Use of 222Rn to trace submarine groundwater discharge in a tidal period along the coast of Xiangshan, Zhejiang, China

222Rn, 210Pb and 210Po in coastal zone groundwater: Activities, geochemical behaviors, consideration of seawater intrusion effect, and the potential radiation human-health risk

Groundwater quality in human-influenced coastal landscapes is receiving novel attention. Radionuclides have been recognized as another important monitoring indicator in many developed countries due to the discovery of extremely high level of natural ²¹⁰Po (up to 10,000 Bq/m³) and radium and radon isotopes. This study aims to evaluate the groundwater quality in the Beibu Bulf-Guangxi coast from radiological point of view. ²¹⁰Po, ²¹⁰Pb and ²²²Rn activities in 20 wells ranged from 0.24 ± 0.05 to 6.96 ± 1.62 Bq/m³, 2.17 ± 0.12 to 13.08 ± 0.74 Bq/m³ and 1500 ± 200 to 31,800 ± 900 Bq/m³, respectively. Compared with research data of other countries, groundwaters in this area have ²¹⁰Po, ²¹⁰Pb and ²²²Rn activity within low levels. The large deficiencies of ²¹⁰Po and ²¹⁰Pb relative to ²²²Rn in groundwaters implied that ²¹⁰Po and ²¹⁰Pb are strong particle-reactive radionuclides and they might be controlled by similar scavenging processes in groundwaters due to a good positive correlation between ²¹⁰Pb and ²¹⁰Po (R² = 0.67, p < 0.01). The concentrations of ²¹⁰Po and ²¹⁰Pb decreased with increasing pH values and salinity, which indicated that geochemical behaviors of ²¹⁰Po and ²¹⁰Pb in groundwater were influenced by seawater intrusion and pH changing. Groundwater ²²²Rn activity concentrations decreased with increasing salinity in coastal zone, which may be caused by dilution due to seawater intrusion or intensified ²²²Rn escaping from well-developed pores in coastal zone. The estimated annual ingestion doses for infants, children and adults were well below the recommended reference dose level (RDL) of 0.2-0.8 mSv/a, suggesting that consumption of analyzed groundwaters is safe from radiological point of view.

Improved Approach for the Investigation of Submarine Groundwater Discharge by Means of Radon Mapping and Radon Mass Balancing

The paper presents an improved approach for investigating submarine groundwater discharge (SGD) based on radon mapping and radon mass balancing in the coastal sea. While the use of radon as an environmental tracer in SGD studies is well-established, we identified based on our longstanding experience six methodical shortcomings of the conventional approach and suggest corresponding developments. The shortcomings include: (1 and 2) inadequate consideration of both detection equipment response delay and influence of tidal stage; (3 and 4) incorrect quantification of radon losses, due to offshore mixing and degassing resulting in a potentially incorrect radon mass balance; (5) inaccurate determination of the terrestrial groundwater endmember, due to inhomogeneous radon distribution in the coastal aquifer; and (6) difficulties in distinguishing between discharged fresh groundwater and recirculated seawater. The improved approach is practically demonstrated in a step by step manner in a large-scale field study, which was carried out in False Bay (South Africa) and which consisted of two parts, namely (i) qualitative SGD localization along the entire False Bay coastline based on coastal radon distribution patterns and (ii) quantitative SGD investigation within a defined coastal area of interest (AOI) based on a radon mass balance (RMB). The plausibility of the AOI related results was evaluated by a hydrogeological model, used for qualitative SGD localization, and a hydrological model, applied for estimating groundwater recharge within the AOI catchment.