Abstract This study integrates geochemical modeling, spatial analysis and several statistical methods including principal component analysis, multivariate regression and cluster analysis to investigate hydrogeologic controls of arsenic and uranium contamination within 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 enriched in arsenic and uranium related to volcanic ash deposits and acts as a geogenic 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.

中文翻译：

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南达科他州 Pine Ridge Reservation 地下水中砷和铀溶解的水文地质控制

摘要 本研究综合了地球化学建模、空间分析和多种统计方法，包括主成分分析、多元回归和聚类分析，以研究松岭保护区 (PRR) 上 Arikaree 含水层地下水中砷和铀污染的水文地质控制。PRR 位于南达科他州西南部，主要是农村地区，许多人依靠在 Arikaree 含水层完成的家庭供水井作为他们的主要饮用水源。在当地，不整合地作为 Arikaree Group 基础的 White River Group 富含与火山灰沉积物有关的砷和铀，并且是地质成因的金属来源。通过与 Oglala Sioux 部落合作，从 250 多个地下水样本中获得地球化学数据。集群空间统计分析描绘了地下水化学具有统计显着变化的四个区域，代表了 Arikaree 含水层的上坡、中坡和下坡部分。地下水在流经 Arikaree 含水层时随着流动路径的碱度、钠和 pH 值的增加而演变。这些化学变化很可能是由于碳酸盐矿物和火山灰的溶解。热力学计算表明地下水相对于方解石的过饱和度越来越高；因此，火山灰溶解可能是碱度的重要次要来源。发现升高的碱度和 pH 值是砷和铀迁移的驱动因素，