Evaluation of lacustrine groundwater discharge and associated nutrients, trace elements and DIC loadings into Qinghai Lake in Qinghai-Tibetan Plateau, using radium isotopes and hydrological methods

Qinghai Lake, the largest lake in China with a surface area of 4260 km ² , lies in the northeastern Qinghai-Tibet Plateau (QTP) at a mean altitude of 3194 m above sea level. The lake has a water volume of 71.6 × 10 ⁹ m ³ , a salinity ranges of 4.18 ± 0.14 ppt to 12.3 ± 0.14 ppt during sampling period which is about a half of typical seawater. The aquifer systems at the lake shoreline have obvious saline water and freshwater mixing zones with similar hydrogeological and biogeochemical settings to coastal aquifers. This study aims to estimate lacustrine groundwater discharge (LGD) and the associated chemical loadings to western Qinghai Lake. Naturally occurring radium isotopes ( ²²³ Ra, ²²⁴ Ra, ²²⁶ Ra and ²²⁸ Ra), nutrients, heavy metals and dissolved inorganic carbon (DIC) were systematically investigated in different water endmembers of western Qinghai Lake. LGD was estimated with radium inventory model and radium eddy diffusion model, leading to a LGD estimate of 7.67 × 10 ⁶ m ³ d ⁻¹ and 8.52 × 10 ⁶ m ³ d ⁻¹ , respectively. The fresh groundwater discharge obtained based on regional water budget is about 5.97 × 10 ⁶ m ³ d ⁻¹ during sampling period, accounting for 73.74% (ranging from 53.22 to 80.43%) of total LGD. This study reveals that the fresh groundwater component is dominant in LGD of Qinghai Lake, which is quite different from most coastal scenarios. Furthermore, the chemical loadings derived from fresh-LGD and recirculated-LGD are evaluated. The preliminary results indicate that fresh-LGD is an important source of nutrient, barium, strontium, uranium, and DIC to the lake, and may significantly impact biogeochemical and ecological processes in the lake. This study presents the first LGD estimate in the largest QTP saline lake in China, and would be instructive to further similar studies in saline lakes in QTP and elsewhere in the world.