Bird, et al., reported in 2020 that they applied geochemical modeling, spatial analysis and several statistical methods including principal component analysis, multivariate regression and cluster analysis to investigate hydrogeologic controls of [naturally induced] arsenic and uranium contamination within the groundwater of the Arikaree aquifer on the Pine Ridge Reservation (PRR). Located in southwestern South Dakota, the PRR is largely rural and many people rely on domestic supply wells completed in the Arikaree aquifer as their primary drinking water source.
Locally, the White River Group, which unconformably underlies the Arikaree Group, is naturally enriched in arsenic and uranium related to volcanic ash deposits and acts as a geogenic [aka naturally induced] metal source. Geochemical data from over 250 groundwater samples were obtained through collaboration with the Oglala Sioux Tribe. Cluster spatial statistics analyses delineated four regions of statistically significant variations in groundwater chemistry that represent upgradient, intermediate, and downgradient portions of the Arikaree aquifer.
Groundwater evolves as it flows through the Arikaree aquifer with increasing alkalinity, sodium, and pH along flow paths. These chemical changes are likely due to dissolution of carbonate minerals and volcanic ash. Thermodynamic calculations suggest increasing supersaturation of the groundwater with respect to calcite; thus, volcanic ash dissolution may be an important secondary source of alkalinity. Elevated alkalinity and pH levels were found to be the driving factors of arsenic and uranium mobility, and downgradient sections of the aquifer in the northern portions of the PRR are most likely to be impacted by metal(loid) contamination, with 73% of the wells in this grouping failing a USEPA maximum contaminant level for arsenic, uranium, and/or gross alpha [.... all naturally induced].
The authors concluded that Here, we explored underlying controls to As and U mobility in 253 groundwater chemistry samples from the PRR using various statistical
and geochemical methods. Their conceptual model indicates that water quality naturally degrades along the flow direction of the Arikaree aquifer. The
southern, upgradient sections of the Arikaree aquifer are influenced by recharge of meteoric water [aka rainfall at updip recharge zones] that keeps groundwater pH and alkalinity relatively low, which limits As and U mobility. As groundwater flows through the Arikaree aquifer, pH and alkalinity rise due to mineral dissolution and, potentially, leaching of volcanic ash from the basal portion of the Arikaree aquifer, which increases As and U mobility.
Human health impacts from high As and U levels are more likely in the down-gradient sections of the Arikaree aquifer, such as the northern extent of the PRR. The dataset obtained from IHS indicated that 32% of wells exceeded the MCL for arsenic, 8% exceeded the uranium MCL, and 23% exceeded the MCL for gross alpha, highlighting the critical need for continued investigation as thousands of people on the PRR rely on the Arikaree aquifer as their drinking water source, [which may require treatment of domestic supplies. This issue is similar to the subsurface conditions in parts of the Houston-Katy, Texas area where groundwater supplies also contain anomalous uranium derived from nearby naturally occurring uranium deposits known just south of Brookshire, Texas. See: http://www.i2mconsultants.com/downloads/corp/JGG-1-015.pdf pp. 14-18.]
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