Groundwater hydrochemical characteristics and evolution of the karst water system in the Feicheng fault block in Shandong Province

The Feicheng area is one of the important industrial bases in Shandong Province with the early development of coal mine, power generation and chemical industry. Besides, vegetable farming is widely distributed. Karst groundwater is the main source of water supply and the only source of drinking water in this area. In recent years, due to human activities such as industrial and agricultural exploitation of groundwater, and reduction of water emissions from coal mine closures, the groundwater dynamic and chemical fields in this area have undergone changes. The current status of groundwater environmental quality needs to be determined. On the basis of hydro-geological survey, this study collected 59 groundwater samples and 6 river water samples in the dry season of 2022, and comprehensively used mathematical statistical methods, hydrochemical methods (Piper three-line diagram, Gibbs model, mineral saturation index, ion proportion analysis) to explore the hydrochemical characteristics and evolution rules of groundwater in the Feicheng fault block.

The results show that, (1) The average pH of groundwater in the study area is 7.34–7.47, all of which are weakly alkaline, and Ca²⁺, Mg²⁺, ${\rm{HCO}}_3^{-}$ and ${\rm{SO}}_4^{2-}$ are the main ions in the water. The hydrochemical composition is mainly controlled by water rock interaction, while the influence of atmospheric precipitation and evaporation concentration is relatively small. The water-rock interaction of calcite and dolomite plays a major role in controlling the chemical composition of karst water and fissure water, and the dissolution of evaporite plays an important role in the chemical composition of pore water. The mineral saturation index shows that most of calcite and dolomite is in a saturated state, while gypsum and halite minerals are in a dissolved but unsaturated state.

(2) Karst hydrochemical types in the area are mainly HCO₃-Ca (Mg) type, accounting for 47.9%, followed by HCO₃·SO₄-Ca type and HCO₃·Cl-Ca type, 16.7% and 20.8%, respectively. Pore water is mainly of SO₄·HCO₃-Ca type and HCO₃·Cl·SO₄-Ca type, with SO₄·NO₃-Ca type appearing locally in the area of Wangguadian town. The chemical types of river water are relatively complex, including SO₄·HCO₃-Ca·Na type, Cl·SO₄-Ca·Na type, etc.

(3) From the perspective of component content, the average TDS content of pore water, karst water, and fissure water is 1,207.13 mg·L⁻¹, 670.96 mg·L⁻¹, 514.5 mg·L⁻¹, respectively. The coefficient of variation of Ca²⁺ and Mg²⁺ in groundwater is relatively small, indicating that they are relatively stable ions. The average values of ${\rm{SO}}_4^{2-}$, Cl⁻ and ${\rm{NO}}_3^{-}$ in pore water are 288.29 mg·L⁻¹, 155.86 mg·L⁻¹, 195.41 mg·L⁻¹, respectively, with high content and small coefficient of variation, indicating that they are mainly influenced by external inputs from human activities. The content of ${\rm{SO}}_4^{2-}$, Cl⁻ and ${\rm{NO}}_3^{-}$ in karst water is relatively low, but the coefficient of variation is large, with uneven concentration distribution, and occurrence of enrichment in local areas. The average ${\rm{SO}}_4^{2-}$ in the Huihe river reaches 345.83 mg·L⁻¹, which is much higher than that of the surrounding groundwater and has a small coefficient of variation, indicating that its source is mainly from external inputs and may be related to the drainage of surrounding coal mines.

(4) Some sources of Ca²⁺ are related to the dissolution of gypsum minerals and the infiltration of pollutants containing Cl⁻ and ${\rm{NO}}_3^{-}$, and the nitrate pollutants mixed into aquifers may promote the dissolution of carbonate rocks. The cation exchange is weak in groundwater, but strong in river water.

(5) From a time-scale perspective, the groundwater quality in the study area has shown a decreasing trend compared to the quality in 1999 and 2013, with significant increases in the content of Cl⁻, ${\rm{SO}}_4^{2-}$, and ${\rm{NO}}_3^{-}$ in groundwater over the years. From the perspective of groundwater types and spatial distribution, the chemical characteristics of groundwater in the Feicheng area are significantly influenced by human activities. Both the concentrations of nitrogen and chlorine are generally low in the fissure water of magmatic rocks, and hence the water quality is relatively the best. The overall quality of karst water is good, and the rise of the water level in the closed pit mine in Feicheng has not caused cross-layer pollution to the surrounding karst water. However, there are local occurrences of "high nitrogen" or "high chlorine" in Wangzhuang, Taoyuan, Shiheng, Dayang, and other places, which may be affected by pollution sources such as agricultural fertilization, livestock breeding, and domestic sewageinfiltration. Both the concentrations of nitrogen and chlorine are high in most of the pore water, and hence the water quality is generally poor, with ${\rm{NO}}_3^{-}$, as the main ion exceeding the permitted level, which may be related to the application of chemical fertilizer for large-scale agricultural vegetable farming in the study area, and the infiltration of surface nitrate and other pollutants into the groundwater with rainwater. The nitrogen concentration in both Kangwang river and the Huihe river is generally low, while those of chlorine and sulfur are high, reflecting the significant impact of urban sewage discharge and coal mine drainage on these two rivers.