Geochemical assessment of soils in districts of fluoride-rich and fluoride-poor groundwater, north-central Sri Lanka

Journal of Geochemical Exploration 114 (2012) 118–125

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Geochemical assessment of soils in districts of fluoride-rich and fluoride-poor groundwater, north-central Sri Lanka D.T. Jayawardana a,⁎, H.M.T.G.A. Pitawala b, 1, H. Ishiga a, 2 a b

Department of Geoscience, Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan Peradeniya University, Department of Geology, 20400, Peradeniya, Sri Lanka

a r t i c l e

i n f o

Article history: Received 15 May 2011 Accepted 11 January 2012 Available online 18 January 2012 Keywords: Geochemistry Fluoride Soil Groundwater

a b s t r a c t High contents of fluoride in groundwater are a controversial issue in the dry zone of Sri Lanka. This study describes the geochemistry of residual soils from relatively fluoride-rich (b 8 mg/L; mean 2.0 mg/L) and fluoride-poor (b 1 mg/L; mean 0.4 mg/L) groundwater sites in the dry zone to identify possible sources for fluoride. Abundances of 22 major and trace elements were determined in 74 soil samples using X-ray fluorescence. The results show that soil fluoride is lower than average upper continental crust and basement rocks in both the fluoride-rich (b 411 mg/kg) and fluoride-poor (b 277 mg/kg) groundwater sites. Negative linear correlation exists between fluoride in the soil and the groundwaters, suggesting that fluoride is readily leached to water rather than being retained in the unconsolidated sandy clay loam soils. Weathering of heavy minerals such as zirconium, apatite, fluorite, monazite and garnet are the main source for the soil in the fluoride-rich groundwater districts. In these areas Zr, Nb and Th are immobile relative to the basement, and F, CaO and P2O5 are depleted, suggesting that the loss of CaO provides favorable conditions for the leaching of F to water. Conversely, soils in the relatively fluoride-poor district are enriched in TiO2, Fe2O3, MnO, Cr, V and Sc, denoting the weathering of biotite, hornblende, garnet and pyroxenes in the basement. Primary minerals present in the soils are the main cause for the enrichment of those elements. Further, fluoride levels in the soils and subsequently in the groundwaters show links with original magmatic contrast between the basement formations in each area. Soil geochemistry suggests that the meta-igneous rocks in the fluoride-rich districts may have been influenced by a fluoride-rich residual melt, whereas the fluoride-poor districts are associated with acidic meta-igneous rocks and meta-sedimentary rocks. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Fluoride is an essential element for both humans and animals, and its behavior in drinking water is vital. Fluorine has high electronegativity and solubility, and hence occurs as F − in natural waters (Liu and Lipták, 2000). Optimum content (b1.5 mg/L) of fluoride in water is essential for growth of bones and formation of dental enamels. Higher contents (>1.5 mg/L) pose a threat to human health, and can cause severe health problems such as dental and skeletal fluorosis, particularly in arid and semi-arid regions of the world. For example, fluoride problems occur in some regions of India, China, the Korean Peninsula, East and North Africa, the United Kingdom, and the Western United States (Ayoob and Gupta, 2006; Bell and Ludwig, 1970; Chae et al., ⁎ Corresponding author at: Department of Geoscience, Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue 690-8504, Japan. Tel.: + 81 08056208290; fax: + 81 852326469. E-mail addresses: [email protected] (D.T. Jayawardana), [email protected] (H.M.T.G.A. Pitawala), [email protected] (H. Ishiga). 1 Tel.: + 94 812389156. 2 Tel.: + 81 852326459. 0375-6742/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2012.01.004

2007; Harrison, 2005; Jacks et al., 2005; Kim and Jeong, 2005; Reddy et al., 2010; Xiong et al., 2007; Zhu et al., 2007). Conversely, water fluoridation is necessary in Australia, New Zealand, Canada, and most South American countries, due to lack of natural fluoride. However, Scandinavian countries where dental awareness is very high use alternative sources, such as fluoride-rich toothpaste (Lennon et al., 2004). Fluorine is available in soils and waters due to the weathering of fluoride-bearing minerals predominantly of igneous origin (Breiter and Kronz, 2004; Breiter et al., 2006; Liu and Zhu, 1991; Lukkari and Holtz, 2007; Reddy et al., 2010; Totsche et al., 2000; Zhu et al., 2007). Apatite [Ca5(PO4)3F] and fluorite [CaF2] are the most common fluoride-bearing minerals. However, zircon [ZrSiO4] is also a possible source for fluoride in water or soil, as igneous zircons contain fluorine complexes (Bailey and MacDonald, 1975; Farges, 1996; Keppler, 1993). Biotite [K(Mg,Fe)3AlSi3O10(F,OH)2], hornblende [(Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8 O22(OH,F)2] and soils that consist mainly of clay minerals such as vermiculite [(MgFe,Al)3(Al,Si)4O10(OH)], kaolinite [Al2Si2O5(OH)4] and montmorillonite [(Na,Ca)0.33 (Al,Mg)2(Si4O10)(OH)2·nH2O] are also major sources of fluoride (Munoz and Ludington, 1974; Wodeyar and Sreenivasan, 1996; Zhu et al., 2007).

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The abundance of fluoride in soil is controlled by several factors. Average fluoride levels in soils are usually relatively low (407 mg/kg; Zhu et al., 2007) due to high mobility, whereas upper continental crust averages 557 mg/kg (Rudnick and Gao, 2005). This discrepancy indicates that fluoride in soil is leached out into groundwater. High groundwater fluoride values seem to be associated with the weathered basement complex in many parts of the world (Msonda et al., 2007). Intense weathering may be the main source for soil fluoride and ultimately for shallow groundwater fluoride. The migration process of fluorine in soil is complex due to absorption and desorption interactions with clay minerals, organic substances and other elements. Lower contents of clay and higher contents of organic matter reduce the amount of fluoride retained in soil (Zhu et al., 2007). Fe and Al are the main fluoride absorbers in soils. Application of agricultural fertilizers can also elevate fluoride levels in soils together with Pb, Zn and Cu (Geeson et al., 1998). The presence of soluble and/or labile fluoride in soils is the major controlling factor for the fluoride levels in groundwater. Soluble fluoride is mainly added by agricultural practices which substantially increase fluoride levels in soils under acidic pH, mainly due to dissolution of metal fluoride complexes (Manoharan et al., 2007; Zhu et al., 2006). Conversely, under alkaline condition soluble fluoride can be retained in the soil (Skjelkvåle, 1994). On the other hand, labile fluoride is contained within mineral phases, and can be readily released to groundwater at any pH (Larsen and Widdowson, 1971). Hence, weathering conditions and the maturity of soils are the main governing factors for the availability of labile fluoride in soils and subsequent transfer to groundwater (Zhu et al., 2007). The problems associated with high fluoride content in groundwater in the dry zone of Sri Lanka are well known (Dissanayake and Weerasooriya, 1986; Lennon et al., 2004). It has been found that both shallow and deep groundwater exceeded the World Health Organization (WHO) recommended levels in drinking water (>1.5 mg/L). Endemic diseases and chronic renal failure are critical health problems in the region (Dissanayake, 1991; Dissanayake and Chandrajith, 1999). Several studies have been carried out to determine water chemistry and its relationship to the basement geology. For example, hydrogeochemical studies infer that higher fluoride content in groundwater is associated with a mineralized belt with a strong enrichment of Cu, Zn and V in meta-igneous rocks such as metabasites and charnockites (Dissanayake and Weerasooriya, 1986). Changing mineral constituents (hornblende and biotite) in rocks imply clear contrast with fluoride availability in water (Dharmagunawardhane and Dissanayake, 1993). Despite the availability of fluoride-rich minerals in the basement rocks, local and regional water movements in regolith aquifers are thought to control the fluoride levels in water (Jayawardana et al., 2010). However, the sources of fluoride-rich water and the mechanism of movement from source to groundwater are still controversial, because the fluoride concentration in water depends on many factors. Although many studies have been carried out, the relationship between fluoride content in water and soil geochemistry have not yet been examined, even though there may be a strong relationship between them, especially in shallow water bodies. Hence, such a study would improve understanding of the relationship of groundwater fluoride with trace elements in soil. Therefore, the scope of this study is to investigate soil geochemistry in fluoride-rich and fluoride-poor groundwater districts in the dry zone of Sri Lanka to determine a possible source and evaluate the relationship of fluoride with trace elements in the soil. 2. Study area and physical setting A sampling campaign was undertaken in the dry zone of Sri Lanka in Madirigiriya, Talawa and Padaviya (Fig. 1). Paddy cultivation is the dominant agricultural practice in the area, and farming continues throughout the year. The vast majority of the populations in these rural districts are dependent on groundwater for domestic use. The

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Fig. 1. Map showing geology of Sri Lanka and location of sampling sites. Dashed lines denote climatic boundaries, crosses denote sample locations.

area has a tropical climate with average annual precipitation below 1000 mm, and average annual evaporation of 1400 mm. The average annual temperature is 33 °C and average 2555 h of sunshine annually (Chandrapala and Wimalasuriya, 2003). 2.1. Hydrological setting The groundwater flow system in Sri Lanka has been widely investigated (Cooray, 1984; Dissanayake and Weerasooriya, 1986; Jayasena et al. 2008; Panabokke and Perera, 2005; Villholth and Rajasooriyar, 2010). Water recharge occurs in the central mountainous forest regions (>1000 ft), and a centrifugal water flow pattern brings water to the dry zone discharge regions through streams and alluvial aquifers present in the flood plains (Herath, 1984). Nevertheless, the study sites contain shallow regolith aquifers which have their own recharge and discharge systems which are sensitive to local rainfall (Cooray, 1984; Jayasena et al., 2008). During the rainy period (November–January) the area receives rain of relatively low pH (5.3–7.5; Ileperuma, 2000). The rain water gently percolates along soil horizons, hence water-soluble ions in the soils such as fluoride are readily released to the groundwater system. The regolith aquifers have no continuous body of groundwater in a single water table, instead separate pockets of groundwater are present. The water-bearing formation of the system is weathered basement (thickness 1–5 m) and deep fractures in the basement rocks (Panabokke and Perera, 2005). The water table fluctuates over a range of a few meters depending on local rainfall. In the dry season it is lower, and in the rainy season it rises dramatically. 2.2. Soil characteristics Dry zone soils differ from the other parts of the country due to different rates of leaching and lower weathering intensity (Chemical Index of

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Alteration b 60; Jayawardana and Izawa, 1994; Fernando et al., 2001). The soils are typically Fe-enriched products deficient in exchangeable cations (Shamshuddin et al., 1995). Residual soils in this zone have been formed from a variety of rocks, mainly metabasites, charnockitic and granitic gneisses, and subordinate metasedimentary rocks. The soils are typically immature (Herath, 1984) and hence there is no clear contrast among the soil horizons. The upper parts of the horizons consist mostly of organic matter rich highly weathered loams. However, the underlying zones are immature, moderately-weathered sandy clay loams. Madirigiriya and Talawa soils are reddish-brown earths which contain some primary quartz, zircon, monazite, garnet, with lesser biotite and hornblende. Padaviya soils are reddish to yellowish soils containing relatively higher quantities of primary biotite and hornblende with quartz and feldspar (Herath, 1973; Navaratne et al., 1996). 2.3. Geology and geochemistry More than 90% of the Precambrian high-grade metamorphic rocks in Sri Lanka belong to three major lithotectonic units known as the Highland, Wanni, and Vijayan Complexes (Fig. 1). The study sites are located in the northern section of the Highland Complex, which is composed of meta-igneous rocks and concordant meta-sedimentary rocks (Cooray, 1984, 1994). The meta-sedimentary rocks are mostly Proterozoic–Archaean meta-pelites, meta-arkoses and meta-greywackes (Dissanayake and Munasinghe, 1984). The meta-igneous rocks comprise granitic, charnockitic and quartzofeldspathic gneisses, along with metabasites and syenitic gneiss (Jayawardena and Carswell, 1976). Metabasites and charnockitic gneisses are the common basement rocks in the Madirigiriya and Talawa districts. Major mineral constituents include hypersthene, diopside, hornblende and biotite. Highly resistant minerals such as ilmenite, zircon, monazite and garnet are common accessories. Conversely, the Padaviya area is characterized by acidic rocks such as quatzofeldspathic gneisses and granitic gneisses, along with meta-pelites (Dissanayake and Munasinghe, 1984). Quartz and feldspar are major mineralogical constituents, along with lesser hornblende, hypersthene and accessory minerals (Cooray, 1984, 1994). The Highland Complex rocks display a marked compositional gap between 57 and 62 wt.% SiO2, and are characterized by pronounced Fe2O3, TiO2, P2O5, MnO, Zr, Pb, Th, Sr and Nb enrichment, especially in alkaline rocks (Cooray, 1984; Pohl and Emmermann, 1991). Major and trace element trends in charnockites and metabasites suggest they belong to igneous differentiation sequences (Munasinghe and Dissanayake, 1980). Further, composition of the rocks reflect that higher levels of fluoride are mostly present in metabasites (0.50 wt.%) and charnockitic gneisses (0.41 wt.%) however granitic gneisses contain relatively lower values (0.07 wt.%) while meta-sedimentary rocks show rather lesser concentrations (Pohl and Emmermann, 1991). 3. Materials and methods Seventy four representative residual soil samples were collected from the zone of accumulation in relatively fluoride-rich groundwater sites in the areas of Madirigiriya and Talawa (depth range 0.6–3.0 m) and fluoride-poor sites at Padaviya (depth range 0.6–4.6 m), using a hand auger. Three replicate samples were collected from each site. Fluoride concentration in the groundwater, geomorphology, basement geology and anthropogenic impacts were the major factors in the selection of sampling sites. Samples were collected from immature, moderately-weathered layers which are mostly sandy clay loams. The samples were analyzed for major and trace elements by X-ray fluorescence spectrometry (RIX 2000) at the Department of Geoscience in Shimane University. Splits of each sample were oven-dried for 48 h at 160 °C. Powdered samples (b63 μm) were compressed into briquettes under a force of 200 kN for 60 s. The briquettes were then analyzed for selected major oxides (TiO2, Fe2O3, MnO, CaO and P2O5) and trace

elements (As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc, Br, I, F and total sulfur). Average error for these elements is less than± 10% relative. Water samples were analyzed during the intermediate season (March to April in 2008). Samples from domestic water production wells at each soil sampling site (74 samples) were analyzed for fluoride using of the 4-pyridine carboxylic acid colorimetric field testing method. Detection limit for the fluoride is 0.2 mg/L (range 0.2–8.0 mg/L). The pH was estimated using a Horiba digital multi-parameter meter D-series instrument. Average accuracy for the analysis is ±0.1 for fluoride and pH. The domestic wells sampled were individual wells which had been constructed by the local community, mainly for private drinking water supply. Most of the wells were shallow, and water is extracted by manual pumping. Samples were collected after 5 min of constant rate pumping. Average values of three measurements were calculated for each sample. The average yield of the wells ranges from 1 to 25 L/s, and typically approximately 500–1000 L of water is extracted daily for domestic purposes (Panabokke and Perera, 2005). 3.1. Principal component analysis The Minitab 14 statistical package (Minitab Inc.) was used for statistical analysis of the soil geochemical data. Multivariate principal component (PC) was used to identify the relationships between the elements, using the following major steps: (1) normality test was used to determine well-modeled data set; (2) the Pearson correlation matrix (p b 0.01) was calculated; (3) any components that only accounted for a small proportion of the variation in the data sets were discarded (Ouyang, 2005); and (4) PC 1, 2 and 3 were calculated, to represent more variation while maintaining an eigenvalue of greater than one. Principal component analysis is one of the most widely used techniques in statistical analysis. It is useful for combining several correlated variables into a single variable. The dimensions of datasets are reduced into uncorrelated principal components based on covariance or correlation, which represents the inter-relationships among the variables (Jolliffe, 2002). This technique has been used to evaluate multi-element geochemical data of various types, because it reveals the correlation structure of the elements, determined by the geological processes which affected the geochemical data (Chandrajith et al., 2005; Jayawardana et al., 2012; Ouyang, 2005; Pattan et al., 1995; Zuo, 2011). 4. Results and discussion Concentration ranges and mean values for the analyzed soil samples from each locality are shown in Table 1. Fluoride concentrations in the shallow groundwater of Madirigiriya and Talawa areas are relatively high, whereas Padaviya is characterized by low fluoride content. Mean fluoride values in the groundwater in Madirigiriya, Talawa and Padaviya are 2.4 mg/L (range 0.2–8.0 mg/L) 1.7 mg/L (range 0.2–4.0 mg/L) and 0.4 mg/L (range 0.2–1.0 mg/L) respectively (Table 1). The groundwater samples represent the shallow regolith aquifers. Average depths at Madirigiriya (average 6.7 m; range 4.6–9.1 m) and Talawa (6.4 m; range 3.0–10.7 m) are somewhat shallower than at Padaviya (9.4 m; range 3.0–13.7 m). Plots of average soils normalized against UCC (Rudnick and Gao, 2005) and basement rock averages (Pohl and Emmermann, 1991) are given in Figs. 2 and 3, respectively. The Madirigiriya and Talawa soils are both characterized by high Br and I, and both elements are significantly enriched relative to UCC, whereas F is strongly depleted relative to both UCC and the basement rocks (Figs. 2, 3a and b). Average TiO2, Fe2O3, MnO, P2O5, Pb, Zn, Cr, Sr and Sc contents of the soils in both areas are comparable with UCC, whereas CaO, As, Cu, Ni, Y and Th are slightly depleted. Average V and Zr contents of the soils in both areas are slightly enriched relative to UCC, but Nb shows variable levels. Compared to the basement rocks, the soils are characterized by (i) comparable levels of TiO2, MnO, Pb and Zn (ii) higher concentrations of Fe2O3

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Table 1 Summary of trace and major element abundances in soils and the hydrogeochemistry of groundwater from the Madirigiriya, Talawa, and Padaviya districts in the dry zone of Sri Lanka. TS—total sulfur. Madirigiriya (N 24) Min Major elements (wt%) TiO2 0.51 Fe2O3 1.90 MnO 0.03 CaO 1.27 P2O5 0.11 Trace elements (mg/kg) As 1 Pb 10 Zn 16 Cu 7 Ni 11 Cr 57 V 62 Sr 200 Y 7 Nb 3 Zr 43 Th 2 Sc 3 Br 2 I 2 F 4 TS 301 Groundwater F (mg/L) pH Depth (m)

0.2 7.2 4.6

Max 1.04 11.46 0.23 4.23 0.33

7 39 100 39 54 189 310 756 23 9 392 11 46 5 36 411 645

8.0 7.9 9.1

Talawa (N 20) Mean 0.71 4.94 0.11 2.17 0.18

3 18 56 20 28 97 129 372 14 6 242 6 15 3 25 121 473

2.4 7.4 6.7

SD 0.16 2.61 0.05 0.99 0.06

1 7 23 9 15 34 65 170 5 2 103 2 12 1 8 93 106

2.4 0.2 1.5

Min 0.40 4.55 0.06 1.02 0.06

2 15 35 13 14 61 118 185 12 7 157 1 6 3 7 47 362

0.2 5.9 3.0

and (iii) slight depletion in CaO and P2O5 (Fig. 3a and b). Copper, Ni and Cr are enriched relative to charnockites. However, the soils have lower or comparable values of those elements compared to metabasites. Average Zr and Th contents are comparable to the basement rocks, and Y is slightly depleted. As at Madirigiriya and Talawa, bromine and iodine in the Padaviya soils are significantly enriched relative to UCC. Fluorine is also depleted relative to both UCC and the basement rock averages (Figs. 2 and 3c), although less so than at Madirigiriya and Talawa. UCC normalization shows the Padaviya soils are somewhat enriched in TiO2, Fe2O3, MnO, P2O5, Cu, Cr, V and Sc, slightly depleted in CaO, As, Nb, and Zr, and strongly depleted in Th. Compared to the basement TiO2, MnO and CaO are slightly enriched with respect to quatzofeldspathic/granitic rocks, but show identical average values relative to metapelites (Fig. 3c). In addition, Cu, Ni and Cr are strongly enriched relative to the quartzofeldspathic/granitic average, whereas Pb, Cu and Cr are slightly enriched relative to metapelites. Zinc and Ni contents are almost similar. Furthermore, compared to the both basement rocks, the Padaviya soil average shows strong Fe2O3 enrichment, slight P2O5 enrichment, and significant Y, Nb, Zr and Th depletion.

Fig. 2. Average major and trace element compositions of the soil samples normalized against average UCC composition (Rudnick and Gao, 2005).

Padaviya (N 30)

Max

Mean

1.17 8.20 0.37 2.08 0.21

8 50 150 34 38 109 237 397 24 24 594 15 25 5 36 222 1096

4.0 7.4 10.7

0.89 6.03 0.13 1.49 0.12

4 24 66 22 26 76 170 309 18 15 337 8 12 4 17 115 563

1.7 6.9 6.4

SD 0.20 1.14 0.07 0.30 0.04

2 9 31 6 6 15 32 53 4 6 109 4 4 1 8 55 191

1.2 0.4 2.1

Min 0.40 5.01 0.01 0.54 0.10

1 7 24 2 11 72 103 11 6 3 59 1 2 2 2 21 260

0.2 6.6 3.0

Max 2.11 20.18 0.34 4.44 0.38

24 163 226 164 111 451 807 558 74 14 291 4 69 6 26 277 730

1.0 7.8 13.7

Mean 1.00 10.26 0.16 2.71 0.21

3 23 81 43 45 165 287 297 19 6 147 2 29 3 12 124 461

0.4 7.1 9.4

SD 0.43 3.67 0.08 1.01 0.08

5 31 39 28 23 88 140 127 12 3 61 1 13 1 7 75 128

0.2 0.3 3.4

Multivariate paired t-tests using Minitab 14 software under 95% confidence interval indicate that there are significant differences between Madirigiriya and Talawa from Padaviya in abundances of TiO2 (t= 2.85 p = 0.008), Fe2O3 (t= 7.17 p = 0.0001), Mn (t= 2.74

Fig. 3. Average bulk concentrations of selected major and trace elements of the soil samples at (a) Madirigiriya, (b) Talawa and (c) Padaviya normalized against average basement rocks (Pohl and Emmermann, 1991).

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p = 0.011), Cu (t= 3.71 p = 0.001), Ni (t= 4.30 p = 0.0001), Cr (t= 4.25 p = 0.0001), V (t= 5.41 p = 0.0001), Zr (t= 4.42 p = 0.0001) and Th (t= 6.17 p = 0.0001). The mean values of Zr and Th are higher in the areas of fluoride-rich groundwater at Madirigiriya and Talawa. In contrast, TiO2, Fe2O3, CaO, P2O5, Zn, Cu, Ni, Cr, V and Sc are higher in the area of low groundwater fluoride at Padaviya. The mean values of F are similar in all regions, but the high groundwater fluoride areas range to considerably greater maximum values (b411 mg/kg). No significant differences were observed in mean values of MnO, As, Pb, Y, Br and total sulfur in all three regions. 4.1. Principal components 1, 2 and 3 for Madirigiriya and Talawa The positive loadings of the elements (F, As, Pb, Nb, Zr, Th, I, Ca, P and Cr) in the components represent that they are derived from mineral phases, but in which the geochemical behavior is mostly governed by the physico-chemical factors involved during weathering. However, variable loadings along with PC 1, PC 2 and PC 3 are mainly due to a wide range of mobility of the elements. Fluorine in PC 1 for the Madirigiriya and Talawa data shows significant positive loadings with Zr, Nb and Th, in PC 2 a positive loading with Ca, Sr, Cr and P2O5, and in PC 3 a significant loading with Zr and Cr (Table 2). Zirconium, apatite, fluorite, monazite and garnet are probably the dominant mineral hosts for those elements (Ahijado et al., 2005; Breiter et al., 2006). Moreover, comparable abundances of Zr and Th in the soils and the basement rocks imply that these elements are stable during weathering due to their lesser mobility, whereas apparent depletion of F, Ca, Sr and P2O5 relative to the metabasites suggests that these elements are mobile during the weathering of metabasite (Fig. 3a, b). This implies that dissolution of fluoride-bearing heavy minerals due to weathering releases both immobile and mobile elements. The immobile elements are retained in the soils, whereas mobile elements such as F are freely released to groundwater. Furthermore, loss of Ca and enrichment of Cr are common features during the weathering of charnockitic and granitic rocks (Sharma and Rajamani, 2001), and consequently lower Ca in soil provides favorable conditions for the release of F to water (Jacks et al., 2005). Yttrium shows significant negative loading with PC 1, 2 and 3 with respect to the heavy mineral components (Zr, Nb and Th) because it is depleted during the formation of charnockites (Hansen et al., 1987; Prame, 1991) (Fig. 3a, b). Hornblende and biotite are common in the basement rocks such as metabasites (Cooray, 1984, 1994), but are less abundant in the soils, indicating that weathering of these minerals may also release labile fluoride to water (Larsen and Widdowson, 1971). However, elements which are associated with these minerals such as, Fe2O3, V, Sc, Cr and Ni are somewhat negatively loaded with soil fluoride. Significant enrichments of Fe2O3, Ni and Cr (Fig. 3a, b) in the soils usually occur during weathering of ferromagnesian heavy minerals, also due to complexing of these elements with clay minerals (Nesbitt and Young, 1996; Sharma and Rajamani, 2001). In addition, clay usually acts as an absorber of F. However, the soils studied here are mainly unconsolidated sandy clay loams which may have lower potential to absorb F and the ferromagnesian elements (Wodeyar and Sreenivasan, 1996; Zhu et al., 2007). Weathering of ferromagnesian minerals also releases iodine to the soils. However, due to lesser reactivity iodine is retained in the soils more than F, but still shows strong positive loading with F due to similar chemical and physical behavior (Fuge and Johnson, 1986). Variable loading of As and Pb with F in PC 1, 2 and 3 implies that sulfide group minerals are less important for the availability of fluoride. Furthermore, the variability of As, Pb, Zn and Cu in the soils with respect to the basement rocks and in PC 1, 2 and 3 suggests that they are mobile during weathering (Fig. 3a, b). 4.2. Principal components 1, 2 and 3 for Padaviya The chemical compositions of the soils in the lower water fluoride area vary from those in the high-fluoride area (Fig. 3c), because

Table 2 Principal component analyses of the soil samples to discriminate major components. Factors are determined from the Pearson product–moment correlation matrix using Minitab 14 software. Variable

PC 1

PC 2

PC 3

Eigenvalue

10.8

5.1

1.4

Proportion

57%

27%

8%

Madirigiriya and Talawa As Pb Ni Cr V Sr Y Nb Zr Th Sc Br I F TiO2 Fe2O3 MnO CaO P2O5

0.245 0.227 − 0.299 − 0.292 − 0.232 − 0.264 − 0.117 0.108 0.250 0.083 − 0.297 − 0.044 0.248 0.181 0.028 − 0.276 − 0.284 − 0.294 − 0.259

− 0.127 − 0.127 − 0.033 0.068 − 0.258 0.033 − 0.345 − 0.401 − 0.193 − 0.407 − 0.047 − 0.355 0.236 0.064 − 0.418 − 0.180 − 0.023 0.064 0.113

− 0.285 − 0.417 − 0.044 0.143 0.206 − 0.379 − 0.324 − 0.166 0.247 − 0.064 0.120 0.382 0.141 0.155 0.123 0.063 0.108 − 0.055 − 0.317

PC 1 8.4 47%

PC 2 5.4 30%

PC 3 2.8 15%

0.158 − 0.276 − 0.253 0.304 0.314 − 0.290 0.279 0.292 0.127 0.043 0.042 − 0.106 0.086 0.316 0.305 0.148 − 0.304 0.225

0.356 0.221 0.234 0.073 0.151 0.195 0.168 − 0.144 − 0.373 − 0.381 0.082 − 0.370 0.281 − 0.058 0.177 − 0.026 0.175 0.286

0.164 0.152 0.205 0.149 0.101 0.068 0.241 0.142 − 0.045 0.217 − 0.571 0.127 − 0.275 0.181 − 0.109 − 0.505 0.075 0.145

Variable Eigenvalue Proportion Padaviya Zn Cu Ni Cr V Sr Y Nb Zr Th Sc I F TiO2 Fe2O3 MnO CaO P2O5

basement rock mineralogy and weathering intensity differ somewhat (Herath, 1973; Jayawardana and Izawa, 1994). Positive weights of F with Fe2O3, V, Sc and Cr for the PC 1 and 2 and for Ca and Ni in PC 2 in Padaviya (Table 2) may reflect the presence of biotite, hornblende, garnet and pyroxene as the dominant mineral hosts in those soils. These primary minerals are abundant in the Padaviya soils, suggesting that the soils are immature and intensity of weathering is lower (Herath, 1973). Stronger enrichment of TiO2, Fe2O3, Ni, Cr, CaO and P2O5 in Padaviya soils than in those at Madirigiriya and Talawa (Fig. 3) further implies that primary minerals such as mica are present in the former (Nesbitt and Young, 1996). This further supports lower weathering intensity at Padaviya, and this may be a controlling factor for the lesser release of fluoride to the groundwater. Moreover, these mica group minerals may have derived from metasedimentary rocks and are mainly hydrous, and hence F contents are low, leading to lower values of fluoride in the soils. Considerably higher weights given to P2O5 and Y in PC 1, 2 and 3; TiO2 and Nb in PC 1 and 3; MnO in PC 1 and the positive weight given to Sc in PC 1 and 2 (Table 2) may indicate that the soils also contain highly resistant minerals such as apatite, monazite and rutile (Chandrajith et al., 2005). The positive loading of Zn in PC 1, 2 and 3 and Cu in PC 2 and 3 may be associated

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with weathering products of sulfide minerals which have been reported from meta-sedimentary rocks in the area (Dissanayake and Munasinghe, 1984; Prame, 1991). Zinc and Cu in the soils are more enriched than in meta-igneous rocks, but are similar to metasedimentary rocks such as metapelites (Fig. 3c). Furthermore, higher abundances of Fe2O3 in the soils and strong positive Pearson correlation coefficients of V with Mn (R=0.78; Pb 0.01) and Cr (R=0.63; Pb 0.01) implies that the Padaviya soils contain Fe–Mn–V rich garnet. 4.3. Relationships of halogens in soil and water Fluoride in water and soil of Madirigiriya, Talawa and Padaviya areas is negatively correlated, with least squares regression coefficients of −0.41 (P b 0.01), −0.80 (P b 0.01) and −0.70 (P b 0.01) respectively (Fig. 4). This implies that fluoride may have leached from the soils into the shallow aquifers. A similar observation was made by Dissanayake and Weerasooriya (1986) and Dharmagunawardhane and Dissanayake (1993), who suggested that labile fluoride in mineral phases is the source for groundwater fluoride. Due to the strong electronegativity and reactivity of fluoride, weathering of rocks/minerals can release fluoride to water under any pH conditions (Liu and Lipták, 2000). Fluoride-bearing minerals are mostly present in the basement rocks in the Madirigiriya and Talawa districts, and their residual soils are mostly unconsolidated sandy clays with low organic matter contents and neutral pH. Consequently, the prevailing conditions are favorable for movement of labile fluoride from the soils to the water, rather than being retained in the soils (Larsen and Widdowson, 1971; Pickering, 1985; Zhu et al., 2007). Although Padaviya soils have similar conditions, fluoride-bearing minerals are less abundant and weathering intensity is less than at Madirigiriya and Talawa, leading to lower water fluoride contents. Iodine abundance of the soil is extremely high (Fig. 2). In general, iodine in rocks is associated with minerals such as biotite, hornblende, hypersthene and fluorite (Fuge and Johnson, 1986). Weathering of these phases increases the iodine content of soils, because iodine does not leach easily into water due to its very low electronegativity and reactivity (Fuge and Johnson, 1986; Liu and Lipták, 2000), and hence iodine concentrations in the groundwater are low (b84 μg/L, Fordyce et al., 2000). On the other hand, bromine is slightly enriched, possibly because it is readily bound to organic substances in soils (Gerritse and George, 1988) (Fig. 2). The electronegativity and reactivity of Br is somewhat greater than that of iodine, and hence significant amounts of Br can leach into groundwater. Although halogen-bearing minerals

Fig. 4. Correlations between groundwater fluoride and soil fluoride. Fitted regressions soil and water samples for each location.

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are present in both the parent rocks and soils, the wide range of electronegativity of these elements means their mobility to water differs, and thus correlations between them are weak [F and Br (R= −0.07; P b 0.01), F and I (R= 0.08; P b 0.01) and Br and I (R= −0.33; P b 0.01)]. 4.4. Relationship between fluoride in water and basement rock lithology Metabasites and charnockitic gneisses in the Madirigiriya and Talawa districts contribute significantly to fluoride concentrations in water. The basement rocks underlying wells with very high groundwater fluoride (b8 mg/L) in the Madirigiriya area are mainly metabasites that contain an average 0.50 wt.% of fluoride (Pohl and Emmermann, 1991). The metabasites consist mainly of hornblende and biotite with accessory apatite and fluorite, and these phases may be the dominant sources for the water fluoride (Breiter et al., 2006; Liu and Zhu, 1991). The Talawa area is underlain by hypersthene and diopside-rich charnockitic gneisses that have an average total fluoride of 0.41 wt.% (Pohl and Emmermann, 1991). This study shows that the fluoride content of shallow groundwater in the Talawa area is slightly lower than that at Madirigiriya. Breakdown of fluoride-bearing minerals such as hornblende and biotite to pyroxenes during the formation of charnockites may lower the fluoride content of such rocks (Hansen et al., 1987). Granitoids, quartzofeldspathic gneisses, acidic charnockites and meta-sedimentary rocks in the Padaviya area contain less than 0.07% fluoride (Pohl and Emmermann, 1991), leading to lower fluoride contents in the groundwater (b1 mg/L). These features imply that the fluoride levels in groundwater depend mainly on the basement lithology. 4.5. Genesis over the high groundwater fluoride The geochemical composition of the in-situ weathered residual soils in the fluoride-rich and fluoride-poor groundwater site districts is mainly inherited from their parental meta-igneous rocks. Most of those rocks are considered as representative of metamorphosed suites of basalts. Some meta-sedimentary rocks in the fluoride-poor site (Padaviya) represent metamorphosed clastic sediments that were originally deposited in a marine basin (Dissanayake and Munasinghe, 1984; Munasinghe and Dissanayake, 1980). Immobile trace element relationships of in-situ weathered soils and sediments provide useful clues on provenance (Bhatia and Crook, 1986; Roser, 2000). A Zr–Ti/100 − Y ∗ 3 diagram for the soils (Fig. 5a) clearly demarcates possible magmatic contrasts of the basement in the higher and lower water fluoride sites. The basements in the fluoride-rich areas (Madirigiriya and Talawa) are mainly discriminated as Zr-rich basaltic compositions generally comparable with Highland Complex metabasites and charnockites (Pohl and Emmermann, 1991; Prame, 1991). In addition, positive loadings of Zr, Th, Cr and Nb in the principal components indicate that significant enrichment of heavy minerals in the high fluoride zone (Table 2) may have been caused by formation of a residual fluoride-rich melt during the original magmatic event (Farges, 1996; Keppler, 1993). The basement rocks in the fluoridepoor area at Padaviya are relatively Ti-rich (Fig. 5a), and are enriched in Fe2O3, MnO, P2O5, Cu, Cr, V and Sc (Table 2). This association of elements may represent hydrous minerals such as hornblende in granitic and quartzofeldspathic gneisses, which coexist with metasedimentary rocks in the area (Pohl and Emmermann, 1991). A Th/Sc–Zr/Sc diagram (Fig. 5b) further illustrates the magmatic contrast (Fig. 5b). Samples in the Madirigiriya and Talawa fluoride-rich sites range from basaltic to felsic compositions with strong Zr enrichment. Padaviya soils show andesitic to felsic volcanic compositions, and may also reflect the compositions of meta-sedimentary rocks in the area (Dissanayake and Munasinghe, 1984; Munasinghe and Dissanayake, 1980). The presence of fluoride in the soils and waters are closely linked to geochemical variations in the soils, and also compare to original

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Fig. 5. Possible igneous contrasts in fluoride-rich and fluoride-poor groundwater districts. (a) Zr–Ti/100 − Y ∗ 3 plot (Pearce and Cann, 1973); basement metabasites, charnockitic and granitic gneisses (Pohl and Emmermann, 1991); (b) Th/Sc and Zr/Sc plot (McLennan et al., 1993; after Roser, 2000). Circles: igneous rock averages from Condie (1993); FEL = Mesozoic–Cenozoic felsic volcanic rock; AND, BAS and FVR = andesite basalt and felsic volcanic rocks, respectively.

magmatic signatures in the parent geological formations (Fig. 5a and b). The results of this study suggest that Zr–F rich magma may have acted as the main fluoride source during the formation of Highland Complex supracrustal rocks (Breiter et al., 2006; Dissanayake and Munasinghe, 1984; Farges, 1996). Anhydrous pyroxene granulites and amphiboles may have originated by replacement of OH groups by fluoride, and also formation of fluoride-rich accessory minerals such as apatite, topaz and fluorite (Prame, 1991). Furthermore, movement of the magma may have strongly influenced the boundary regions of the Highland Complex (Fig. 1), and hence fluoride-rich waters have mainly been reported in that area. In particular, Madirigiriya lies close to the boundary between the Highland and Vijayan complexes (Fig. 1). This boundary is regarded as a mineralization zone related to magmatic events occurring during Highland and Vijayan complex tectonic collision (Dissanayake and Munasinghe, 1984). Groundwater in the central part of the Highland Complex is usually fluoride-poor, possibly due to lesser influence of primary F-rich fluid. Hydrous hornblende granulites are thus common, and meta-sedimentary formations are also dominant in that area.

5. Conclusions The geochemistry of soils from the Madirigiriya, Talawa, and Padaviya districts suggests that fluoride is not readily retained, leading to high values in groundwater. The high mobility of fluoride from soil to water is facilitated by immature soils which have sandy clay loam texture containing some primary minerals and rock fragments. The results suggest that weathering of heavy minerals including zirconium, apatite, fluorite, monazite and garnet are the dominant mineral hosts in the soils in fluoride-rich groundwater districts. Relative to the basements rocks, Zr, Nb and Th are stable, whereas F, CaO and P2O5 are depleted. Loss of CaO provides favorable conditions for the leaching of fluorine to groundwater. Conversely, in fluoride-poor groundwater districts the soils are enriched in TiO2, Fe2O3, MnO, Cr, V and Sc, reflecting the weathering of biotite, hornblende, garnet and pyroxenes in the basement. The primary minerals present in these soils are the main cause for the enrichment of those elements. The soil geochemistry may also reflect original magmatic contrasts in the meta-igneous rocks, which then led to variations in fluoride in the dry zone of Sri Lanka. The greater influence of the primary fluoride-rich fluid that formed the metaigneous rocks in the fluoride-rich groundwater districts caused enrichment of fluoride-bearing heavy minerals.

Acknowledgements We thank Professor Yoshihiro Sawada of Shimane University for access to the XRF facilities. Dr. Barry Roser of Shimane University is acknowledged for his constructive review, which improved the manuscript considerably. We also acknowledge the head of Department of Geology and staff of Peradeniya University for their great support by providing laboratory facilities, and help with collection and handling of samples. This study was supported by a MEXT (Monbukagakusho) graduate scholarship to DTJ.

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