Characterizing 226Ra and its daughters in coastal zone groundwater of a typical human-activity affected bay: occurrence, safety, and source evaluation

Abstract

Due to their extremely toxic properties, ²²⁶Ra and it daughters (²²²Rn, ²¹⁰Pb, and ²¹⁰Po) in drinking groundwater require monitoring. Recent studies have reported exceptionally high levels of natural ²¹⁰Po (up to 10,000 Bq/m³), ²²⁶Ra, and ²²²Rn isotopes in groundwater. This study aims to provide background data on ²²⁶Ra and its daughter radionuclides in the typical agricultural-industrial Dongshan Bay (DSB) before the construction of Zhangzhou Nuclear Power Plant (Zhangzhou NPP). The measurement results indicate that no abnormally high activities of ²¹⁰Po and ²¹⁰Pb were detected in the investigated wells. Strong positive correlations between ²¹⁰Pb and ²¹⁰Po, as well as between ²²²Rn and ²¹⁰Pb activities, suggest that the origins of ²¹⁰Pb and ²¹⁰Po in groundwater are strongly influenced by the decay of the parent radionuclides ²²²Rn and ²¹⁰Pb, respectively. In the DSB coastal zone groundwater, significant deficiencies of ²¹⁰Po relative to ²¹⁰Pb and ²¹⁰Pb relative to ²²²Rn were observed, providing further evidence that ²¹⁰Po and ²¹⁰Pb are also effectively scavenged due to their geochemical properties (specifically particle affinity) within the groundwater-aquifer system. A systematic comparison among all relevant water bodies in the DSB revealed that the activity concentrations of ²¹⁰Pb and ²¹⁰Po in groundwater were the highest, except for rainwater. Based on the evaluation of ²¹⁰Pb sources, the results imply that submarine groundwater discharge (SGD) is an important pathway for transferring radionuclides (such as ²¹⁰Pb) from land to the nearshore marine environment, even though the study area has a lower ²¹⁰Pb background groundwater. By considering all the ²¹⁰Pb's sources in the DSB, we found low ²¹⁰Pb background groundwater discharge still needs to be taken into account for small-scale bays. This is because SGD was calculated to be one of the most important ²¹⁰Pb sources in the bay during observation season. Regardless of whether the system is in a normal state or a nuclear accident emergency state, greater attention should be paid to the groundwater discharge of radionuclides into the ocean.

Keywords: 210Pb-222Rn disequilibrium; 210Po-210Pb disequilibrium; Coastal zone groundwater; Radiation risk; Radionuclide transport pathway.