干旱区生态环境知识资源中心

Inland basins show various redox conditions in the groundwater system, which control the distribution and transportation of redox-sensitive radionuclides. However, the origin and fate of groundwater radionuclides in those basins are still unclear. High levels of naturally occurring uranium (U) and radon (Rn) have been observed in low total dissolved solid and oxic groundwater in the recharge area of the Hetao basin, a typical inland basin. Around 80% of investigated groundwater had U concentrations above the international drinking water standards (30 mu g/L), and 97% exceeded the United States Environmental Protection Agency (U.S. EPA) guideline of 11.1 Bq/L Rn. Groundwater U was mobilized by oxic weathering and complexation with carbonate, while Rn-222 was mainly provided by alpha recoil of adsorbed Ra on the aquifer solids. Relatively high radium (Ra) activities were frequently found in reducing groundwater of the transition area and the flat plain, although combined Ra-226 and Ra-228 did not exceed the international drinking water standard. Radium isotopes showed that Ra was mainly mobilized via reductive dissolution of Fe/Mn oxides and alpha recoil of parent Th isotopes on aquifer sediments. As a result of saturation of barite in groundwater, co-precipitation would be a significant removal mechanism for groundwater Ra, which restrains Ra activities within the U.S. EPA guideline of 0.185 Bq/L. This study suggested that, in other analogue arid/semi-arid basins of northwestern China with high U backgrounds, where both SO42- and Ba concentrations are low under reducing conditions, activities of groundwater Ra isotopes deserve evaluation for their potential health risks. As a result of severe health impacts of drinking water U, the drinking water standard for the U level is needed in China.