Data for Radium Mobility and the Age of Groundwater in Public-drinking-water Supplies from the Cambrian-Ordovician Aquifer System, North-Central USA

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）的水质数据与用于估算地下水年龄的环境示踪剂（3H、3Hetrit、SF6、14C及4Herad），对寒武纪-奥陶纪（Cambrian-Ordovician, C-O）含水层系统可饮用区段的高镭（Ra）浓度展开了调查。
地下水年龄通过示踪剂与集总参数模型的校准得到估算，其取值范围为现代至大于1百万年（1 Myr），其中最下游的承压饮用区段的地下水年龄可达1百万年。
平均地下水年龄超过1000年与50000年的样本占比分别超过80%与40%。
含水层系统随时间推移逐渐形成缺氧铁还原环境与更高的矿化度，这一过程将镭解吸进入水溶液，导致结合镭（Rac=226Ra+228Ra）的浓度频繁超过美国环境保护署（USEPA）制定的185毫贝可每升（185 mBq/L，即5皮居里每升，5 pCi/L）最大污染物限值（Maximum Contaminant Level, MCL）。
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的浓度呈强相关性，在区域非承压区的样本中，二者占总镭浓度的比例更高；该区域的长石砂岩为钍-232（232Th）衰变链的子体产物提供了更为充足的来源。
而在下游承压区的样本中，226Ra占总镭浓度的比例更高。
与区域非承压区相比，含水层系统区域承压区的样本总镭浓度显著更高，这是由于该区域的226Ra浓度更高。
226Ra的分配系数随缺氧环境与地下水离子强度（即矿化度）的升高而显著降低，这表明当吸附容量下降时，含水层固相中的镭会被解吸进入水溶液。
随着吸附容量与钡（Ba）浓度的降低，通过α反冲机制从固相释放并保留在溶液中的226Ra量，相较于通过吸附过程或与重晶石共沉淀而被固定的镭量有所增加。
尽管226Ra的浓度高于224Ra与228Ra，但由于228Ra的毒性更强，其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饮用水样本带来了显著的α粒子放射性活度。
因此，在区域非承压的上游至下游区段开展镭同位素与总α放射性（gross-alpha-activity, GAA）监测具有重要意义；且需在样本采集后72小时内完成总α放射性测量，以最准确地捕获短寿命核素224Ra产生的α粒子辐射。