Data for Radium Mobility and the Age of Groundwater in Public-drinking-water Supplies from the Cambrian-Ordovician Aquifer System, North-Central USA: Table 1. Dissolved gas modeling results, environmental tracer concentrations (tritium, tritiogenic helium-3, sulfur hexafluoride, carbon-14, and radi

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.

研究团队利用80口公共供水井（PSWs）的地下水年龄相关水质数据与环境示踪剂（氚（³H）、氚化氢（³H_{trit}）、六氟化硫（SF₆）、碳十四（¹⁴C）及放射性氦（⁴He_{rad}）），对寒武纪-奥陶纪（C-O）含水层系统可饮用区域内的高镭（Ra）浓度情况展开了调查。通过将示踪剂校准至集总参数模型，研究人员估算得到地下水年龄，其年龄跨度为从现代至1百万年（1 Myr），覆盖可饮用系统中最靠近下游的承压区域。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%。含水层系统中，缺氧铁还原环境与升高的矿化度随时间推移逐渐形成，促使镭解吸进入溶液，导致复合镭（Rac = ²²⁶Ra + ²²⁸Ra）的浓度频繁超过美国环境保护署（USEPA）设定的185毫贝可每升（185 mBq/L，即5皮居里每升，5 pCi/L）的最大污染物浓度限值（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，且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。而在下游承压区域的样本中，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中总镭浓度显著更高，这是由于该区域的²²⁶Ra浓度有所升高。²²⁶Ra的分配系数随缺氧环境与地下水离子强度（即矿化度）升高而显著降低，这表明当含水层固相的吸附容量下降时，镭会从固相解吸进入溶液。随着吸附容量与钡（Ba）浓度降低，通过α反冲机制从固相释放并保留在溶液中的²²⁶Ra量，相较于通过吸附作用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或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可饮用样本带来了可观的α粒子放射性活度。因此，在区域非承压的上游区域以及下游区域中，对镭同位素与总α活度（GAA）进行监测具有重要意义；且在样本采集后72小时内开展总α活度测量，可最精准地捕获短寿命²²⁴Ra产生的α粒子辐射。