Unravelling the Natural and Anthropogenic Controls on Karst Groundwater Hydrochemistry in the Zaozhuang Area, Northern China: A Combined Hydrogeochemical and PMF Source Apportionment Study

Karst groundwater is the principal water supply for Zaozhuang City and an essential safeguard for the local environment. In May 2025, 131 karst groundwater samples were collected and analysed using Piper, Durov, and Gibbs diagrams; ion ratios; mineral saturation indices; and multivariate statistics, and the sources of the dissolved constituents were quantified with positive matrix factorisation (PMF). The water is weakly alkaline, with pH between 7.04 and 7.78 (mean 7.33) and total dissolved solids (TDSs) between 288 and 2578 mg/L (mean 609.10 mg/L). HCO3− and Ca2+ are the dominant ions, and the water is mainly of the HCO3−Ca type. Rock weathering and mineral dissolution, together with anthropogenic inputs, control the water chemistry: Ca2+, Mg2+ and HCO3− are derived mainly from carbonate weathering and dissolution, SO42− from the dissolution of sulfate minerals such as gypsum, Na+, K+ and Cl− from evaporite dissolution and anthropogenic inputs, and NO3− mainly from agricultural activities; cation exchange modifies the ionic balance but does not add solute mass. The PMF model resolves four sources—carbonate weathering, agricultural nitrogen input, sulfate mineral dissolution, and halite dissolution together with anthropogenic salt input—which account for 36.85%, 27.41%, 24.81% and 10.92% of the solute mass, respectively. Carbonate weathering supplies most of the HCO3− and Mg2+, the agricultural nitrogen factor accounts for 95.62% of the NO3−, sulfate mineral dissolution for 83.92% of the SO42−, and the halite–anthropogenic salt factor for 71.32% of the Na+ and 43.89% of the Cl−. Mineral dissolution and water–rock interaction are the main sources of the solutes, while agricultural nitrogen and salt input are the principal anthropogenic contributions. Carbonate and sulfate mineral dissolution together carry about 62% of the solute mass and agricultural nitrogen and salt input about 38%, so the protection of this aquifer should focus on nitrate from farming and salt and sulfate inputs in the urban and mining districts.