Study on the groundwater quality and its influencing factor in Songyuan City, Northeast China, using integrated hydrogeochemical method.

Clean groundwater resources are important for the health of human. In Songyuan City, Northeast China, anthropogenic activities have led to changes in groundwater circulation, thereby depleting the aquifer system and causing water quality deterioration. To evaluate the genesis of water quality, we analyzed the hydrochemical and stable isotope compositions of shallow and deep groundwater. According to drinking water quality standards, the concentrations of Ca2+, Mg2+, and NO3- in 23.0, 30.2 and 35.4% of the samples, respectively, exceeded the acceptable ranges. The groundwater chemistry in these samples was related to geochemical processes and agricultural pollution. The hydrochemical analysis explained the reaction mechanisms in each aquifer, showing that the main source of ions in both deep and shallow groundwater is the weathering of silicate rock. In addition, the dissolution of carbonate minerals and artificial pollutants is greater in the shallow groundwater. The stable isotope results showed that long-term extraction is the cause of the diffusion of pollutants in shallow aquifers. Moreover, because most of the well-drilling techniques are backward, the aquifer structure is destroyed, and the deep groundwater is mixed with the shallow groundwater during the process of artificial extraction. The study also analyzed the conditions of the water-rock reaction. Combined with the geological background, it was found that the deep CO2 gas reservoir could provide the necessary material source for the reactions. Owing to frequent tectonic activities, deep CO2 could be discharged to the surface through the fault zone, which promotes the water-rock reaction in this area.