| Summary: | The hydrogeochemical characteristics of groundwater are an important element in the study of the spatial and temporal variation of groundwater resources, which is crucial to water resources utilization, ecological environmental protection, and human development. Water samples were collected at eight observation of Nandong Karst Water System (NKWS) sites in each month of 2019, and the main ions and isotopes of the water samples were examined. The hydrogeochemistry characteristics of groundwater and its differences with surface water were explored by using the methods of multivariate statistics, Gibbs model. Results showed that the water chemistry types of groundwater were mainly HCO<sub>3</sub>–Ca and HCO<sub>3</sub>–Ca·Mg. The analysis of hydrogen and oxygen isotope showed that the initial recharge source of surface water and groundwater were atmospheric precipitation, and the measured hydrogen and oxygen isotopes of surface water were heavier due to the strong evaporation effect. The natural and anthropogenic processes contributed to the chemical composition of surface water and groundwater in the study area. However, the main factor affecting the quality of surface water and groundwater was the input of anthropogenic contaminants. In terms of natural factors, the main chemical ions of surface water and groundwater were mainly controlled by water-rock action originating from weathering and hydrolysis of rocks and soils. Ca<sup>2+</sup>, Mg<sup>2+</sup>, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">H</mi><mi mathvariant="normal">C</mi><mi mathvariant="normal">O</mi></mrow><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula> mainly originated from natural dissolution of carbonate rocks. K<sup>+</sup>, Na<sup>+</sup>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">S</mi><mi mathvariant="normal">O</mi></mrow><mn>4</mn><mrow><mn>2</mn><mo>−</mo></mrow></msubsup></semantics></math></inline-formula>, and Cl<sup>−</sup> were partly from atmospheric precipitation. For human activities, Na<sup>+</sup> and Cl<sup>−</sup> were partly from domestic water for local residents. <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">S</mi><mi mathvariant="normal">O</mi></mrow><mn>4</mn><mrow><mn>2</mn><mo>−</mo></mrow></msubsup></semantics></math></inline-formula> in surface water mainly came from mining. <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">N</mi><mi mathvariant="normal">O</mi></mrow><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula> in groundwater mainly came from chemical fertilizers, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">N</mi><mi mathvariant="normal">O</mi></mrow><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula> in surface water were mainly from human waste and domestic sewage.
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