Geochemical behavior of high-level radium contamination in representative coastal saltworks

The coexistence of salt production with coastal and residential areas presents significant environmental pollution risks. This study explores the hydrochemical characteristics and radium isotopic signatures of saline water through case studies of groundwater and surface water in representative coastal salt fields, supplemented by additional data. The water in salt field regions is predominantly of the Na-Cl type, significantly influenced by evaporation and seawater intrusion, with elevated levels of ammonium-nitrogen pollution. Compared to non-salt field areas, groundwater in salt field regions exhibits significantly higher radium radiation levels and annual effective doses, exceeding WHO standards and industrial emission limits, which threaten the limited freshwater resources and nearby ecosystems. Distinct variations in radium activities and their ratios reveal multiple supply and removal mechanisms, including adsorption–desorption, recoil, decay, and co-precipitation. Salinity primarily controls the mobilization of exchangeable radium through constrained desorption and co-precipitation. Seawater intrusion significantly increases groundwater salinity, facilitating the desorption of radium from particles and sediments, thereby elevating associated risks. Evaporation in surface water within salt ponds further promotes the coprecipitation of all four radium isotopes with potential host mineral (barite), at consistent salinity threshold, partially mitigating radiological risks. However, poor management practices observed in salt field operations substantially increase the risk of radioactive water leakage into surrounding environments. Geochemical modeling suggests that short-lived radium isotopes are more effectively incorporated into precipitated minerals compared to their long-lived counterparts. A limitation of this study is the inability to exclude the potential influence of fine colloids on radium transport in high-salinity solutions. These findings provide critical insights for the sustainable management of salt field operations, particularly in regions where freshwater resources are scarce.