Stable isotopes of water as a tracer for revealing spatial and temporal characteristics of groundwater recharge surrounding Qinghai Lake, China

Studying spatial and temporal characteristics of regional groundwater recharge will guide the scientific management and sustainable development of regional water resources. This study investigated stable isotopes (δ¹⁸O and δ²H) of precipitation, groundwater, river water and lake water during 2019-2020 in Qinghai Lake Basin to reveal the spatial and temporal characteristics of groundwater recharge. The local meteoric water line was simulated using ordinary least squares regression (δ²H = 7.80 δ¹⁸O +10.60). The local evaporation lines of the river water, lake water and groundwater were simulated as δ²H = 6.21 δ¹⁸O - 0.72, δ²H = 5.73 δ¹⁸O - 3.60 and δ²H = 6.59 δ¹⁸O + 1.76, respectively. The δ²H and δ¹⁸O of river water and groundwater were in more depleted values due to the recharge by precipitation at high altitudes or precipitation effects, and the δ²H and δ¹⁸O of the lake water were in more enriched values because of evaporation. The relationship between the δ²H and δ¹⁸O of groundwater and river water was not significantly different, indicating a strong hydrological connection between the groundwater and river water surrounding Qinghai Lake. Additionally, the maximum values of δ¹⁸O and the minimum values of lc-excess of groundwater in most regions were both in August, and the minimum values of δ¹⁸O and the maximum values of lc-excess of groundwater in most regions were both in October. Therefore, the groundwater was recharged by soil water with strong evaporation in August and recharged by precipitation at high altitudes in October. The recharge rate of groundwater was relatively fast in areas with large slopes and large hydraulic gradients (e.g., south of Qinghai Lake), and in areas with strong hydrological connections between the groundwater and river water (e.g., the Buha River Valley). Those results can provide data support for protection and utilization of water resources in Qinghai Lake Basin, and provide reference for groundwater research in closed lake basins on the Qinghai-Tibet Plateau.