[Dynamic Mechanisms of Groundwater Quality by Residual Contaminants of the Tanghe Wastewater Reservoir in Xiong'an New Area]

Tanghe wastewater reservoir(TWR) is located on the west side of Baiyangdian Lake in Xiong'an New Area, where sewage infiltration and irrigation has been taking place for 40 years, and a large number of contaminants have accumulated in the unsaturated zone. Identifying the mechanisms by which this combined system contributes to groundwater hydrochemical dynamics is important for the protection of the water environment in the area. Hydrogeochemical methods such as tracing and improved chlor-alkali index are used to analyze the spatial and temporal characteristics and evolution mechanisms of shallow groundwater. The study shows that the groundwater chemistry in the sewage reservoir area is SO₄·HCO₃-Na type, with an average sewage fraction of 48.4%, and the contribution of Na⁺ from ion exchange and halite dissolution is 29.9% and 8.6%, respectively. The chemical type of groundwater in the sewage irrigation area is SO₄·HCO₃-Na·Mg, the average sewage fraction is 58.3%, and Na⁺ consumption of ion exchange is 8.1%. The mix dilution of precipitation and irrigation leads to a reduction in the effluent fraction and saturation index in the groundwater, and promotes the adsorption of Na⁺ from groundwater into the soil. Denitrification in aquifers can effectively reduce groundwater nitrate pollution. In addition, the sewage fraction before and after the restoration of the reservoir was 61.5% and 49.3%, respectively. Pollutants retained in the sewage infiltration and irrigation combined system will continue to affect the quality of shallow groundwater with varying degrees of mixing and water-rock interaction driven by rainfall and irrigation.