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

The Nandagang Wetland is important in North China. In recent years, the ecosystem health of this wetland has not been optimal. To explore the water cycle process in Nandagang area, the river water, lake water, seawater, and groundwater samples were collected in May 2023, and analyzed to reveal the characteristics of hydrogen (δ²H) and oxygen (δ¹⁸O) isotopes in different water bodies, evaporation of lake water, and the exchange between surface water and groundwater in the Nandagang Wetland, as well as to explore the water cycle process in the Nandagang Wetland. The evaporation of lake water was estimated based on isotope fractionation and deuterium surplus (d value) levels. The exchange between surface water and groundwater was evaluated using the mixing model of mass balance. The results showed that most of the shallow groundwater in the Nandagang Wetland was distributed along the local meteoric water line, indicating that precipitation was the main source of groundwater. The groundwater in the study area was mainly brackish and saline water. The averaged δ¹⁸O values of the different water bodies were in the decreasing order of lake water > seawater > river water > shallow groundwater, whereas the averaged δ²H values were in the decreasing order of sea water > lake water > river water > shallow groundwater. The averaged d values were in the increasing order of lake water < river water < shallow groundwater. The δ¹⁸O and δ²H values of surface water were generally higher than those of groundwater under the influence of evaporation. Compared with surface water, groundwater had a larger variation in isotope values, indicating that groundwater in different regions are affected to different degrees by surface water. The following evaporation line equation of lake water was obtained through δ¹⁸O and δ²H regression analysis: δ²H=6.1498δ¹⁸O−14.774 (n=12, R²=0.9814). If the influence of recharge, leakage, drainage, and transpiration is ignored, the evaporation loss of lake water estimated by the deuterium surplus would be 27%−37%, increasing with a decrease in d value. From inland to coast, the δ²H and δ¹⁸O values increased whereas d values of groundwater decreased as the groundwater depth became shallower and evaporation became stronger. The movement of lake water was strongly affected by human activities. The wetland water source comes from the southeast. The water bodies in the west and north of the wetland have longer retention times, higher δ²H and δ¹⁸O values, and smaller d value. The northern and western parts of the lake have higher chloride contents owing to the influence of the northern salt ponds. According to the end-member mixing model based on Cl⁻ concentration and δ¹⁸O value, half of the shallow groundwater samples were located on the mixing line between seawater and inland groundwater. Although there is leakage on the east side of the lake, the extent of leakage affecting shallow groundwater is only 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 leakage. These research results help us better understand the mechanisms of evaporation and surface water-groundwater exchange and provide a reference on the water resources and ecological protection in the Nandagang Wetland for future studies.