Hyrogeochemistry Of U-Th Series Radionuclides In Groundwater Of The Hetao Basin,Inner Mongolia

Since radionuclides are carcinogenic and teratogenic,the health risk of groundwater radionuclides(uranium,radium and radon)has received much concern.In Northern China,where groundwater is used as major drinking water resource,it is essential to study groundwater radionuclides.However,few data are available for groundwater radionuclides in the Hetao basin,P.R.China.Therefore,it is necessary to investigate the distribution and migration characteristics of nature radionuclides in groundwater from the Hetao basin.From alluvial fans,via transition area,to flat plain in the Hetao basin,82 groundwater samples were taken from irrigation and drinking wells,and 16 sediment samples from two boreholes and 15 bedrock samples with different lithology in the Langshan mountain were also collected.Radionuclides and major components and trace elements were analyzed in the lab.Results showed that 40.3% of investigated samples had uranium concentrations greater than 30 ?g/L(WHO drinking water standards).High U groundwater was mainly found in alluvial fans and transition area.Of analysed samples,52 groundwater samples had radon activity >11.1 Bq/L(EPA drinking water standards),which was mostly observed in alluvial fans.Although an increasing trend in Ra concentrations was observed from alluvial fans to flat plain,no samples had Ra concentrations greater than 0.185 mBq/L(EPA drinking water standards).Groundwater in alluvial fans was mainly recharged from fracture water in mountain areas,which was present under oxic conditions.Uranium existed in the form of U(VI),and mainly occurred as complexes with bicarbonate in groundwater,which was related to high mobility of U.In transition area and flat plain,U(VI)was reduced into U(IV),and consequently sequetrated as UO2.The Ra isotopes in groundwater were mainly derived from alpha recoil process of the parent nucleus in the aquifer rocks and on the surface coatings of clay minerals and Fe/Mn oxides.In the alluvial fans,Ra was readily adsorbed on the clay minerals neighbouring aquifers,and Fe/Mn oxides,which were the sinks of groundwater Ra.In the transition area and flat plain,groundwater Ra mainly originated from alpha recoil process of the parent nucleus in the aquifer sediments,dissolution of radium sulfate minerals and reduction of Fe/Mn oxides.Groundwater Rn was mainly from decay of adsorbed Ra by ?-recoil process.In alluvial fans,Ra-226 decayed from U-238 was adsorbed on sediment surfaces,which was the reason for high activity of Rn in groundwater.In transition area,although U was immobilized in sediments,the decay product Ra-226 was released during reductive dissolution of Fe/Mn oxides,which led to a low proportition of samples >11.1 Bq/L.In flat plain,both U and Ra-226 were low in aquifer sediments,which led to less production of Rn-222 from ?-recoil process.