Groundwater Hydrochemical Characteristics in a Plain River Network Region: Effects of Dissolved Organic Carbon and Possible Traceability of Pollution Sources

Abstract The chemical composition of groundwater indicates the water quality and provides useful information for identifying pollution sources. The aim of this study was to explore the effects of dissolved organic matter inputs on the ionic composition of groundwater and identify ions that can be used as indicators of pollution sources. Descriptive statistics, a Piper diagram, a Gibbs plot, major ion ratios, and Pearson’s correlation coefficients were used to analyze the chemical data of 40 groundwater samples collected from the shallow aquifer surrounding Lake Taihu. The results showed that the water quality index values of most sampling points were less than 50 (excellent water quality), except for one point in the southeast direction of the lake basin (good water quality). The dominant hydrochemical type of groundwater was Ca–Mg–HCO3 type, and rock dominance was the major mechanism controlling the groundwater chemistry. With an increasing concentration of dissolved organic carbon, the Na+, Mg2+, and HCO3- concentrations all showed a sharp increase followed by a slow increase, while the NO3– concentration initially decreased sharply and then decreased slowly. The K+ concentration was positively correlated with total dissolved nitrogen and phosphorus, nitrate, As, and Cd concentrations (p < 0.05). The Ca2+, Na+, Mg2+, Cl-, HCO3-, and CO32- concentrations were all positively correlated with Pb concentration (p < 0.001). The results indicated that high organic matter inputs can directly or indirectly change the hydrochemical type of groundwater, and K+ can be used as a tracer ion for the sources of As and Cd in groundwater in the study area.