Low 210Pb background groundwater discharge still need to be considered for small-scale bay: a case study of Dongshan Bay

Abstract

Groundwater quality is receiving more and more attention due to the increasing human-impact on earth environment. Due to theirs extremely toxic chemical properties, the 210Po and 210Pb activities in drinking groundwater are needed to be monitored, besides some previous studies have reported extremely high levels of natural 210Po (up to 10,000 Bq/m3), 226Ra and 222Rn isotopes. This study aims to evaluate the groundwater quality of a typical agricultural-industrial bay (Dongshan Bay) in the Taiwan Strait from radiological point of view and provide background data of naturally occurring radionuclides (226Ra and its daughter nuclides) before the construction of Zhangzhou Nuclear Power Plant, and further to discuss the relationships between 226Ra and its daughter radionuclides (222Rn, 210Pb and 210Po) and their geochemical behaviors in groundwater environment of this important coastal zone of Taiwan Strait. 210Po, 210Pb, 222Rn and 226Ra activities in 12 selected wells surrounding the bay were measured and theirs activities ranged from 0.09 ± 0.01 to 1.09 ± 0.14 Bq/m3, from 0.41 ± 0.03 to 6.28 ± 0.25 Bq/m3 and from 1180 ± 530 to 177000 ± 2200 Bq/m3, and from 2.43±0.28 to 93.7±0.9 Bq/m3, respectively. No abnormally high 210Po and 210Pb activities were detected for the investigated 12 wells surrounding the Dongshan Bay. The evaluated results show that the potential radiation doses of 226Ra and its daughters are much lower than the recommended reference dose level (RDL) of 200-800 μSv/a. Strong positive correlations between 210Pb and 210Po and between 222Rn and 210Pb activities were observed in the Dongshan Bay coastal zone groundwater, which indicates that the origins of 210Pb and 210Po in groundwater are strongly controlled by decay of the parent radionuclides 222Rn and 210Pb, respectively. We observed large deficiencies of 210Po relative to 210Pb and 210Pb relative to 222Rn occurred in the Dongshan Bay coastal zone groundwaters, which supported that 210Po and 210Pb were strongly scavenged due to their geochemical properties (particle affinity) in the groundwater-aquifer system. Among all relevant water bodies (river water, groundwater, seawater, and rainwater) in Dongshan Bay, the activity concentrations of 210Pb and 210Po in groundwater were the highest except for rainwater. However, submarine groundwater discharge (SGD) is a possible pathway that transfer radionuclides (like 210Pb) from land to nearshore marine environment. By constraining all the 210Pb’s sources in Dongshan Bay, we found groundwater discharge was the most important 210Pb source in this bay during dry season (December, 2020). Therefore, no matter in normal state or nuclear accident emergency state, more attention should be paid to the groundwater discharge of radionuclides into the ocean.