Fluorine geochemistry in bedrock groundwater of South Korea.

Fluoride in drinking water has both positive and negative effects on human health. Small concentrations of fluoride have beneficial effects on the teeth by hardening the enamel and reducing the incident of caries (Fung et al., 1999, Shomar et al., 2004). On the other hand, concentrations of fluoride above 1.5 mg/L may cause an endemic disease called dental fluorosis (Handa, 1975, WHO (World Health Organization), 1984, USPHS (United States Public Health Services), 1987, Ripa, 1993). Many people in countries such as Algeria, Australia, China, Egypt, Ghana, India, Iran, Iraq, Israel, Japan, Jordan, Kenya, Libya, Mexico, Morocco, New Zealand, South Africa, Spain, Sri Lanka, Tanzania, Turkey and USA have been reported to suffer from fluorosis due to intake of fluoride-rich groundwater (Teotia et al., 1981, Dissanayake, 1991, Ramamohana Rao et al., 1993, Grimaldo et al., 1995, Apambire et al., 1997, Zhang et al., 2003, Shomar et al., 2004, Binbin et al., 2005, Meenakshi and Maheshwari, 2006). Recently in Korea, dental fluorosis has also been reported in children who drink water with elevated fluoride concentrations over a long period of time (Shin et al., 1998, Yi et al., 2001, Choi et al., 2004).

Evaluating the causes for elevated fluoride concentrations in natural waters entails the identification of the fluoride source, but also requires an understanding of solubility, transport and sinks for fluoride ion. Thus, a variety of geochemical studies have been performed on various aspects of fluoride in groundwater. In particular, some researchers have studied the relationship between fluoride concentration and water–rock interaction in various aquifers with different geologic settings (Nordstrom and Jenne, 1977, Edmunds et al., 1984, Nordstrom et al., 1989, Gaciri and Davies, 1993, Saxena and Ahmed, 2003). Fluoride concentrations frequently are proportional to the degree of water–rock interaction because fluoride primarily originates from the geology (Gizaw, 1996, Banks et al., 1995, Dowgiallo, 2000, Frengstad et al., 2001, Carrillo-Rivera et al., 2002). It is also reported that fluoride in groundwater is negatively related to anthropogenic contaminants that may infiltrate from the land surface (Pertti and Backman, 1995, Kim and Jeong, 2005). Potential sources of fluoride in groundwater include various minerals in rocks and soils, such as topaz (Al₂(F,OH)SiO₄), fluorite (CaF₂), fluorapatite (Ca₁₀(PO₄)₆F₂), cryolite (Na₃AlF₆), amphiboles (Ca,Na,K)_0–1(Ca,Fe,Li,Mg,Mn,Na)₂(Al,Cr,Fe,Mg,Mn,Ti)₅(Al,Si,Ti)₈O₂₂(OH,F,Cl)₂ and micas (K,Na,Ca,Ba)(Al,Cr,Fe,Li,Mg,Mn,V,Zn)_2–3(Al,Be,Fe,Si)₄O10(OH,F)₂ (Handa, 1975, Pickering, 1985, Wenzel and Blum, 1992, Bardsen et al., 1996, Subba Rao and Devadas, 2003). Fluorite (CaF₂) has generally been considered a dominant source of groundwater fluoride, especially in granitic terrains (Deshmukh et al., 1995, Shah and Danishwar, 2003). However, its solubility in fresh water is low and its dissolution rate is remarkably slow (Nordstrom and Jenne, 1977). Therefore, some researchers suggest that high fluoride concentrations in groundwater are more likely to result from the dissolution of biotite, which may contain fluorine at the OH^– sites of the octahedral sheet (Nordstrom et al., 1989, Li et al., 2003, Chae et al., 2006b). Positive correlations between fluoride and SiO₂ as well as between fluoride and Na ion also support the silicate origin of fluoride (Koritnig, 1992, Chae et al., 2006a). It has been also reported that low calcium and high alkalinity favor high fluoride content (Meenakshi and Maheshwari, 2006). However, quantitative assessments of fluoride enrichment in natural waters, especially through the study of fluid–mineral equilibria, are sparse (Saxena and Ahmed, 2001).

In South Korea, excessive fluoride concentrations are frequently encountered in deep groundwater (Kim and Choi, 1998, Yum and Kim, 1999, Kim et al., 2000, Jeong et al., 2003, Kim and Jeong, 2005), and have created a serious problem by limiting the use of this groundwater. The fluoride concentration in deep groundwaters in Korea tends to be dependent upon aquifer lithology and well depth (Lee et al., 1997); the higher fluoride concentrations are typically observed in deep wells, as well as in granitic and gneissic aquifers, a trend which has been recognized in many studies on groundwaters in granitic settings (White et al., 1963, Yun et al., 1998a). Hwang, 2001, Hwang, 2002 reported that the high fluoride concentration is related to the distribution of Cretaceous granitoids in Korea. Results of batch experiments with Mesozoic granite and biotite by Chae et al. (2006b) also indicated that the dissolution of F-bearing biotite might be the primary source of high fluoride concentrations in groundwater. Therefore, we suggest that fluoride concentration in bedrock groundwater can be effectively used as an indicator of the subsurface geology and depth of groundwater circulation. Positive correlation between fluoride concentration and well depth is caused by the increase of temperature and residence time with increasing depth, which enhances the dissolution of fluorine-bearing minerals in rocks (Nordstrom et al., 1989, Saxena and Ahmed, 2003).

The purpose of this study was to outline the relationships between the fluoride concentration in groundwater and geology and to explain the geochemical behavior of fluoride in deep bedrock groundwater using the hydrogeochemical study of 377 collected groundwater samples. The results of this study will provide a better understanding of high fluoride concentrations in groundwater in an attempt to develop management plans for developing the deep bedrock groundwater. In addition, these results can be used for developing cost-effective treatment of high fluoride groundwater.