Evaporation of lake water and exchange of surface water and groundwater in the Nandagang Wetland

The Nandagang wetland is one of the important wetlands in North China. In recent years, the ecological environment of the wetland and its health condition are not optimistic. To reveal the characteristics of hydrogen (δ²H) and oxygen (δ¹⁸O) isotopes for different water bodies, the evaporation of lake water and the exchange between surface water and groundwater in the Nandagang Wetland, and to explore the water cycle process in Nandagang area, river water, lake water, seawater and groundwater samples were collected and analyzed in May 2023. The evaporation of lake water was estimated based on isotope fractionation and deuterium surplus (d). The exchange between surface water and groundwater was evaluated through the mass balance mixing model. The results showed that most of the shallow groundwater in Nandagang is distributed along the local meteoric water line, indicating that precipitation is the main source of groundwater. The groundwater in the study area is mainly brackish water and saline water. The order of averaged δ¹⁸O values of different water bodies is lake > seawater > river > shallow groundwater, and the order of the averaged δ²H values is sea water > lake water > river water > shallow groundwater. Averaged d values are ranked as: lake water < river water <groundwater. The δ¹⁸O and δ²H values of surface water are generally higher than those of groundwater under the influence of evaporation. Compared with surface water, the variation range of isotope values in groundwater is larger, indicating that groundwater in different regions is affected by surface water to different degrees. The evaporation line equation of lake water is obtained by δ¹⁸O and δ²H regression analysis: δ²H=6.1498δ¹⁸O−14.774 (n=12, R²=0.981). If the influence of recharge, leakage, drainage and transpiration is ignored, the evaporation loss of lake water estimated by deuterium surplus is 27%−37%, and it increases with the decrease of d values. From inland to coast, as the groundwater depth becomes shallower and evaporation becomes stronger, the δ²H and δ¹⁸O values get larger and the d values of groundwater get smaller. The movement of lake water is strongly affected by human activities. The water source of wetland comes from southeast. The water bodies in the west and north of the wetland have longer retention time, higher δ²H and δ¹⁸O values and smaller deuterium surplus. The northern and western parts of the lake have higher chloride content due to the influence of the northern salt ponds. According to the end-member mixing model based on Cl⁻ and δ¹⁸O, half of shallow groundwater samples are located on the mixing line between seawater and freshwater. There is a leakage on the east side of the lake, but the extent of the leakage affecting shallow groundwater is within 1 km. The analysis shows that the lake water in the Nandagang Wetland is replenished by precipitation, Nanpai River and Liaojiawa Channel, and discharged by evapotranspiration and seepage. The research results are helpful to understand the mechanism of evaporation and surface water-groundwater exchange, and provide reference for water resources and ecological protection in Nandagang.