Associations between private well water and community water supply arsenic concentrations in the conterminous United States

Abstract

Geogenic arsenic contamination typically occurs in groundwater as opposed to surface water supplies. Groundwater is a major source for many community water systems (CWSs) in the United States (US). Although the US Environmental Protection Agency sets the maximum contaminant level (MCL enforceable since 2006: 10 μg/L) for arsenic in CWSs, private wells are not federally regulated. We evaluated county-level associations between modeled values of the probability of private well arsenic exceeding 10 μg/L and CWS arsenic concentrations for 2231 counties in the conterminous US, using time invariant private well arsenic estimates and CWS arsenic estimates for two time periods. Nationwide, county-level CWS arsenic concentrations increased by 8.4 μg/L per 100% increase in the probability of private well arsenic exceeding 10 μg/L for 2006-2008 (the initial compliance monitoring period after MCL implementation), and by 7.3 μg/L for 2009-2011 (the second monitoring period following MCL implementation) (1.1 μg/L mean decline over time). Regional differences in this temporal decline suggest that interventions to implement the MCL were more pronounced in regions served primarily by groundwater. The strong association between private well and CWS arsenic in Rural, American Indian, and Semi Urban, Hispanic counties suggests that future research and regulatory support are needed to reduce water arsenic exposures in these vulnerable subpopulations. This comparison of arsenic exposure values from major private and public drinking water sources nationwide is critical to future assessments of drinking water arsenic exposure and health outcomes.

Keywords: Arsenic; Drinking water; Environment and Public Health; Water pollution.

Figure 1.. County-level high probability private well water arsenic (wAs) > 10 μg/L across the conterminous US (N=3,109 counties)¹.

NA = Not available. ¹Map data attributable to Lombard 2021a, © OpenStreetMap contributors.

Figure 2.. County-level mean probability private well water arsenic (wAs) > 10 μg/L across the conterminous US¹

NA = Not available. ¹Map data attributable to Lombard 2021a, © OpenStreetMap contributors.

Figure 3.. County-level community water system (CWS) arsenic (μg/L) in 2006-2011 by high probability private well water arsenic (wAs) > 10 μg/L across the conterminous US, by region and sociodemographic county cluster.

Dots represent counties. Lines represent the relationship between private well and CWS arsenic based on linear regression models. Shades represent 95% confidence intervals.

Figure 4.. County-level community water system (CWS) arsenic concentrations (μg/L) in 2006-2011 by high probability private well water arsenic (wAs) > 10 μg/L across the conterminous US, by majority water source.

Dots represent counties. Lines represent the relationship between private well and CWS arsenic based on linear regression models. Shades represent 95% confidence intervals.

Figure 5.

Linear associations (95% confidence interval (CI)) between high probability private well water arsenic (wAs) > 10 μg/L and community water system (CWS) arsenic concentrations comparing counties with high probability wAs >5-15%, >15-30%, >30-50%, and >50%, versus counties with high probability wAs ≤5% (reference), 2006-2011.

Figure 6.. County-level community water system (CWS) arsenic (μg/L) in 2006-2008, 2009-2011 by high probability private well water arsenic (wAs) > 10 μg/L across the conterminous US

on the original scale using a linear regression model (left), and on the natural log scale using a Loess model (right). Dots represent counties. Lines represent the relationship between private well and CWS arsenic based on linear (left) and loess (right) models. Shades represent 95% confidence intervals.