Sociodemographic inequalities in uranium and other metals in community water systems across the USA, 2006-11: a cross-sectional study

Abstract

Background: The US Environmental Protection Agency (EPA) currently sets maximum contaminant levels (MCLs) for ten metals or metalloids in public drinking water systems. Our objective was to estimate metal concentrations in community water systems (CWSs) across the USA, to establish if sociodemographic or regional inequalities in the metal concentrations exist, and to identify patterns of concentrations for these metals as a mixture.

Methods: We evaluated routine compliance monitoring records for antimony, arsenic, barium, beryllium, cadmium, chromium, mercury, selenium, thallium, and uranium, collected from 2006-11 (2000-11 for uranium; timeframe based on compliance monitoring requirements) by the US EPA in support of their second and third Six-Year Reviews for CWSs. Arsenic, barium, chromium, selenium, and uranium (detectable in >10% records) were included in the main analyses (subgroup and metal mixture analyses; arsenic data reported previously). We compared the mean, 75th percentile, and 95th percentile contaminant concentrations and the percentage of CWSs with concentrations exceeding the MCL across subgroups (region, sociodemographic county-cluster, size of population served, source water type, and CWSs exclusively serving correctional facilities). We evaluated patterns in CWS metal concentration estimate profiles via hierarchical cluster analysis. We created an online interactive map and dashboard of estimated CWS metal concentrations for use in future analyses.

Findings: Average metal concentrations were available for a total of 37 915 CWSs across the USA. The total number of monitoring records available was approximately 297 000 for arsenic, 165 000 for barium, 167 000 for chromium, 165 000 for selenium, and 128 000 for uranium. The percentage of analysed CWSs with average concentrations exceeding the MCL was 2·6% for arsenic (MCL=10 μg/L; nationwide mean 1·77 μg/L; n=36 798 CWSs), 2·1% for uranium (MCL=30 μg/L; nationwide mean 4·37 μg/L; n=14 503 CWSs), and less than 0·1% for the other metals. The number of records with detections was highest for uranium (63·1%). 75th and 95th percentile concentrations for uranium, chromium, barium, and selenium were highest for CWSs serving Semi-Urban, Hispanic communities, CWSs reliant on groundwater, and CWSs in the Central Midwest. Hierarchical cluster analysis revealed two distinct clusters: an arsenic-uranium-selenium cluster and a barium-chromium cluster.

Interpretations: Uranium is an under-recognised contaminant in CWSs. Metal concentrations (including uranium) are elevated in CWSs serving Semi-Urban, Hispanic communities independent of location or region, highlighting environmental justice concerns.

Funding: US National Institutes of Health Office of the Director, US National Institutes for Environmental Health Sciences, and US National Institute of Dental and Craniofacial Research.

Figure 1:. County-level weighted average of water contaminant concentrations in CWSs (n=37 915) from 2006–11 for barium (A), chromium (B), selenium (C), and uranium (D; 2000–11)

Average concentrations were weighted by the population served by each CWS to estimate the county-level weighted average CWS concentrations. Counties which were not represented by any CWSs in the SYR3 database are labelled as having no data available. Counties with inadequate data did not have CWS data representing at least 50% of the public water reliant population (appendix 2 pp 5–6). Estimates for uranium are derived from both the second (2000–05) and third (2006–11) SYR. For barium, chromium, and selenium, the lowest concentration category corresponds to less than or equal to the SYR3 minimum reporting level (100 μg/L for barium, 1 μg/L for chromium, 5 μg/L for selenium), and the other three categories reflect tertiles of the remaining distribution of county-level estimates. For uranium, the lowest concentration category corresponds to less than or equal to 1 μg/L, and the other three categories reflect cut-points that might be considered in future regulatory decisions. CWS=community water system. SYR=Six-Year Review.

Figure 2:. Dendrogram of hierarchical cluster analysis for 6-year averages (2006–11) of regulated metal concentrations in CWSs across the USA (n=12 756).

Analysis was restricted to CWSs with no missing concentration estimates for arsenic, selenium, uranium, barium, and chromium. Values for uranium are averaged from 2000–11. We used Ward’s method for Euclidean distances and normalised each metal to unit variance and zero mean before analysis. CWS=community water system.