The uranium disequilibrium of groundwater at Gobi Desert in the arid Alxa Plateau, Inner Mongolia, China

Full text: The disequilibrium of uranium series of groundwaters within Alxa Plateau with annual mean precipitation of about 50 mm is used as a means of providing information related to the hydrogeological framework of its aquifer system. Sampling area including Gobi, dune desert and grassland covers approximately 60 000 km2 since 1997. Variation of uranium content and the environment of groundwaters The concentration of dissolved uranium (UC) in phreatic groundwater ranges from 0.65 μg/l to 37.5 μg/l while that in confined groundwater including springs ranges from 2.7 μg/l to 28.76 μg/l. The highest values are found from the phreatic groundwater in Badain Jaran Dune Desert. For the variation of activity ratio 234U/238U (or AR) which is a measure of the fractionation of 234U from 238U, different from the case of UC, both confined and phreatic groundwater hold a similar range from 0.96 to 2.92 of deep groundwater and 0.94 to 2.81 of phreatic groundwater. For the inflow Black River water, UC ranges from 1.20 to 5.55 μg/l and AR from 0.58 to 1.22 according to the water stages. The changing of the above isotopic characters is mainly due to the differing aquifer histories and its environment, the occurrence of isotopic re-equilibrium. It leads to the possibility for grouping the regional circulation patterns of groundwaters. Three circulation patterns from the UC versus AR relationship are grouped including (a) Group of active circulation and oxidizing-like environment characterized by high UC values more than 20 μg/l accompanied by lower AR values. It reveals an intense exchange with relative high transmissivity. (b) Group of weak circulation and reducing-like environment characterized by lowermost UC values less than 2 μg/l accompanied by a varied AR values. It reveals a stagnant-like environment. The low UC is due principally to the increasing insoluble quadrivalent ion of uranium in reducing-like environment. The relative high values of AR is due to the recoil transfer process. However, the mechanism of the lowermost AR of 0.94 which is lower than the secular equilibrium, is still not clear. (c) Group of moderate circulation characterized by mediocre UC of 2-20 μg/l with large variation of AR, nearly all the deep groundwater and spring water fall into this group. It reveals a mediocre mixing mechanism, the paleowater accepts with varying degrees the meteoric recharge water from its overlaying formations. For example, the palaeowater with 14C age of about 10 000 BP and AR near to the secular equilibrium, but it holds a moderate UC of 7.29 μg/l. Sources of groundwaters A measure of the disequilibrium of uranium series i.e., the excess of 234U, 234Uex = (UC)(AR-1) is used. Three sources of confined groundwater are identified from the 234Uex versus UC and pMC relationships. For that of the Gurinai grassland within Gobi desert covering an area of about 4500 km2, they are: (a) The meteoric water originated from percolation of shallow groundwater; (b) The palaeowater with 14C age of 9575±165 BP and TU less than 2.2 sourced from the Badain Jaran Dune Desert to the south of Gurinai and (c) Another kind of palaeowater with 14C age of 12435±295 BP and TU < 2.1 sourced from the dune desert to the east. For that of the Ejin area, they are: (a) The recharge from the vast Gobi area situated to the east; (b) The palaeowater with 14C age of 12000±270 BP, TU < 2.1; and (c) underground recharge from Gobi desert area of Mongolia situated to the north. Using mixing ratios, contributions of these source waters are evaluated quantitatively. Local systems of phreatic groundwater are differentiated also from 234Uex, together with water isotopes, the precipitation recharge and the leakage of palaeowater are identified as the main sources even another source originated from the percolation of Black River and Gobi area is deduced. (author)