Analysis of hydrochemical characteristics and controlling factors of groundwater in the covered karst area of northern Guangzhou

Guangzhou is clearly positioned as the core engine of the Guangdong−Hong Kong−Macao Greater Bay Area and shoulders the responsibility of leading the high-quality development of the Greater Bay Area. The northern Guangzhou serves as a demonstration area for urban-rural integration, featuring a distinctive ecological agriculture industry and a functional area for rural tourism. However, the covered karst in this area is highly developed. Consequently, the environmental problems, such as karst ground collapse and the deterioration of groundwater quality, have limited to some extent the local construction and development. In order to study the hydrochemical characteristics and formation of groundwater in the covered karst area of northern Guangzhou. We utilized fourteen groups of karst water samples and fourteen groups of rock chemical composition test data collected during the dry season from 2017 to 2018. We conducted a qualitative analysis of the hydrochemical characteristics and controlling factors of groundwater using mathematical statistics, Piper diagrams, the Gibbs model, ion ratio diagrams, the and chlor-alkali index. Additionally, using the hydrogeochemical reaction simulation software PHREEQC, we performed a quantitative analysis of the hydrogeochemical processes, with a particular emphasis on the dissolution−precipitation equilibrium of minerals in groundwater.

The results show, (1) The karst water in the covered karst area of northern Guangzhou is characterized as extremely soft to slightly hard, neutral, and fresh. The total dissolved solids (TDS) and total hardness of the karst water are low, and the pH remains relatively stable. The dominant anion and cation are ${\rm{HCO}}_3^{-}$ and Ca²⁺, respectively. The trend of ion concentrations is Ca²⁺>K⁺+Na⁺>Mg²⁺ and ${\rm{HCO}}_3^{-}$ >Cl⁻>${\rm{SO}}_4^{2-}$ , respectively. The hydrochemical types of karst water are primarily of the HCO₃ type. (2) The ρ(K⁺+Na⁺)/ρ(K⁺+Na⁺+Ca²⁺) of karst water ranges from 0.1 to 0.6, and the ρ(Cl⁻)/ρ (Cl⁻+${\rm{HCO}}_3^{-}$ ) ranges from 0.1 to 0.5. The chlor-alkali indices (CAI1 and CAI2) mainly fall within the ranges of -2 to 1 and -0.25 to 0.25, respectively, indicating weak cation exchange adsorption. In 78.57% of the water samples, the ratio of γ(K⁺+ Na⁺) to γ(Cl⁻) ions is at or above the 1∶1 line, indicating that K⁺, Na⁺ and Cl⁻ primarily originate from the dissolution of rock salt. The ratios of γ(Ca²⁺+Mg²⁺) to γ(${\rm{HCO}}_3^{-}$ + ${\rm{SO}}_4^{2-}$ ) ions are concentrated near or above the 1∶1 line. Furthermore, 85.71% of the water samples display ratios of γ(Ca²⁺) to γ(${\rm{HCO}}_3^{-}$) ions that are either near the 1∶1 line or between the 1∶1 line and 2∶1 line. Additionally, 85.71% of the water samples exhibit ratios of γ(Ca²⁺) to γ(Mg²⁺) ions above the 1∶1 line. These findings suggest that Ca²⁺, Mg²⁺ and ${\rm{HCO}}_3^{-}$ ions are derived from the dissolution of carbonate minerals, while ${\rm{SO}}_4^{2-}$ ions originate from the dissolution of carbonate rocks and evaporitic salts, such as gypsum. (3) The lithology of covered karst is dominated by limestone, followed by dolomite limestone. The ratio of CaO to MgO in the rock chemical composition of limestone (19.36−119.73) is much larger than that of dolomitic limestone (1.43−4.71), and the dissolution ability of limestone is obviously stronger than that of dolomitic limestone. (4) PHREEQC software has been used to establish a reverse hydrogeochemical model in the rock sampling points from SY20 to SY19 in the Caopu area of Lyutian Town . The simulation results quantitatively confirmed the dissolution of calcite, dolomite, gypsum, and rock salt in karst water, with respective dissolution amounts of 2.599×10⁻⁴ mol·L⁻¹, 8.474×10⁻⁵ mol·L⁻¹, 4.165×10⁻⁶ mol·L⁻¹, and 3.446×10⁻⁵ mol·L⁻¹. (5) The saturation indices of calcite and dolomite in karst water show a good correspondence with karst development. Among the water sampling points where calcite is in a good dissolved state, 85.71% of the points exhibit karst development. In contrast, among the water sampling points where dolomite is in a dissolved state, 66.67% of the points exhibit karst development. The saturation indices of calcite and dolomite in groundwater indicate the trend of karst development and can serve as criteria for fine evaluation of karst development.

This study reveals the hydrochemical characteristics of groundwater, as well as its formation and evolution in the covered karst area of northern Guangzhou. It also explores the relationship between groundwater mineral saturation indices and karst development. The research findings can provide a scientific basis for the exploitation and utilization of groundwater and for the protection of geological environment in the covered karst area of Guangzhou, which holds significant practical implications.