Evaluating heterogeneous lacustrine groundwater discharge of Hulun Lake using radium isotopes in Inner Mongolia, China

Study region: Hulun Lake Basin, Northern China Study focus: Understanding lacustrine groundwater discharge (LGD) is important for elucidating the hydrological cycle and sustainable use of water resources. Hulun Lake is the largest fresh water lake and has prominent ecological function in North China. However, the effect of groundwater discharge on the water balance and spatial distribution of LGD under different aquifers have not yet been evaluated in detail. Herein, radium (223Ra, 224Ra, and 228Ra) and stable isotopes (delta D, delta 18O) were used to identify the LGD locations and quantify the spatial LGD flux into the lake. New hydrological insights for the region: The Ra isotope activities in groundwater from the fissured bedrock in the western part of the Lake are depleted relative to those from the east, underlain by the Quaternary unconsolidated sediments aquifer overlying granitic rocks. The spatial distribution of LGD under heterogeneous aquifers was revealed and groundwater discharge from fractured bedrock aquifer is the predominant source of the lake. The LGD flux is estimated to range from (1.07 +/- 1.36)x 107 to (6.54 +/- 1.89)x 107 m3/d, which exceeds that in the Quaternary aquifer by one order of magnitude due to the high permeability enlarged via tectonic faults and fractures. These findings promote scientific understanding of water balance in Hulun Lake and are instructional for LGD studies of freshwater lakes with complex water systems in cold and arid regions.