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.
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.
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
2025, 44(3): 477-487, 518.
doi: 10.11932/karst20250303
2025, 44(3): 510-518.
doi: 10.11932/karst20250305
Founded in 1982 bimonthly
ISSN: 1001-4810
CN 45-1157/P
Editor-in-chief: Jiang Zhongcheng
Sponsor: Chinese Academy of Geological Sciences
Sponsored by: Institute of Karst Geology, Chinese Academy of Geological Sciences
Cosponsor: International Research Center on Karst under the Auspices of UNESCOKarst Geology Committee of Geological Society of ChinaSpeleology Committee of Geological Society of China
Wechat
