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Article

Seasonal Differences in the Hydrochemical Characteristics of Karst Wetlands and the Associated Mechanisms in Huixian, China

1
Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China
2
International Research Center on Karst under the Auspices of UNESCO, National Center for International Research on Karst Dynamic System and Global Change, Guilin 541004, China
3
Chengdu Center, China Geological Survey, Chengdu 610081, China
*
Author to whom correspondence should be addressed.
Water 2022, 14(15), 2362; https://doi.org/10.3390/w14152362
Submission received: 16 June 2022 / Revised: 26 July 2022 / Accepted: 28 July 2022 / Published: 30 July 2022
(This article belongs to the Section Hydrology)

Abstract

:
The present study aimed to identify the seasonal changes in the hydrochemical characteristics of the Huixian karst wetland and the underlying mechanism. Conventional ions and isotopes of 130 groundwater samples collected during the wet and dry seasons were analyzed. The hydrochemical characteristics of groundwater in the Huixian karst wetland were clarified using mathematical statistics and hydrochemical methods, including Durov diagrams, ionic ratios, Gibbs diagrams, and H and O isotopes. The hydrochemical evolution and sources of major ions in the Huixian karst wetland were also investigated. The results showed that cations and anions in groundwater in the study area were dominated by Ca2+ and HCO3, respectively, sourced mainly from calcite weathering. The ions of some groundwater samples were regulated by weathering of dolomite, dolomitic limestone, and pyrite, resulting in relatively high concentrations of Mg2+ and SO42−. K+, Na+, SO42−, NO3, and Cl in groundwater originated from precipitation, Na+ and Cl originated from domestic sewage, K+ was related to the application of potassium fertilizer in agriculture, and NO3 mainly originated from chemical fertilizer. Groundwater ions were primarily controlled by rock weathering, followed by evaporative concentration. The sources of major ions were dependent on the dissolution and precipitation of carbonate rock, as well as the chemical weathering of silicate rock and evaporite. Samples from the various water sources were scattered on both sides of the local meteoric water line (δD = 3.13δ18O − 13.9), which indicated isotopic composition was affected by evaporation and precipitation.

1. Introduction

Wetlands represent important global ecosystems that provide ecological goods and services to humans. Unfortunately, intensive socioeconomic development in recent centuries has had a destructive effect on wetlands [1,2,3]. The Huixian wetland karst basin in Guilin is the largest subtropical low-altitude karst wetland in China [4,5]. Groundwater is an important water resource and effectively supports the development of agriculture and animal husbandry. Nevertheless, previous studies on groundwater have mainly focused on lake hydrology, the relationship between rivers and the ecological environment, and the evaluation of regional water resources [6,7,8,9,10]. Therefore, knowledge of the hydrochemical characteristics of groundwater and its evolution remains limited. The impacts of anthropogenic activities and climate change over the past 70 years have resulted in the gradual shrinking of the Huixian wetland, whereas the resident population and area of arable land have increased by several orders of magnitude over the same period [11,12]. Anthropogenic activities will not only result in continual deterioration of the wetland ecological environment [13,14,15], but also in water pollution to different degrees.
The environment of surface water in Huixian wetland was alarming. In densely populated areas, surface waters were usually the main places to receive the pollutants from industrial and agricultural production activities. The pollutants entering the surface water along the runoff direction and cause complex biogeochemical effects, which will probably cause the pollution of the hydrological system. This will exacerbate the water resources crisis. The non-point source pollution caused by intensive agricultural planting and livestock and poultry aquaculture in Huixian wetland was aggravating the deterioration of the wetland water environment [16]. At present, nitrate and organochlorine pesticides had been detected in the groundwater of the Huixian karst wetland [7], and eight antibiotics had also been detected in the surface water [17]. Recent research showed that the maximum concentration of Hg2+ in groundwater (1.08 μg L−1) had exceeded China’s water quality standards. To date, the research on the Huixian karst wetland system mainly focuses on groundwater environmental pollution [18], heavy metal pollution in soil sediments, and wetland hydrological effects [19]. With regard to the wetland surface water environment, the existing research focuses on the concentration distribution and pollution of nitrate water nutrients, organochlorine pesticides and sulfa antibiotics [14,20], only a few studies had quantified the hydrochemical characteristics and causes of the formation of water bodies in this region in the wet and dry seasons.
Scientific understanding of the hydrochemical characteristics of groundwater in this area and the underlying mechanism is of great significance for ensuring sustainable utilization of groundwater resources of this wetland and other similar wetlands [21,22,23]. Therefore, the present study aimed to analyze the seasonal changes in the hydrochemical evolution of the Huixian karst wetland to identify the factors regulating the hydrochemical characteristics of groundwater. The knowledge gained can guide the rational allocation, utilization, and protection of water resources. The present study adopted the Huixian karst wetland basin as a study area and comprehensively applied statistical analysis, ionic ratio, hydrodynamic parameters, and other methods to identify the hydrochemical characteristics of different water bodies and the underlying mechanisms.

2. Study Area

The Huixian wetland is in the karst core area of East Asia, representing one of three major karst distribution areas globally. The study area is −21 km southwest of the Guilin urban area with geographic coordinates of 25°04′58″–25°08′25″ north latitude and 110°09′23″–110°14′36″ east longitude [24,25,26,27]. The Huixian wetland has a mid-subtropical monsoon climate and falls in the central zone of subtropical peak forest landforms in China. The annual average temperature of the study area is 18.9 °C, with extreme maximum and minimum temperatures over multiple years of 38.8 °C and −3.3 °C, respectively. Data measured by the Guilin meteorological station from 1951 to 2012 indicate annual precipitation and potential evaporation to be 1886 mm and 1378.3 mm, respectively. The wet season occurred from April to August, with precipitation over this period accounting for 70% of annual precipitation, whereas the dry season extends from September to March, accounting for the rest of annual precipitation [28,29,30,31,32,33,34,35].
The overall terrain of the Huixian wetland area is high in the north and south, low in the central region, and with some low mountains and hills in the east and west. Surface river water and groundwater are generally replenished by the mountainous areas in the north and south. Recharge water collects in the central plain, basin, and wetland core areas. The surface water and groundwater in the wetland are drained to the east and west due to the north-south inclination of the watershed slope in the middle of the wetland [36,37]. The Liangfeng and Luoqing rivers occur on the eastern and western sides of the study area, respectively. The ancient Guiliu Canal runs through the center of the study area, thereby connecting the two rivers. Recharge to the wetland during the wet season is mainly from the mountainous areas to the north and south. This recharge is mainly routed as surface runoff through the ancient Guiliu Canal, which enters the Liangfeng and Lijiang rivers in the east and the Luoqing River to the west, which converge into the Liujiang River. Recharge to the wetland during the dry season is mainly through underground runoff discharged from the rivers to the east and west (Figure 1). The area of the Huixian wetland has decreased from 42 km2 to 15 km2 over the past 40 years, indicating that the ecological structure of the wetland has gradually transformed from a natural wetland to an artificial wetland [38].

3. Materials and Methods

3.1. Water Sample Collection and Testing

The present study collected 56 groups and 34 groups of water samples in September 2021 and October 2020, respectively to represent the dry season, whereas 40 groups of water samples were collected in May 2021 to represent the wet season. Among these samples, 35, 4, 24, and 50 samples were collected from wells, subterranean streams, blue holes, and surface rivers, respectively. Figure 1 shows the distribution of sample sites for 2H and 18O isotope samples. This distribution of sampling points represented the main water body types utilized for water resources in the study area. Groundwater samples were mainly collected from blue holes and well. Water in the wells was mostly Quaternary pore water and surface karst water, whereas water in the blue holes was generally connected to underground pipes. The subterranean stream samples were used to control the water quality characteristics of underground river inlets, river sections, and outlets.
Samples for measurement of conventional ions were collected in 500 mL polyethylene bottles. These bottles were sterilized beforehand by rinsing with deionized (DI) water. Before sampling, the bottle was rinsed with the sample source water three times. Each set of samples consisted of two 500 mL water samples and a 50 mL sample for the measurement of 15N-NO3, 18O-NO3, and 13CDIC in nitrate. Water samples were filtered with a 0.45 μm filter before collection. All water samples were stored at 4 °C. pH, dissolved oxygen (DO), Eh, and conductivity were measured on-site using portable field meters. Ca2+ and HCO3 in samples were analyzed by hardness and alkalinity meters. Mg2+, Na+, K+ and SO42−, Cl in samples were measured using a Dionex ICS1500 ion chromatograph(DIONEX, Sunnyvale, CA, USA) and Metrohm MIC ion chromatograph(Metrohm, Herisau, Switzerland), respectively. 2H and 18O isotopes in samples were measured using high-temperature heat transfer element-isotope ratio mass spectrometry with accuracies of ±0.3‰ and ±0.16‰, respectively. All laboratory tests were conducted at the Institute of Karst Geology, Chinese Academy of Geological Sciences. Quality control was realized by reference material provided by the National Standard Center, and the relative standard deviations of measurements were all less than 10%. The accuracies of all obtained data met the study requirements, the water sample testing process was established using a blank control, and all chemical reagents used were analytically pure. Deuterium (D), 18O, and 3H isotopes were quantitatively determined using a stable isotope gas mass spectrometer MAT-253(Finnigan Mat, Bremen, Germany) and ultra-low background liquid scintillation spectrometer Quantulus 1220(PerkinElmer, Waltham, MA, USA). No special pretreatment of samples before measurements was required and all test results were based on the VSMOW national standard.

3.2. Methods

MapGIS 6.7 software(Wuhan, China University of Geosciences) was used to plot the distribution of sample testing results. Statistical testing of hydrochemical parameters was conducted in SPSS 22(IBM, New York, NY, USA). Each water sample was classified by Shukarev hydrochemical type to clarify the spatial characteristics of hydrochemical components in the watershed. Piper’s three-line diagrams were plotted in AquaChem software(WaterlooHydrogeologic, lnc., Waterloo, ON, Canada), including Durov and Schoeller diagrams to identify the drivers of the distribution of hydrochemical ions in the study area. Origin 10.0(OriginLab, Northampton, MA, USA) was used to plot Gibbs, karst groundwater ionic ratio, and isotope analysis diagrams, following which the correlations between ionic ratios were numerically identified. The main drivers of the hydrochemical characteristics of water in the study area were then identified. In order to understand the genesis and geological characteristics of deep karst caves, the methods of well core analysis, regional hydrogeological survey, groundwater geochemistry, isotope testing, and geophysical exploration are mainly used.

4. Results and Analysis

4.1. Hydrochemical Compositions of Groundwater Samples

Studies on the hydrochemical characteristics of groundwater and their evolution are based on the statistical analysis of hydrochemical parameters, which can reflect the state of groundwater components in a study area over a certain period [38,39].
As shown in Table 1, the analysis of the 110 water samples indicated the pH of Huixian karst wetland water to be 6.61–7.82 (mean = 7.37); the concentration of Ca2+ was 24.45–352.68 mg·L−1 (mean = 76.73 mg·L−1); Mg2+ was 0.89–311.31 mg·L−1 (mean = 8.31 mg·L−1); K+ + Na was 0.43–110.36 mg·L−1 (mean = 12.78 mg·L−1); Cl was 1.43–49.55 mg·L−1; sulfate was 3.22–1813.50 mg·L−1 (mean = 30.28 mg·L−1). The results indicated that groundwater Na+, Ca2+, HCO3, and Cl were relatively stable. The total concentrations of major groundwater ions during the wet season exceeded that during the dry season. The major ions concentrations in groundwater all exceeded those in surface river water. This result can be attributed to the rapid flow and rapid renewal of surface river water by mainly rainfall-runoff.
Besides well water with a total dissolved solids (TDS) concentration exceeding 1000 mg·L−1, the water of the Huixian wetland showed a low TDS, with means of between 160.76–455.53 mg·L−1. The mean ionic concentrations displayed in the piper three-line diagram (Figure 2) indicated the cation and anion with the highest concentrations among the different types of water samples to be Ca2+ and HCO3, respectively. This result indicates similarities in groundwater compositions among the different aquifers, suggesting hydraulic connections between them. The concentrations of major ions in water samples of subterranean streams, blue holes, and wells all exceeded those of surface river water samples. This result can be attributed to the rapid flow and rapid renewal of surface river water, mainly through rainfall-runoff. Table 1 summarizes the results of testing of Huixian karst wetland water for various parameters.
The order of major cations in the water of surface rivers, subterranean streams, blue holes, and wells in terms of concentration was: Ca2+ > Mg2+ ≈ Na+ > K+, whereas that of main anions was: HCO3 > SO42− > Cl. The relationship between main anion and cation concentrations was similar between surface river water, blue hole, and subterranean stream, indicating closely related sources of recharge. Due to the longtime water-rock interaction in wells, the concentration of hydrochemical ions was relatively high.
The results showed that besides Cl and Na+ in surface river water, well water showed the highest mean concentrations of ions in the Huixian wetland. The hydrochemical groundwater type in the study area did not change between the wet and dry seasons. The hydrochemical type of sampling point GL09 was SO4-Ca·Mg due to the impact of upstream pyrite and strata, whereas the hydrochemical water type of the remaining samples was mainly HCO3-Ca. However, differences in the hydrochemical characteristics of the four water sources remained. This result indicated that while the hydrochemical types of subterranean stream water and blue hole water showed relatively concentrated spatial distributions, those of surface river water and well water were relatively scattered. Comprehensive analysis showed that the hydrochemical compositions of well water and surface river water were greatly affected by external conditions, while external conditions had relatively little effect on the water of blue holes and subterranean streams. As shown in Figure 3, the concentrations of the cations Ca2+ and Mg2+ were relatively high, whereas those of K+ and Na+ were low; and the concentrations of the anion Cl were relatively high, whereas those of sulfate were low.
Chemical oxygen demand (COD) represents the quantity of reducing substances oxidized in water, and mainly includes organics, nitrite, sulfide, and other reducing substances [40,41]. COD is regarded as an important indicator of water quality [42]. COD in the water of the Huixian karst wetland ranged between nd and 0.83 mg·L−1 (mean of 0.05 mg·L−1), below the threshold of 1.0 mg·L−1, indicating good quality groundwater. However, the mean CODs of water of surface rivers and blue holes were 3.3 mg·L−1 and 4.5 mg·L−1, respectively, exceeding the threshold of 1.0 mg·L−1 and indicating the impact of anthropogenic activities on water quality.
The Schoeller diagram is a graphical method commonly used to represent the hydrochemical characteristics of water and can be used to analyze trends of major ions in water samples [15]. As shown in Figure 4, trends in ions were basically the same between groundwater (blue holes, subterranean streams, wells) and surface water, further demonstrating closely related sources of supply between groundwater and surface water.
The maximum concentrations of the three nitrogen species, NO3, NO2, and NH4+ in surface river water were 27.63, 1.23, and 5.26 mg·L−1, respectively, which all exceeded the standard limits of Class III groundwater of 20, 1.0, and 0.5 mg·L−1 by 6.67%, 3.33%, and 23.33%, respectively [43]. The maximum concentrations of NO3, NO2 and NH4+ in subterranean stream water were 17.53, 1.90, and 1.93 mg·L−1, respectively, which all exceed the standard limit of Class III groundwater by 25%, 25%, and 25%, respectively. The maximum concentrations of NO3, NO2 and NH4+ in blue hole water were 76.50, 1.56, and 12.96 mg·L−1, which all exceeded the standard limit of Class III groundwater by 33.33%, 4.16%, and 12.5%, respectively. The maximum concentrations of NO3 and NO2 in well water were 74.74 and 1.04 mg·L−1, respectively, which exceeded the standard limits of Class III groundwater by 51.42% and 28.57%, respectively, whereas the concentration of NH4+ did not exceed the corresponding limit.
The present study identified good linear fitting positive relationships between concentrations of NO3 and TDS in different types of groundwater (Figure 5), with the rank of the different types of water according to the strength of this relationship being: wells > blue holes > subterranean streams > surface rivers. Meanwhile, the concentration of NO3 in the study area exceeded the Class III limit to a much higher degree than NO2 and NH4+. NO3 in groundwater typically originates from the degradation of natural organic nitrogen or humus, nitrification, and anthropogenic activities (application of chemical fertilizers and farmyard manure, industry and agriculture, domestic sewage). High concentrations of NO3 were concentrated in the watershed discharge area and gradually increased from the recharge area in the north to the discharge area in the south.
High concentrations of NO3 mainly occurred in the main residential areas and on agricultural land, suggesting that the application of agricultural fertilizer is a driver of high concentrations of groundwater NO3, which further resulted in increases in groundwater TDS. Poor hydrodynamic conditions result in a slow turnover of groundwater and a corresponding higher concentration of groundwater NO3.

4.2. Water-Rock Interaction

4.2.1. Ionic Ratio

The Gibbs diagram is an analytical method reflecting the factors regulating major ions in water. The method generally divides dominant driving factors into the categories of evaporation and crystallization, rock weathering, and precipitation [44]. The Gibbs diagram applies a logarithmic coordinate system to groundwater hydrochemical components in which the ordinate is the logarithmic coordinate of total TDS in groundwater and the abscissa are the Na+/(Na+ + Ca2+) and Cl/(Cl + HCO3) ratios in groundwater, respectively [45].
Carbonate rock is typically prone to karstification with the participation of CO2 and H2O. The maximum concentrations of Ca2+ in karst groundwater (subterranean streams, blue holes, wells) in the study area ranged between 104.25–352.68 mg·L−1 (mean = 91.74 mg·L−1); that of Mg2+ was 4.50–311.31 mg·L−1 (mean = 13.06 mg·L−1); HCO3 was 277.18–371.86 mg·L−1 (mean = 263.94 mg·L−1). As shown in the Gibbs diagram (Figure 6), there were moderate concentrations of TDS in groundwater and surface water of the Huixian karst wetland, and the Na+/(Na+ + Ca2+) or Cl/(Cl + HCO3) ratio was generally less than 0.4. Outliers of groundwater were mainly affected by rock chemical weathering, whereas outliers of surface river water were affected by atmospheric deposition. Therefore, the hydrochemical components of water in the Huixian karst wetland were mainly affected by the chemical weathering of rock minerals. The hydrochemical components of the Huixian karst wetland were mainly affected by minerals, such as dolomite (CaMg(CO3)2), calcite (CaCO3), and gypsum (CaSO4:2H2O).
As shown in Figure 6, besides the sampling points GL09 (well water) and WS35 (surface river water), the water samples of the Huixian karst wetland during both the wet and dry seasons were all located in the middle of the Gibbs diagram, indicating highly consistent chemical compositions, migration, and sources of ions in water in the study area between the wet and dry seasons. Meanwhile, water-rock interaction was identified as the main factor regulating the hydrochemical composition of the water of the Huixian karst wetland. This result can mainly be attributed to the Huixian karst wetland being dominated by continuous carbonate rock interspersed with carbonate rock. There was a positive relationship between the duration of interaction between groundwater and carbonate rock and the rate of dissolution weathering of carbonate rock [32,40].

4.2.2. Process of Water-Rock Interaction

There were moderate concentrations of TDS in surface rivers and groundwater (subterranean streams, blue holes, wells) of the Huixian karst wetland. The overall Na+/(Na+ + Ca2+) or Cl/(Cl + HCO3) ratios were less than 0.4, with outliers mainly affected by rock chemical weathering and some well water outliers affected by evaporation concentration. Therefore, the hydrochemical components of water of the Huixian wetland were mainly affected by the chemical weathering of rocks. Evaporative concentration also affected the hydrochemical components of water in the study area due to the shallow depths of buried bedrock and fissure water in some areas. There were high concentrations of Cl in some water samples, indicating the influences of anthropogenic activities. The Gibbs diagram indicated that the hydrochemical characteristics of surface river water were mainly influenced by rock weathering.
Figure 7a shows the relationship between SO42− + Cl and HCO3 in which the areas above and below the 1:1 line represent the dissolution of carbonate rock and dissolution of evaporites (e.g., gypsum, NaCl), respectively. The groundwater of the Huixian karst wetland was dominated by the dissolution of carbonate rock with an absence of dissolution of evaporite (e.g., gypsum, NaCl), consistent with the hydrogeological conditions of the Huixian karst wetland.
Ca2+ and Mg2+ in groundwater of the Huixian karst wetland mainly originated from the dissolution of carbonate or silicate and evaporite. Therefore, the mg equivalent ratio between (Ca2+ + Mg2+) and (SO42− + HCO3) can be used to identify the dominant sources of Ca2+ and Mg2+. Figure 7b shows the relationship between SO42− + HCO3 and Ca2+ + Mg2+. The karst groundwater samples were approximately linearly distributed near the line γ (Ca2+ + Mg2+)/γ (SO42− + HCO3) = 1, suggesting that water in the study area contains both carbonate minerals and dissolved aluminosilicate minerals. These minerals can affect the formation of hydrochemical components in groundwater.
The mg equivalent ratio of Na+ and Cl can be used for determining the occurrence of dissolution of aluminosilicate minerals. The ratio of γ(Na+)/γ(Cl) = 1 showed that Na+ originated from rock salt dissolution or dissolution of aluminosilicate minerals. As shown in Figure 8a, groundwater of the Huixian karst wetland was mainly located above and below the line γ(Na+)/γ(Cl) = 1, suggesting that the hydrochemical characteristics of water in the study area are affected by both rock salt and silicate dissolution. Besides, the concentration of Cl in well water exceeded the concentration of Na+, indicating that Cl in groundwater originated from both rock salt dissolution and other anthropogenic sources.
Figure 8b shows the relationship between Na+ + K+ − Cl and Ca2+ + Mg2+ − SO42− − HCO3. As observed, some sampling points were located near the cation exchange line, suggesting that hydrochemical characteristics of groundwater in the Huixian karst wetland are related to the dissolution of calcite (aragonite), dolomite and gypsum, and cation exchange.
Figure 9 shows the relationship between Ca2+/Na+ and HCO3/Na+, revealing the influence of different sources of substances (evaporite, silicate rock, precipitation, and carbonate rock) in the groundwater cycle. Molar concentration ratios of Ca2+/Na+, Mg2+/Na+, and HCO3/Na+ are commonly used to study the reaction between water and different rocks. The ratios of carbonate rock weathering control endmembers were close to 50, 10, and 120, respectively, while the ratios of silicate rock control endmembers were close to 0.35 ± 0.15, 0.24 ± 0.12, and 2 ± 1, respectively.
Besides water samples from blue holes, the hydrochemical characteristics of water samples from different water types of the Huixian karst wetland were mainly influenced by weathering of carbonate rock, followed by weathering of carbonate and silicate rock and weathering of carbonate rock and evaporites. This result suggested that the hydrochemical characteristics of water in the Huixian karst wetland are mainly regulated by the chemical weathering of carbonate rock, but also by silicate rock weathering and precipitation. Furthermore, the overall effect of evaporite chemical weathering was weak, consistent with the results of earlier analysis (see Figure 9).

4.2.3. Sources of Ca2+, Mg2+, and HCO3

The hydrochemical characteristics of groundwater in the study area were mainly regulated by the chemical weathering of carbonate rock, including limestone and dolomite, which are composed of calcite (CaCO3) and dolomite [CaMg(CO3)2], respectively. The process under which calcite and dolomite dissolve into groundwater follows Equations (1) and (2). The groundwater Mg2+/Ca2+ ratio is 0 when dissolved pure calcite reaches an equilibrium. The groundwater Mg2+/Ca2+ ratio is 1 when dissolved pure dolomite reaches equilibrium [37,43]. The groundwater Mg2+/Ca2+ ratio is 0.5 when dissolved calcite and dolomite reaches equilibrium (Equation (3)) [32,44].
CaCO3 + CO2 + H2O → Ca2+ + 2HCO3
CaMg(CO3)2 + 2CO2 + 2H2O → Ca2+ + Mg2+ + 4HCO3
CaCO3 + CaMg(CO3)2 + 3CO2 + 3H2O → 2Ca2+ + Mg2+ + 6HCO3
Figure 10 shows the relationship between Mg2+/Ca2+ and HCO3 in surface river water and groundwater (subterranean streams, blue holes, wells) in the Huixian karst wetland. Most sampling points were in areas A and B, and the Mg2+/Ca2+ ratios of all samples were below 0.5, suggesting that most groundwater hydrochemical components in the study area were dominated by the dissolution of calcite. The Mg2+/Ca2+ ratio of the well water sample GW17 in Area C was 0.53, close to the y = 0.5 line, suggesting that the hydrochemical characteristics of groundwater at this point were dominated by the dissolution of calcite and dolomite over different periods. This result can be attributed to the C1y and C1d strata near this point and the presence of dolomite and dolomitic limestone upstream. The Mg2+/Ca2+ ratio of well water sample GL09 in the C area was relatively high at 1.5. This result could be attributed to the hydrodynamic conditions and local content of dolomite during the rainy season, leading to the neutralization of Ca2+ by other anions, especially under non-organic ionization disequilibrium (NICB) conditions. The temporal scale of the effect of carbonate dissolution was not obvious at the other sites, indicating that Ca2+, Mg2+, and HCO3 in groundwater of the Huixian karst wetland originated from the dissolution of calcite, followed by dolomite. Moreover, sources of dissolved ions were consistent among surface river water and groundwater.
TDS can act as a comprehensive indicator of water quality. The TDS of karst groundwater (subterranean streams, blue holes, wells) in the study area ranged between 120.03–2674.07 mg·L−1 (mean = 120.03 mg·L−1), with the maximum TDS in well sample GL09. The order of different types of water according to mean TDS concentration was: surface river water < subterranean streams < blue holes < wells. The ranking of the different parts of the catchment according to TDS concentration was: supply area < runoff and the discharge area < retention area. As shown in Figure 11a, there was a strong correlation between TDS and sulfate (coefficient R2 of 0.83), indicating that sulfate is the main factor regulating TDS. However, SO42− in karst groundwater in the area mainly originated from the dissolution of gypsum, acid coal mine water, and sulfate in precipitation [46,47,48].
When rock weathering is the main source of ions and carbonate rock is the main factor regulating Ca2+, the molar ratios of SO42−/NO3 and SO42−/Ca2+ can be used to analyze the influence of anthropogenic activities on major ions in groundwater. As shown in Figure 11b, the NO3/Ca2+ ratio exceeded the SO42−/Ca2+ ratio, suggesting that groundwater in the Huixian wetlands is greatly affected by agricultural activities and domestic sewage. This result can be attributed to the underdevelopment of industrial activities around the wetland, with agricultural activities exceeding industrial activities. K+, Na+, SO42−, NO3, and Cl originated from precipitation, Na+ and Cl originated from domestic sewage, whereas K+ was related to the application of potassium fertilizer. NO3 mainly originated from chemical fertilizer.
δD of karst groundwater in the study area ranged from −33.5 to −15.7‰ (mean = −28.55‰). δ18O of karst groundwater in the study area ranged from −5.66 to −0.1‰ (mean = −4.62‰) (Table 2). As shown in Figure 12, both the intercept and slope of the δD-δ18O relationship line were smaller than the global meteoric water line (δD = 8δ18O + 10), indicating that the region has an inland arid climate. In summary, precipitation represented the main source of water for the Huixian karst wetland. H and O isotopes in surface river water were consistent with those in well water in the runoff zone, indicating that karst groundwater also received seepage recharge from surface river water.
Huixian karst wetland was affected by groundwater circulation conditions, natural geographical environment, and human factors. There were certain differences in the isotopic distribution of surface water and groundwater in the wetland basin. On the whole, the δ2D and δ18O isotopes concentrations in the surface river were higher than that of groundwater samples (blue hole water, well water), indicating that the evaporation of surface water is more intense.
There were certain differences in isotope values of different water bodies from δ2D, δ18O statistical analysis and their relationship (Figure 12), and the degree of isotope enrichment was shown as surface river > Blue Hole Water > well water. Due to the low temperature of groundwater, there was no significant exchange of δ18O.
The blue hole water sample points fell between the surface water supply area and deep groundwater, reflecting the characteristics of mixed water. Two well water sampling points were more strongly affected by surface water mixing, which was related to the proximity of well water sampling points to surface rivers. For most other well water samples, the δ2D, δ18O changed little and was relatively stable, indicating that the well water was supplied from the same source and was less affected by external conditions.

5. Conclusions

(1)
The groundwater of the Huixian karst wetland was dominantly weak alkaline fresh water in which the main ions were Ca2+ and HCO3. The contents of major ions of groundwater in the wet season exceeded that in the dry season. Groundwater Na+, Ca2+, HCO3, and Cl were relatively stable.
(2)
The influences of carbonate rock resulted in consistency in groundwater hydrochemical characteristics between the wet and dry seasons, with the dominant water type being HCO3-Ca water. The GL09 sample point was affected by upstream pyrite and strata, with SO4-Ca·Mg water distributed in both the wet and dry seasons. Overall, the highest cation concentrations of water of the Huixian karst wetland were for calcium and magnesium, whereas the lowest was for potassium and sodium; as well, the highest anion concentration was for sulfate and the lowest was for chloride.
(3)
The different water body types in the study area showed different degrees to which nitrogen species (NO3, NO2, NH4+) exceeded the Class III standards. The concentration of NO3 exceeded the standard in the study area to a higher degree than NO2 and NH4+. NO3 in groundwater typically originated from the degradation of natural organic nitrogen or humus, nitrification, and anthropogenic activities (application of chemical fertilizer and farmyard manure, industry and agriculture, domestic sewage).
(4)
The hydrochemical characteristics of groundwater were mainly regulated by rock weathering, with water of some shallow fissures also affected by evaporative concentration. Major ions in groundwater originated from the dissolution and precipitation of minerals from carbonate rocks, such as calcite and dolomite. The leaching of silicate and evaporite also contributed to major ions in groundwater.
(5)
The degree to which hydrochemical components of groundwater in the study area were regulated by water-rock interaction was consistent between the wet and dry seasons. Ca2+ and HCO3 in groundwater originated from calcite weathering, although weathering of dolomite, dolomitic limestone, and pyrite regulated the hydrochemical characteristics of some samples, resulting in high concentrations of Mg2+ and SO42− in groundwater. K+, Na+, SO42−, NO3, and Cl originated from precipitation, Na+ and Cl originated from domestic sewage, whereas K+ was related to the application of potassium fertilizer in agriculture. NO3 mainly originated from chemical fertilizer.
(6)
The sources of recharge to surface river water and groundwater were closely related, and groundwater quality was easily affected by the surface river water quality. Groundwater was greatly affected by anthropogenic activities, among which the impacts of domestic sewage and agricultural activities exceeded that of industrial activities, particularly fertilizer application. Therefore, there should be further regulation of domestic sewage discharge and application of agricultural fertilizers to ensure sustainable use of wetland groundwater resources.

Author Contributions

Conceptualization, J.B.; methodology, J.B.; formal analysis, F.L.; investigation, C.T. and C.P.; writing––original draft preparation, J.B.; writing––review and editing, J.B.; project administration, J.B. and Y.D.; all the co-authors performed a critical revision of the intellectual content of the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Guilin Scientific Research and Technology Development Program Projects (20180101-3), the Guangxi Natural Science Foundation (No. 2018GXNSFAA294046 and No. 2020GXNSFAA297025), the National Key Research and Development Program of China (No. 2018YFC0604301 and No. 2017YFC0406104), the Geological Survey Program of China Geological Survey (No. DD20221658).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Compliance with MDPI Research Data Policies.

Acknowledgments

Du Wen-Yue and Yang Hui of the Institute of Karst Geology of the Chinese Academy of Geological Sciences were of great support during sample testing and analysis processes.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the Huixian wetland and distribution of sampling sites.
Figure 1. Location of the Huixian wetland and distribution of sampling sites.
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Figure 2. Piper diagram of major ions in different water types of the Huixian karst wetland.
Figure 2. Piper diagram of major ions in different water types of the Huixian karst wetland.
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Figure 3. Durov diagram of ionic concentrations of different samples of the Huixian karst wetland.
Figure 3. Durov diagram of ionic concentrations of different samples of the Huixian karst wetland.
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Figure 4. Schoeller diagram of major ions in surface river water and groundwater (subterranean stream water, blue hole water, well water) of the Huixian karst wetland.
Figure 4. Schoeller diagram of major ions in surface river water and groundwater (subterranean stream water, blue hole water, well water) of the Huixian karst wetland.
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Figure 5. Relationship between NO3 and total dissolved solids (TDS) in groundwater of the Huixian karst wetland.
Figure 5. Relationship between NO3 and total dissolved solids (TDS) in groundwater of the Huixian karst wetland.
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Figure 6. Gibbs diagram representing hydrochemical characteristics of subterranean streams, blue holes, wells, and surface rivers of the Huixian karst wetland.
Figure 6. Gibbs diagram representing hydrochemical characteristics of subterranean streams, blue holes, wells, and surface rivers of the Huixian karst wetland.
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Figure 7. Relationships between major ions in the water of the Huixian karst wetland. (a) SO42− + Cl vs HCO3, (b) SO42− + HCO3 vs Ca2+ + Mg2+.
Figure 7. Relationships between major ions in the water of the Huixian karst wetland. (a) SO42− + Cl vs HCO3, (b) SO42− + HCO3 vs Ca2+ + Mg2+.
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Figure 8. Relationships between major ions in the water of the Huixian karst wetland. (a) Cl vs. Na+, (b) Na+ + K+ − Cl vs. Ca2+ + Mg2+ − SO42− − HCO3−.
Figure 8. Relationships between major ions in the water of the Huixian karst wetland. (a) Cl vs. Na+, (b) Na+ + K+ − Cl vs. Ca2+ + Mg2+ − SO42− − HCO3−.
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Figure 9. Relationships between major ions in the water of the Huixian karst wetland. (a) Ca2+/Na+ vs. HCO3/Na+, (b) Ca2+/Na+ vs. Mg2+/Na+.
Figure 9. Relationships between major ions in the water of the Huixian karst wetland. (a) Ca2+/Na+ vs. HCO3/Na+, (b) Ca2+/Na+ vs. Mg2+/Na+.
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Figure 10. Relationship between HCO3 and Mg2+/Ca2+ in groundwater of the Huixian karst wetland.
Figure 10. Relationship between HCO3 and Mg2+/Ca2+ in groundwater of the Huixian karst wetland.
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Figure 11. Major ionic ratios within the karst groundwater of the Huixian karst wetland. (a) Total dissolved solids (TDS) vs. SO42−, (b) NO3/Ca2+ vs. SO42−/Ca2+.
Figure 11. Major ionic ratios within the karst groundwater of the Huixian karst wetland. (a) Total dissolved solids (TDS) vs. SO42−, (b) NO3/Ca2+ vs. SO42−/Ca2+.
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Figure 12. δ18O vs. δD in karst groundwater of the Huixian karst wetland.
Figure 12. δ18O vs. δD in karst groundwater of the Huixian karst wetland.
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Table 1. Hydrochemical characteristics of groundwater of the Huixian karst wetland.
Table 1. Hydrochemical characteristics of groundwater of the Huixian karst wetland.
TypeParameterpHMass Concentrations of Components/mg·L−1
K+Na+Ca2+Mg2+ClSO42−HCO3NO3NO2NH4+CODMnTDS
Surface river water samples (n = 50)mean7.345.563.7944.894.557.4910.83149.757.200.200.803.30160.76
maximum7.6322.4535.8293.219.5244.5523.93277.1827.631.235.269.60295.73
minimum6.61
0.22
1.36
4.98
0.4927.741.992.463.4392.600.880.00nd0.7735.29
standard deviation6.9614.311.988.266.1941.576.940.391.451.8353.14
Coefficient of variation0.030.901.840.320.441.100.570.280.961.921.820.550.33
Subterranean stream samples (n = 4)mean7.262.811.7378.123.494.5912.01226.7511.860.961.931.51225.26
maximum7.357.143.09104.254.507.5219.12277.1817.531.901.932.23286.63
minimum7.130.340.7352.582.032.208.40164.913.350.02nd0.83158.03
standard deviation0.113.120.9921.211.132.224.8246.757.501.33nd0.7055.83
Coefficient of variation0.011.110.570.270.320.480.400.210.631.38nd0.470.25
Blue hole water samples (n = 24)mean7.432.512.1888.176.625.9717.89256.3317.700.412.154.50269.21
maximum7.8227.2813.11126.1015.6927.3775.63317.5376.501.5612.9610.96410.45
minimum7.080.050.2934.830.891.433.22142.101.450.00nd0.83156.02
standard deviation0.205.682.5623.834.195.3616.1457.5816.280.674.465.6142.72
Coefficient of variation0.032.261.170.270.630.900.900.220.921.642.071.250.16
Well water samples (n = 35)mean7.3916.878.1695.7518.5715.0580.38273.4028.430.210.040.05455.53
maximum7.7286.9023.46352.68311.3149.551813.50371.8674.741.040.140.832674.07
minimum7.080.121.0124.451.433.026.2880.841.920.000.02nd120.03
standard deviation0.1421.614.9549.6451.3510.20301.9360.9223.030.350.03nd404.64
Coefficient of variation0.021.280.610.522.760.683.760.220.811.680.81nd0.89
Exceeding rate/%0001.1009.700.1
Quality standards of groundwater6.5–8.520025025088.573.290.643.01000
Hygienic standard for drinking water6.5–8.5200300.30088.570.645.01500
Notes: Min: minimum; Max: maximum; Aver: mean; nd: not detected; TDS; total dissolved solids.
Table 2. Statistics for δD and δ18O values.
Table 2. Statistics for δD and δ18O values.
Sample No.Sample CategoryδD(V-SMOW)‰STDC.Vδ18O(V-SMOW)‰STDC.V
GL03well water−23.81.28 0.03−3.770.130.02
GL05well water−33.52.20 0.05−5.630.370.05
GL08well water−31.22.30 0.06−5.350.290.04
GL09well water−30.10.79 0.02−5.400.120.02
HX05well water−31.31.65 0.03−5.450.070.009
HX06well water−21.20.84 −0.02−2.960.110.01
HX10well water−32.13.19 0.11−5.450.31−0.08
HX11well water−28.31.58 0.12−4.780.320.13
HX13well water−30.62.94 0.07−5.400.150.07
HX15well water−31.63.37 −0.01−5.540.16−0.01
HX16well water−31.01.76 0.06−5.460.240.05
HX18well water−32.61.170.03−5.660.220.04
HX14blue hole water−26.71.210.08−4.420.190.07
HX17blue hole water−25.71.190.04−4.460.270.11
HX23blue hole water−29.51.380.06−4.990.150.08
HX40blue hole water−31.32.330.07−5.470.240.18
HX41blue hole water−28.90.69−0.01−3.890.120.01
HX01Surface river−15.71.510.09−0.100.160.03
HX12Surface river−26.51.580.07−3.790.220.04
HX21Surface river−26.71.790.02−3.470.360.05
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Ba, J.; Dan, Y.; Luo, F.; Tang, C.; Peng, C. Seasonal Differences in the Hydrochemical Characteristics of Karst Wetlands and the Associated Mechanisms in Huixian, China. Water 2022, 14, 2362. https://doi.org/10.3390/w14152362

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Ba J, Dan Y, Luo F, Tang C, Peng C. Seasonal Differences in the Hydrochemical Characteristics of Karst Wetlands and the Associated Mechanisms in Huixian, China. Water. 2022; 14(15):2362. https://doi.org/10.3390/w14152362

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Ba, Junjie, Yong Dan, Fei Luo, Chunlei Tang, and Cong Peng. 2022. "Seasonal Differences in the Hydrochemical Characteristics of Karst Wetlands and the Associated Mechanisms in Huixian, China" Water 14, no. 15: 2362. https://doi.org/10.3390/w14152362

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