The groundwater characteristics in two areas, Tushan town (TS) in Laizhou County and Puzhuang town (PZ) in Changyi County, were compared to discuss the dynamics of fluorine enrichment. Groundwater has average fluorine levels of 2.33 mg/L in PZ town ranging from 0.65 to 9.87 mg/L and of 0.91 mg/L ranging from 0.27 to 1.25 mg/L in TS town. The geochemical indexes indicate seawater intrusion in the both areas, the more serious seawater intrusion in PZ town causes the higher pH, electrical conductivity (EC), total dissolved solids (TDS), HCO3 and Na+ than TS town, which can accelerate fluorine leach ability from rock/soil and result in higher fluorine than in TS town. So, the seawater intrusion is related to groundwater fluorine levels. Moreover, the groundwater shows the contrary process of cation exchange. That is, the exchange of Na+ to Ca2+ in TS town occurs and that of Ca2+ to Na+ occurs in PZ town during seawater intrusion. As a result, PZ town has relatively higher Na+ and lower Ca2+ than TS town, and the groundwater types transform from Ca-Cl type in TS town to Na-Cl type in PZ town with the ongoing seawater intrusion. Such a process decreases fluorine solubility in groundwater and contributes to lower fluorine levels in TS town. Therefore, the seawater intrusion and the special cation exchange explain well the different fluorine levels in the two areas, which should be considered when the fluorine enrichment is discussed in coastal regions.

INTRODUCTION

Fluorine is a widespread element in the natural environment. It is also an essential trace element for plants, animals and humans (Liu et al. 2007), but it is toxic if taken in excess. Exceeding the tolerable upper in take level can cause fluorosis, such as dental, skeletal disease and osteosclerosis (WHO 1984; Dhiman & Keshari 2006). Fluoridation of drinking water or other methods of fluorine supplementation may reduce fluorosis, and thus research on the relationship between fluorine concentration in drinking water and fluorosis has been conducted in recent years (Hinman Sterrit & Reeves 1996; Hileman 1998; Fantong et al. 2009).

Severe endemic fluorosis of the drinking water occurs widely in the northern coastal region of Laizhou Bay, China (Chen et al. 2011, 2012a, b). About 640,000 of residents along Laizhou Bay have fluorosis due to the long-term consumption of fluoride-contamination water (Han 1997; Chen et al. 2011, 2012a, b). Moreover, the literature and our investigations suggested the new fluorosis areas are spreading (Gao et al. 2007a; Chen et al. 2012a, b).

In this area, a series of investigations have been made mainly related to groundwater fluorine levels and epidemiology. But so far, there were seldom studies focusing on fluorine source, fluorine enrichment genesis and dynamics. Rocks and sediments determine groundwater fluorine levels in two ways: its contents and its fluorine releasing conditions. The previous research concluded that the sediment did not show high levels of fluorine, and noticed seawater intrusion in this area enhances fluorine leach ability from sediment and is the important dynamics of fluorine enrichment (Chen et al. 2011, 2012a, b, 2014). But the relationship between the process of seawater intrusion and groundwater fluorine levels is still undefined.

Na-Ca cation exchange, which has been widely detected, is an important hydrological process during seawater intrusion (Wu et al. 1996; Zhang & Peng 1998; Su et al. 2012), and obviously, the balance between Na+ and Ca2+ levels is upset. Moreover, the levels of Na+ and Ca2+ in water deeply affect fluorine releasing ability and groundwater fluorine levels (Faure 1991; Gao et al. 2007b). Logically, the process of Na-Ca cation exchange should be related to fluorine.

In this research work, two areas, which have different levels of groundwater fluorine and different development degree of seawater intrusion, were selected along Laizhou Bay, and the process of Na-Ca cation exchange during seawater intrusion and its effect on groundwater fluorine levels were detected in detail. The present study is conducted to: (1) reveal the difference of groundwater fluorine and other properties in two areas; (2) analyze the seawater intrusion process of Na-Ca cation exchange and correlate it with groundwater fluorine levels; and (3) discuss the possible dynamics and origin of groundwater fluorine enrichment in coastal areas.

SAMPLING SITE AND ANALYSIS METHODS

Site description

The studied area, Laizhou Bay, is located in the north coast of Shandong Province, China. The studied area is mainly distributed by Quaternary sand sediment, whose thickness changes greatly from 10–50 m in the southeast to 100–300 m in the northwest (Chen et al. 2012a, b). The Quaternary sediments consist of many layers, among which are three large marine deposits at depth of 8–10, 23–35, and 43–60 m, respectively (Xue et al. 1998). There are original saline water deposits in the marine aquifer, and the other layers contain mainly alluvial sediments. Besides, Early Cretaceous Qingshan Formation, Eocene Huangxian Formation and basalt scarcely expose and sporadically distribute on the border of this area (Xue et al. 1998).

Seawater intrusion occurs in this area because it is adjacent to the Bohai Sea and Yellow Sea (Xue et al. 1998). It is estimated that a square of 2,500 km2 has been intruded in this regions (Chen et al. 2012a, b). Two seawater intrusion types, modern seawater intrusion and paleo-seawater intrusion, were involved along Laizhou Bay. The two selected towns, Puzhuang (PZ) town in Changyi County and Tushan (TS) town in Laizhou County, were located in paleo-seawater intrusion area. Paleo-seawater intrusion, also usually called brine water, brackish water or saline water intrusion, is found on a large scale and has been recognized as an important seawater intrusion types in this area (Wu et al. 1996; Xue et al. 1998; Zhang & Peng 1998; Qiu et al. 2007; Chen et al. 2014). The occurrence of paleo-seawater intrusion is related to the geological processes and sea level fluctuation during the Quaternary. The Bohai sea became an enormous closed salt lake with the sea level descending during 70–39 thousand years before present. Marine transgression and regression occurred since the Late Pleistocene period, and the lowest sea level reached 160 m below the current level between 20,000 and 15,000 years before present, after which it rose to the current level with some fluctuations (Zhao et al. 1982). The less-permeable layers were formed by marine sediments, and the seawater was retained in the sediment pores. Part of the seawater flushed out when the sea level descended during the glacial periods. Such intrusion process has also been found in other coastal aquifers (Kooi et al. 2000).

The TS town lies in the transition belts of intrusion and the PZ town lies in the intrusion belt (Chen et al. 2014). Besides, the two towns have different fluorosis situation. PZ town has more fluorosis patients and the more severe fluorine contamination than TS town.

The two towns are characterized by the Quaternary loose deposits. Our previous work revealed that the two towns have the same sedimentary lithology of sediment cores. The sediments in the two towns are mainly composed of clay, silt clay, sand and gravel (Chen et al. 2014). Documents indicated that the aquifers in the two towns are mainly constituted of sand and gravels, and the sand and gravels are mainly the weathering detritus of granite. That is, the parent rock of the gravels in the aquifers is granite, and the details can be referenced by Chen et al. (2014). The determined fluorine levels of sediment in the two cores show low values, ranging from 130 to 528 mg/kg, which is equal to, or even lower than, those in the soil or sediment (Chen et al. 2014). Chen et al. (2014) also reported that the sediments in the aquifer show the lowest levels of fluorine. The lithology and geochemistry of the sediments in the two towns indicate there is no great difference, and thus the groundwater geochemistry difference cannot be contributed to the sediment.

Sampling and analysis method

The groundwater sample was gained for the unit with the village between May and July of the same year. Sixteen villages in PZ town and six villages in TS town were involved. The sampling locations were shown in Figure 1. More than five samples were randomly obtained in every village, and the exact sample number is shown in Table 1. Those samples were taken once in the autumn of 2010. Among which, the water in Liujia and Bajia villages of PZ town is supplied water which is not local, and the water in Zhuodong village of TS town is treated, and so the samples in the three villages were excluded in this research.

Table 1

Groundwater characteristics in two areas of Tushan town and Puzhuang town villages

F (mg/L)pHEC (mS/cm)TDS (mg/L)HCO3 (mg/L)Cl (mg/L)Br (mg/L)NO3 (mg/L)
TownVillage (depth, sample number)MeanRangeMeanRangeMeanRangeMeanRangeMeanRangeMeanRangeMeanRangeMeanRange
PZ Jinshan (12 m, 5) 0.65 0.53–0.74 7.252 7.019–7.509 4.56 4.24–5.09 2.28 2.12–2.55 12.13 9.15–14.63 587.57 427.03–670.37 1.40 1.07–1.98 81.67 24.80–188.17 
 Baimiao (10 m, 6) 1.05 0.78–1.40 7.539 7.301–7.816 4.60 3.49–5.61 2.11 1.38–2.80 8.60 7.30–10.17 717.36 455.66–938.85 0.83 0.49–1.15 212.51 1.07–581.25 
 Majia (9 m, 5) 1.99 1.14–2.51 7.34 7.14–7.657 4.47 3.13–5.13 2.04 1.25–2.56 8.72 7.16–9.86 707.87 251.11–999.55 0.84 0.51–1.61 191.91 23.26–406.46 
 Dongzhao (16 m, 5) 9.87 9.23–10.34 8.108 8.054–8.259 2.54 1.95–3.50 1.37 0.97–1.75 11.44 6.19–9.56 515.00 333.14–668.01 1.04 0.65–1.82 1.36 0.94–1.65 
 Yaojia (12 m, 5) 2.25 0.45–5.37 7.728 7.257–8.218 2.69 2.36–3.26 1.35 1.18–1.63 7.09 5.60–8.34 475.47 372.48–595.98 0.97 0.83–1.18 2.35 1.50–3.18 
 Wanglu (10 m, 7) 1.15 1.02–1.32 7.455 7.254–7.622 4.67 3.33–5.43 2.12 1.22–2.72 10.10 9.35–11.59 878.58 395.28–1109.07 1.73 1.07–2.16 1.58 1.10–3.00 
 Yingzi (13 m, 5) 0.81 0.65–0.96 7.597 7.476–7.731 2.98 2.45–3.82 1.49 1.22–1.92 7.97 6.45–8.87 433.35 311.39–582.50 1.05 0.66–1.46 37.47 0.99–80.21 
 Xinsheng (16 m, 5) 4.53 2.02–7.17 8.288 8.08–8.454 2.32 2.17–2.51 1.16 1.09–1.26 6.64 4.18–9.31 403.83 350.95–470.99 0.84 0.61–0.95 0.92 0.45–1.32 
 Qianlu (13 m, 5) 1.50 1.12–2.00 7.902 7.615–8.048 2.61 2.13–3.59 1.30 1.06–1.80 5.30 4.89–5.76 440.86 367.24–554.49 0.83 0.51–1.17 28.77 0.81–138.07 
 Xujiazhuang (32 m, 6) 2.71 1.41–4.72 7.923 7.802–8.03 1.30 1.14–1.57 0.65 0.57–0.79 6.34 5.84–7.02 81.37 74.98–89.77 0.25 0.14–0.41 3.50 1.67–5.25 
 Lvjiazhuang (18 m, 5) 1.19 0.94–1.40 7.76 7.504–8.128 1.21 0.83–1.83 0.60 0.41–0.91 3.90 2.90–5.44 165.10 2103.59–67.39 0.26 0.13–0.45 16.41 5.83–28.60 
 Fu'an (19 m, 5) 2.21 1.94–2.54 7.781 7.362–8.169 1.44 1.15–1.87 0.72 0.58–0.93 7.12 6.72–8.44 131.66 77.39–169.49 0.30 0.17–0.37 1.00 0.81–1.20 
 Dahebei (30 m, 5) 1.35 1.32–1.38 7.884 7.596–8.171 1.86 1.76–1.96 0.93 0.88–0.98 7.86 7.63–8.10 258.78 253.09–264.47 0.68 0.64–0.72 11.87 5.35–18.39 
 Liuzhuang (10 m, 5) 1.40 1.18–1.64 7.424 7.35–7.5 3.16 2.49–4.04 1.58 1.25–2.02 8.38 7.87–9.05 557.35 370.50–809.84 1.06 0.79–1.47 7.64 1.03–27.16 
TSa Taipingzhuang (26 m, 8) 0.63 0.36–0.81 7.608 7.410–7.73 1.43 1.28–1.74 0.72 0.63–0.87 3.47 1.77–3.57 218.61 138.82–329.40 0.28 0.22–0.41 118.20 27.55–161.18 
 Xiaohuibu (11 m, 5) 0.27 0.24–0.30 7.593 7.433–7.754 1.37 1.06–1.72 0.69 0.53–0.88 3.91 2.70–4.85 144.87 92.32–185.35 0.39 0.20–0.49 237.26 177.03–283.72 
 Huibu town (32 m, 7) 1.19 0.46–1.61 7.506 7.400–7.580 1.62 1.50–1.83 0.81 0.75–0.91 3.94 2.42–6.03 216.31 196.29–245.63 0.36 0.28–0.57 196.71 190.74–214.16 
 Jiaojiazhuangzi (9 m, 9) 1.25 0.80–1.98 7.266 7.130–7.390 2.51 1.87–3.01 1.26 0.93–1.50 4.39 2.29–9.25 412.84 275.61–534.79 0.76 0.55–1.06 390.38 278.42–558.98 
 Lvjiaji (16 m, 5) 1.21 0.96–1.53 7.228 7.120–7.320 2.88 2.51–3.31 1.44 1.26–1.66 5.19 2.33–7.69 510.64 441.96–585.30 0.77 0.60–0.89 515.75 394.47–632.16 
  SO42– (mg/L) Li+ (mg/L) Na+ (mg/L) K+ (mg/L) Mg2+ (mg/L) Ca2+ (mg/L) SAR 
Town Village (depth, sample number) Mean Range Mean Range Mean Range Mean Range Mean Range Mean Range Mean Range 
PZ Jinshan (12 m, 5) 624.09 592.90–682.97 0.0047 0.0041–0.0053 634.91 541.76–692.77 15.85 12.26–21.36 103.88 83.04–116.57 60.24 49.23–78.34 11.43 10.88–13.49 
 Baimiao (10 m, 6) 433.42 304.68–623.16 0.0156 0.0083–0.0290 506.25 300.69–687.30 96.49 24.73–227.92 96.33 72.06–113.24 56.76 39.76–72.63 9.44 5.96–12.92 
 Majia (9 m, 5) 615.71 499.10–760.68 0.0224 0.0124–0.0334 516.59 288.45–752.26 60.90 37.61–103.09 100.91 65.99–124.02 31.07 20.03–42.33 10.06 6.89–13.47 
 Dongzhao (16 m, 5) 99.93 71.25–131.94 0.0068 0.0055–0.0076 498.90 359.25–669.08 15.21 10.41–20.03 13.38 9.83–18.13 8.68 5.98–10.99 24.65 18.88–29.36 
 Yaojia (12 m, 5) 397.18 131.07–697.10 0.0150 0.0112–0.0200 363.94 264.94–465.62 31.53 18.84–48.36 80.13 36.20–124.07 62.73 23.84–92.74 7.14 4.87–12.89 
 Wanglu (10 m, 7) 621.38 413.65–785.75 0.0179 0.0156–0.0219 588.49 392.94–735.96 50.24 36.27–61.64 117.33 89.03–130.57 32.80 13.94–50.10 10.71 8.32–12.11 
 Yingzi (13 m, 5) 569.57 450.68–701.61 0.1620 0.0194–0.3250 369.99 340.48–430.90 35.93 28.46–44.17 57.19 47.73–72.21 27.27 15.97–38.94 9.19 8.32–18.99 
 Xinsheng (16 m, 5) 78.05 48.28–117.15 0.0079 0.0060–0.0089 443.62 409.15–465.94 13.76 9.54–21.29 4.45 3.20–6.12 2.28 1.19–4.23 39.18 31.29–45.63 
 Qianlu (13 m, 5) 331.23 166.96–616.45 0.0980 0.0129–0.2175 317.94 228.67–402.08 44.95 30.59–63.62 51.47 23.78–92.69 44.56 22.64–85.18 7.66 4.37–11.77 
 Xujiazhuang (32 m, 6) 132.39 122.44–146.64 0.1270 0.0094–0.2970 200.68 160.53–256.72 29.20 14.39–35.17 26.32 10.74–41.77 14.89 10.38–18.45 7.20 5.10–17.90 
 Lvjiazhuang (18 m, 5) 115.88 94.03–157.14 0.0088 0.0060–0.0128 91.08 71.33–119.09 12.49 6.62–24.22 38.10 21.58–64.96 59.33 43.58–82.07 2.26 1.76–2.79 
 Fu'an (19 m, 5) 194.58 117.12–376.08 0.0081 0.0071–0.0101 237.28 205.59–267.54 4.43 4.06–4.87 17.20 13.93–20.56 18.70 13.65–20.11 9.58 9.39–10.22 
 Dahebei (30 m, 5) 134.64 107.16–162.12 0.0113 0.0085–0.0140 302.93 302.80–303.05 6.52 4.12–8.91 32.33 25.94–38.72 18.82 11.97–25.67 9.77 9.53–10.03 
 Liuzhuang (10 m, 5) 389.32 273.30–513.94 0.0125 0.0092–0.0172 380.81 283.66–500.19 18.11 13.91–21.36 76.50 54.40–97.27 59.84 54.02–65.10 7.65 6.40–9.18 
TSa Taipingzhuang (26 m, 8) 81.54 61.32–101.34 0.0127 0.0085–0.0188 80.75 61.51–101.39 2.64 1.02–5.72 28.78 24.87–37.04 168.86 150.80–216.61 1.51 1.21–1.93 
 Xiaohuibu (11 m, 5) 103.44 80.73–123.11 0.0065 0.0050–0.0082 40.25 35.23–44.59 1.63 0.99–1.98 31.69 23.23–37.21 168.86 99.41–153.71 0.81 0.70–0.87 
 Huibu town (32 m, 7) 111.34 94.52–152.28 0.0089 0.0078–0.0097 88.88 59.26–100.54 1.02 0.78–1.32 43.27 39.23–46.11 172.25 157.59–207.43 1.56 1.01–1.71 
 Jiaojiazhuangzi (9 m, 9) 135.13 80.76–191.17 0.0254 0.0173–0.0389 161.71 105.83–191.05 1.52 0.73–3.07 45.20 37.26–59.43 272.59 204.81–321.31 2.38 1.77–2.91 
 Lvjiaji (16 m, 5) 107.86 53.62–265.59 0.0369 0.0325–0.0427 182.10 130.40–248.72 3.20 1.41–6.55 62.27 52.96–76.00 280.85 229.54–317.04 2.55 1.80–3.35 
F (mg/L)pHEC (mS/cm)TDS (mg/L)HCO3 (mg/L)Cl (mg/L)Br (mg/L)NO3 (mg/L)
TownVillage (depth, sample number)MeanRangeMeanRangeMeanRangeMeanRangeMeanRangeMeanRangeMeanRangeMeanRange
PZ Jinshan (12 m, 5) 0.65 0.53–0.74 7.252 7.019–7.509 4.56 4.24–5.09 2.28 2.12–2.55 12.13 9.15–14.63 587.57 427.03–670.37 1.40 1.07–1.98 81.67 24.80–188.17 
 Baimiao (10 m, 6) 1.05 0.78–1.40 7.539 7.301–7.816 4.60 3.49–5.61 2.11 1.38–2.80 8.60 7.30–10.17 717.36 455.66–938.85 0.83 0.49–1.15 212.51 1.07–581.25 
 Majia (9 m, 5) 1.99 1.14–2.51 7.34 7.14–7.657 4.47 3.13–5.13 2.04 1.25–2.56 8.72 7.16–9.86 707.87 251.11–999.55 0.84 0.51–1.61 191.91 23.26–406.46 
 Dongzhao (16 m, 5) 9.87 9.23–10.34 8.108 8.054–8.259 2.54 1.95–3.50 1.37 0.97–1.75 11.44 6.19–9.56 515.00 333.14–668.01 1.04 0.65–1.82 1.36 0.94–1.65 
 Yaojia (12 m, 5) 2.25 0.45–5.37 7.728 7.257–8.218 2.69 2.36–3.26 1.35 1.18–1.63 7.09 5.60–8.34 475.47 372.48–595.98 0.97 0.83–1.18 2.35 1.50–3.18 
 Wanglu (10 m, 7) 1.15 1.02–1.32 7.455 7.254–7.622 4.67 3.33–5.43 2.12 1.22–2.72 10.10 9.35–11.59 878.58 395.28–1109.07 1.73 1.07–2.16 1.58 1.10–3.00 
 Yingzi (13 m, 5) 0.81 0.65–0.96 7.597 7.476–7.731 2.98 2.45–3.82 1.49 1.22–1.92 7.97 6.45–8.87 433.35 311.39–582.50 1.05 0.66–1.46 37.47 0.99–80.21 
 Xinsheng (16 m, 5) 4.53 2.02–7.17 8.288 8.08–8.454 2.32 2.17–2.51 1.16 1.09–1.26 6.64 4.18–9.31 403.83 350.95–470.99 0.84 0.61–0.95 0.92 0.45–1.32 
 Qianlu (13 m, 5) 1.50 1.12–2.00 7.902 7.615–8.048 2.61 2.13–3.59 1.30 1.06–1.80 5.30 4.89–5.76 440.86 367.24–554.49 0.83 0.51–1.17 28.77 0.81–138.07 
 Xujiazhuang (32 m, 6) 2.71 1.41–4.72 7.923 7.802–8.03 1.30 1.14–1.57 0.65 0.57–0.79 6.34 5.84–7.02 81.37 74.98–89.77 0.25 0.14–0.41 3.50 1.67–5.25 
 Lvjiazhuang (18 m, 5) 1.19 0.94–1.40 7.76 7.504–8.128 1.21 0.83–1.83 0.60 0.41–0.91 3.90 2.90–5.44 165.10 2103.59–67.39 0.26 0.13–0.45 16.41 5.83–28.60 
 Fu'an (19 m, 5) 2.21 1.94–2.54 7.781 7.362–8.169 1.44 1.15–1.87 0.72 0.58–0.93 7.12 6.72–8.44 131.66 77.39–169.49 0.30 0.17–0.37 1.00 0.81–1.20 
 Dahebei (30 m, 5) 1.35 1.32–1.38 7.884 7.596–8.171 1.86 1.76–1.96 0.93 0.88–0.98 7.86 7.63–8.10 258.78 253.09–264.47 0.68 0.64–0.72 11.87 5.35–18.39 
 Liuzhuang (10 m, 5) 1.40 1.18–1.64 7.424 7.35–7.5 3.16 2.49–4.04 1.58 1.25–2.02 8.38 7.87–9.05 557.35 370.50–809.84 1.06 0.79–1.47 7.64 1.03–27.16 
TSa Taipingzhuang (26 m, 8) 0.63 0.36–0.81 7.608 7.410–7.73 1.43 1.28–1.74 0.72 0.63–0.87 3.47 1.77–3.57 218.61 138.82–329.40 0.28 0.22–0.41 118.20 27.55–161.18 
 Xiaohuibu (11 m, 5) 0.27 0.24–0.30 7.593 7.433–7.754 1.37 1.06–1.72 0.69 0.53–0.88 3.91 2.70–4.85 144.87 92.32–185.35 0.39 0.20–0.49 237.26 177.03–283.72 
 Huibu town (32 m, 7) 1.19 0.46–1.61 7.506 7.400–7.580 1.62 1.50–1.83 0.81 0.75–0.91 3.94 2.42–6.03 216.31 196.29–245.63 0.36 0.28–0.57 196.71 190.74–214.16 
 Jiaojiazhuangzi (9 m, 9) 1.25 0.80–1.98 7.266 7.130–7.390 2.51 1.87–3.01 1.26 0.93–1.50 4.39 2.29–9.25 412.84 275.61–534.79 0.76 0.55–1.06 390.38 278.42–558.98 
 Lvjiaji (16 m, 5) 1.21 0.96–1.53 7.228 7.120–7.320 2.88 2.51–3.31 1.44 1.26–1.66 5.19 2.33–7.69 510.64 441.96–585.30 0.77 0.60–0.89 515.75 394.47–632.16 
  SO42– (mg/L) Li+ (mg/L) Na+ (mg/L) K+ (mg/L) Mg2+ (mg/L) Ca2+ (mg/L) SAR 
Town Village (depth, sample number) Mean Range Mean Range Mean Range Mean Range Mean Range Mean Range Mean Range 
PZ Jinshan (12 m, 5) 624.09 592.90–682.97 0.0047 0.0041–0.0053 634.91 541.76–692.77 15.85 12.26–21.36 103.88 83.04–116.57 60.24 49.23–78.34 11.43 10.88–13.49 
 Baimiao (10 m, 6) 433.42 304.68–623.16 0.0156 0.0083–0.0290 506.25 300.69–687.30 96.49 24.73–227.92 96.33 72.06–113.24 56.76 39.76–72.63 9.44 5.96–12.92 
 Majia (9 m, 5) 615.71 499.10–760.68 0.0224 0.0124–0.0334 516.59 288.45–752.26 60.90 37.61–103.09 100.91 65.99–124.02 31.07 20.03–42.33 10.06 6.89–13.47 
 Dongzhao (16 m, 5) 99.93 71.25–131.94 0.0068 0.0055–0.0076 498.90 359.25–669.08 15.21 10.41–20.03 13.38 9.83–18.13 8.68 5.98–10.99 24.65 18.88–29.36 
 Yaojia (12 m, 5) 397.18 131.07–697.10 0.0150 0.0112–0.0200 363.94 264.94–465.62 31.53 18.84–48.36 80.13 36.20–124.07 62.73 23.84–92.74 7.14 4.87–12.89 
 Wanglu (10 m, 7) 621.38 413.65–785.75 0.0179 0.0156–0.0219 588.49 392.94–735.96 50.24 36.27–61.64 117.33 89.03–130.57 32.80 13.94–50.10 10.71 8.32–12.11 
 Yingzi (13 m, 5) 569.57 450.68–701.61 0.1620 0.0194–0.3250 369.99 340.48–430.90 35.93 28.46–44.17 57.19 47.73–72.21 27.27 15.97–38.94 9.19 8.32–18.99 
 Xinsheng (16 m, 5) 78.05 48.28–117.15 0.0079 0.0060–0.0089 443.62 409.15–465.94 13.76 9.54–21.29 4.45 3.20–6.12 2.28 1.19–4.23 39.18 31.29–45.63 
 Qianlu (13 m, 5) 331.23 166.96–616.45 0.0980 0.0129–0.2175 317.94 228.67–402.08 44.95 30.59–63.62 51.47 23.78–92.69 44.56 22.64–85.18 7.66 4.37–11.77 
 Xujiazhuang (32 m, 6) 132.39 122.44–146.64 0.1270 0.0094–0.2970 200.68 160.53–256.72 29.20 14.39–35.17 26.32 10.74–41.77 14.89 10.38–18.45 7.20 5.10–17.90 
 Lvjiazhuang (18 m, 5) 115.88 94.03–157.14 0.0088 0.0060–0.0128 91.08 71.33–119.09 12.49 6.62–24.22 38.10 21.58–64.96 59.33 43.58–82.07 2.26 1.76–2.79 
 Fu'an (19 m, 5) 194.58 117.12–376.08 0.0081 0.0071–0.0101 237.28 205.59–267.54 4.43 4.06–4.87 17.20 13.93–20.56 18.70 13.65–20.11 9.58 9.39–10.22 
 Dahebei (30 m, 5) 134.64 107.16–162.12 0.0113 0.0085–0.0140 302.93 302.80–303.05 6.52 4.12–8.91 32.33 25.94–38.72 18.82 11.97–25.67 9.77 9.53–10.03 
 Liuzhuang (10 m, 5) 389.32 273.30–513.94 0.0125 0.0092–0.0172 380.81 283.66–500.19 18.11 13.91–21.36 76.50 54.40–97.27 59.84 54.02–65.10 7.65 6.40–9.18 
TSa Taipingzhuang (26 m, 8) 81.54 61.32–101.34 0.0127 0.0085–0.0188 80.75 61.51–101.39 2.64 1.02–5.72 28.78 24.87–37.04 168.86 150.80–216.61 1.51 1.21–1.93 
 Xiaohuibu (11 m, 5) 103.44 80.73–123.11 0.0065 0.0050–0.0082 40.25 35.23–44.59 1.63 0.99–1.98 31.69 23.23–37.21 168.86 99.41–153.71 0.81 0.70–0.87 
 Huibu town (32 m, 7) 111.34 94.52–152.28 0.0089 0.0078–0.0097 88.88 59.26–100.54 1.02 0.78–1.32 43.27 39.23–46.11 172.25 157.59–207.43 1.56 1.01–1.71 
 Jiaojiazhuangzi (9 m, 9) 135.13 80.76–191.17 0.0254 0.0173–0.0389 161.71 105.83–191.05 1.52 0.73–3.07 45.20 37.26–59.43 272.59 204.81–321.31 2.38 1.77–2.91 
 Lvjiaji (16 m, 5) 107.86 53.62–265.59 0.0369 0.0325–0.0427 182.10 130.40–248.72 3.20 1.41–6.55 62.27 52.96–76.00 280.85 229.54–317.04 2.55 1.80–3.35 
Figure 1

The geographical map of sample sites.

Figure 1

The geographical map of sample sites.

The water sample was collected in pre-cleaned liter polyethylene containers and kept at low temperature. The groundwater was divided into two groups after it was sent to immediately to the laboratory. One was used to analyze TDS, total hardness (TH), electrical conductivity (EC), CO32–, , pH and mineralization within 48 hours, and another was frozen to analyze F, Cl, Br, Na+, Li+, K+, Ca2+ and Mg2+. DDS-320 conductivity meter, produced by Shanghai REX Instrument Factory, was used to analyze EC, pH, TDS, TH and mineralization. and were determined by 0.01 mol/L HCl titration with phenolphthalein and helianthin B indicators. The analysis of F, Cl, Br, Na+, Li+, K+, Ca2+ and Mg2+ follows ICS-90 ion chromatography by Dionex. The parallel samples and standard samples were used for quality control (QC), and the error was less than 5%.

The data statistics analysis was executed by Excell and Spss 10.0 software.

RESULTS AND DISCUSSION

Groundwater fluorine levels in the two areas

There is considerable disagreement on the appropriate drinking water standards for fluoride due to many factors, including climate, diet, and the characteristics of target population. The US Environmental Protection Agency has set 4 mg/L as the maximum contaminant level (Ozsvath 2009). The World Health Organization (WHO) recommends an appropriate standard of 1.5 mg/L (WHO 1999). The National Sanitary Standard for drinking water in China established standards of 1.0 mg/L for its naturally high fluoride districts (GB5749-85).

The statistical values of groundwater fluorine levels and other characteristics are presented in Table 1. On comparing the results with the limits prescribed by GB5749-85, it was observed that 14.5% of samples in PZ town and 65.8% of samples in TS town fall under the safe limits. The average fluorine levels in PZ town is 2.33 mg/L ranging from 0.65 to 9.87 mg/L, while that in TS town is 0.91 mg/L, with a scope of 0.27–1.25 mg/L. All the villages in PZ town except Jinshan and Yingzi have groundwater out of the safe average fluorine levels, while the average fluorine levels in Huibu, Jiaojiazhuangzi and Lvjiaji villages are slightly out of the safe limits. Such a distribution of groundwater fluorine levels also indicates that the PZ town has a more serious problem of fluorine contamination than TS town.

Groundwater hydro-geochemical characteristics and seawater intrusion

The dissolution and depletion of fluorine in groundwater is controlled by chemical type. Hence, Piper diagrams have sometimes been used to infer the relationships between groundwater composition and fluorine distribution (Piper 1994; Krishnaraj et al. 2012), and the results are presented in Figure 2. It reveals most of the groundwater in PZ town is classified as Na-Cl and Na-HCO3 type water, mainly as Na-Cl. Only four samples plot outside the Na-Cl and Na-HCO3 field. Researchers have also indicated that the high fluorine levels are generally associated with those samples having high Na contents (Chae et al. 2007; Gao et al. 2007b; Chen et al. 2012a, b). The majority of samples in TS town clusters in the Ca-Cl type except two samples. The dominant Cl anion in the two areas may be due to the seawater intrusion, but its dominant cation show great difference from PZ town although the sediments in the two areas are similar, which means different hydrological process occurs.

Figure 2

Piper graph of groundwater (the left indicates PZ town and the right indicates TS town).

Figure 2

Piper graph of groundwater (the left indicates PZ town and the right indicates TS town).

The groundwater characteristics also imply seawater intrusion in the two areas. The China Geological Survey has prescribed the seawater intrusion grades (DD2008-03). All the Cl, Br and TDS concentrations of groundwater in all the villages of PZ town except Xujiazhuang, Lvjiazhuang and Fu'an have exceeded the limits of 250, 0.55 and 1 mg/L, respectively, which means the groundwater is mixed into seawater. Meanwhile, the Cl, Br and TDS in the Jiaojiazhuangzi and Lvjiaji villages with high fluorine groundwater also indicate seawater intrusion and those in Huibu town show a slight intrusion in TS town, which means the distribution of high fluorine groundwater is closely related to seawater intrusion.

Sodium adsorption ratio (SAR) is usually used to assess water quality of agricultural irrigation (Emdad et al. 2004). It is also used as a hydrochemical index of seawater intrusion and comprehensive judgment on intrusion degree (Zhao 1999; Zhang et al. 1998), and even ranked as geochemical standard of seawater intrusion by China Geological Survey (DD2008-03). The proportion of Na+, Ca2+ and Mg2+ is expressed in terms of SAR and can be calculated by the following formula (Biswas & Mukherjee 1987; Tahir et al. 2009): 
formula
1
where concentrations are in me/L. The calculated results are listed in Tables 1 and 3. All the samples in PZ town have the SAR values exceeding the limit of 2.0, which means a strong seawater intrusion and ion exchange of Ca2+ and Mg2+ to Na+. Especially, although Cl, Br and TDS in Xujiazhuang, Lvjiazhuang and Fu'an villages of PZ town are within the limits, the SAR show high average values of 7.2, 2.26 and 9.58, respectively. Such a fact indicates that the complex process occurs after intrusion. Similarly, the SARs in Jiaojiazhuangzi and Lvjiaji villages of TS town are also more than 2.0, and the SAR in Huibu town is less than 2.0 but more than other villages, which also indicates a slight seawater intrusion.

The difference of groundwater fluorine and other characteristics in the two areas and its discussion on fluorine enrichment

The average values of groundwater characteristics are presented in Table 2. Generally, the samples in PZ town has higher fluorine levels, together with the higher pH, EC, TDS, , Cl, Br, Li+, Na+, K+, Mg2+, SAR, Na+/Ca2+ and lower Ca2+, than those in TS town. Moreover, the groundwater in PZ town has higher levels of Cl, Br, TDS and SAR than that in TS town, which indicates the more serious seawater intrusion in PZ town, which also results in the higher pH, EC, TDS, and Na+. A series of research concluded that fluorine is favorite of the conditions of alkaline, soft groundwater that is depleted in Ca2+ and enriched in Na+ water (Robertson 1986; Gupta et al. 1999; Woo et al. 2000; Earle & Krogh 2004; Chae et al. 2006; Dhiman & Keshari 2006; Chae et al. 2007; Walna et al. 2007). Obviously, such groundwater properties change occurs because of the mixing of seawater, which potentially results in the acceleration of fluorine leach ability from rocks/soil and causes more fluorine levels in groundwater. Therefore, the more serious seawater intrusion is responsible for the higher levels of groundwater fluorine in PZ town than in TS town. In fact, the investigation demonstrated the fluorosis along Laizhou Bay and seawater intrusion are distributed in the same manner (Chen et al. 2012a, b). Experiment has confirmed that salt lake water intrusions, a process similar to seawater intrusion, elevated groundwater fluorine levels and causes fluososis in Yuncheng, Shanxi Province of China and in Nagar Parker, Sindh Province of Pakistan (Gao et al. 2007b; Tahir et al. 2009). Moreover, fluorosis in seawater intrusion areas such as Gai County, Yingkou City of Niaoning Province (Kou & Wang 2000) and Chaoyang of Guangdong (Chen & Zheng 1995) has been widely documented. In general, these results are in agreement with our findings and strongly support the correlation between seawater intrusion and fluorosis.

Table 2

The difference of average values for groundwater characteristics in two areas and its comparisons with seawater

F (mg/L)pHEC (mS/cm)TDS (mg/L)HCO3 (mg/L)Cl (mg/L)Br (mg/L)
PZ town 2.33 7.713 2.89 1.41 7.75 453.87 0.86 
TS town 0.91 7.44 1.96 0.98 4.18 300.65 0.51 
Seawatera 1.3 8.25 – 35.154 142 19353 66 
  (mg/L) Li+ (mg/L) Na+ (mg/L) K+ (mg/L) Mg2+ (mg/L) Ca2+ (mg/L)  (mg/L) 
PZ town 338.38 0.037 389.53 31.12 58.25 35.57 42.78 
TS town 107.86 0.018 110.74 2.0 42.24 199.35 291.66 
Seawatera 2712 – 10760 387 1294 413 – 
F (mg/L)pHEC (mS/cm)TDS (mg/L)HCO3 (mg/L)Cl (mg/L)Br (mg/L)
PZ town 2.33 7.713 2.89 1.41 7.75 453.87 0.86 
TS town 0.91 7.44 1.96 0.98 4.18 300.65 0.51 
Seawatera 1.3 8.25 – 35.154 142 19353 66 
  (mg/L) Li+ (mg/L) Na+ (mg/L) K+ (mg/L) Mg2+ (mg/L) Ca2+ (mg/L)  (mg/L) 
PZ town 338.38 0.037 389.53 31.12 58.25 35.57 42.78 
TS town 107.86 0.018 110.74 2.0 42.24 199.35 291.66 
Seawatera 2712 – 10760 387 1294 413 – 
In order to identify the process of seawater intrusion and its effect on groundwater fluorine levels, ratio values of variables were compared in Table 3. The average of γNa+/γCl in PZ town is 1.31, which is higher than the seawater (0.846). However, the average γNa+/γCl in TS town is observed to be 0.56, which is lower than the seawater. This indicates groundwater in the two areas is influenced by the following cation exchange due to seawater intrusion: 
formula
2
 
formula
3
In TS town, the exchange of Na+ to Ca2+ occurs, that is, the Na+ ions in groundwater were absorbed by the clay of water-bearing median due to the replacement of Ca2+ of the collide clay. While in PZ town, the inverse exchange of Ca2+ to Na+ occurs, namely, the clay releases the absorbed Na+ into water and absorbs the Ca2+. This inverse process of cation exchange in the two areas may be due to their different belts of seawater intrusion. The TS town lies in transition belts and the PZ town lies in slat water intrusion belts. Besides, the exchange ratio for waters is presented by the ratio of (γNa+ − 0.65γCl)/(γCa2+ + γMg2+) (Boyle & Chagnon 1995). The ratio of water in PZ town is positive and that in TS town is negative, which also illustrates different cation exchange processes occur in the two areas.
Table 3

The comparisons of ratio values of groundwater variables in two areas (γ in me/L)

SARNa+/Ca2+γNa+/γCl(γNa+ − 0.65γCl)/(γCa2+ + γMg2+)γHCO3/γCa2+
PZ town 9.30 28.63 1.306 1.283 0.00143 
TS town 1.85 0.51 0.561 −0.057 0.000188 
Seawater 58.37 26.05 0.846 0.844 0.00225 
 Mg2+/Ca2+ Na+/Mg2+ γCl/γCa2+ (Na+ + K+)/(Ca2+ + Mg2+γMg2+/γCl 
PZ town 1.638 6.687 7.291 4.484 0.0312 
TS town 0.212 2.622 0.862 0.467 0.0341 
Seawater 3.133 8.315 26.777 6.53 0.0163 
SARNa+/Ca2+γNa+/γCl(γNa+ − 0.65γCl)/(γCa2+ + γMg2+)γHCO3/γCa2+
PZ town 9.30 28.63 1.306 1.283 0.00143 
TS town 1.85 0.51 0.561 −0.057 0.000188 
Seawater 58.37 26.05 0.846 0.844 0.00225 
 Mg2+/Ca2+ Na+/Mg2+ γCl/γCa2+ (Na+ + K+)/(Ca2+ + Mg2+γMg2+/γCl 
PZ town 1.638 6.687 7.291 4.484 0.0312 
TS town 0.212 2.622 0.862 0.467 0.0341 
Seawater 3.133 8.315 26.777 6.53 0.0163 

In fact, cation exchange is a common process of seawater intrusion and has been confirmed to be reversible. Wu et al. (1996) observed that Ca2+ sharply increases and Na+ decreases in the seawater intrusion transition belts of Longkou City, Shandong Province and confirmed the conclusion by experimental simulation of water–rock interaction. Zhang & Peng (1998) and Su et al. (2012) all stated that cation exchange influenced the balance between Na+ and Ca2+. Obviously, the contrary cation exchange processes in the two areas cause the higher levels of Na+ and lower levels of Ca2+ in PZ town than those in TS town. Therefore, such ratios as Na+/Ca2+, SAR, , (Na+ + K+)/(Ca2+ + Mg2+), Na+/Mg2+, γCl/γCa2+ and Mg2+/Ca2+ of groundwater in PZ town are lower than those in TS town. While the groundwater in the two areas has almost the equal ratios of γMg2+/γCl because the exchange of Mg2+ and Na2+ is secondary and hardly occurs (Wu et al. 1996).

Generally, although the two areas is adjacent and have the same sedimentary lithology, the geochemical characteristics of groundwater in the two areas show different process of seawater intrusion, which influences the balance of Na+ and Ca2+. The levels of groundwater Na+ and Ca2+ in the two areas are extremely different and the water types transform from Ca-Cl in TS town to Na-Cl in PZ town.

However, the levels of Na+ and Ca2+ are of great importance to maintain the fluorine levels in groundwater. Fluorine levels are promoted when Na+ increases because of the two reasons: Na+ is more reactive to combine with F than Ca2+ and Mg2+, and NaF has higher solubility than CaF2. Experiments and field investigations also confirmed that fluorine levels increase with the increasing of Na+ and Na+/Ca2+ (Krainov & Petrova 1976; Krainov & Zakutin 1994; Gao et al. 2007b). Gao et al. (2007b) even found the NaF complexes increases and CaF+ or HF decreases when more Na+ is mixed into water. Hyndaman (1985) and Faure (1991) noted that high fluorine levels of groundwater forms by contact with the underlying intrusive igneous rocks, which typically enrich plagioclase composition, the sodium-rich end-member. Meanwhile, the fluorine maximum concentration is generally restricted by CaF2 dissolution (Nordstrom & Jenne 1977; Apambire et al. 1997; Cronin et al. 2000; Saxena & Ahmed 2003; Chae et al. 2007). As a result, conditions leading to low calcite solubility also lead to high fluorine levels in groundwater (Rafique et al. 2008; Tahir et al. 2009). A negative correlation of Ca2+ and F generally appears and such results have been widely observed (Apambire et al. 1997; Saxena & Ahmed 2003). Consequently, the higher levels of Na+ and low Ca2+ due to different processes of cation exchange contribute to the different fluorine levels in the two areas, and such a process should be noticed when discussing the dynamics of fluorine enrichment in seawater intrusion areas.

Additionally, this is just a primary discussion about the fluorine enrichment in coastal areas, and the materials we collected support our ideas. Still, more proof should be hunted for and we are eager for other wise opinions.

CONCLUSIONS

The groundwater samples from two areas with different situation of fluorosis along Laizhou Bay, China, were gained to discuss the possible dynamics and origin of fluorine enrichment. The following results were found.

  • The average fluorine levels in PZ town is 2.33 mg/L ranging from 0.65–9.87 mg/L, while that in TS town is 0.91 mg/L, with a scope of 0.27–1.25 mg/L. The fluorine situation in PZ town is more serious than that in TS town. The geochemical indexes indicate seawater intrusion in the two areas. PZ town has more serious seawater intrusion, resulting in higher pH, EC, TDS, and Na+. Such geochemical changes accelerate fluorine leach ability from rocks/soil and enhance groundwater fluorine solubility.

  • The groundwater in the two areas indicates cation exchange. Geochemical characteristics indicate that TS town lies in transition zone of seawater intrusion and the exchange of Na+ to Ca2+ occurs, which results in lower ratio of Na+/Ca2+. However, the groundwater in PZ town shows the exchange of Ca2+ to Na+, which leads to the relatively higher Na+ level and lower Ca2+. The groundwater transforms from Ca-Cl type in TS town to Na-Cl type in PZ town with the ongoing of seawater intrusion. Such a fact decreases fluorine solubility in groundwater and contributes to lower fluorine levels in TS town. Therefore, seawater intrusion and the special cation exchange can explain the difference of groundwater fluorine levels in the two areas, which should be considered when discussing the fluorine enrichment in coastal regions.

ACKNOWLEDGEMENTS

This work was supported by the Natural Science Fund of China (Nos 40901027, 41106036 and 71303140), the Natural Science Fund of Shandong Province (ZR2011DQ006), the International Partnership Creative Group, the Chinese Academy of Sciences ‘Typical Environmental Process and Effects of Coastal Zone Resources’, the research grant of the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (SKLIG-KF-14-02), and the research grant of Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals (DMSM201408).

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