Hydrodynamic Groundwater Modeling and Hydrochemical Conceptualization of the Closure Mining Area of the WuMa River Watershed of China

Abstract

The WuMa River (WMR) watershed is located in Renhuai City, Guizhou Province of China, which is a first-class tributary of the Chishui River. The geochemical investigation mainly included the determination of groundwater pH, total hardness, total dissolution solid, major cationic and anionic, and the geochemical groundwater modeling. The principal component analysis (PCA) and Gibbs model were used to analyze the pollution type and geochemical composition. The geochemical investigation results show that the cations of groundwater are dominated by Ca²⁺ and the anions are dominated by HCO₃^-; therefore, two main hydrochemical types in the study area are identified as Ca²⁺-Mg²⁺-HCO₃^- and Ca²⁺-Mg²⁺-SO₄^2-. The chemical composition of groundwater in this area is mainly controlled by weathering of the carbonate rocks. The ion concentration of groundwater in the study area exhibited significant spatial variability between dry and wet seasons, while temporal changes of cationic and anionic concentrations exhibited irregularities. In PCA and FA analysis, PC1, PC2, and PC3 were extracted, which could explain 51.92, 26.98, and 12.61% of the total information, respectively. F1 explained 67.44% of the total variance, among which Ca²⁺, Mg²⁺, K⁺, SO₄^2-, and Cl^- contributed the most among the factors and were the main factors controlling the chemical composition of groundwater. The relative error between the measured water level and the simulated water level is less than 2%, which meets the requirements of simulation accuracy. During the simulation period of the model, a total recharge of 339.05 × 10⁴ m³ was observed in the simulated area, primarily attributed to infiltration from rainfall. The total excretion amounted to 330.78 × 10⁴ m³, primarily through evaporation, with a minor amount of lateral outflow. The migration pathway of pollutants in groundwater primarily follows the direction of groundwater flow while diffusing vertically. The migration range of the pollutant is in accordance with the direction of groundwater flow and extends along the larger hydraulic gradient, demonstrating consistency. The findings of this study serve as a reminder that the closure of coal mines can constitute a significant source of water pollution. Simultaneously, they offer empirical data and theoretical references for the simulation and prediction of groundwater contamination in enclosed coal mines.

Figure 1

Analysis of major anion concentration distribution in groundwater.

Figure 2

Percentage of ionic and anion concentrations in groundwater.

Figure 3

Piper chart of the chemical composition of groundwater.

Figure 4

Gibbs map of groundwater samples.

Figure 5

Ca²⁺/Na⁺ vs. Mg²⁺/Na⁺ and Ca²⁺/Na⁺ vs. HCO₃^–/Na⁺ in groundwater.

Figure 6

Fitting results of observed and calculated water level values.

Figure 7

Pollution distribution of Fe in the Longtan Formation.

Figure 8

Pollution Distribution of Fe in the Changxing Formation.

Figure 9

Pollution dispersion diagram of the Yelang group.

Figure 10

Pollution range map of limestone in the Maokou Formation and Qixia Formation.

Figure 11

Scope of contaminant pollution in the A–A profile.

Figure 12

Scope of contaminant pollution in the B–B′ profile.

Figure 13

Concentration of pollutants from the Changxing Formation flowing into WMR after 20 years.