Data for Radium Mobility and the Age of Groundwater in Public-drinking-water Supplies from the Cambrian-Ordovician Aquifer System, North-Central USA: Table 2. Detailed information on the calibration of dissolved gas models to dissolved gas concentrations (neon, argon, krypton, xenon, and nitrogen).

High radium (Ra) concentrations in potable portions of the Cambrian-Ordovician (C-O) aquifer system were investigated using water-quality data and environmental tracers ( 3H, 3Hetrit, SF6 , 14C and 4Herad) of groundwater age from 80 public-supply wells (PSWs). Groundwater ages were estimated by calibration of tracers to lumped parameter models and ranged from modern (1 Myr) in the most downgradient, confined portions of the potable system. More than 80 and 40 percent of mean groundwater ages were older than 1000 and 50,000 yr, respectively. Anoxic, Fe-reducing conditions and increased mineralization develop with time in the aquifer system and mobilize Ra into solution resulting in the frequent occurrence of combined Ra (Rac = 226Ra + 228Ra) at concentrations exceeding the USEPA MCL of 185 mBq/L (5 pCi/L). The distribution of the three Ra isotopes comprising total Ra (Rat = 224Ra + 226Ra + 228Ra) differed across the aquifer system. The concentrations of 224Ra and 228Ra were strongly correlated and comprised a larger proportion of the Rat concentration in samples from the regionally unconfined area, where arkosic sandstones provide an enhanced source for progeny from the 232Th decay series. 226Ra comprised a larger proportion of the Rat concentration in samples from downgradient confined regions. Concentrations of Rat were significantly greater in samples from the regionally confined area of the aquifer system because of the increase in 226Ra concentrations there as compared to the regionally unconfined area. 226Ra distribution coefficients decreased substantially with anoxic conditions and increasing ionic strength of groundwater (mineralization), indicating that Ra is mobilized to solution from solid phases of the aquifer as sorption capacity is diminished. The amount of 226Ra released from solid phases by alpha-recoil mechanisms and retained in solution increases relative to the amount of Ra sequestered by adsorption processes or co-precipitation with barite as sorption capacity and the concentration of Ba decreases. Although 226Ra occurred at concentrations greater than 224Ra or 228Ra, the ingestion exposure risk was greater for 228Ra owing to its greater toxicity. In addition, 224Ra added substantial alpha-particle radioactivity to potable samples from the C-O aquifer system. Thus, monitoring for Ra isotopes and gross-alpha-activity (GAA) is important in upgradient, regionally unconfined areas as downgradient, and GAA measurements made within 72 h of sample collection would best capture alpha-particle radiation from the short-lived 224Ra.

本研究借助80口公共供水井（public-supply wells, PSWs）的水质数据与地下水年龄环境示踪剂（氚（³H, tritium）、六氟化硫（SF₆）、碳十四（¹⁴C）与放射成因氦（⁴He(rad)）），对寒武纪-奥陶纪（Cambrian-Ordovician, C-O）含水层系统可开采段的高镭（radium, Ra）浓度展开调查。地下水年龄通过示踪剂与集总参数模型（lumped parameter models）校准得到，范围为现代水至该可开采含水层最下游承压段的1百万年（1 Myr），超80%与40%的平均地下水年龄分别大于1000年与5万年。含水层系统中，缺氧铁还原环境与矿化度随时间逐渐增强，促使镭溶入溶液，导致复合镭（Rac=²²⁶Ra+²²⁸Ra）浓度频繁超过美国环境保护署（United States Environmental Protection Agency, USEPA）的最大污染物限值（MCL）185毫贝克勒尔每升（mBq/L，即5皮居里每升pCi/L）。构成总镭（Rat=²²⁴Ra+²²⁶Ra+²²⁸Ra）的三种镭同位素在含水层系统中的分布存在差异：²²⁴Ra与²²⁸Ra的浓度呈显著正相关，在区域非承压区的样品中二者占总镭浓度的比例更高，该区域的长石砂岩（arkosic sandstones）为钍232（²³²Th）衰变系列的子体核素提供了更充足的来源；而在下游承压区的样品中，²²⁶Ra占总镭浓度的比例更高。与区域非承压区相比，含水层区域承压区的样品总镭浓度显著更高，这源于该区域²²⁶Ra浓度的升高。镭226的分配系数随缺氧环境增强与地下水离子强度（矿化度）升高而显著降低，表明当含水层固体相的吸附能力减弱时，镭会从固相溶入溶液；随着吸附能力与钡（Ba）浓度降低，通过α反冲机制从固相释放并留存于溶液中的²²⁶Ra量，相较于通过吸附作用或与重晶石共沉淀固存的镭量有所增加。尽管²²⁶Ra的浓度高于²²⁴Ra与²²⁸Ra，但²²⁸Ra因毒性更强，经口摄入的暴露风险更高。此外，²²⁴Ra为寒武纪-奥陶纪含水层系统的可开采水样贡献了大量的α粒子放射性活度。因此，在上游区域非承压区与下游区域开展镭同位素与总α放射性（gross-alpha-activity, GAA）监测至关重要，且需在样品采集后72小时内完成总α放射性测量，以准确捕获短寿命核素²²⁴Ra产生的α粒子辐射。