Hydrogeochemistry of Fluorine in Groundwater in Humid Mountainous Areas: A Case Study at Xingguo County, Southern China

The understanding of F − concentration in groundwater in humid areas is limited although there are lots of research on high-ﬂuoride groundwater in arid areas. In this paper, with controlling factors of F − concentrations in humid areas as the focus, 130 groundwater samples, obtained from four subsystems in Northwest Xingguo County, Jiangxi Province, China, were investigated to demonstrate the controlling factors of F − concentrations in humid areas. According to analytical results, the following hydrogeochemical characteristics of the ﬂuorine in humid mountainous areas were determined: (1) F − concentration is positively correlated with total dissolved solids (TDS), Ca 2+ , HCO 3 − , and pH; (2) the groundwater features a high ﬂow rate and low TDS; (3) the equilibrium constant of CaF 2 is less than its solubility product constant, and the ﬂuorine-bearing minerals in rocks are in a dissolved state; and (4) the dissolved ﬂuoride-bearing minerals constitute the main sources of F − in the groundwater. Fluorine mainly comes from groundwater ﬂuorine-bearing minerals in metamorphic rocks. Moreover, the low F − concentration in the groundwater mainly results from the fast ﬂow rate of groundwater. Fluoride in groundwater has great potential hazards in humid areas.


Introduction
Fluorine is an indispensable trace element in the human body. It is an essential component to maintain the normal development of human bones. However, excessive fluoride in the human body can lead to endemic fluorosis [1]. It is stipulated in Standards for Drinking Water Quality (GB 5749-2006) and Quality Standard for Groundwater (GB/T 14848-2017) that F − concentration in drinking water should be less than 1.0 mg/L. Water with F − concentration greater than 1.0 mg/L is defined as high-fluoride water [2].
ere is a remarkable positive correlation between Na + and F − , and the enrichment of Ca 2+ and Mg 2+ will inhibit the enrichment of F − [30]. With respect to groundwater chemistry, the fluoride concentration is usually high in Na-HCO 3 type groundwater and low in Ca-HCO 3 type groundwater [6,20,[31][32][33][34][35]. e deficiency of calcium ion concentration in the groundwater from calcite precipitation favors fluorite dissolution leading to excess fluoride concentration. e groundwater is oversaturated with respect to calcite and undersaturated with respect to fluorite [36][37][38][39][40][41]. Evapotranspiration leads to precipitation of calcite, lowering of Ca activity, and increase in Na/Ca ratios, and this allows an increase in F − levels in the arid area [39,[42][43][44].
In a previous study on fluorine dissolved in groundwater, most scholars focused on the source, migration, and enrichment of fluorine in arid and semiarid areas and alkaline and weak alkaline environment, and they achieved many good results. e research results of this paper have important theoretical and practical significance for people to understand the migration and enrichment characteristics of fluorine in weak acidic and acidic groundwater in humid areas. e groundwater is widely distributed and the flow rate is fast; the F − in groundwater will migrate to any areas where the groundwater flows. erefore, fluorine in groundwater has great potential harm in humid areas. e hydrogeochemistry of fluorine in humid mountainous areas is still important. In this paper, the hydrogeochemistry of groundwater and factors controlling the distribution of fluoride in groundwater of Northwest Xingguo County, southern China, have been evaluated. e main objectives of this paper are as follows: hydrogeochemical characteristics of fluorine in groundwater in humid areas, including the source of fluorine and its influencing factors.

Geographical Setting.
e study area, Northwest Xingguo County, Jiangxi Province (also referred to as the area) lies in E115°00′-E115°15′ and N26°20′-N26°30′, with an area of about 460 km 2 . It features a humid subtropical monsoon climate, with an average annual temperature of 18.8°C and an average annual rainfall of 1560 mm. e surface water is composed of Suishui River, Jianshui River, Shuicha River, and Wushu River. In the low-middle mountains, it mainly develops metamorphic rocks and granite. Hilly terrain of granite is distributed in the southeastern. It is a low mountainous area in general with an altitude less than 1000 m. e terrain is steep, V-shaped valleys are commonly developed, and the elevation of the highest peak is 1176 m in the area.

Geology.
e area is mainly comprised of strata of Sinian, Cambrian, Devonian, Carboniferous, and Quaternary with an outcrop area of 402 km 2 , accounting for 87.3% of the total survey area. e strata of Sinian and Cambrian are well developed and fully exposed. Magmatic rocks, mainly including plutonic intrusion and vein rocks, are generally distributed in the southeastern part of the area. Granite is common in the area, with an outcrop area of 58.67 km 2 . e vein rocks are mainly composed of quartz veins, granite veins, lamprophyre veins, and diabase veins ( Figure 1).

Characteristics of Water-Bearing Formations.
According to the geological and hydrogeological characteristics, the water-bearing formations in the area can be divided into three types: (1) porous water-bearing formation of loose rocks, (2) porous-fissured water-bearing formation of clastic rocks, and (3) fissured water-bearing formation of magmatic rocks and metamorphic rocks. e distribution areas of these three types are 20.78 km 2 , 86.56 km 2 , and 352.66 km 2 , accounting for 4.52%, 18.82%, and 76.66% of the total area, respectively. e porous water-bearing formations of loose rocks are mainly distributed along river banks, and the lithology of the aquifer is mainly characterized by sand, gravel, and pebble. e burial depth of the water table is 0.5-2.5 m, the TDS of groundwater samples is 0.029-0.145 g/L, and the hydrochemical type is HCO − 3 Ca. e porous-fissured water-bearing formations of clastic rocks are distributed in the northeast, northwest, and southwest of the area, with a spring flow of 0.014-9.328 L/s and a single well water yield greater than 100 m 3 /d. Hydrochemical types are mainly HCO − 3 Ca or HCO − 3 Ca·Mg, and TDS is 0.090-0.284 g/L for this type. As for the granite bearing weathering-fissure water, the single well water yield is 4.73-12.66 m 3 /d and the hydrochemical type is HCO − 3 Na. However, regarding the fissured water-bearing formations of metamorphic rocks of Cambrian and Sinian, single well water yields are 0.92-60.93 m 3 /d and 2.65-3.82 m 3 /d and share a common hydrochemical type of HCO − 3 Ca.

Groundwater
System. e area is located in the upper reaches of the Ganjiang River, the third-scale (III) groundwater subsystem in the Yangtze River basin. ere are fourth-scale groundwater subsystems in the area, precisely including Pingjiang River (III 2-1 ), Liangkou River (III 3-1 ), Yuntingshui River (III 4-1 ), and Wushu River (III 5-1 ). According to the Groundwater System Division Guideline (GWI-A5) issued by the China Geological Survey, the area is divided into four fifth-scale groundwater subsystems (Table 1).

Recharge, Runoff, and Discharge.
According to the distribution characteristics of the topography, geomorphology, and surface water in the area, the groundwater is replenished through atmospheric precipitation. e groundwater flows from west to east in the Suishui groundwater system and from north to south in the Jianshui River and Wushu river groundwater systems. e groundwater discharges into rivers, springs, and wells.

Methods and Materials
A total of 130 groundwater samples were collected from the outcrops of the four groundwater subsystems: Suishui River III 2-1-2 , Jianshui River III 3-1-1 , Shuicha River III 4-1-1 , and Wushu River III 5-1-1 , from June 23 to August 23, 2017 ( Figure 2). Major hydrochemical parameters (e.g., F − , Ca 2+ , HCO − 3 , pH, and TDS) in these water samples were analyzed in the field. Hash DR2800 spectrophotometer was employed for F − testing, reagent titration method was adopted for Ca 2+ and HCO − 3 testing, and acidimeter and test pen were used for pH and TDS testing, respectively. e results are shown in Table 2.

Hydrogeochemistry of Groundwater.
e chemical composition of 15 representative groundwater outcrops (springs, wells) was tested in order to further determine the hydrogeochemical characteristics of groundwater in the  Ca·Na, and the second is HCO − 3 Ca·Mg (Table 3). According Sinian fissured water-bearing formation of metamorphic rock; 2: Cambrian fissured water-bearing formation of metamorphic rock; 3: Devonian porous-fissured water-bearing formation of clastic rock; 4: carboniferous porous-fissured water-bearing formation of clastic rock; 5: Jurassic fissured water-bearing formation of granite; 6: code of groundwater system; 7: boundary of fifth-scale groundwater subsystem; 8: surface water system; 9: sampling location; 10: village; 11: peak; and 12: groundwater flow direction. Journal of Chemistry to the Piper triple-variation diagram, all groundwater in the study area is located in the bicarbonate type water area and is mainly located in the Ca type water area with low TDS hydrogeochemical characteristics (Figure 3).  [18,19,26,45]. Previous studies show that pH value varies from 7.3 to 9.0 in high-fluoride water [46,47]. Fluorine-bearing minerals in carbonate areas of weakly alkali environments are more liable to dissolve, resulting in high fluorine concentration in water [48]. e pH in groundwater is generally 6.3-6.7.

Factors
e F − concentration is positively correlated with pH in the groundwater of the three subsystems (Suishui River III 2-1-2 , Jianshui River III 3-1-1 , and Wushu River III 5-1-1 ) (Figure 4). It is not conductive to the dissolution of fluorine-bearing minerals owing to its low pH, and thus the F − concentration in it is low.

4.2.2.
e Impact of TDS. High fluoride groundwater generally has high TDS and high HCO − 3 in the weak alkaline environment [29]. e relationship between F − and TDS in the groundwater samples collected from different groundwater subsystems in the studied area is shown in Figure 5. It is indicated that the F − concentration is positively correlated with TDS in each groundwater subsystem. e reason is that ions in groundwater cannot be saturated owing to the fast water cycle rate in bedrock mountainous areas.

F − and Ca 2+ Balance.
e relationship between F − and Ca 2+ in the groundwater is shown in Figure 6. It is indicated that F − concentrations are positively correlated with Ca 2+ concentrations in the groundwater subsystems of Suishui River III 2-1-2 , Jianshui River III 3-1-1 , and Wushu River III 5-1-1 . e groundwater features the temperature of 16-25°C, low TDS (14.3-64.7 mg/L), and low Ca 2+ concentration (2.5-15.4 mg/L). erefore, the Ca 2+ and F − concentrations can be considered as their activity values. e equilibrium constant of CaF 2 (K � [Ca 2+ ] [F − ] 2 ) in the samples was calculated and compared to its solubility product constant (K sp ) ( Table 4). e result indicates that, under natural conditions, the solubility product constants are higher than the equilibrium constants, whereas the saturation indexes (SI) of CaF 2 are negative. is further confirms that the fluorine-bearing minerals in rocks are in a dissolving state and the increase of Ca 2+ will not lead to the precipitation of CaF 2 . However, the F − concentration tends to be negatively correlated with Ca 2+ concentration in arid areas [2,12,14]. is is because Ca 2+ and F − are saturated in high-fluorine groundwater in arid areas and, therefore, the increase of Ca 2+ concentration will lead to CaF 2 precipitation and the decrease of F − concentration accordingly.

4.2.4.
e Dissolution of Minerals. As mentioned above, high F − groundwater mostly locates in Na-HCO 3 type. F − concentrations in the groundwater are also positively correlated with HCO − 3 concentrations (Figure 7), indicating HCO − 3 concentration is another important factor affecting F − concentration in the study area. e anions in the groundwater are mainly HCO − 3 (8.4-34.4 mg/L), and CaF 2 is in a dissolved state as mentioned above. e dissolutionprecipitation equilibrium relationship between CaF 2 and CaCO 3 is expressed as follows: (1) e number of F − and HCO − 3 increases with the dissolution of CaCO 3 and CaF 2 in the groundwater subsystems. In other words, the dissolution will lead to a positive correlation between F − and HCO − 3 . is is another hydrochemical characteristic of F − in bedrock mountainous areas in a humid climate.

e Dissolution of Biotite.
Fluorine concentration in granite ranges from 0.044% to 0.216%. e biotite [K(Mg, Fe 2+ ) 3 (Al, Fe 3+ )Si 3 O 10 (OH, F) 2 ], which accounts for 15%-30% in metamorphic rocks, constitutes the main fluorinebearing minerals. Firstly, the fluorine-bearing minerals in these rocks dissolve and thus F − is released into the groundwater. Secondly, a faster flow rate of groundwater occurs in the area since it is located in the low-middle mountainous area. is probably leads to low F − concentration in the groundwater. irdly, the groundwater in the area shows weakly acidic characteristics, which may promote the dissolution of fluorine-bearing minerals. According to the results obtained from the hydrogeological survey in the area, it can be concluded that fluorine in the groundwater mainly comes from the interaction between the groundwater and the fluorine-bearing minerals in metamorphic rocks.

Gibbs Diagram.
Gibbs diagram built a simple and effective diagram that can be used to compare TDS∼(Na + / (Na + + Ca 2+ )) or TDS∼(Cl − /(Cl − + HCO 3 )); this can be used to identify the influencing factors of the groundwater hydrochemistry. For example, the chemical composition is primarily affected by rock weathering, evaporation, and crystallization of soluble salts [50,51].
According to the chemical composition data of groundwater, the Gibbs diagram of the study area is shown in Figure 8. e TDS of groundwater in the study area is located in 47.7-172.3 mg/L, and the ratio of Na/(Na + Ca) and Cl/(Cl + HCO 3 ) is between 0.056-0.554 and 0.010-0.114, respectively. It shows that the groundwater is characterized by high Na+ and Ca 2+ and suggests a geochemical source of granite and metamorphic rocks. erefore, it can be considered that fluorine in groundwater comes from the dissolution of fluorine-containing minerals in these rocks. e Gibbs analysis of groundwater also indicates that the formation of groundwater chemical Journal of Chemistry      Journal of Chemistry components in Xingguo County is dominated by rock weathering reactions; that is, water-rock interaction plays an important role in the migration and enrichment of fluoride in groundwater.

Conclusion
In this paper, the migration, enrichment characteristics, and influencing factors of fluorine in weak acidic and acidic groundwater in humid mountainous areas are described. e F − concentration is positively correlated with the contents of TDS, Ca 2+ , and HCO − 3 and pH value in the groundwater subsystems in the area owing to the intensive exchange of groundwater with aquifer minerals. In the study area, TDS of groundwater is low and the fluorine-bearing minerals (mainly CaF 2 ) in the rocks are in a dissolved state, with solubility product constant less than its equilibrium constant. Fluoride mainly originates from the interaction between the groundwater and fluorine-bearing minerals. Moreover, the F − concentration is low in the area due to the fast flow rate of groundwater. Fluoride in groundwater has great potential hazards in humid areas.
is study has important theoretical and practical significance for understanding the hydrogeochemical characteristics of fluorine in humid areas.
Data Availability e data have been published in the Geological Cloud.

Conflicts of Interest
e authors declare that they have no conflicts of interest.  Journal of Chemistry 9