Spatial relationship between well water arsenic and uranium in Northern Plains Native lands

Abstract

BACKGROUND AND AIM: Arsenic and uranium in unregulated private wells affect many rural populations across the US, including American Indian communities. The current EPA maximum contaminant level (MCL) is 10 µg/L for arsenic and 30 µg/L for uranium. Here, we evaluate the association between drinking water arsenic and uranium levels in three tribal regions in North Dakota and South Dakota participating in the Strong Heart Water Study. METHODS: Well water samples were collected in 2014-2020. Metal concentrations from 441 wells were dichotomized by their MCL for a graduated visualization across the regions. Spearman correlations and region-specific semivariograms were computed. Spatial interpolation using inverse distance weighting was conducted by region. RESULTS:Groundwater contamination was extensive; 29% and 7% of wells exceeded maximum contaminant levels for arsenic and uranium respectively. 81% of wells had both arsenic and uranium concentrations at one-tenth of their human-health benchmark (arsenic, 1 μg/L; uranium 3 μg/L). Well arsenic and uranium concentrations were uncorrelated (rs=0.06); however, there appeared to be a spatial correlation of wells co-contaminated by arsenic and uranium associated with flow along a geologic contact. CONCLUSIONS:Elevated arsenic and uranium in drinking water are common across Northern Plains Tribal Lands in the US. More than a third of all wells sampled contained either arsenic or uranium at levels exceeding the MCL, but co-contamination was more common in Region 3. These findings indicate the importance of measuring multiple metals in well water, and to understand underlying hydrogeological conditions. The underlying mechanisms for the prevalence of arsenic and uranium across Northern Plains Tribal Lands in the US, and in particular the occurrence of both elevated arsenic and uranium in drinking water wells in this region, demands further study. KEYWORDS: water quality, multi-pollutant, environmental disparities