Delineation of groundwater flow and estimation of lake water flushing time using radium isotopes and geochemistry in an arid desert: The case of Badain Jaran Desert in western inner Mongolia (CHN)

Abstract

The Badain Jaran Desert, located in western Inner Mongolia in northwestern China, is characterized by the coexistence of high megadunes and numerous lakes. The spatial distribution of groundwater systems and supply sources of lakes have yet to be elucidated. Ra isotopes and geochemistry of water samples from 17 lakes and their peripheral groundwater were investigated. The characteristics of the geochemical behaviors of Ra isotopes, including activities, activity ratios, main sources and sinks, and geochemistry both of groundwater and lake water, suggest that there are two groundwater flow systems in the studied area: the southern-central and the northern groundwater flow systems. The southern-central groundwater is supplied from mountains farther to the south and the northern groundwater supplied from areas farther to the north. We developed mass balance models for 228Ra in fresh and saline lakes. The equation for the saline lakes has three input terms: groundwater discharge, atmospheric deposition and diffusion from bottom sediments; and two outputs: radioactive decay and coprecipitation. The freshwater lake equation has one input term: radionuclide supply from groundwater; and two output terms: adsorption onto particles in the lake and radioactive decay. Mass balance calculations for 228Ra give discharge fluxes of 0.03 × 106–7.72 × 107 m3 yr−1 for the lakes. The flushing times of the lakes were estimated to be 0.05–9.23 yrs. The results of this study promote scientific understanding of the hydrologic cycle and water resource evaluation in the Badain Jaran Desert and provide a paradigm for radium isotope application in hydrological research in arid areas.