Uranium Hydrogeochemistry Of Shallow Groundwater System In Datong Basin Of Northern China

In this dissertation, hydrogeochemical studies were carried out in Datong basin of northern China, including hydrogeochemical investigation by integrating methods of stable isotopes (18O,2H),87Sr/86Sr ratios,234U/238U activity ratios, Cl/Br ratios, chloride-mass balance, mass balance and hydrogeochemical modeling; in addition to geochemical investigations of uranium (U) in the environment. The objectives of this study were:(1) to interpret geochemical evolution of groundwater,(2) to investigate U occurrence in the environment and to understand geochemical behavior and distribution of U in shallow aquifers, and (3) to identify water-rock interactions related to the mobilization and transport of U at basin scale.The major results and conclusions of the studies are as follows.The H, O isotopic compositions in precipitation exhibit a local meteoric water line of δ2H=6.4δ18O-5(R2-0.94), while those in groundwater suggest their meteoric origin in a historically colder climatic regime. A speculated recharge rate is overall less than20.5mm per year throughout the basin. Besides, recharge may be also from a component of residual saline water or brackish water associating with organic-rich sediments enriched in Br. According to a Sr isotope binary mixing model, a possible mixing mechanism of recharges from the Shentou karst springs (24%), the western margins (11%) and the eastern margins (65%) account for the groundwater in the deep aquifers of the down-gradient parts of the central basin.In Datong basin, hydrolysis of silicate minerals is the most important hydrogeochemical process responsible for groundwater chemistry, in addition to dissolution of carbonate minerals and evaporites. In the recharge zones, silicate chemical weathering is typically at the bisiallitization stage, while that in the central basin is mostly at the monosiallitization stage with limited evidence of being in equilibrium with gibbsite. Na exchange with bound Ca, Mg prevails at basin scale, and intensifies with groundwater salinity, while Ca, Mg exchange with bound Na locally occurs in the east pluvial and alluvial plains. Although groundwater salinity increases with the progress of water-rock/sediment interactions along the flow paths, as a result of carbonate solubility control and continuous evapotranspiration, Na-HCO3and Na-Cl-SO4types of water are usually characterized respectively in the deep and the shallow aquifers of an inland basin with a silicate terrain in an arid climatic regime.U contents were generally<1mg kg-1in the igneous and metamorphic rocks, typically2-5mg kg-1in the Carboniferous and Permian sedimentary rocks and around3mg kg-1in the sediments and topsoil, respectively. U may be primarily associated with carnotite (K2(UO2)2(VO4)2in the Quaternary aquifer sediments from the Carboniferous and Permian coal-bearing clastic rocks around the basin. Shallow groundwater had U concentrations of<0.02-288μg L-1(average24μg L-1), with24%of the investigated boreholes above the WHO provisional guideline of30μg L-1for U in drinking water. Average U concentration in surface water was5.8μg L-1. In oxidizing waters, uranyl (UO22+) species is dominant and strongly adsorbed onto iron (hydro)xides, while would be preferentially complexed with carbonate in the alkaline groundwater, forming highly soluble uranyl-carbonate complexes in Datong. Under reducing conditions, uranous (U(IV)) species is readily to precipitate or bind to organic matter, therefore having a low mobility.Groundwater containing of high levels of U (>30μg L-1) occurs in the alluvial plains due to intermediate redox and enhanced alkaline conditions. In contrast, groundwater containing of abnormally high levels of U (>100μg L-1) occurs locally in the west alluvial plains, probably owing to the prevailing of U co-precipitation as secondary carbonate minerals such as Ca2UO2(CO3)3in the predominantly Ca-Mg-Na-HC03type groundwater in the east alluvial plains. In addition, bedrocks such as Carboniferous and Permian sedimentary rocks, especially the coal-bearing strata in the west mountain areas which have higher U contents may also account for the abnormally high levels of U in groundwater.The recharge zones and the deeper aquifers of the flow-through zones had relatively high234U/238U activity ratios (average around1.60-1.69), while the shallow aquifers of the central parts of the basin and the west alluvial plains illustrated relatively low ratios (average around1.30-1.40). The influence of atmospheric input of U during ancient recharge period is negligible, regarding the very low dissolved U concentrations (<0.02μg L-1) in the deeper aquifers. Meteoric contribution of U from the weathered zone is insufficient, while atmospheric contribution for U-series radioactive disequilibria may be also relatively insignificant. Water-rock/sediments interactions account for the U-series radioactive disequilibria in the groundwater, during which the contribution of direct recoil to an elevated234U/238U activity ratio is less significant relative to preferential leaching of234U occurring at very early stages of U minerals (primary and newly precipitated) weathering/dissolution. Mixing of end-members with distinct U isotopic signatures is responsible for the observed groundwater U-series radioactive disequilibria and chemistry.Functional groups of NO3reducers, Fe(III) reducers and SO4reducers may simultaneously function under anaerobic conditions in the study area, facilitating the fractionations and the resulting radioactive disequilibria via reduction of U(VI) to U(IV). This newly precipitated U leads to higher ratios in groundwater, owing to preferential leaching of234U from U mineral that probably has an amorphous structure, although this leaching has not resulted in significant increase of dissolved U concentrations. The234U/238U activity ratios are expected to increase along the flow path in a carbonate-hosting groundwater system, and a faster increase rate in the redox front zones. However, in a silicate-hosting groundwater system, the234U/238U activity ratios decrease along the flow path under oxidizing/suboxic conditions, while the precipitated U results in an increase of the ratios that could be generally at a higher level relative to the secular equilibrium value under reducing conditions. The variation patterns of dissolved U concentrations may be only dependent on redox conditions.