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

Authors: Jinhui Liu; Xinfeng Wang; Weidong Xu; Mian Song; Lei Gong; Mengnan Zhang; Linbo Li; Lishan He
Journal: Journal of Chemistry
 (2021)

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## Abstract

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), Ca2+, HCO3−, and pH; (2) the groundwater features a high flow rate and low TDS; (3) the equilibrium constant of CaF2 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.


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## Body

## 1. 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].In recent years, fluorine hydrogeochemical study indicates that dissolved fluorine in groundwater mainly originates from fluorine-bearing minerals [3–8]. Climate, topography, hydrogeological conditions, and hydrochemical environment take the major control of the migration and enrichment of fluorine [9–15]. At the same mineralization level, the higher the hardness of water is, the higher the Ca2+ concentration is and the lower the F− content is in arid areas [10]. In semiarid areas of China (e.g., North China Plain), F− concentration in groundwater is positively correlated with Na+ concentration and pH, while negatively correlating with Ca2+ contents [16–22]. Meanwhile, Ca2+ is the main hydrochemical factor that controls the formation of high-fluoride water in deep groundwater [11]. In shallow groundwater in the North China Plain, Ca2+ concentration exerts the greatest influence on the F− content, which is significantly positively correlated with pH [23]. Fluorine is more liable to be enriched in the weakly alkaline groundwater environment (pH = 7–9) [6, 20–29]. There is a remarkable positive correlation between Na+ and F−, and the enrichment of Ca2+ and Mg2+ will inhibit the enrichment of F− [30]. With respect to groundwater chemistry, the fluoride concentration is usually high in Na-HCO3 type groundwater and low in Ca-HCO3 type groundwater [6, 20, 31–35]. The deficiency of calcium ion concentration in the groundwater from calcite precipitation favors fluorite dissolution leading to excess fluoride concentration. The groundwater is oversaturated with respect to calcite and undersaturated with respect to fluorite [36–41]. 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, 42–44].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. The 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. The groundwater is widely distributed and the flow rate is fast; the F− in groundwater will migrate to any areas where the groundwater flows. Therefore, fluorine in groundwater has great potential harm in humid areas. The 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. The 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.


## 2. Geology and Hydrogeology

### 2.1. Geographical Setting

The 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 km2. It features a humid subtropical monsoon climate, with an average annual temperature of 18.8°C and an average annual rainfall of 1560 mm. The 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. The terrain is steep, V-shaped valleys are commonly developed, and the elevation of the highest peak is 1176 m in the area.


### 2.2. Geology

The area is mainly comprised of strata of Sinian, Cambrian, Devonian, Carboniferous, and Quaternary with an outcrop area of 402 km2, accounting for 87.3% of the total survey area. The 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 km2. The vein rocks are mainly composed of quartz veins, granite veins, lamprophyre veins, and diabase veins (Figure 1).Figure 1
Geological map of Northwest Xingguo County. 1: quaternary; 2: early carboniferous; 3: late Devonian; 4: early and middle-late Cambrian; 5: middle Sinian; 6: granite in late Jurassic; 7: fault; 8: stratigraphic boundary; 9: river.


### 2.3. Hydrogeology

#### 2.3.1. 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. The distribution areas of these three types are 20.78 km2, 86.56 km2, and 352.66 km2, accounting for 4.52%, 18.82%, and 76.66% of the total area, respectively.The 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. The 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 isHCO3− Ca.The 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 m3/d. Hydrochemical types are mainly HCO3− Ca or HCO3− 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 m3/d and the hydrochemical type is HCO3− Na. However, regarding the fissured water-bearing formations of metamorphic rocks of Cambrian and Sinian, single well water yields are 0.92–60.93 m3/d and 2.65–3.82 m3/d and share a common hydrochemical type of HCO3− Ca.


#### 2.3.2. Groundwater System

The area is located in the upper reaches of the Ganjiang River, the third-scale (III) groundwater subsystem in the Yangtze River basin. There are fourth-scale groundwater subsystems in the area, precisely including Pingjiang River (III2-1), Liangkou River (III3-1), Yuntingshui River (III4-1), and Wushu River (III5-1). 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).Table 1
Groundwater system division in Northwest Xingguo County.
Name and code of third-scale subsystemName and code of fourth-scale subsystemName and code of fifth-scale subsystemArea (km2)Upper reaches of Ganjiang III1Gongshui III2Pingjiang River III2-1Suishui River III2-1-2213Gan III3Liangkou River III3-1Jianshui River III3-1-1152Gan III4Yuntingshui River III4-1Shuicha River III4-1-133Gan III5Wushu River III5-1Wushu River III5-1-162


#### 2.3.3. 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. The 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. The groundwater discharges into rivers, springs, and wells.


## 2.1. Geographical Setting

The 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 km2. It features a humid subtropical monsoon climate, with an average annual temperature of 18.8°C and an average annual rainfall of 1560 mm. The 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. The terrain is steep, V-shaped valleys are commonly developed, and the elevation of the highest peak is 1176 m in the area.


## 2.2. Geology

The area is mainly comprised of strata of Sinian, Cambrian, Devonian, Carboniferous, and Quaternary with an outcrop area of 402 km2, accounting for 87.3% of the total survey area. The 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 km2. The vein rocks are mainly composed of quartz veins, granite veins, lamprophyre veins, and diabase veins (Figure 1).Figure 1
Geological map of Northwest Xingguo County. 1: quaternary; 2: early carboniferous; 3: late Devonian; 4: early and middle-late Cambrian; 5: middle Sinian; 6: granite in late Jurassic; 7: fault; 8: stratigraphic boundary; 9: river.


## 2.3. Hydrogeology

### 2.3.1. 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. The distribution areas of these three types are 20.78 km2, 86.56 km2, and 352.66 km2, accounting for 4.52%, 18.82%, and 76.66% of the total area, respectively.The 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. The 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 isHCO3− Ca.The 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 m3/d. Hydrochemical types are mainly HCO3− Ca or HCO3− 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 m3/d and the hydrochemical type is HCO3− Na. However, regarding the fissured water-bearing formations of metamorphic rocks of Cambrian and Sinian, single well water yields are 0.92–60.93 m3/d and 2.65–3.82 m3/d and share a common hydrochemical type of HCO3− Ca.


### 2.3.2. Groundwater System

The area is located in the upper reaches of the Ganjiang River, the third-scale (III) groundwater subsystem in the Yangtze River basin. There are fourth-scale groundwater subsystems in the area, precisely including Pingjiang River (III2-1), Liangkou River (III3-1), Yuntingshui River (III4-1), and Wushu River (III5-1). 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).Table 1
Groundwater system division in Northwest Xingguo County.
Name and code of third-scale subsystemName and code of fourth-scale subsystemName and code of fifth-scale subsystemArea (km2)Upper reaches of Ganjiang III1Gongshui III2Pingjiang River III2-1Suishui River III2-1-2213Gan III3Liangkou River III3-1Jianshui River III3-1-1152Gan III4Yuntingshui River III4-1Shuicha River III4-1-133Gan III5Wushu River III5-1Wushu River III5-1-162


### 2.3.3. 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. The 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. The groundwater discharges into rivers, springs, and wells.


## 2.3.1. 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. The distribution areas of these three types are 20.78 km2, 86.56 km2, and 352.66 km2, accounting for 4.52%, 18.82%, and 76.66% of the total area, respectively.The 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. The 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 isHCO3− Ca.The 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 m3/d. Hydrochemical types are mainly HCO3− Ca or HCO3− 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 m3/d and the hydrochemical type is HCO3− Na. However, regarding the fissured water-bearing formations of metamorphic rocks of Cambrian and Sinian, single well water yields are 0.92–60.93 m3/d and 2.65–3.82 m3/d and share a common hydrochemical type of HCO3− Ca.


## 2.3.2. Groundwater System

The area is located in the upper reaches of the Ganjiang River, the third-scale (III) groundwater subsystem in the Yangtze River basin. There are fourth-scale groundwater subsystems in the area, precisely including Pingjiang River (III2-1), Liangkou River (III3-1), Yuntingshui River (III4-1), and Wushu River (III5-1). 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).Table 1
Groundwater system division in Northwest Xingguo County.
Name and code of third-scale subsystemName and code of fourth-scale subsystemName and code of fifth-scale subsystemArea (km2)Upper reaches of Ganjiang III1Gongshui III2Pingjiang River III2-1Suishui River III2-1-2213Gan III3Liangkou River III3-1Jianshui River III3-1-1152Gan III4Yuntingshui River III4-1Shuicha River III4-1-133Gan III5Wushu River III5-1Wushu River III5-1-162


## 2.3.3. 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. The 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. The groundwater discharges into rivers, springs, and wells.


## 3. Methods and Materials

A total of 130 groundwater samples were collected from the outcrops of the four groundwater subsystems: Suishui River III2-1-2, Jianshui River III3-1-1, Shuicha River III4-1-1, and Wushu River III5-1-1, from June 23 to August 23, 2017 (Figure 2). Major hydrochemical parameters (e.g., F−, Ca2+, HCO3−, 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 Ca2+ and HCO3− testing, and acidimeter and test pen were used for pH and TDS testing, respectively. The results are shown in Table 2.Figure 2
Sampling locations of groundwater in Northwest Xingguo County. 1: 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.Table 2
Summary of Ca2+, HCO3−, pH, TDS, and F− in groundwater of different groundwater subsystems.
Groundwater subsystemNumber of samplesCa2+ (mg/L)HCO3− (mg/L)pHTDS (mg/L)F− (mg/L)RangeAverageRangeAverageRangeAverageRangeAverageRangeAverageSuishui River III2-1-2791–329.23.05–73.2224.65.19–8.486.35–12541.70.02–1.320.19Jianshui River III3-1-1363–4615.46.10–67.1226.15.60–7.326.515–15559.50.02–0.510.15Shuicha River III4-1-141–52.56.10–12.208.45.37–6.836.28–1914.30.02–0.090.06Wushu River III5-1-1112–5415.26.10–140.3534.45.85–7.426.716–19464.70.01–0.280.08


## 4. Results and Discussion

### 4.1. Hydrogeochemistry of Groundwater

The chemical composition of 15 representative groundwater outcrops (springs, wells) was tested in order to further determine the hydrogeochemical characteristics of groundwater in the study area. The groundwater chemical components were tested in various groundwater systems in this survey, and the test results show that the main ion components in groundwater under natural conditions are Ca2+, Na+, and HCO3−. The chemical type of groundwater is mainly HCO3− Ca·Na, and the second is HCO3− Ca·Mg (Table 3). According 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).Table 3
Chemical composition of various groundwater systems in the study area.
Groundwater systemSample numberWater chemical composition (mg/L)Chemical type of groundwaterK+Na+Ca2+Mg2+HCO3−CO32−SO42−Cl−NO3−TDSSuishui River (III2-1-2)SJ0513.108.847.130.3043.010.002.233.5420.5588.7HCO3− Na·CaSJ0141.214.495.010.1137.630.002.060.570.1251.2HCO3− Ca·NaGJ0021.096.9910.340.3853.760.003.981.740.1778.5HCO3− Ca·NaSJ0700.562.484.061.3037.630.002.600.610.9050.1HCO3− Ca·NaGJ0030.782.4815.422.6169.890.003.691.120.1096.1HCO3− CaSJ1040.772.036.582.0837.630.002.501.030.1152.7HCO3− Ca·Mg·NaSJ0581.271.375.030.4526.880.001.803.477.4547.7HCO3− Ca·NaSJ0366.433.7721.013.5483.332.646.621.801.52130.7HCO3− Ca·NaSJ1541.693.547.352.6648.385.293.771.110.1173.9HCO3− Ca·Na·MgJianshui River (III3-1-1)GJ0010.893.928.262.9359.130.005.430.611.1282.3HCO3− Ca·MgSJ1137.644.5918.454.9686.010.008.238.197.91146.0HCO3− Ca·NaSJ1841.3811.5218.324.0794.082.647.241.350.10140.7HCO3− Ca·NaSQ0490.562.405.042.0143.010.001.610.550.1055.3HCO3− Ca·MgSJ02910.805.1723.863.26102.140.008.654.7113.68172.3HCO3− Ca·NaWushu River (III5-1-1)SQ0520.910.7712.972.1156.452.647.880.911.3286HCO3− CaNote: the data in the table is tested by the Nanchang Supervision Center of Mineral Resources, Ministry of Land and Resources, 2017.Figure 3
The water chemical pipe triple-variation figure of Northwest Xingguo County.
(a)(b)(c)


### 4.2. Factors Affecting the Distribution of Fluoride in Groundwater

#### 4.2.1. pH Condition

The fluoride contents are in positive correlation with pH [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]. The pH in groundwater is generally 6.3–6.7. The F− concentration is positively correlated with pH in the groundwater of the three subsystems (Suishui River III2-1-2, Jianshui River III3-1-1, and Wushu River III5-1-1) (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.Figure 4
Relationship between F− and pH in the groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)


#### 4.2.2. The Impact of TDS

High fluoride groundwater generally has high TDS and highHCO3− in the weak alkaline environment [29]. The 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. The reason is that ions in groundwater cannot be saturated owing to the fast water cycle rate in bedrock mountainous areas.Figure 5
Relationship between F− and TDS in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)


#### 4.2.3. F− and Ca2+ Balance

The relationship between F− and Ca2+ in the groundwater is shown in Figure 6. It is indicated that F− concentrations are positively correlated with Ca2+ concentrations in the groundwater subsystems of Suishui River III2-1-2, Jianshui River III3-1-1, and Wushu River III5-1-1. The groundwater features the temperature of 16–25°C, low TDS (14.3–64.7 mg/L), and low Ca2+ concentration (2.5–15.4 mg/L). Therefore, the Ca2+ and F− concentrations can be considered as their activity values.Figure 6
Relationship between F− and Ca2+ in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)The equilibrium constant of CaF2 (K = [Ca2+] [F−]2) in the samples was calculated and compared to its solubility product constant (Ksp) (Table 4). The result indicates that, under natural conditions, the solubility product constants are higher than the equilibrium constants, whereas the saturation indexes (SI) of CaF2 are negative. This further confirms that the fluorine-bearing minerals in rocks are in a dissolving state and the increase of Ca2+ will not lead to the precipitation of CaF2. However, the F− concentration tends to be negatively correlated with Ca2+ concentration in arid areas [2, 12, 14]. This is because Ca2+ and F− are saturated in high-fluorine groundwater in arid areas and, therefore, the increase of Ca2+ concentration will lead to CaF2 precipitation and the decrease of F− concentration accordingly.Table 4
Equilibrium constant (K), solubility product constant (Ksp), and SI of CaF2 in groundwater.
Sample no.F− (mg/L)Ca2+ (mg/L)KKSP (t = 18°C)KSP (t = 25°C)SI (t = 18°C)SJ1570.0921.122 × 10−153.40 × 10−113.95 × 10−11−4.482SQ0660.0251.385 × 10−16−5.390SJ1480.62246.389 × 10−13−1.726SJ1560.03543.366 × 10−15−4.004SJ0970.76208.000 × 10−13−1.628SJ1380.2601.662 × 10−13−2.311SJ1031.18898.582 × 10−12−0.598SJ1840.321309.219 × 10−13−1.567


#### 4.2.4. The Dissolution of Minerals

As mentioned above, high F− groundwater mostly locates in Na-HCO3 type. F− concentrations in the groundwater are also positively correlated with HCO3− concentrations (Figure 7), indicating HCO3− concentration is another important factor affecting F− concentration in the study area. The anions in the groundwater are mainly HCO3− (8.4–34.4 mg/L), and CaF2 is in a dissolved state as mentioned above. The dissolution-precipitation equilibrium relationship between CaF2 and CaCO3 is expressed as follows:(1)CaF2=Ca2++2F−KCaF2=1019.8CaCO3+H+=Ca2++HCO3−KCaCO3=101.98Figure 7
Relationship between F− and HCO3− in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)The number of F− and HCO3− increases with the dissolution of CaCO3 and CaF2 in the groundwater subsystems. In other words, the dissolution will lead to a positive correlation between F− and HCO3−. This is another hydrochemical characteristic of F− in bedrock mountainous areas in a humid climate.


#### 4.2.5. The Dissolution of Biotite

Fluorine concentration in granite ranges from 0.044% to 0.216%. The biotite [K(Mg, Fe2+)3(Al, Fe3+)Si3O10(OH, F)2], which accounts for 15%–30% in metamorphic rocks, constitutes the main fluorine-bearing 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. This probably leads to low F− concentration in the groundwater. Thirdly, 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.


#### 4.2.6. Gibbs Diagram

Gibbs diagram built a simple and effective diagram that can be used to compare TDS∼(Na+/(Na+ + Ca2+)) or TDS∼(Cl−/(Cl− + HCO3)); 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 Figure8. The 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 + HCO3) is between 0.056–0.554 and 0.010–0.114, respectively. It shows that the groundwater is characterized by high Na+ and Ca2+ and suggests a geochemical source of granite and metamorphic rocks. Therefore, it can be considered that fluorine in groundwater comes from the dissolution of fluorine-containing minerals in these rocks. The Gibbs analysis of groundwater also indicates that the formation of groundwater chemical 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.Figure 8
Gibbs diagram for groundwater samples.
(a)(b)


## 4.1. Hydrogeochemistry of Groundwater

The chemical composition of 15 representative groundwater outcrops (springs, wells) was tested in order to further determine the hydrogeochemical characteristics of groundwater in the study area. The groundwater chemical components were tested in various groundwater systems in this survey, and the test results show that the main ion components in groundwater under natural conditions are Ca2+, Na+, and HCO3−. The chemical type of groundwater is mainly HCO3− Ca·Na, and the second is HCO3− Ca·Mg (Table 3). According 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).Table 3
Chemical composition of various groundwater systems in the study area.
Groundwater systemSample numberWater chemical composition (mg/L)Chemical type of groundwaterK+Na+Ca2+Mg2+HCO3−CO32−SO42−Cl−NO3−TDSSuishui River (III2-1-2)SJ0513.108.847.130.3043.010.002.233.5420.5588.7HCO3− Na·CaSJ0141.214.495.010.1137.630.002.060.570.1251.2HCO3− Ca·NaGJ0021.096.9910.340.3853.760.003.981.740.1778.5HCO3− Ca·NaSJ0700.562.484.061.3037.630.002.600.610.9050.1HCO3− Ca·NaGJ0030.782.4815.422.6169.890.003.691.120.1096.1HCO3− CaSJ1040.772.036.582.0837.630.002.501.030.1152.7HCO3− Ca·Mg·NaSJ0581.271.375.030.4526.880.001.803.477.4547.7HCO3− Ca·NaSJ0366.433.7721.013.5483.332.646.621.801.52130.7HCO3− Ca·NaSJ1541.693.547.352.6648.385.293.771.110.1173.9HCO3− Ca·Na·MgJianshui River (III3-1-1)GJ0010.893.928.262.9359.130.005.430.611.1282.3HCO3− Ca·MgSJ1137.644.5918.454.9686.010.008.238.197.91146.0HCO3− Ca·NaSJ1841.3811.5218.324.0794.082.647.241.350.10140.7HCO3− Ca·NaSQ0490.562.405.042.0143.010.001.610.550.1055.3HCO3− Ca·MgSJ02910.805.1723.863.26102.140.008.654.7113.68172.3HCO3− Ca·NaWushu River (III5-1-1)SQ0520.910.7712.972.1156.452.647.880.911.3286HCO3− CaNote: the data in the table is tested by the Nanchang Supervision Center of Mineral Resources, Ministry of Land and Resources, 2017.Figure 3
The water chemical pipe triple-variation figure of Northwest Xingguo County.
(a)(b)(c)


## 4.2. Factors Affecting the Distribution of Fluoride in Groundwater

### 4.2.1. pH Condition

The fluoride contents are in positive correlation with pH [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]. The pH in groundwater is generally 6.3–6.7. The F− concentration is positively correlated with pH in the groundwater of the three subsystems (Suishui River III2-1-2, Jianshui River III3-1-1, and Wushu River III5-1-1) (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.Figure 4
Relationship between F− and pH in the groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)


### 4.2.2. The Impact of TDS

High fluoride groundwater generally has high TDS and highHCO3− in the weak alkaline environment [29]. The 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. The reason is that ions in groundwater cannot be saturated owing to the fast water cycle rate in bedrock mountainous areas.Figure 5
Relationship between F− and TDS in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)


### 4.2.3. F− and Ca2+ Balance

The relationship between F− and Ca2+ in the groundwater is shown in Figure 6. It is indicated that F− concentrations are positively correlated with Ca2+ concentrations in the groundwater subsystems of Suishui River III2-1-2, Jianshui River III3-1-1, and Wushu River III5-1-1. The groundwater features the temperature of 16–25°C, low TDS (14.3–64.7 mg/L), and low Ca2+ concentration (2.5–15.4 mg/L). Therefore, the Ca2+ and F− concentrations can be considered as their activity values.Figure 6
Relationship between F− and Ca2+ in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)The equilibrium constant of CaF2 (K = [Ca2+] [F−]2) in the samples was calculated and compared to its solubility product constant (Ksp) (Table 4). The result indicates that, under natural conditions, the solubility product constants are higher than the equilibrium constants, whereas the saturation indexes (SI) of CaF2 are negative. This further confirms that the fluorine-bearing minerals in rocks are in a dissolving state and the increase of Ca2+ will not lead to the precipitation of CaF2. However, the F− concentration tends to be negatively correlated with Ca2+ concentration in arid areas [2, 12, 14]. This is because Ca2+ and F− are saturated in high-fluorine groundwater in arid areas and, therefore, the increase of Ca2+ concentration will lead to CaF2 precipitation and the decrease of F− concentration accordingly.Table 4
Equilibrium constant (K), solubility product constant (Ksp), and SI of CaF2 in groundwater.
Sample no.F− (mg/L)Ca2+ (mg/L)KKSP (t = 18°C)KSP (t = 25°C)SI (t = 18°C)SJ1570.0921.122 × 10−153.40 × 10−113.95 × 10−11−4.482SQ0660.0251.385 × 10−16−5.390SJ1480.62246.389 × 10−13−1.726SJ1560.03543.366 × 10−15−4.004SJ0970.76208.000 × 10−13−1.628SJ1380.2601.662 × 10−13−2.311SJ1031.18898.582 × 10−12−0.598SJ1840.321309.219 × 10−13−1.567


### 4.2.4. The Dissolution of Minerals

As mentioned above, high F− groundwater mostly locates in Na-HCO3 type. F− concentrations in the groundwater are also positively correlated with HCO3− concentrations (Figure 7), indicating HCO3− concentration is another important factor affecting F− concentration in the study area. The anions in the groundwater are mainly HCO3− (8.4–34.4 mg/L), and CaF2 is in a dissolved state as mentioned above. The dissolution-precipitation equilibrium relationship between CaF2 and CaCO3 is expressed as follows:(1)CaF2=Ca2++2F−KCaF2=1019.8CaCO3+H+=Ca2++HCO3−KCaCO3=101.98Figure 7
Relationship between F− and HCO3− in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)The number of F− and HCO3− increases with the dissolution of CaCO3 and CaF2 in the groundwater subsystems. In other words, the dissolution will lead to a positive correlation between F− and HCO3−. This is another hydrochemical characteristic of F− in bedrock mountainous areas in a humid climate.


### 4.2.5. The Dissolution of Biotite

Fluorine concentration in granite ranges from 0.044% to 0.216%. The biotite [K(Mg, Fe2+)3(Al, Fe3+)Si3O10(OH, F)2], which accounts for 15%–30% in metamorphic rocks, constitutes the main fluorine-bearing 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. This probably leads to low F− concentration in the groundwater. Thirdly, 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.


### 4.2.6. Gibbs Diagram

Gibbs diagram built a simple and effective diagram that can be used to compare TDS∼(Na+/(Na+ + Ca2+)) or TDS∼(Cl−/(Cl− + HCO3)); 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 Figure8. The 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 + HCO3) is between 0.056–0.554 and 0.010–0.114, respectively. It shows that the groundwater is characterized by high Na+ and Ca2+ and suggests a geochemical source of granite and metamorphic rocks. Therefore, it can be considered that fluorine in groundwater comes from the dissolution of fluorine-containing minerals in these rocks. The Gibbs analysis of groundwater also indicates that the formation of groundwater chemical 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.Figure 8
Gibbs diagram for groundwater samples.
(a)(b)


## 4.2.1. pH Condition

The fluoride contents are in positive correlation with pH [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]. The pH in groundwater is generally 6.3–6.7. The F− concentration is positively correlated with pH in the groundwater of the three subsystems (Suishui River III2-1-2, Jianshui River III3-1-1, and Wushu River III5-1-1) (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.Figure 4
Relationship between F− and pH in the groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)


## 4.2.2. The Impact of TDS

High fluoride groundwater generally has high TDS and highHCO3− in the weak alkaline environment [29]. The 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. The reason is that ions in groundwater cannot be saturated owing to the fast water cycle rate in bedrock mountainous areas.Figure 5
Relationship between F− and TDS in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)


## 4.2.3. F− and Ca2+ Balance

The relationship between F− and Ca2+ in the groundwater is shown in Figure 6. It is indicated that F− concentrations are positively correlated with Ca2+ concentrations in the groundwater subsystems of Suishui River III2-1-2, Jianshui River III3-1-1, and Wushu River III5-1-1. The groundwater features the temperature of 16–25°C, low TDS (14.3–64.7 mg/L), and low Ca2+ concentration (2.5–15.4 mg/L). Therefore, the Ca2+ and F− concentrations can be considered as their activity values.Figure 6
Relationship between F− and Ca2+ in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)The equilibrium constant of CaF2 (K = [Ca2+] [F−]2) in the samples was calculated and compared to its solubility product constant (Ksp) (Table 4). The result indicates that, under natural conditions, the solubility product constants are higher than the equilibrium constants, whereas the saturation indexes (SI) of CaF2 are negative. This further confirms that the fluorine-bearing minerals in rocks are in a dissolving state and the increase of Ca2+ will not lead to the precipitation of CaF2. However, the F− concentration tends to be negatively correlated with Ca2+ concentration in arid areas [2, 12, 14]. This is because Ca2+ and F− are saturated in high-fluorine groundwater in arid areas and, therefore, the increase of Ca2+ concentration will lead to CaF2 precipitation and the decrease of F− concentration accordingly.Table 4
Equilibrium constant (K), solubility product constant (Ksp), and SI of CaF2 in groundwater.
Sample no.F− (mg/L)Ca2+ (mg/L)KKSP (t = 18°C)KSP (t = 25°C)SI (t = 18°C)SJ1570.0921.122 × 10−153.40 × 10−113.95 × 10−11−4.482SQ0660.0251.385 × 10−16−5.390SJ1480.62246.389 × 10−13−1.726SJ1560.03543.366 × 10−15−4.004SJ0970.76208.000 × 10−13−1.628SJ1380.2601.662 × 10−13−2.311SJ1031.18898.582 × 10−12−0.598SJ1840.321309.219 × 10−13−1.567


## 4.2.4. The Dissolution of Minerals

As mentioned above, high F− groundwater mostly locates in Na-HCO3 type. F− concentrations in the groundwater are also positively correlated with HCO3− concentrations (Figure 7), indicating HCO3− concentration is another important factor affecting F− concentration in the study area. The anions in the groundwater are mainly HCO3− (8.4–34.4 mg/L), and CaF2 is in a dissolved state as mentioned above. The dissolution-precipitation equilibrium relationship between CaF2 and CaCO3 is expressed as follows:(1)CaF2=Ca2++2F−KCaF2=1019.8CaCO3+H+=Ca2++HCO3−KCaCO3=101.98Figure 7
Relationship between F− and HCO3− in groundwater of different subsystems. (a) Groundwater subsystem of Suishui River. (b) Groundwater subsystem of Jianshui River. (c) Groundwater subsystem of Wushu River.
(a)(b)(c)The number of F− and HCO3− increases with the dissolution of CaCO3 and CaF2 in the groundwater subsystems. In other words, the dissolution will lead to a positive correlation between F− and HCO3−. This is another hydrochemical characteristic of F− in bedrock mountainous areas in a humid climate.


## 4.2.5. The Dissolution of Biotite

Fluorine concentration in granite ranges from 0.044% to 0.216%. The biotite [K(Mg, Fe2+)3(Al, Fe3+)Si3O10(OH, F)2], which accounts for 15%–30% in metamorphic rocks, constitutes the main fluorine-bearing 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. This probably leads to low F− concentration in the groundwater. Thirdly, 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.


## 4.2.6. Gibbs Diagram

Gibbs diagram built a simple and effective diagram that can be used to compare TDS∼(Na+/(Na+ + Ca2+)) or TDS∼(Cl−/(Cl− + HCO3)); 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 Figure8. The 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 + HCO3) is between 0.056–0.554 and 0.010–0.114, respectively. It shows that the groundwater is characterized by high Na+ and Ca2+ and suggests a geochemical source of granite and metamorphic rocks. Therefore, it can be considered that fluorine in groundwater comes from the dissolution of fluorine-containing minerals in these rocks. The Gibbs analysis of groundwater also indicates that the formation of groundwater chemical 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.Figure 8
Gibbs diagram for groundwater samples.
(a)(b)


## 5. 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. The F− concentration is positively correlated with the contents of TDS, Ca2+, and HCO3− 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 CaF2) 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. This study has important theoretical and practical significance for understanding the hydrogeochemical characteristics of fluorine in humid areas.



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*Source: 5567353-2021-07-15.xml*