Title: Arsenic Exposure in US Drinking Water
Description: Reducing population exposure to inorganic arsenic (iAs), a known carcinogen and highly toxic metalloid of great public health concern, remains an ongoing challenge worldwide and in the United States (US). In the US, the Environmental Protection Agency (EPA) regulates the maximum contaminant level (MCL) for total arsenic in public drinking water supplies through the Safe Drinking Water Act. In 2001, the US EPA implemented the Final Arsenic Rule, which lowered the MCL for arsenic in public drinking water supplies from 50 to 10 µg/L. Reductions in iAs exposure and subsequent related disease associated with this important regulatory change have not been quantified. Currently, no national-level exposure estimates of iAs drinking water exposure are available for US residents reliant on public drinking water. There is a critical need to identify susceptible subgroups of the US population who remain at risk for elevated iAs drinking water exposure. This dissertation aimed to quantify the reduction in drinking water iAs exposure resulting from the US EPA MCL regulatory change, to estimate drinking water iAs exposure for US residents reliant on public drinking water, to identify susceptible subgroups across the US whose water iAs remains high, and to determine if iAs exposure was associated with heart disease mortality in the general US population.

Chapter 1 provides background information necessary to contextualize the work contained in this dissertation. In Chapter 2, we conducted a cross-sectional analysis of dietary sources of iAs exposure in the Strong Heart Family Study, a cohort of American Indian adults followed primarily for cardiovascular disease, using a self-reported food frequency questionnaire and urinary iAs measurements. Self-reported intake of rice, organ meat, processed meat, and non-alcoholic drinks was associated with increased urinary iAs concentrations. Diet alone explained only 3% of total variability in urinary iAs concentrations, indicating that the majority of iAs exposure for SHFS participants occurs from drinking water. Second, (in Chapter 3), we explored trends in water iAs exposure in the general US population associated with the EPA’s MCL change using the National Health and Nutrition Examination Survey (NHANES) from 2003-2014, separately for participants reliant on public drinking water vs. private well water (which is not subject to US EPA regulation).

We estimated that implementation of the new US EPA MCL was associated with a 17% reduction in drinking water iAs exposure for all participants reliant on public drinking water; the corresponding reduction was 32% for Mexican-American participants. No reduction was observed for participants reliant on private wells. Third (in Chapter 4), we estimated drinking water iAs exposure at the community water system and county-level across the entire US from 2006-2011 using the US EPA’s Six Year Review of Contaminant Occurrence database. We estimated that nationwide public drinking water iAs concentrations decreased by 8.5% and 21.6% at the 80th and 99th percentiles of the water iAs distribution in accordance with the MCL implementation, with significant differences across US subgroups. Greater decreases in iAs concentrations were reported for systems reliant on groundwater, systems serving smaller populations, and systems in the Northeast, Central Midwest, and Southwestern regions of the US. Susceptible subgroups whose public drinking water iAs exposure remains high include populations served by small community water systems reliant on groundwater, communities in the Southwestern US, Semi-Urban, Hispanic communities, and Rural, American Indian communities.

Fourth (in Chapter 5), we assessed six-year average arsenic concentrations in community water systems exclusively serving correctional facilities in the US (e.g. prisons, jails, detention centers) compared to other community water systems. Average arsenic concentrations were twice as high in correctional facility community water systems located in the Southwest (6.41 µg/L, 95% CI 3.48, 9.34) compared to all other community water systems in the Southwest (3.11 µg/L, 95% CI 2.97, 3.24). Over a quarter of correctional facility systems in the Southwest reported a six-year average arsenic concentration exceeding the 10 µg/L MCL. Persons incarcerated in the Southwestern US were at disproportionate risk of drinking water arsenic exposure and related disease from 2006-2011. Fifth (in Chapter 6), we multiply imputed urinary arsenic concentrations below the limit of detection (LOD) in NHANES 2003-2016 using a Bayesian Tobit regression model. Epidemiological analyses of urinary arsenic data in NHANES are limited by the relatively high analytical LODs and large proportion of participants with undetectable values.

Distributions of urinary arsenic originally reported in NHANES, which replace values below the LOD with the LOD divided by the square root of two, likely overestimate iAs exposure at the lowest exposure levels and may introduce significant bias. Bayesian-multiply imputed datasets may improve the assessment of iAs exposure in cohorts with high analytical LODs for arsenic species. Finally (in Chapter 7), we evaluated the association between urinary iAs concentrations (internal dose) and heart disease mortality as recorded in the National Death Index in NHANES 2003-2014 participants. We found a positive but non-significant prospective association between increasing iAs exposure and heart disease mortality for all participants (hazard ratio 1.15, 95% CI 0.77, 1.70), and a significant positive association for non-Hispanic white participants using flexible spline models. Geometric mean ratios of iAs exposure were higher among cases compared to non-cases, especially for Mexican-American participants (1.30, 95% CI 0.90, 1.88).

These findings further support the potential association between low- to moderate- iAs exposure and cardiovascular disease in the US population, and indicate that further high-quality prospective studies of Hispanic and Latino Americans are needed to investigate the potential increased susceptibility of Mexican-Americans to iAs-related cardiovascular disease. Taken together, these studies suggest that while the implementation of the US EPA’s 10 µg/L MCL has reduced drinking water arsenic exposure for many Americans reliant on public drinking water systems, these reductions were not uniform across all US populations. Populations who remain at risk of elevated drinking water arsenic exposure include those reliant on domestic wells, those located in the Southwest, persons incarcerated in the Southwest, tribal communities, and Hispanic communities. Further high-quality epidemiologic research is needed to evaluate the association between low- to moderate iAs exposure and cardiovascular disease in these populations.

Stronger federal regulations, targeted compliance enforcement and technical assistance, and other public health interventions are needed to reduce drinking water arsenic exposure in these communities.