Hydrogeology of an arid endorheic basin (Tsagaan Els, Dornogobi, Mongolia): field data and conceptualization, three-dimensional groundwater modeling, and water budget

The Tsagaan Els basin is a ~16,300 km² endorheic basin located in the Gobi Desert in Mongolia. This Cretaceous sedimentary basin is crossed by the North Zuunbayan fault, which determines two partially connected hydrogeological sub-basins: Zuunbayan and Unegt. Two roll-front uranium ore deposits have been discovered in these sub-basins by Orano Mining and its subsidiary COGEGOBI LLC, and the regional aquifers have been surveyed for several years. Based on that knowledge, this study presents the regional hydrogeological functioning of this multi-layered aquifer system supported by a regional groundwater model. The latter was implemented with MODFLOW and calibrated through PEST using piezometric levels. Recharge rates were adjusted to between 0.6 and 3.1 mm/year. The resulting water budget indicates an important transfer of water from Unegt sub-basin toward Zuunbayan sub-basin, occurring at a few main springs located along the North Zuunbayan fault. Evaporation loss is potentially high during these transfers but the remaining water seeps through and later evaporates in the depression located at the center of Zuunbayan sub-basin, which is the terminal discharge playa of the entire basin. These results can be helpful for future water management plans.