Association of Radionuclide Isotopes with Aquifer Solids in the Midwestern Cambrian–Ordovician Aquifer System,ACS Earth and Space Chemistry

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Association of Radionuclide Isotopes with Aquifer Solids in the Midwestern Cambrian–Ordovician Aquifer System
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2021-01-20 , DOI: 10.1021/acsearthspacechem.0c00279
Madeleine Mathews ₁ , Sean R. Scott ₂ , Madeline B. Gotkowitz ₃ , Matthew Ginder-Vogel ₁

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Groundwater, an important source of drinking water globally, is susceptible to contamination by naturally occurring metals and radionuclides. Regional trends in groundwater quality are useful in predicting the occurrence of contaminants but are difficult to translate to local scales due to complex contaminant–solid phase associations. Here, the aqueous phase of sequential extractions is analyzed using multicollector inductively coupled plasma mass spectrometry techniques to quantify ultratrace radium (Ra) levels in operationally defined fractions of the aquifer solids. Results demonstrate that local-scale geochemistry drives Ra partitioning to groundwater in the U.S. Midwestern Cambrian–Ordovician aquifer system. Analysis of whole-rock extractions indicates that parent and daughter isotope activity ratios indicate that Ra remains close to the site of ²³⁸U decay in most stratigraphic units, where the ²³⁸U/²²⁶Ra ratio is similar to the equilibrium value; however, other stratigraphic units display isotope ratios indicative of Ra leaching. Additionally, Ra varies in prevalence across examined stratigraphy, in both whole-rock and sequential extractions; the average whole-rock ²²⁶Ra activity is 70 ± 10 mBq/cm³ in the Maquoketa shale in comparison to 6 ± 1 mBq/cm³ in the St Peter sandstone. This suggests that Ra mobilization depends on both the reactive solid phases present in the stratigraphy and the influence of local geochemical conditions on solid phase–contaminant interactions. Variation in geochemical conditions, such as redox or competitive ion exchange, affects Ra partitioning to groundwater differently across stratigraphy, depending on initial solid-phase associations.

中文翻译：

中西寒武纪-奥陶纪含水层系统中放射性核素同位素与含水层固体的联系

地下水是全球重要的饮用水来源，容易受到天然金属和放射性核素的污染。地下水水质的区域趋势可用于预测污染物的发生，但由于污染物与固相之间的复杂联系，因此很难转化为当地规模。在此，使用多收集器电感耦合等离子体质谱技术分析顺序萃取的水相，以量化含水层固体的有效定义馏分中的超痕量镭（Ra）含量。结果表明，在美国中西部寒武纪-奥陶纪含水层系统中，局部地球化学驱动Ra分配至地下水。在大多数地层单位中²³⁸ U衰减，其中²³⁸ U / ²²⁶ Ra比与平衡值相似；但是，其他地层单位显示的同位素比值表示Ra浸出。此外，在整个岩层和连续抽采中，Ra在整个检查地层中的发生率都不同。Maquoketa页岩的平均全岩²²⁶ Ra活度为70±10 mBq / cm ³，而6±1 mBq / cm ³在圣彼得砂岩中。这表明Ra的动员既取决于地层中存在的反应性固相，也取决于局部地球化学条件对固相-污染物相互作用的影响。地球化学条件的变化（例如氧化还原或竞争性离子交换）会影响Ra分配给整个地层的地下水，具体取决于初始固相联系。

更新日期：2021-02-18

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