Comparison of the hydrochemical characteristics of shallow groundwater in the Huaihe River Basin during a ten-year period and its significance to environmental change

In the Huaihe River Basin, the shallow groundwater plays a significant role in rural scattered water supply and farmland irrigation. The analysis on hydrochemical characteristics changes of the shallow groundwater in the region will provide scientific basis for ensuring local water and food security. In order to summarize the variation rule of the shallow groundwater chemical properties, the study adopted numerical statistics, Shukarev classification, Piper three-line graph, and water quality evaluation to compare and analyze the hydrochemical data of a ten-year interval between 2009 and 2019. The results show that the shallow groundwater of Huaihe Basin has weakly alkaline pH values, rising mean values of total dissolved solids and total hardness but decreasing median and maximum values of these variables. There is no order change of the main ions, which are still dominated by Na⁺and HCO^-₃ in the shallow Huaihe groundwater with further enhancement of spatial variability of the hydrochemical components. Na-Ca-Mg·HCO₃ and Ca-Mg·HCO₃ continued to be the main hydrochemical types, but the proportion of Na-Ca-Mg·HCO₃ decreased apparently while Ca-Mg·HCO₃ increased slightly, showing the diversification and complication of the hydrochemical types. Comparatively speaking, the hydrochemical changes of the shallow groundwater in the Western Huaibei Plain are mainly reflected in the proportion increase of HCO^-₃ type and the water quality improvement; while those in the Central Huaibei Plain are characterized with the decrease of the Ca type-HCO₃ type and obvious proportion decrease of I-III type water. The proportion of Na-Ca-Mg·HCO₃ dramatically decreased while the ratio of Na·SO₄-Cl significantly increased in the the Nansihu Plain, and the proportion of Na·Cl and Na·SO₄-Cl increased evidently in the Lubei River drainage plain, on both of the two plains the V type water account for a larger part.