Inequalities in Public Water Arsenic Concentrations in Counties and Community Water Systems across the United States, 2006–2011

Background: In the United States, nationwide estimates of public drinking water arsenic exposure are not readily available. We used the U.S. Environmental Protection Agency’s (EPA) Six-Year Review contaminant occurrence data set to estimate public water arsenic exposure. We compared community water system (CWS) arsenic concentrations during 2006–2008 vs. after 2009–2011, the initial monitoring period for compliance with the U.S. EPA’s 10 μg/L arsenic maximum contaminant level (MCL). Objective: Our objective was to characterize potential inequalities in CWS arsenic exposure over time and across sociodemographic subgroups. Methods: We estimated 3-y average arsenic concentrations for 36,406 CWSs (98%) and 2,740 counties (87%) and compared differences in means and quantiles of water arsenic (via quantile regression) between both 3-y periods for U.S. regions and sociodemographic subgroups. We assigned CWSs and counties MCL compliance categories (High if above the MCL; Low if below) for each 3-y period. Results: From 2006–2008 to 2009–2011, mean and 95th percentile CWS arsenic (in micrograms per liter) declined by 10.3% (95% CI: 6.5%, 14.1%) and 11.5% (8.3%, 14.8%) nationwide, by 11.4% (4.7%, 18.1%) and 16.3% (8.1%, 24.5%) for the Southwest, and by 36.8% (7.4%, 66.1%) and 26.5% (12.1%, 40.8%) for New England, respectively. CWSs in the High/High compliance category (not MCL compliant) were more likely in the Southwest (61.1%), served by groundwater (94.7%), serving smaller populations (mean 1,102 persons), and serving Hispanic communities (38.3%). Discussion: Larger absolute declines in CWS arsenic concentrations at higher water arsenic quantiles indicate declines are related to MCL implementation. CWSs reliant on groundwater, serving smaller populations, located in the Southwest, and serving Hispanic communities were more likely to continue exceeding the arsenic MCL, raising environmental justice concerns. These estimates of public drinking water arsenic exposure can enable further surveillance and epidemiologic research, including assessing whether differential declines in water arsenic exposure resulted in differential declines in arsenic-associated disease. https://doi.org/10.1289/EHP7313


Introduction
Racial/ethnic, socioeconomic, and geographic inequalities in U.S. drinking water contaminant exposures reflect structural inequality ingrained in land-use patterns, regulatory policy, facility siting, underlying geologic processes, and municipal zoning decisions (Gibson et al. 2020;Morello-Frosch et al. 2002;Nigra 2020;Wilson et al. 2008). Yet, systematic studies of inequalities in public drinking water contaminant exposures are lacking. Previous studies have identified inequalities in public drinking water exposure estimates for nitrate (higher among Hispanic populations) and arsenic (higher among incarcerated populations in the Southwest) (Nigra and Navas-Acien 2020;Schaider et al. 2019). These studies highlight the critical need to systematically identify subgroups of the U.S. population with elevated public drinking water contaminant exposures in order to identify environmental justice concerns and to inform public health interventions and regulatory action needed to eliminate exposure inequalities.
Inorganic arsenic is a human carcinogen (IARC 2004), highly toxic metalloid (Kuo et al. 2015(Kuo et al. , 2017Milton et al. 2017;Moody et al. 2018;Moon et al. 2017;Sanchez et al. 2018), and water contaminant present in many aquifers in the United States. In 2001, the U.S. Environmental Protection Agency (EPA) Final Arsenic Rule lowered the arsenic maximum contaminant level (MCL) in public water systems from 50 to 10 lg=L (U. S. EPA 2001aS. EPA , 2001b. The Final Arsenic Rule went into effect on 23 January 2006 and included an initial monitoring period for compliance with the new MCL (2006)(2007)(2008). Reducing the MCL from 50 to 10 lg=L has prevented an estimated 200-900 cancer cases per year (Nigra et al. 2017). Evaluating the extent to which MCL implementation reduced public water arsenic exposure, and identifying population subgroups disproportionately exposed by geography, race/ethnicity, and socioeconomic status (SES), remains critical.
Some evidence indicates that reductions in public water arsenic exposure were not uniform across all communities and that some public water systems remain in violation of the 10 lg=L MCL, especially in the southwestern United States (Environmental Integrity Project 2016;Foster et al. 2019;Nigra et al. 2017;Welch et al. 2018). Systems serving smaller populations were more likely to be in violation of the new MCL than larger systems, likely because of the increased relative cost in treating, mixing, or switching source water per population-served size (Foster et al. 2019). Although racial/ethnic inequalities in urinary arsenic concentrations are well documented (with higher internal dose estimates for Hispanic and Asian-American subgroups), evaluating the relative contribution of diet vs. drinking water to total inorganic arsenic exposure remains challenging because no national-level estimates of public drinking water arsenic exposure are available (Awata et al. 2017a(Awata et al. , 2017bCubadda et al. 2017;Jones et al. 2019;Kurzius-Spencer et al. 2014). Estimating public drinking water arsenic exposure for sociodemographic and geographic subgroups is needed to evaluate whether inequalities in arsenic exposure and compliance with the MCL persist across the United States, to inform future national-and state-level arsenic MCL deliberations, and to investigate whether inequalities in drinking water arsenic exposure by subgroup contributes to disparities in arsenic-related disease.
Our objective was to estimate public drinking water arsenic exposure in community water systems (CWSs) across the United States and to identify subgroups whose public water arsenic concentrations have remained above 10 lg=L after the new arsenic MCL implementation and are, therefore, at disproportionate risk of arsenic-related adverse health outcomes. We estimated arsenic exposure at the CWS level (primary analysis) and at the county level (secondary analysis), comparing arsenic concentrations during 2006-2008 vs. after 2009-2011, the initial monitoring period for compliance with the MCL, using the national contaminant occurrence database supporting the U.S. EPA's Third Six-Year Review of drinking water regulations. This database contains monitoring records for arsenic in public drinking water systems across the United States from 2006 to 2011. We considered the following subgroups in our analysis: a) U.S. region (CWS and county level); b) sociodemographic county cluster (CWS and county level); and c) population-served size and source water type (CWS level).

U.S. EPA Public Water Databases
The U.S. EPA compiles compliance monitoring data from public water supplies for regulated drinking water contaminants every 6 y as required by the Safe Drinking Water Act (SDWA) (U.S. EPA 2016a, 2016b). Data are obtained through the Information Collection Request process (voluntarily sent in from states, territories, and tribal authorities). The U.S. EPA works directly with agencies to collect records for a given monitoring period. We used arsenic monitoring data from the Third Six-Year Review (SYR3) period (2006)(2007)(2008)(2009)(2010)(2011), which included approximately 13 million analytical records from 139,000 public water systems serving 290 million people annually. Data from 46 states, Washington DC, the Navajo Nation, and American Indian tribes from U.S. EPA Regions 1, 4, 5, 8, and 9, representing 95% of all public water systems and 92% of the total population served by public water systems nationally, were included (U.S. EPA 2016a, 2016b). Agencies with primacy for implementing the SDWA at public water systems in the states of Colorado, Delaware, Georgia, and Mississippi and U.S. EPA Regions 2, 6, 7, and 10 for water systems serving American Indian tribes did not submit data for SYR3 (U.S. EPA 2016a). The U.S. EPA conducted extensive quality assurance and quality control assessments prior to publishing the final SYR3 data (U.S. EPA 2016a(U.S. EPA , 2016b(U.S. EPA , 2016c. From an initial 297,354 arsenic monitoring records from 54,845 public water systems, we excluded 67,089 records from 17,747 systems categorized as transient or nontransient noncommunity water systems. Of the remaining 230,265 records, 104,730 reported arsenic detections and 125,535 reported nondetections. We replaced arsenic values below the limit of detection (LOD) with the LOD divided by the square root of 2. Although the U.S. EPA established the maximum LOD of 0:5 lg=L, many systems reported lower and higher LODs or did not report record-specific LODs. When records reported the laboratory LOD as <5 lg=L, we replaced arsenic values below the LOD with the LOD divided by the square root of 2 (N = 30,256 records); for 1,202 records from Wisconsin and West Virginia that likely reported LODs in incorrect units (milligrams per liter instead of micrograms per liter), we imputed the corrected LOD divided by the square root of 2 (West Virginia Department of Health and Human Services 2019; Wisconsin Department of Natural Resources 2019). We imputed the value of 0:35 lg=L (the U.S. EPA LOD of 0:5 lg=L divided by the square root of 2) for a) 88,640 records with arsenic levels reported below the LOD without record-specific LOD; and b) the remaining 5,437 records for which LODs were reported as ≥5 lg=L because these were considered unreliable.
To assign counties-served for each CWS, we merged the SYR3 arsenic monitoring data with system inventory information extracted from the U.S. EPA Safe Drinking Water Information System (SDWIS), including counties served, number of people served, type of system, and source water type (U.S. EPA 2017). Because only county served was reliably reported in SDWIS for each CWS, we could not aggregate to smaller geographic scales (e.g., census-tract levels). Of the 37,098 CWSs, we successfully merged 36,372 to SDWIS. Of the remaining CWSs, we manually assigned county-served to 452 of these systems based on ZIP code (439 via U.S. Department of Housing and Urban Development Crosswalk files, and 13 via GoogleMaps) ( Figure S1 presents a flowchart of the data cleaning, data merging, and inclusion/exclusion criteria). The final sample size included 36,406 CWSs (98% of CWSs in the SYR3) serving a total of 2,740 counties (87% of 3,141 U.S. counties and county-equivalents) across 47 states and serving 254,610,301 people.

Water Arsenic Exposure Estimates
We averaged available water arsenic monitoring records to 3-y periods (2006-2008 and 2009-2011) because these sampling periods differentiated between during (2006-2008) and after (2009-2011) the initial compliance monitoring period. We also averaged to 3-y periods because the number of records reported by CWSs was differential by prior MCL exceedances and source water type and varied substantially across CWSs (range: 1-1,506; mean: 4) (U.S. EPA 2004). Averaging available records to 3-y periods also minimized missing data and facilitated the comparison of arsenic exposure estimates across CWSs (for diagnostics regarding this analytical decision, see Tables S1 and S2). Few CWSs reported records of both raw and finished (i.e., treated) water samples within the same year (N = 1,182). When the 3-y average of arsenic in finished water samples was lower than in raw concentrations (N = 336 CWSs), we calculated the 3-y average with only the finished water samples.
At the county level (secondary analyses), we estimated weighted average 3-y (2006-2008 and 2009-2011) water arsenic concentrations, accounting for the number of people served by each CWS (population served). For each 3-y period, the population-served weight for each CWS was calculated as the population served by that CWS divided by the total population served by all CWSs serving that county. CWSs not reporting data for a particular 3-y period did not contribute to that county's total population served or 3-y average (Equation S1). To avoid reporting a county-level average derived from CWSs that served only small populations relative to the entire county population, the average water arsenic concentration for a given county and 3-y period was estimated only if the CWSs serving that county reported serving at least 50% or more of the public water-reliant population in the entire county. We estimated the public water-reliant population for each county using the latest nationwide U.S. Census statistic on county-level household tap water source from the 1990 U.S. Census (Ruggles et al. 2019), which was also recently used by the U.S. Geologic Survey (USGS) (Ayotte et al. 2017). For descriptive purposes, we also calculated county-level 6-y average water arsenic concentrations as the average of the two 3-y period estimations. We mapped 3-and 6-y county-level estimates of water arsenic averages across the conterminous United States using the maps package in R version 3.5.3 (Becker and Wilks 2018).

Statistical Analysis: Water Arsenic Exposure Estimates
Primary analyses were conducted at the CWS level and secondary analyses at the county level. We calculated the distributionincluding percentiles, arithmetic means, and geometric meansof 6-and 3-y average water arsenic concentrations at the CWS (N = 36,406) and county level (N = 2,740) for the entire United States. We also compared mean differences between the two time periods. Because the reductions in average water arsenic concentrations over time occurred at the highest end of the distribution, we also performed quantile regression at the 75th, 80th, 85th, 95th, and 99th percentiles to estimate the difference in water arsenic concentration over time at these quantiles. For CWS-level models, we evaluated both crude models and models adjusting for source water type and the size of the population served to determine whether differences in water arsenic concentrations over time were related to these variables.

Statistical Analysis: Analyses Stratified by Subgroup
To identify subgroups of the U.S. population whose estimated public drinking water arsenic exposures were relatively high, we stratified our analyses by the following population subgroups: region (CWS-and county-level analyses), sociodemographic countycluster (CWS-and county-level analyses), source water type (CWSlevel analyses), and size of the population served by CWSs (CWSlevel analyses). Region groupings were based on USGS-identified areas with similar groundwater arsenic patterns: Pacific Northwest ( (Ayotte et al. 2017). Sociodemographic countyclusters (N = 8 distinct clusters) were derived by Wallace et al. (2019) to enable the direct comparison of county-level health outcomes and behaviors while accounting for the sociodemographic makeup of a county's population. Briefly, Wallace et al. (2019) used k-means analysis to identify groups of counties with similar sociodemographic profiles (e.g., race and ethnicity, age, educational attainment, employer status, health insurance status). In the present study, we stratified public water arsenic exposure estimates by these sociodemographic county-clusters to identify characteristics of population subgroups exposed to elevated public water arsenic exposure. These sociodemographic county-clusters are as follows: Semi-Urban, High SES; Semi-Urban, Mid/Low SES; Semi-Urban, Hispanic; Mostly Rural, Mid SES; Rural, Mid/Low SES; Young, Urban, Mid/High SES; Rural, American Indian; and Rural, High SES. We also stratified CWS-level analyses by source water type (groundwater vs. surface water, as reported in SDWIS) and by the size of the population served (≤500, 501-3,300, 3,301-10,000, and >10,000 persons) (these are not relevant at the county level).
We conducted sensitivity analyses for the comparison of water arsenic concentrations over the two 3-y periods. First, we replaced all arsenic concentration values below the LOD with 0:35 lg=L (the standard U.S. EPA LOD divided by the square root of 2), regardless of the LOD reported in each record. Second, we excluded records with a reported LOD of ≥5 lg=L. Third, we estimated county average water arsenic concentrations for a given 3-y period only if the CWSs serving that county reported serving at least 70% and 80% or more of the public water-reliant population in the entire county (instead of 50% or more as in our main analysis).

MCL Compliance Evaluation
We assigned all CWSs and counties to one of four MCL compliance categories using the 10 lg=L MCL cut point based on the average water arsenic concentration in the first ( . We also categorized compliance categories using cutoff values of 5 lg=L (the current MCL for New Hampshire and New Jersey) and 1 lg=L (the MCL for the Netherlands and close to the U.S. EPA's MCL goal of 0 lg=L). We compared characteristics of CWSs and counties (region, sociodemographic countycluster, source water type, and population served) by compliance category, and we mapped these county-level compliance categories for the conterminous United States. Finally, we identified all counties and CWSs that served counties classified into the High/ Low (i.e., successful in reducing water arsenic below the new MCL) and High/High (i.e., not successful in reducing water arsenic below the new MCL) compliance categories to identify counties that were successful vs. unsuccessful in reducing water arsenic concentrations. We also assessed the odds ratio (OR) of average water arsenic concentration exceeding the 10 lg=L MCL at either time period (2006-2008 and 2009-2011) for CWSs in a given subgroup, compared with all CWSs not in that subgroup (reference), using logistic regression models with generalized estimating equations to account for two 3-y averages per CWS.

Interactive Map and Publicly Available Data
We also created an online interactive map of county-level water arsenic concentrations for all three time periods (2006-2008, 2009-2011, and 2006-2011) and MCL compliance categories to improve accessibility and results dissemination (https://annenigra.github. io/ColumbiaArsenicMap.html). Data sets of the 3-y and 6-y average arsenic concentrations at the CWS and county level and a reproducible R archive of analyses and data sets are available in the Supplemental Material and via GitHub (https://github.com/ annenigra/epa-public-water-arsenic).   Excel Table S1. Distributions of average water arsenic from 2006-2008 were markedly bimodal in the Southwest and New England regions ( Figure 1). Mean difference estimates did not change with adjustment for source water type and size of population served (Table S3).   (Table S6). County-level average arsenic estimates for 2006-2008 and 2009-2011 displayed similar regional patterns to those at the CWS level, with a general pattern of markedly higher water arsenic concentrations in western vs. eastern counties (Figure 3; Excel Table S2). When restricting our analysis to counties with CWSs serving at least 70% or 80% of the public water-reliant population, overall effect estimates remained similar, but sample size was reduced (Table S7).

Spatial Patterns and ORs of MCL Exceedances
The percentage of CWSs with average concentrations of arsenic above the 10 lg=L MCL was 3.2% in 2006-2008 vs. 2.3% in 2009-2011 (

Characteristics of CWSs by MCL Compliance Categories
When evaluating compliance categories applying the 10 lg=L cut point, the large majority of CWSs were categorized into the Low/Low category (N = 25,846, 96.1%), followed by the High/ High category (N = 493, 1.8%), the High/Low category (N = 397, 1.5%), and the Low/High category (N = 159, <1%) ( Table 3) (2006-2008 and 2009-2011). CWS arsenic concentrations were weighted by the population served by each system to estimate the county-level weighted average CWS arsenic concentrations. Counties that were not represented by any CWSs in the SYR3 database are labeled as "No data available." Counties with "Inadequate data" did not have CWS data representing at least 50% of the public water-reliant population for either time period (2006-2008 and 2009-2011). SYR, Six-Year Review. 0.01) in the Low/Low category, −12:84 lg=L (95% CI: −15:36, −10:32) in the High/Low category, 6:81 lg=L (95% CI: 5.17, 8.46) in the Low/High category, and −0:97 lg=L (95% CI: −1:62, −0:33) in the High/High category. CWSs in the High/ High and Low/High categories were more likely to be in the Southwest, to serve smaller populations, to serve Semi-Urban, Hispanic counties, and to be served by groundwater compared with those in Low/Low and High/Low categories. CWSs in the Low/Low category were more likely to rely on surface water, to serve a larger population size, to serve Semi-Urban, High SES counties, and to be located in the Southeast and Mid-Atlantic, and less likely to be located in the Southwest and Pacific Northwest compared with CWSs in other categories. CWSs in the High/Low category (those who successfully reduced water arsenic below the 10 lg=L MCL) were more likely to rely on groundwater, to serve a smaller population size, to be tribal systems, to serve Semi-Urban Hispanic counties, and to be located in the New England, Eastern Midwest, Pacific Northwest, and Southwest regions. When applying the 5 lg=L cut point (MCL for New Jersey and New Hampshire), 7.4% of CWSs (N = 1,993) were in the High/High category. The majority of these systems were in the Southwest (N = 874, 43:9%), Pacific Northwest  Table S5). Patterns were similar for CWSs in the High/ High category applying the 1 lg=L cut point (MCL for the Netherlands).
At the county level, counties were most likely categorized into the Low/Low category (N = 2,228, 98:5%), followed by the High/High category (N = 18, < 1%), the High/Low category (N = 12, < 1%), and the Low/High category when applying the 10 lg=L cut point (N = 4, < 1%) (Figure 4; Excel Table S6). When evaluating county compliance categories applying the 5and 1 lg=L cut points, the majority of counties in the Central Midwest and Southwest United States had weighted average public drinking water arsenic concentrations exceeding these thresholds (Figure 4). In addition to the static maps presented here, an interactive map of county-level water arsenic concentrations and MCL compliance categories is available at https://annenigra. github.io/ColumbiaArsenicMap.html. Characteristics of counties by compliance category applying the 10-, 5-, and 1 lg=L cut points were similar to findings at the CWS level (Excel Table S6) and support that the new MCL likely reduced water arsenic exposure for many communities, with water arsenic levels from 2006-2008 being well below the 10 lg=L standard.
A total of 18 counties (served by a total of 81 CWSs) were categorized into the High/High compliance category (water arsenic >10 lg=L during both time periods) (Excel Table S7

Discussion
Community water arsenic concentrations decreased significantly in the New England, Eastern Midwest, and Southwest regions of the United States from 2006 through 2011. Declines in CWS arsenic concentrations were largest at the higher end of the distribution and generally nonsignificant at quantiles below the 80th percentile, indicating that the decline occurred in places with elevated water arsenic concentrations, likely because of specific interventions to comply with the 10 lg=L MCL. Significant declines also occurred for CWSs that were reliant on groundwater, serving smaller populations, and serving counties classified as Mostly Rural, Mid SES, Semi-Urban, High SES, and Rural, High SES. Despite the observed decline in CWS arsenic concentrations, several sociodemographic subgroups remained more likely exposed to arsenic concentrations in public drinking water exceeding the arsenic 10 lg=L MCL. CWSs more likely to continue exceeding the arsenic MCL were those serving Hispanic communities, located in the Southwestern United States, reliant Table 2. Number (%) of 3-y average arsenic concentrations (lg=L) equal to or exceeding the maximum contaminant level (MCL, 10 lg=L), and ORs (95% CIs) of MCL exceedance (>10 lg=L) in either time period (2006-2008and 2009-2011ORs  Categories of population served are standard U.S. EPA categories. Population served is adjusted total population served, which accounts for systems that sell or purchase water and avoids overcounting. c A total of 172 CWSs served more than one county; of these, approximately half served counties categorized to different sociodemographic county-clusters (e.g., NY7003493 serves New York, New York (Young, Urban, Mid/High SES) and Bronx, New York (Semi-Urban, Hispanic). These CWSs are represented for each county that they serve in the sociodemographic county-cluster analyses (N = 36,674).
on groundwater, and serving smaller populations. These results could have important implications for future efforts aimed at reducing the arsenic MCL further and for the regulation of similar drinking water contaminants. We previously estimated a 17% reduction in public water arsenic exposure nationwide (corresponding to a decline in water arsenic exposure of 1:42 lg=L) from 2003 to 2014, using urinary arsenic concentrations measured in the National Health and Nutrition Examination Survey (NHANES) (corresponding to an estimated 200-900 avoided excess cases of lung and bladder cancer per year), but we were not able to assess differences by geographic subgroups and were limited by self-reported dietary and water source data (Nigra et al. 2017). In the present study, the average change in CWS arsenic concentration differed across MCL compliance categories using the 10 lg=L cut point, from no change in Low/Low and High/ High CWSs to −12:84 lg=L in High/Low CWSs, indicating that CWSs in the High/Low compliance category reported relatively large reductions in accordance with MCL implementation compared with those in the High/High compliance category. At both the CWS and the county level, our findings indicate that a large majority of CWSs and U.S. counties (especially those in the western United States) had average public drinking water arsenic concentrations exceeding the more health protective regulatory standards of 5 lg=L (the MCL for New Jersey and New Hampshire) and 1 lg=L (the MCL for the Netherlands).
The present study indicates mean public drinking water arsenic concentrations decreased from 2006 to 2011 by 10% nationwide, with significant declines ranging from 11.4% in the Southwest to 36.6% in New England, and from 9.7% in Semi-Urban, High SES counties to 14.7% in Rural, High SES counties. These direct estimates of arsenic concentrations as reported in CWS monitoring records are not subject to potential confounding by diet and other arsenic exposure sources and further affirm that public health benefits resulting from the new arsenic MCL implementation were not uniform across the United States. Thus, any subsequent declines in arsenic-associated adverse health outcomes, which have not been adequately characterized, are likely differential by region and sociodemographic subgroup. Moreover, subgroups with the highest exposure from 2006 to 2008 (e.g., the Southwest and Semi-Urban Hispanic counties) could see greater reductions in arsenicassociated disease, even with smaller absolute reductions in water arsenic exposure, compared with subgroups with relatively low arsenic exposure from 2006 to 2008. Future work should evaluate whether MCL implementation has reduced arsenic-associated disease separately for these relevant subgroups.
The present study revealed substantial environmental justice concerns for largely Hispanic communities in the Southwest region that are served by CWSs that rely on groundwater and serve smaller populations. Of 18 counties categorized into the High/ High compliance category using the 10 lg=L cut point, 7 of these were Semi-Urban, Hispanic counties (38.9%) and 12 were in the Southwest region (66.7%). In addition, a recent study also found that Southwestern CWSs that exclusively serve correctional facilities reported arsenic concentrations that were twice as high as other Southwestern CWSs from 2006 to 2011 (Nigra and Navas-Acien 2020). Together, these findings highlight significant environmental Table 3. Characteristics of community water systems (CWSs) by compliance category applying 10 lg=L cut point for categorization (N = 26,895 CWSs with average arsenic estimates from both 3-y time periods available).

CWS Categories
All CWSs justice concerns for drinking water arsenic exposure in the United States, even among communities reliant on public water systems. Additional financial and technical resources and interventions are needed to ensure that communities served by these CWSs are protected from elevated drinking water arsenic exposure. The present analysis is likely not representative of tribal community water systems or of counties in the Rural, American Indian sociodemographic county-cluster given that less than 50% of the more than 700 tribal systems currently operating were included in our analysis, limiting our ability to draw conclusions about whether tribal CWSs in the United States remain at risk for elevated water arsenic exposure. The present study highlights the importance of collecting tribal public water systems data in the future SYR so that analyses can evaluate tribal water systems and Rural, American Indian counties, especially because the establishment of CWSs in some tribal communities has likely been critical for reducing water  We also found higher estimated water arsenic concentrations for CWSs and counties in the Rural, High SES sociodemographic cluster, which could reflect that these counties are predominately located in the Central Midwest where water arsenic concentrations were high (see Wallace et al. 2019). We present CWS-and county-level estimates of arsenic exposure in public drinking water across the United States for 2006-2011, leveraging two of the largest U.S. EPA databases of public water available: SYR3 and SDWIS. The time period analyzed in this study (2006)(2007)(2008)(2009)(2010)(2011) coincides with the U.S. EPA's MCL regulatory change, which was enacted in 2001 and went into effect in 2006. Although not formalized in the Final Arsenic Rule, training documents indicate that an initial monitoring period gave systems time to comply with the new MCL (U.S. EPA 2002). Depending on whether the primary source of water was surface or groundwater, systems had until December 2006 or 2007 to collect initial monitoring samples, respectively. To allow time to address noncompliance (typically through installing treatment systems or switching or mixing water sources), an additional year of sampling was allowed for systems whose initial monitoring samples exceeded the MCL (U.S. EPA 2002EPA , 2004. SYR data beyond 2011 is not yet available. The U.S. EPA estimates that data from the Fourth Six-Year Review (SYR4) will be made available in 2023 and future work should analyze these data to evaluate continued compliance with the 10 lg=L MCL.
Because of the additional cost of installing and operating arsenic treatment systems, small CWSs were anticipated to experience difficulty reducing water arsenic concentrations below the 10 lg=L MCL. Although we found the largest decreases in 3-y average CWS arsenic concentrations among CWSs that served the smallest population sizes, we also found that CWSs in the High/ High and Low/High compliance categories were more likely to serve smaller populations than CWSs in other compliance categories. Therefore, although the largest decreases in average arsenic concentrations have occurred among smaller CWSs, numerous smaller CWSs were still struggling to reduce water arsenic concentrations to <10 lg=L as of 2009-2011. These findings are consistent with the focus of federal and state programs (Indian Health Service, U.S. EPA, U.S. Department of Agriculture, New Mexico, California) that provide financial and technical assistance to small public water systems. Federal and state efforts are still needed to support those systems not in compliance.
Data quality limitations of the SYR3 must be considered. First, due to the voluntary data submission, states and U.S. EPA Regional offices that did not submit monitoring records for the SYR3 (e.g., Colorado and Mississippi; U.S. EPA for tribal public water systems) had significantly higher arsenic violation rates reported in SDWIS compared with states and agencies that did contribute monitoring records (U.S. EPA 2016a). Second, reported LODs differed substantially across CWSs and were often missing from monitoring records; moreover, we found several reported LODs that were likely reported with incorrect units. We assigned county-served based on reported ZIP code for 418 CWSs that could not be matched automatically to our SDWIS database; however, these ZIP codes could represent administrative addresses and may not reflect the distribution system and the population served. Still, the quality assurance and quality control measures implemented by the U.S. EPA prior to the publication of the SYR3 were extensive and addressed many known data quality-related challenges (U.S. EPA 2016b). We categorized CWS source water type using reported source water type in SDWIS (categorized as surface vs. groundwater). It is possible that some CWSs may have changed source water type over time, which could influence our findings, which are stratified by source water type.
Unlike epidemiologic research on air quality, which often relies on national monitoring networks for exposure assessments (e.g., the National Air Toxics Assessment), epidemiologic research on drinking water contaminants remains limited by the lack of a centralized, national monitoring network of public drinking water. The merging of SDWIS with SYR3 enables nationwide epidemiologic studies assigning estimated water arsenic exposures to populations reliant on public drinking water. CWS-level estimates of exposure may be more relevant for populations residing in counties with County compliance categories, 1 µg/L cut point significant heterogeneity of public water arsenic concentrations. The association between estimates of CWS arsenic concentrations and biomarkers of inorganic arsenic exposure can also be evaluated in epidemiologic cohorts with biomarker and dietary recall data available, including national surveys (e.g., NHANES) and other studies. The quality of the SYR databases could be improved and used for this purpose by including higher-resolution geographical distribution data beyond county-served (e.g., census-tract served) and implementing additional efforts and compensation for obtaining a more complete reporting of regulated contaminant monitoring records and rigorous oversight of data entry and quality. Some monitoring records were excluded from the present analysis because of missing data, data quality concerns, and difficulties in merging a small number of records in SYR3 with SDWIS.
The present study indicates that the U.S. EPA's Arsenic Rule and MCL change was successful in reducing public drinking water arsenic exposure for most subgroups of the U.S. population and provides environmental exposure estimates for further epidemiologic investigation, although certain subgroups remain more highly exposed. Future work should validate these CWS-and countylevel public water arsenic exposures with biomarkers of inorganic arsenic exposure in epidemiologic cohorts and should evaluate whether communities and counties with decreased estimated public water arsenic concentrations experienced a subsequent reduction in arsenic-associated health outcomes (e.g., cardiovascular disease, related cancers, adverse birth outcomes). Stronger regulations, targeted compliance enforcement, and continued state and federal funding for infrastructure and technical assistance support for public water systems is needed to reduce inequalities and to further protect numerous communities in the United States affected by elevated drinking water arsenic exposure.