5.2%), NO₃-N (4.4%), or U (1.7%) (DeSimone et al., 2015). An earlier nationwide study similarly found that inorganic contaminants exceeded public supply MCLs more often than organic contaminants, with As, NO₃-N, and U above standards in roughly 11%, 8%, and 4% of wells, respectively (Focazio et al., 2006). Statewide testing in Nevada, North Carolina, Pennsylvania, West Virginia, and Wisconsin has found comparable results, with some variation in the contaminants of most concern (Arienzo et al., 2022; Eaves et al., 2022; Knobeloch et al., 2013; Law et al., 2017; Swistock et al., 2013). Risk assessment studies in other countries have similarly found substantial health risks from heavy metals contamination of groundwater (Cui et al., 2021; Rajput et al., 2024; Ravindra and Mor, 2019; Wang et al., 2021; Zhang et al., 2024). Multiple studies found that consumption of home well water can pose serious public health risks, including neurodevelopmental effects, birth defects, and hearing loss, particularly from toxic metal exposures (Balazs and Ray, 2014; Gavino-Lopez et al., 2022; Gibson et al., 2020; Langley et al., 2015; Leker and Gibson, 2018; Marsh et al., 2010; Sanders et al., 2014). The American Academy of Pediatrics judged the risks to child health sufficiently serious to have recently issued a technical report and policy statement on well water consumption (Woolf et al., 2023a; Woolf et al., 2023b).

Most well testing studies (Arienzo et al., 2022; DeSimone, 2009; Eaves et al., 2022; Focazio et al., 2006; Knobeloch et al., 2013; Law et al., 2017; Swistock et al., 2013) have assessed drinking water-contaminant health risks based on the percentage of individual exceedances of EPA enforceable MCLs for public supply drinking water (U.S. Environmental Protection Agency, 2021b). MCLs are established based on public health goals as well as technical and financial considerations of drinking water treatment (U.S. Environmental Protection Agency, 2023b). In contrast, studies based on the EPA’s more protective health-only Maximum Contaminant Level Goals (MCLGs), defined as the level of a contaminant in drinking water below which there is no known or expected risk to health (U.S. Environmental Protection Agency, 2024a), are limited (Bradley et al., 2020b; Bradley et al., 2018b; Bradley et al., 2021b; Bradley et al., 2022b). MCLGs allow for a margin of safety, are non-enforceable (U.S. Environmental Protection Agency, 2021b), and are set at zero for human carcinogens (U.S. Environmental Protection Agency, 2023f). Arsenic (Agency for Toxic Substances and Disease Registry, 2007), radium (Ra) (Agency for Toxic Substances and Disease Registry, 2014), and U (Agency for Toxic Substances and Disease Registry, 2023b) are known human carcinogens, and lead (Pb) is classified as a probable carcinogen (Agency for Toxic Substances and Disease Registry, 2020); all four have MCLGs of zero (U.S. Environmental Protection Agency, 2009).

In addition, some inorganics have non-regulatory EPA Drinking Water Health Advisories (HAs) that lack MCLs, including boron (B), Mn, molybdenum (Mo), nickel (Ni), Sr, and zinc (Zn) (U.S. Environmental Protection Agency, 2004; U.S. Environmental Protection Agency, 2018). Although MCLs are available for just under 100 inorganic and organic contaminants, and non-enforceable federal advisories and benchmarks are available for another ? 800 contaminants (Smith and Nowell, 2024; U.S. Environmental Protection Agency, 2009; U.S. Environmental Protection Agency, 2021a; U.S. Environmental Protection Agency, 2022; U.S. Environmental Protection Agency, 2023c), these thresholds only cover about 0.2% of the more than 350,000 chemicals and chemical mixtures now registered for commercial production and use globally (Wang et al., 2020). Hence, studies comparing well water quality to public supply MCLs (DeSimone, 2009; Focazio et al., 2006; Knobeloch et al., 2013; Swistock et al., 2013) do not capture the full scope of known drinking water contaminant health risks.

Further, assessing risk by individual contaminants is inadequate because humans are typically exposed to combinations of chemical as well as non-chemical stressors (Meek et al., 2011; Sexton, 2012; Toccalino et al., 2012). In 2009, the U.S. National Research Council of the National Academies acknowledged this in defining cumulative risk assessment (CRA) as, “…the combination of risks posed by aggregate exposure to multiple agents or stressors in which aggregate exposure is exposure by all routes and pathways and from all sources of each given agent or stressor” (National Research Council, 2009, p. 213). In 2011, the EPA piloted a CRA methodology with the publication of its National-Scale Air Toxics Assessment (NATA) (U. S. Environmental Protection Agency, 2011). NATA defined a hazard quotient (HQ) as, “…the ratio of the potential exposure to a substance and the level at which no adverse effects are expected… A hazard quotient of 1 or lower means chronic adverse noncancer effects are unlikely, and thus can be considered to have negligible hazard. For HQs greater than 1, the potential for adverse effects increases, but we do not know by how much” (U.S. Environmental Protection Agency, 2024). In a useful overview of the first 25 years of CRA methodology, Sexton (2012, p. 2) defines CRA as “a science-policy tool for organizing and analyzing relevant scientific information to examine, characterize, and possibl[y] quantify the combined adverse effects on human health from exposure to a combination of environmental stressors,” citing the EPA’s Framework for Cumulative Risk Assessment (Callahan and Sexton, 2007; U.S. Environmental Protection Agency, 2003). In 2023, referring to the sum of contaminant HQs as the Hazard Index (HI), the EPA described it as their “long-established tool” that they “regularly use… to understand health risk from chemical mixtures,” and applied it to establish an HI MCL for the combination of four PFAS compounds (U. S. Environmental Protection Agency, 2023).

This shift from single-chemical risk assessment to CRA has been endorsed by many global organizations, including the EPA, the National Research Council, the World Health Organization, the UK Environment Agency, the European Union, the Canadian Environmental Assessment Agency, and more (Callahan and Sexton, 2007; Canadian Environmental Assessment Agency, 2007; European Food Safety Authority, 2019; Hegmann et al., 1999; Løkke, 2010; Meek et al., 2011; Moretto et al., 2017; National Environmental Justice Advisory Council, 2004; National Research Council, 2009; Sexton, 2012; Stephens et al., 2007; Tulve et al., 2024; Wolf et al., 2016; World Health Organization, 2009). Although there is no definitive methodology for conducting CRAs (Moretto et al., 2017), useful new approaches have been proposed and some are being implemented (Alfredo et al., 2017; Bradley et al., 2022b; Evans et al., 2019; Goumenou and Tsatsakis, 2019; Moretto et al., 2017; Stoiber et al., 2019; Teuschler et al., 2004; Toccalino et al., 2012; Xiao et al., 2019). Challenges remain, such as combining carcinogenic and non-carcinogenic risks, addressing uncertainties associated with interactions among chemical exposures, and especially, accounting for both chemical and nonchemical stressors (Alfredo et al., 2017; Sexton, 2012; Stoiber et al., 2019). Most recently, the EPA stated that their “…Office of Research and Development (ORD) has prioritized cumulative impacts in its research portfolio, and is leading the Agency’s efforts to work with overburdened communities to develop actionable science aimed at informing decisions to make their lives better, healthier, and longer” (Tulve et al., 2024, p. 1).

Cumulative risk assessments of contaminants in home well water, which adapt the EPA’s NATA CRA methodology (Goumenou and Tsatsakis, 2019; U. S. Environmental Protection Agency, 2011), have been piloted by the U.S. Geological Survey (USGS) (Bradley et al., 2020a; Bradley et al., 2018b; Bradley et al., 2021a; Bradley et al., 2021b; Bradley et al., 2022b) and others (Eggers et al., 2018; Eggers, 2014). The latter team used EPA MCLs and the HA for Mn as benchmarks to assess health risks from well water on the Crow Reservation in Montana (Eggers et al., 2018). The USGS assesses tapwater health risks in locations around the United States and uses a precautionary screening level approach that is based on the most protective human health benchmarks (e.g., MCLGs, WHO Guidelines, USGS Health-Based Screening Levels and state drinking water MCLs or HAs) (Bradley et al., 2021a). Broad CRAs of both regulated and unregulated contaminants in home drinking water are rarely conducted in the United States or globally (Bradley et al., 2022b), hence the USGS research program is providing new insights. To date, the USGS CRAs have documented wide-spread human health concerns, especially to the most vulnerable including pregnant women, infants, children, elderly, and the immune-compromised, in both public and private drinking water supplies (Bradley et al., 2020b; Bradley et al., 2018b; Bradley et al., 2021a; Bradley et al., 2021b; Bradley et al., 2022b).

This research represents Montana’s first state-wide screening-level (i.e., Tier 1) (U.S. Environmental Protection Agency, 2023d) CRA of the nature, degree, and spatial distribution of private well drinking water inorganics associated with human health risks. To our knowledge, it is the first such CRA in the United States. The approach adapts the USGS CRA methodology (Bradley et al., 2022b) to the Montana Ground Water Information Center’s large and disparate publicly available database (Montana Bureau of Mines and Geology, 2024). In the process, we identified the primary inorganic analytes driving health risks in the 51 watersheds with sufficient existing data. Although this CRA does not address drinking water risks associated with any individual well, it provides useful local guidance on contaminants of concern for environmental health professionals and well owners, identifies watersheds where the paucity of groundwater data warrants increased well testing and, importantly, highlights watersheds where poor groundwater quality merits more detailed (Tier 2) risk assessments (U.S. Environmental Protection Agency, 2023d) and risk mitigation.

In this work we address the questions: What are the magnitude and spatial distribution of cumulative human health risks from groundwater and which contaminants are the primary drivers of that risk? How does an application of CRA, based on technically and economically feasible public water-supply standards (MCLs), compare to an application based on the more protective health thresholds (MCLGs and HAs)? What does this comparison mean for home well water testing?