[Provenance of Groundwater Solute and Its Controlling Factors in Yancheng Area]

Affected by human activities and climate change, the deterioration of groundwater quality could continue to intensify, and it is difficult to repair after being polluted. In order to reduce the vulnerability of groundwater safety, it is urgent to strengthen research on groundwater quality. To analyze the characteristics of hydrochemical composition and control factors and to explore the impact of human activities, groundwater sampling was conducted in the alluvial and marine plains of Yancheng area, where industrial and agricultural activities were intense. The results showed that:① the TDS value of shallow groundwater was between 211 and 3790 mg·L^-1, of which, brackish water samples accounted for 30.28% in Yancheng. The order of the concentration of anions and cations was:HCO₃^->Cl^->SO₄^2->NO₃^- and Na⁺>Ca²⁺>Mg²⁺>K⁺. However, from Type Ⅰ/Ⅱ water to Type Ⅴ water, with the increase in TDS value, the dominant anions and cations evolved from HCO₃ type to Cl/SO₄ type and from Ca type to Na type, respectively. ② In the initial Type Ⅰ/Ⅱ water bodies, Na⁺ should have mainly originated from the weathering erosion of albite, whereas Ca²⁺, Mg²⁺, and HCO₃^- should have had a common rock weathering source. In the subsequent water quality evolution process, the factors that led the TDS value to increase, or even to exceed the standard, were not limited to rock weathering, and the effects of evaporation-concentration and ion exchange were also very worthy of attention. ③ The enrichment factor analysis showed that the Cl^- and SO₄^2- were mainly from the input of sea salt sources, and their proportions were gradually increasing from the freshwater areas of Type Ⅰ/Ⅱ in the west to the brackish water areas in the east. However, Ca²⁺, K⁺, and HCO₃^- were mainly derived from crustal sources, and Ca²⁺ was precipitated due to saturation. ④ The NO₃^- derived from agricultural fertilizers was limited to water of category Ⅰ/Ⅱ and Ⅲ-1 (500 mg·L^-1 < TDS ≤ 750 mg·L^-1). Discharge of urban sewage mainly affected Type Ⅲ water bodies. The nitrates formed by agricultural organic fertilizers were mainly distributed in the water of Type Ⅳ and Ⅴ, which was the main rice-producing area of Sheyang, Jiangsu, and the SO₄^2- formed by human activities in this area could be ignored. These should be one of the achievements in the development of regional ecological agriculture.