Analysis of chemical substance sources in the groundwater of karst-fissure groundwater system in the Qinglian River, Guangdong Province, China

The Qinglian River Basin is a typical basin unit within the ecological security pattern. Therefore, clarifying its groundwater quality is crucial for strengthening the ecological security barrier of the hilly and mountainous areas in South China and for maintaining the ecological security of the Guangdong−Hong Kong−Macao Greater Bay Area. The chemical composition of water is significantly related to water potability, availability for agriculture and tourism, and interaction with biological systems. However, the lack of understanding regarding the nature of groundwater has presented some challenges for the scientific management of groundwater in the Qinglian Basin, particularly concerning irrational spatial exploitation of groundwater. To address these challenges, methods such as hydrochemical parameters and multivariate statistical techniques-including Durov diagram, Gibbs plot, Schoeller diagram and Positive Matrix Factorization (PMF) model-were employed to trace the sources of chemical substances in groundwater and quantify the contribution rates of various factors affecting groundwater quality.

The main land use types in the study area include forest land, cultivated land, construction land, and water bodies, which account for 87.30%, 11.31%, 1.14%, and 0.27%, respectively. Based on the deposit conditions of groundwater and the characteristics of water-bearing media, the types of groundwater are mainly classified as karst water, igneous-fissure water, clastic-fissure water, and pore water. According to the extended Durov diagram, the pore water in Quaternary sediments predominantly exhibits HCO₃·SO₄−Ca characteristics, while the igneous-fissure water is classified as HCO₃−Ca·Na (Na). The groundwater in areas with dolomitic limestone and clastic rocks is primarily of HCO₃−Ca·Mg types, whereas the other karst water is mainly classified as HCO₃−Ca type.

In the PMF model, a total of 53 groundwater samples were used for the identification and apportionment of groundwater chemical sources. Five major factors of groundwater chemical sources within the basin were identified. Factor 1 (F1) is characterized by Mg²⁺, ${\rm{HCO}}_3^{-}$, and total dissolved solids (TDS), originating from weathering and dissolution of dolomitic limestone and Mg-containing silicate minerals, with a contribution rate of 18%. Factor 2 (F2) is characterized by ${\rm{NO}}_3^{-}$, Cl⁻, and Na⁺, which are derived from anthropogenic activities such as domestic and agriculture practices, with a contribution rate of 20%. In addition, agricultural activities on sloping farmland can cause substances such as ${\rm{NO}}_3^{-}$ to be discharged into the karst basin through underground runoff, further increasing the concentrations of chemical substances in the groundwater of the karst basin. Factor 3 (F3), is characterized by ${\rm{HCO}}_3^{-}$, Ca²⁺, and TDS, resulting from the weathering and dissolution of carbonate minerals, and it is the factor with the highest contribution rate (27%). The dominance of F3 among the five factors corresponds to the largest proportion of karst area in the basin (55%), making it the primary source of chemical substances in the groundwater of the Qinglian River Basin. The most relevant parameter of Factor 4 (F4) is mainly ${\rm{SO}}_4^{2-}$, and its apportioned contribution rate is 16%. The majority of ratios of ${\rm{SO}}_4^{2-}$ to ${\rm{NO}}_3^{-}$− in groundwater fall between one and four, indicating that the primary source is the use of sulfur-containing fertilizers in agricultural activities. Notably, the pore water in loose rock formations within the karst basin has a relatively high concentration of ${\rm{SO}}_4^{2-}$, with a maximum value of 81.30 mg·L⁻¹, showing a significant impact from human activities. However, the underlying karst water has a lower concentration of ${\rm{SO}}_4^{2-}$ (6.90 mg·L⁻¹), indicating less influence from human activities. F5 is characterized by Na⁺, K⁺, Cl⁻, and TDS, which are derived from the weathering of silicate minerals and the dissolution of a small amount of halite, with a contribution rate of 19%.

The distribution areas with high concentrations of chemical parameters are significantly correlated with the spatial distribution of contribution rates of source factors, which indicates that water chemistry responds to the spatial distribution of lithology and land use. F1, F3 and F5, belonging to natural factors such as rock weathering and water−rock interaction, contribute 64% and have a significant correlation with lithology. F2 and F4 are significantly correlated with the distribution of cultivated land and construction land, and are considered to be influenced by human activities, with a contribution rate of 36%. In general, the main sources of the chemical substances in karst water are the weathering and dissolution of carbonate, and silicate and halite minerals. In some areas, human activities such as domestic wastewater and the use of fertilizers areas also have an impact. The main source of igneous-fissure water is the weathering and dissolution of silicate minerals. The quantitative analysis of the contribution rates of groundwater ion sources helps to deepen the understanding of fissures and karst aquifers in the study area, and provide a basis for the scientific management of groundwater.