Environ Earth Sci DOI 10.1007/s12665-015-4225-x

ORIGINAL ARTICLE

Occurrence and formation of high fluoride groundwater in the Hengshui area of the North China Plain Haiyan Liu • Huaming Guo • Lijin Yang Lihan Wu • Fulan Li • Shanyang Li • Ping Ni • Xing Liang

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Received: 1 October 2014 / Accepted: 20 February 2015 Ó Springer-Verlag Berlin Heidelberg 2015

Abstract Although high F- groundwater was observed in the Hengshui of the North China Plain, the depth dependence of groundwater F- and its relation to aquifer sediments remains unknown. Both shallow groundwater and deep groundwater were collected for chemical and isotopic characterization. Sediments were taken from different depths down to 130 m for total element analysis and Fform determination with sequential extraction procedure. Results show that F- concentration ranges from 0.37 to 3.28 mg/L, and from 0.28 to 3.6 mg/L in shallow and deep groundwater, respectively. High F- water from shallow aquifers are of Cl–SO4–Na–Mg, HCO3–Cl–SO4–Na and SO4–Cl–Na–Mg types, while HCO3–SO4–Cl–Na and Cl– SO4–Na types are from deep aquifers. High F- waters are mainly found at the depths of 50, 200 and 300 m. Along the flow path, groundwater shows an increasing trend in Fconcentration from the northwest to the southeast. Isotopes of 18O and D in deep groundwater are more depleted in

L. Yang Beijing Institute of Geological and Prospecting Engineering (BIGPE), Beijing 100048, People’s Republic of China

comparison with shallow groundwater, suggesting that shallow and deep groundwater replenish through different ways with longer retention time of deep groundwater. d13C of dissolved inorganic carbon are between -11.9 and -8.8 % in shallow groundwater and between -10.6 and -7.5 % in deep groundwater, respectively, which indicates that groundwater DIC comes from both rock weathering and biodegradation of organic matter. Fluoride content in the sediment ranges from 140 to 1690 mg/kg, showing a decreasing trend with depth. Good correlations between F- and Al2O3 and Fe2O3 are observed, demonstrating that F- contents in sediments are significantly influenced by the minerals containing Fe2O3 and Al2O3 in terms of adsorption. Sequential extraction procedure shows that exchangeable F- form (F1) and Fe–Mn oxides-bound F- (F3) generally decrease with depths, while organic matter or sulfide-bound F- (F4) keeps relatively stable and carbonate-bound F- (F2) exhibits highly variable. The relationships between F- concentration and Fe or Mn imply that F- in F1 is more inclined to be scavenged by Fe and Mn oxides/hydroxides which have a stronger affinity for F-. Although a variety of hydrogeochemical processes affected F- concentrations, dissolution–precipitation is a vital process in the study area. In comparison with shallow groundwater, cation exchange may exhibit more significantly effect on F- enrichment in deep groundwater. Competitive adsorption of HCO3- and OH- with F- leads to the release of F- from aquifer matrix into solution, which increases groundwater F- concentration. Evaporation is another control on F- concentration, especially in shallow groundwater.

X. Liang School of Environmental Studies, China University of Geosciences (Wuhan), Wuhan 430074, People’s Republic of China

Keywords Fluoride
Groundwater
Sequential extraction procedure
Hydrochemical characteristics
Isotopes

H. Liu
H. Guo
L. Wu
F. Li
S. Li
P. Ni State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, People’s Republic of China H. Liu
H. Guo (&)
L. Wu
F. Li
S. Li
P. Ni School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, People’s Republic of China e-mail: [email protected]

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Introduction High F- groundwater has received much concern from both scientists and administrative institutions because of its considerable impact on human health (Ayenew 2008; Su et al. 2013; Abdelgawad et al. 2009; Li et al. 2014a, b; Machender et al. 2014). It is well known that long-term intake of high F- groundwater is the main cause of fluorosis (Guo et al. 2007, 2012). In the world, 25 nations, including India, Mexico, Korea, Japan, Pakistan and China, are being at risk of fluorosis, including dental and skeleton diseases (Carrillo-Rivera et al. 2002; Jacks et al. 2005; Anil and Mishra 2007; Naseem et al. 2010; Rao et al. 2013; Chidambaram et al. 2013; Avtar et al. 2013; Su et al. 2013; Li et al. 2014a, b; Daessle et al. 2014). The North China Plain (NCP) has been one of the typical regions with semi-arid climate widely hosting high F- groundwater (Xing et al. 2012; He et al. 2013). According to the survey, 8783 villages are affected by fluorosis, with Hengshui and Cangzhou cities being the worst (Dong and Yao 2002). Previous investigations were carried out to characterize hydrochemical property and reveal controls on high F- groundwater in different regions of the NCP, by means of hydrogeochemical plots, statistical analysis and geochemical modeling of groundwater chemical data (Xing et al. 2012). Since F- mainly comes from the dissolution of fluorite (Su et al. 2013), Zeng and Liu (1996) evaluated the dissolution–precipitation conditions of F- containing minerals in soil phases, and suggested that source of fluoride was directly controlled by soil lithology and chemical compositions (Zeng et al. 1997). Hengshui area is located in the center of the NCP, where high F- groundwater is widely distributed. High Fgroundwater poses a significant health risk to more than half of residents living in Fucheng, Jingxian, Gucheng and Zaoqiang county (Yang 2010). It was reported that those suffering from dental and skeleton fluorosis account for 50 and 3.8 % of the survey population, respectively. Groundwater F- concentration is highly variable in Hengshui area, as well as in shallow and deep aquifers. In shallow aquifers, high F- groundwater was zonally or scatteredly distributed from Raoyang Dayin to Anping Houzhangzhuang, while groundwater F- concentration in deep groundwater generally increased from northwest to southeast (Yang 2010; Han 2012). However, the depth dependence of groundwater F- and its relation to aquifer sediments remains unknown. To better understand the formation of high F- groundwater, the objectives of this study are to (1) investigate vertical distribution of groundwater F- and groundwater chemistry, (2) characterize F- forms in the aquifer sediments and their relation to groundwater F-, and (3)

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evaluate geochemical processes controlling F- concentrations in both deep groundwater and shallow groundwater.

Materials and methods Study area The Hengshui city, covering approximately 8815 km2, is one of the largest emerging cities in a relatively flat diluvial and alluvial terrain of the NCP. The city belongs to the continental monsoon region with a semi-arid climate, of which the highest temperature reaches 42.7 °C, and the lowest temperature -23 °C. In the study area, an average annual precipitation is 438.7 mm with 70–80 % in July, August and September. The average annual evaporation is [2000 mm. There are several rivers running from southwest to northeast, including Fuyang River, Fuyangxin River, Fudongpai River and Sunlu River. For many years ([40 years), groundwater has been used as water sources for drinking water supply and intensive agriculture activities in Hengshui area. Accessible groundwater in the Hengshui area mainly occurs in the Quaternary aquifer system. The regional Quaternary aquifers belong to fluvial deposit, which chiefly consist of clay, loam, sandy loam and sand. Vertically, the Quaternary aquifer system could be divided into four groups: the Holocene aquifer, the upper, the middle and the lower Pleistocene aquifers (Zhao 2011). The depth of Holocene aquifer ranged from 4 to 6 m, with sandy loam and fine silt. The upper Pleistocene aquifer with singlelayer thickness of 3–10 m was composed of sandy loam in the upper layer and loam in lower layer. The upper Pleistocene aquifer was the main source of water supply for agricultural irrigation because of the sounded penetrability and large single well capacity. The middle Pleistocene aquifer, which was the third aquifer group, had a buried depth of 60–70 m with total dissolved solid concentration of 0.5–0.7 g/L. The aquifer was mainly loam, fine sand and medium sand. The lower Pleistocene aquifer, with a thickness of 40–50 m, mainly consisted of red–brown clayey silt, loam and coarse, fine sand. Groundwater with (HCO3, Cl)–Na type occurring in the fourth group was an essential water source for potable water supply. According to groundwater exploitation and aquifer distribution, groundwater that occurs in the Holocene aquifer and the upper Pleistocene aquifer belongs to shallow groundwater, while deep groundwater in the other two aquifers (Zhang et al. 2009). Groundwater generally flows from the northwest to the southeast. Meteoric water and river water recharge shallow groundwater, which is discharged through artificial ways and evaporation. In deep aquifer, groundwater is mainly

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recharged by the lateral run-off and shallow groundwater, while artificial mining is the major discharge pathway (more than 90 % of total discharge). Sample collection Sixty-three groundwater samples were collected from electric-powered public water supply wells in August 2013 (Fig. 1), including thirteen shallow groundwater samples, and fifty deep groundwater samples. Depths of the investigated wells range between 20 and 527 m. All groundwaters were sampled after pumping for more than 15 min until water temperature, pH, electrical conductivity (Ec), oxidation–reduction potential (ORP), total dissolved solids (TDS) were stabilized. All samples were filtered through 0.45 lm membrane filters. Samples for testing major cations and trace elements were collected in polyethylene bottles and acidified to pH \2 by adding the solution of 6 M HNO3. Samples for analysis of major anions were collected without acidification. Samples for total organic carbon (TOC) analysis were stored in 30 mL amber glass bottles, and acidified by 1:9 (volume) H2SO4. Groundwaters for analysis of hydrogen and oxygen isotopes were stored in polyethylene bottles without headspace without filtration. All groundwater

samples were stored at 4 °C and brought to laboratory within 3 days. One representative borehole was drilled to take sediment samples from different depths down to 130 m below land surface (BLS) (Fig. 1). Fifty sediment samples were taken during drilling. Immediately after being taken from the boreholes, sediment samples were wrapped in foil and sealed by N2-filled homogeneous bag, and then transported to the laboratory at 4 °C. In the laboratory, they were stored at -20 °C in the refrigerator until analysis. Lithologic composition of the sediments includes clay, silty clay, clayey silt, sandy silt and coarse sand. Analytical methods Physiochemical parameters, including temperature (T), pH, Ec, ORP and TDS, were on-site monitored in an in-line flow cell under minimal atmospheric contact using a multiparameter portable meter (HANNA, HI 9828), which was calibrated using standard solution before use. Concentrations of NH4–N, S2-, and Fe (II) of groundwaters were measured in the field using a portable spectrophotometer (HACH, DR2800). Alkalinity was measured using a Model 16900 digital titrator (HACH) using bromocresol greenmethyl red indicator.

Fig. 1 Location of sampling sites

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Major cations and trace elements were measured by ICP-AES (iCAP6300, Thermo) and ICP-MS (7500C, Agilent), respectively. The analysis precision of ICP-AES and ICP-MS was 0.5 %. Concentrations of Cl-, NO3-, and SO42- were determined using an ion chromatography system (ICS2000, Dionex), with the analysis precision less than 3.0 %. Fluoride concentrations were qualified employing a fluoride ion selective electrode (CHN090, Thermo Fisher Scientific, USA). To eliminate the interference of coexisting anions, 20 mL solution was mixed with 10 mL total ionic strength adjustment buffer (TISAB) solution (142 g of (CH2)6N4, 85 g of KNO3 and 9.97 g of C6H4Na2O8S2
H2O in 1000 mL) and diluted to 50 mL with deionized water for pretreatment before analysis. According to the lithologic variation, 30 sediment samples were selected to perform sequential extraction experiments for F- forms in sediments [including exchangeable form (F1), carbonate-bound form (F2), ironmanganese oxides-bound form (F3), and organic matter or sulfide-bound form (F4)]. Briefly, exchangeable phase (F1) was extracted with 0.5 M Mg (NO3)2 at room temperature, carbonate-bound phase (F2) with 1 M NaOAc at room temperature, Fe–Mn oxide-bound phase (F3) with 0.08 M NH2OH HCl at 96 °C, and organic matter and sulfidebound phase (F4) with 0.02 M HNO3 and 300 mL/L H2O2 at 85 °C (Li 2011). Prior to sequential extraction experiments, sediment mineral composition was determined by a URD-6 X-ray powder diffractometer (Cu Ka radiation, graphite monochromator, 2h range 2–70°, step 0.01°, counting time 5 s per step). The weight percentage of the major mineral phases was determined from XRD results using program MDI Jade 6.5

Results Hydrochemical characteristics In shallow groundwater, major cations were predominant by Na?, accounting for between 40 and 95 % of total cations. Concentrations of Ca2? and Mg2? generally accounted for less than 40 %, of which the lowest near 5 %. Major anions were generally dominated by Cl- and HCO3-, accounting for between 20 and 80 % of total anions. Concentration of SO42- mostly accounted for more than 30 %. High F- groundwaters (F- [1.0 mg/L) occurred in the right side of the piperline plot. Calculation of meq % of major ions indicated that high F- groundwaters were of Cl–SO4–Na–Mg, HCO3–Cl–SO4–Na and SO4–Cl– Na–Mg types. Deep groundwater had different aqueous chemistry from shallow groundwater (Fig. 2). In deep groundwater, Na?

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was the dominant cation, which made up between 40 and 98 % of total cations. Sum of Ca2? and Mg2? accounted for less than 40 %. No dominant anions were apparent in deep groundwater samples. However, samples with high F- concentration were distributed in the lower right part of the piperline. Meq % calculation indicated that high Fgroundwaters were mostly of HCO3–SO4–Cl–Na and Cl– SO4–Na types. Isotopes Values of d18O in shallow groundwater ranged from -11.0 to -7.5 %, with an average value of -9.3 %, and dD from -83.8 to -66.3 %, with an average of -78 %. In comparison with shallow groundwater, less variations in d18O and dD were observed in deep groundwater, with d18O between -12.1 and -10.1 % (average -10.9 %) and dD between -92 and -75 % (average -82.3 %). Obviously, deep groundwater showed the depletion in 18O and D, demonstrating different origins for shallow and deep groundwater. As depicted in the plot of d18O and dD (Fig. 3), all samples were deviated from the curve of global meteoric water line (GMWL), and closed the local meteoric water line (LMWL). It was suggested that groundwaters were mainly derived from local meteoric water. Actually, values of d18O and dD of shallow groundwater were linearly regressed as dD = 7.1d18O–7.56, and dD = 6.6d18O–9.83 for deep groundwater. The point of intersection between the shallow groundwater regression line and the LMWL (dD = 6.25d18O–7.50) was at dD = -7.06 % and d18O = 0.07 %, while the point where the deep groundwater regression line intersected the LMWL was at dD = -3.4 % and d18O = -28.75 %. It indicated that deep groundwater was recharged from the higher altitude than shallow groundwater (Guo et al. 2014). During the long-term water–rock interaction, significant fractionation in hydrogen and oxygen isotope occurred. In addition, more depletion in 18O and D of deep groundwater indicated that deep groundwater was probably recharged from meteoric water at a cold period of paleoclimate, with much older age than shallow groundwater (Chen et al. 2005; Xing et al. 2012). Carbon isotope of dissolved inorganic carbon (DIC) in groundwater showed that values of d13C ranged between -12.8 and -7.5 % (median -9.4 %). The d13C values of shallow groundwaters ranged between -11.9 and -8.8 % (median -10.4 %), while deep groundwater ranged between -10.6 and -7.5 % (median -9.1 %). More significantly depletion in 13C was observed in deep groundwater than shallow groundwater. Previous studies revealed that d13C value lower than -12 % in groundwater would result from present-day life (Hoefs 2012; Chen et al. 2010). In other words, the values of d13C in the study area

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Fig. 2 Piper diagram of shallow groundwater (a) and deep groundwater (b)

Fig. 3 Plots of dD and d18O in shallow and deep groundwaters

(low to -12.8 %) implied that DIC derived from both rock weathering and biodegradations of organic matter. The relatively lower values of d13C in deep groundwater were likely due to an important role of the microbial degradation in regulating carbon isotope fractionation in groundwater systems. Shallow groundwater recharging directly from external settings led to relatively higher 13C content. Distribution of high F- groundwater Lateral distribution Lateral distribution of high F- groundwater was plotted in Fig. 4. Although groundwater samples were collected from the village of Wangjiajing, through Zhaojiaquan, Hengshui city and the village of Longhua, to the county of Fucheng, high F- groundwaters (F- [1.0 mg/L) were generally found in the southern area. In shallow groundwater, high

F- concentrations mainly occurred around the village of Zhaojiaquan, in the south of Wangjiajing, the north of Jizhou city and the west of Fucheng county. High Fgroundwater from deep aquifers was mainly distributed in the area of Taocheng district, Dengjiadian village and Qingliangdian village, from the eastern county of Fucheng to the southeastern village of Longhua. Overall, an increasing trend in F- concentration was observed from northwest to southeast in groundwater (Fig. 4), which was generally consistent with the groundwater flow direction. It should be noted that groundwater was recharged from northwest, and flowed along Qianmotou, Zhaojiaquan, Hengshui city, Longhua, Houliufu, and finally discharged in the southeastern area. Similarly, Xing et al. (2012) observed that F- concentrations were relatively low in the piedmont zone. Slow flowing and strong evaporation increased F- concentration of shallow groundwater in the central plain. Depth dependence distribution Figure 5 shows that high F- groundwaters mainly occurred at the depths around 30, 200 and 300 m. In shallow high Fgroundwater (depth \100 m), F- concentration increased with depths. Concentrations of F- in deep groundwaters were mostly lower than 1.0 mg/L. At the depth of about 200 m, F- concentrations ranged between 1.1 and 2.8 mg/ L, and 1.05 and 2.6 mg/L at around 300 m. The deep groundwater sample taken at the depth of 345 m had Fconcentration up to 2.05 mg/L. Fluoride in sediments Results of XRD analysis indicated that quartz was the major mineralogical component in aquifer sediments,

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Environ Earth Sci Fig. 4 Lateral distribution of groundwater F- in the study area

Fig. 5 Plot of F- concentration versus well-depth

ranging from 31.2 to 40.3 wt%, with an average of 38.2 wt%. Calcite content showed a relatively wide range, from 10.2 to 29.2 wt%, with an average of 15.5 wt%. Feldspar and fluorite had an average of 13.2 and 7.9 %, respectively, and clay minerals accounted for 23.1 wt% averagely. Contents of Fe–Mn oxide minerals and sulfide minerals were relatively low. The abundance of Al2O3 and Fe2O3 (as shown later) was probably associated with clay minerals other than Fe and Al oxide minerals.

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Fluoride content was highly variable in sediments, which ranged between 14 and 1690 mg/kg (median 780 mg/kg). Sediments mostly had higher F- contents than average China soil fluoride value of 478 mg/kg. Fluoride content in sediments was closely related to their lithologies and buried depths. Generally, higher F- content was observed in clay and silty clay, in comparison with silty-fine sand, fine sand and medium sand. A decrease trend was found for F- content along the sampling depth (Fig. 6a), which was possibly due to the effects by paleo-climate during sedimentary or hydrogeological settings later. Specifically, under the influence of the longterm water–rock interactions and chemical weathering, sediment F- at greater depth was leached out more intensively. Fluoride contents in sediments were positively correlated with contents of Al2O3 and Fe2O3, with correlation coefficients of 0.56 and 0.67, respectively (Fig. 6b, c), showing that contents of Al2O3 and Fe2O3 in sediments had a significant impact on F- content. Minerals containing Al and Fe oxides had a strong affinity for F-. Higher content of these oxides resulted in elevated F- contents, and vice versa. This was in good agreement with the relatively higher F- concentration in fine-grained sediments like clay. In addition, SiO2 contents were negatively correlated

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Fig. 6 Depth dependence of F- contents in sediments (a), F- content versus Al2O3 content (b), F- content versus Fe2O3 content (c)

Fig. 7 Variations of contents of fluoride forms with depths (a F1 exchangeable form, b F2 carbonates-bound form. c F3 Fe–Mn oxides-bound form, d F4 organic matters or sulfides-bound form)

with F- contents, suggesting that sediments with more sandy had lower F- content. In addition to total F- in sediments, four F- forms were also evaluated. Results showed that exchangeable form (F1) ranged between 0.53 and 19.6 mg/kg, with average value of 1.99 mg/kg. Generally, F1 content decreased with depths, though an abnormally high value was observed at the depth of 32.1 m (Fig. 7a). Carbonates-bound form (F2) ranged between 0.64 and 31.5 mg/kg, with average of 9.12 mg/kg, showing a big variation (Fig. 7b). Iron-Mn oxides-bound form (F3) ranged from 13.9 to 76.4 mg/Kg with the average content of 41.51 mg/kg, which generally decreased with depths (Fig. 7c). Organic matter or sulfidebound form (F4) exhibited the lowest at the depth of

around 120 m, and kept relatively constant of 8.0 mg/kg for most sediment samples (Fig. 7d). Although the exchangeable F- (F1) was relatively lower, it was well correlated with exchangeable manganese, whereas no obvious relationships were found with exchangeable iron. It demonstrated that the exchangeable F-, which was believed to be easily mobilized during water–rock interaction, was more inclined to be scavenged by manganese and manganese oxides/hydroxides with a stronger affinity of F-. High F- contents were expected to correspond to high Mn and Fe2O3 contents because of the high affinity of F- to Fe/Mn minerals. However, no significant correlation was found between Fe–Mn oxidesbound form (F3) and iron or manganese. The other two F-

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phases (F2 and F4) did not show good correlations with the specifically matched manganese and iron.

Discussion The formation of high F- groundwater is systematically controlled by dissolution–precipitation, cation exchange, desorption and competitive adsorption and evaporation (Carrillo-Rivera et al. 2002; Guo et al. 2007, 2012; Lei et al. 2007; Machender et al. 2014). In combination with hydrogeological conditions and chemical and isotopic characteristics, mechanisms of F- enrichment in groundwater are discussed. Dissolution and precipitation Fluoride in groundwater generally originates from fluorine minerals (Handa 1975; Nordstrom et al. 1989; Shah and Danishwar 2003; Guo et al. 2012; Avtar et al. 2013), especially fluorite (CaF2). Fluoride concentrations in aquifer sediments in the study area were commonly higher than 478 mg/kg, with the maximum of 1690 mg/kg. XRD results showed that fluorite had an average content of 7.9 wt% in sediments, which was a plausible source of Fin groundwater. Dissolution of fluorite could play an important role in the supply of F- from the recharge area and by dissolution in the sediment derived from northwestern shallow borehole within the Hengshui area. By and large, F- concentration was negatively correlated with Ca2? concentration both in deep groundwater and shallow groundwater, as depicted in Fig. 8a, though Ca2? concentrations were mostly lower than 100 mg/L in deep groundwater, and between 50 and 450 mg/L for shallow groundwater. The negative correlation suggested that the fluorite solubility limited F- content (pKfluorite = 10-6 from Parkhurst and Appelo 1999) in groundwater. When [Ca2?][F-]2 \ Kfluorite, fluorite dissolution led to the Fig. 8 The relationship between concentrations of Fand Ca2? (a), and activities of F- and Ca2? (b). Line 1 shows the path of fluorite dissolution; line 2 shows the path of calcite and fluorite dissolution with a ratio of 200 (calcite: fluorite); line 3 shows that the path of Ca2? decrease associated with calcite precipitation and/or cation exchange

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increase in both F- and Ca2? concentrations, while [Ca2?][F-]2 [ Kfluorite, F- concentration would decrease by increasing concentration of Ca2? via fluorite precipitation, restraining F- enrichment in groundwater. The same situation occurred for Mg2? because of their resemble chemistry. Apart from dissolution of fluorite, calcite dissolution was another source of the Ca2? in groundwater. As shown in Fig. 8b, when fluorite dissolved congruently, the variation of the F- and Ca2? activities was expected to follow line 1. However, the activities of F- and Ca2? of all shallow and deep high F- groundwater samples (F- [1.0 mg/L) were located to right side of line 1, and were under the line of pKfluorite. It demonstrated that Ca2? was added to the groundwater from a source other than fluorite dissolution. Both calcite and feldspar could be plausible contributors to groundwater Ca2? in the system, as it was found the presence of those minerals in the aquifer sediments. When calcite coexisted with fluorite in the aquifer sediments and was dissolved all together with the ratio of 200:1 (calcite:fluorite), the variation of the F- and Ca2? activities was predicted to follow line 2 (Guo et al. 2012; Xing et al. 2012). Following the line 3, the activity of Ca2? might decrease in case of mineral precipitation or cation exchange (Guo et al. 2012). Xing et al. (2012) found that all groundwater samples from NCP were oversaturated with respect to calcite. Precipitation of calcite lowers Ca2? concentration and facilitates fluorite dissolution, thus increasing F- concentration (Li et al. 2014a, b). High Fwater samples (F- [1.0 mg/L) in deep aquifers were mostly located between line 1 and line 3 (Fig. 8b), which indicated that mineral precipitation (or/and) cation exchange exhibited significant impact on the F- enrichment. In comparison with deep groundwater, shallow high Fgroundwaters were mostly plotted between line 2 and line 3 (Fig. 8b), suggesting that relatively stronger dissolution of fluorite played an important role in elevating F- concentration. The other shallow high F- groundwaters fell

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between line 1 and line 3 due to the mineral precipitation (or/and) cation exchange.

exhibit more significantly in deep groundwater than shallow groundwater (Qin and Li 2012).

Cation exchange

Desorption and competitive adsorption

Figure 9 shows the good relationship between F- concentration and the molar ratio of Na? to (Na? ? Ca2?), indicating that enrichment mechanism for high F- concentration was probably related to cation exchange between calcium and sodium ions, which removed Ca2? from groundwater by replacing with Na? on mineral surface (Rango et al. 2009; Su et al. 2013; Avtar et al. 2013; Li et al. 2014a, b). The exchange particularly occurred in the aquifer sediment matrixes consisting of fine particles like clay and silty clay (Qin and Li 2012). Su et al. (2013) also found that ion exchange was an important factor responsible for the elevated F- groundwater at Datong basin. Thus, the removal of Ca2? from solution created a favorable setting for fluorite dissolution, which led to high concentration of F-. The intensive cation exchange led to the molar ratio of Na? to (Na? ? Ca2?) approaching to one and high F- concentration (Fig. 9.). It meant that groundwaters with elevated Na? and low Ca2? concentrations favored the release of F- from the aquifer matrix into groundwater during water–rock interactions, which was consistent with high F- groundwater hydrogeochemical facies shown in the piperline plot. Systematically, F- concentration was positively correlated with the molar ratio of Na? to (Na? ? Ca2?) (Fig. 9). Fluoride concentration substantially increased with the value of mNa?/ m (Na? ? Ca2?) getting close to 1 (Fig. 9). Additionally, the mNa?/m (Na? ? Ca2?) values were generally higher in deep groundwater than shallow groundwater (Fig. 9). The relationship between F- concentration and mNa?/ m (Na? ? Ca2?) demonstrated that cation exchange might

It should be noted that pH value is an important factor influencing F- adsorption on mineral surface in the aquifers (Guo et al. 2012; Su et al. 2013; Patel et al. 2014). According to the lithologic composition of the aquifer sediments in study area, clay minerals accounted for a high percentage in sediments. Fluoride would be strongly adsorbed on clay at relatively lower pH, and be released into groundwater by replacement with hydroxyl, due to the similar ionic radii of F- and OH- (Sreedevi et al. 2006). Our results showed that the exchangeable F- ranged between 0.53 and 19.6 mg/kg with the average value of 1.99 mg/kg in the sediments. In study area, F- concentrations exhibited two trends with pH (Fig. 10a). One was that F- concentration increased dramatically with the increase in solution pH. The other was that F- concentration slightly increased. The majority of high F- groundwaters (F- [1.0 mg/L) had pH values between 7.5 and 8.5 (Fig. 10a), suggesting that adsorption–desorption would be achieved in weaker alkaline conditions (Wang and Cheng 2001). Isotope of 18O and D demonstrated that deep groundwater had longer retention time than shallow groundwater, during which favorable conditions for Fadsorption–desorption could be created. The positive correlation was observed between Fconcentration and HCO3- concentration, particularly in deep groundwater (Fig. 10b). It indicated that competitive adsorption between F- and HCO3- accounted for F- enrichment in groundwater. Bicarbonate concentration is expected to be remarkable in high F- groundwater (Gomez et al. 2009). Groundwater with high dissolved F- in our study area had relatively high concentrations of HCO3-, with HCO3–Cl–SO4–Na type. Carbon isotope of DIC suggested that high concentration of HCO3- originated from oxidative decomposition of organic matters (Guo et al. 2014). On the one hand, the elevated HCO3- decreased the concentration of Ca2? by forming low solubility minerals such as calcite and dolomite, which would promote the fluorite dissolution under the control of Kfluorite. On the other hand, high HCO3- content favorably led to competitive adsorption between F- and HCO3- (or/ and CO32-), which released the previously captured F- on aquifer matrix into groundwater, and thus increasing more F- concentrations in solution (Shen et al. 2010). Evaporation

Fig. 9 Molar ratio of Na? to (Na? ? Ca2?) as a function of Fconcentration

Ratio of Na? to (Na? ? Ca2?) as a function of TDS in all samples is plotted in Fig. 11. All samples were plotted in

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Environ Earth Sci Fig. 10 The relationship between concentration of Fand pH (a), concentration of Fand HCO3- (b)

Fig. 11 Ratio of Na? to (Na? ? Ca2?) as a function of TDS in all samples from this study. The gray shaded area reflects weathering dominance. TDS values below the shaded area indicate precipitation dominance, and TDS values above the shaded area by evaporation and mineral precipitation

the area of evaporation and mineral precipitation in Gibbs diagram, showing evaporation and mineral precipitation as the mechanisms controlling the major ion groundwater chemistry. Many investigations note that the relationship between F- and TDS is due to the considerable influence of evaporation on F- enrichment as well as other major components (Handa 1975; Zhou and Liu 1997; Wang et al. 2009; Wang and Cheng 2001; Rao et al. 2013; Li et al. 2014a, b; Machender et al. 2014). Evaporation increased the TDS and pH of the groundwater because of concentration of various components and loss of CO2 and, to certain degrees, elevated F- concentration. The TDS content varied over almost two orders of magnitude in shallow and deep groundwater. The Fconcentrations of shallow groundwater were positively

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correlated with TDS contents, which increased from 704 to 11,200 mg/L (Fig. 12a). However, no good correlation between F- concentration and TDS was observed in deep groundwater with TDS mainly between 500 and 1200 mg/ L (Fig. 12b). It indicated that evaporation exerted a more significant influence on shallow groundwater than deep groundwater (Guo et al. 2012), which was in good agreement with oxygen and hydrogen isotopes’ characteristics discussed above. Actually, concentrations of Ca2? and Mg2? under the control of evaporation were a considerable factor affecting F- activity and dissolution (Wang and Cheng 2001), because high concentrations of Ca2? and Mg2? provided a favorable condition for F- fixation by means of precipitation of CaF2 and MgF2. This would lead to the weak correlation between F- concentration and TDS in groundwater. Overall, evaporation would play an important role in F- enrichment in shallow groundwater.

Conclusions Fluoride concentrations of groundwater from shallow and deep aquifers were investigated in Hengshui area of the NCP. Concentrations of F- in shallow groundwater ranged between 0.37 and 3.28 mg/L (median 1.30 mg/L) while between 0.28 and 3.6 mg/L (median 1.1 mg/L) in deep groundwater. Shallow high F- groundwater (F-[1.0 mg/L) belonged to Cl–SO4–Na–Mg, HCO3–Cl–SO4–Na and SO4– Cl–Na–Mg types, while deep high F- groundwater to HCO3–SO4–Cl–Na and Cl–SO4–Na types. Different origins for shallow and deep groundwater resulted in distinct values of d18O and dD, showing that deep high F- groundwater was recharged from the higher altitude with longer retention time. Average d13CDIC values both in shallow and deep groundwater were lower than -12 % with deep groundwater being more depleted, implying that DIC in high Fgroundwater derived from both rock weathering and

Environ Earth Sci Fig. 12 The relationship between F- concentration and TDS (a shallow groundwater, b deep groundwater)

biodegradation of organic matter. Lateral distribution of groundwater F- was generally consistent with the groundwater flow direction. Groundwater F- increased from northwest to southeast. High F- groundwaters mainly occurred at the depths around 30, 200, and 300 m. Total Fcontent in sediments ranged between 14 and 1690 mg/kg (median 780 mg/kg), with a decrease trend with depth. Sequential extraction procedure of four F- forms in sediments showed that content of exchangeable form (F1) and Fe–Mn oxides-bound form (F3) decreased along the sampling depth, while carbonates-bound form (F2) showed highly variable at different depths. Dissolution–precipitation was a vital process for F- enrichment in groundwater. Compared to shallow groundwaters, cation exchange might exhibit more significant in deep groundwater. In weakly alkaline environment, OH- desorbed F- into groundwater due to their similar ionic radii. Besides, HCO3- would be competitively adsorbed to release F- from aquifer matrix into groundwater. Evaporation was another control on F- concentration, especially in shallow groundwater. Acknowledgments The study was financially supported by the National Basic Research Program of China (No. 2010CB428804), the Geological Survey Program of China Geological Survey (No. 12120113103700), the Fundamental Research Funds for the Central Universities (No. 2652013028), and the Fok Ying-Tung Education Foundation, China (Grant No. 131017).

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