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Geochemical controls of high fluoride groundwater in Umarkot Sub-District, Thar Desert, Pakistan
Science of the Total Environment 530–531 (2015) 271–278
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Geochemical controls of high fluoride groundwater in Umarkot Sub-District, Thar Desert, Pakistan Tahir Rafique a,⁎, Shahid Naseem b, David Ozsvath c, Riaz Hussain d, Muhammad Iqbal Bhanger e, Tanzil Haider Usmani a a
Applied Chemistry Research Centre, PCSIR Laboratories Complex, Karachi 75280, Pakistan Department of Geology, University of Karachi, Karachi 75270, Pakistan Department of Geography/Geology, University of Wisconsin-Stevens Point, USA d Geophysical Division, Geological Survey of Pakistan, Karachi, Pakistan e HEJ Research Institute of Chemistry, ICCBS, University of Karachi, Karachi, Pakistan b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Fluoride in groundwater ranged from 0.06 to 44.40 mg/L (mean value of 5.22 mg/L). • 84% of samples exceeded 1.5 mg/L (recommended WHO limit for fluoride). • Silicate mineral weathering and evaporation controlled overall groundwater quality. • Weathering of minerals derived from Type-A granite released fluoride. • Water influenced by calcite precipitation and base ion exchange contained high F−.
a r t i c l e
i n f o
Article history: Received 28 December 2014 Received in revised form 10 May 2015 Accepted 10 May 2015 Available online 3 June 2015 Editor: Barcelo D. Keywords: Fluoride ions Mineral weathering Evaporation Calcite precipitation Base ion exchange Umarkot Thar Desert
a b s t r a c t Groundwater samples (n = 152) were collected in the Thar Desert of the Umarkot Sub-District, Pakistan to evaluate the geochemical controls on the occurrence of high fluoride (F−) levels within the study area. Fluoride concentrations range from 0.06 to 44.4 mg/L, with mean and median values of 5.22 and 4.09 mg/L, respectively; and roughly 84% of the samples contain fluoride concentrations that exceed the 1.5 mg/L drinking water standard set by WHO. The overall groundwater quality reflects the influences of silicate mineral weathering and evaporation. Fluoride originates from the weathering of minerals derived from Type-A granite and possibly anion exchange (OH− for F−) on clays and weathered micas under high pH conditions. High fluoride levels are associated with Na–HCO3 type water produced by calcite precipitation and/or base ion exchange. Depleted calcium levels in groundwater allow higher fluoride concentrations to occur before the solubility limit for fluorite is reached. © 2015 Elsevier B.V. All rights reserved.
⁎ Corresponding author. E-mail address: tahirrafi[email protected] (T. Rafique).
http://dx.doi.org/10.1016/j.scitotenv.2015.05.038 0048-9697/© 2015 Elsevier B.V. All rights reserved.
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1. Introduction
2. Study area
Fluorine (F) is the lightest member of the halogen family and the most electronegative and reactive of all elements. Although trace quantities (0.05–0.1% or 500–1000 mg/kg) of fluorine occur in all crustal rocks, it is a strong lithophile and thus concentrated in silicate phases, especially in rocks which crystallize at later stages of magmatic differentiation (Lahermo and Backman, 2000). As a result, fluorine is found most abundantly in granites, alkali granites, and hydrothermal deposits (Apambire et al., 1997; Ayoob and Gupta, 2006; Naseem et al., 2010). Some of the more common fluorine-bearing minerals include fluorite (CaF2), sellaite (MgF2), fluorapatite (Ca5(PO4)3F), cryolite (Na3AlF6), villiaumite (NaF), and topaz (Al2(SiO4)F2). However, micas and amphiboles can also contain F−, which substitutes for OH− in the mineral structures (Edmunds and Smedley, 2005; Westrich, 1982). In natural waters, fluorine occurs as the monovalent fluoride (F−) anion (WHO, 2006), which is capable of forming a number of organic and inorganic complexes (Yamin et al., 2011; Hamzaoui-Azaza et al., 2009). Fluoride concentrations in surface waters are usually low, ranging from 0.01 to 0.3 mg/L (UNICEF, 1999). Concentrations in groundwater are typically less than 1 mg/L but can be much higher where the subsurface mineralogy and geochemical conditions are appropriate (Carrillo-Rivera et al., 2002; Frencken, 1992; Frengstad et al., 2001; Gizaw, 1996). Trace quantities of fluoride originate from the dissolution of amphiboles and micas (Kearns et al., 1980), and higher concentrations usually occur where minerals such as fluorite, fluorapatite, or cryolite are present; however, studies have shown that even biotite weathering can produce concentrations greater than 10 mg/L when groundwater is depleted in calcium (Chae et al., 2007). Higher levels of dissolved fluoride are generally associated with high pH and Na– HCO3 type waters, although the temperature and depth of groundwater and climatic factors can also play important roles (Chae et al., 2007; Gupta et al., 2006). Ion exchange has been found to influence fluoride levels through both base exchange, which lowers calcium concentrations (Apambire et al., 1997; Earle and Krogh, 2004; Edmunds and Smedley, 2005; Chae et al., 2006), and anion exchange, in which OH− in groundwater replaces F− on certain clay minerals or weathered micas (Apambire et al., 1997; Smedley et al., 2002; Guo et al., 2007). Fluorine is one of the unique elements that have negative effects on human health when its ingestion levels are either too high or too low. Fluoride is an essential micronutrient needed in trace quantities to strengthen the hydroxyapatite matrix of the skeletal tissues and teeth, and dental caries are a common problem among populations with insufficient fluoride intake. Prolonged exposure of fluoride at higher levels leads to fluorosis, a silent geogenic disease that presently affects about 200 million people in 28 developed and developing countries, including India, China, Pakistan, Sri Lanka, Turkey, Iran, East Africa's Rift Valley, Scandinavia, Algeria, Libya, Iraq, USA, Canada, Thailand, New Zealand and Japan (WHO, 2006). Areas of Thar Desert in the Sindh Province of Pakistan have recently been reported to have high fluoride concentrations in groundwater resources (Naseem et al., 2010; Rafique et al., 2008, 2009, 2013). There are various manifestations of fluorosis, including dental and skeletal abnormalities, crippled limbs, the calcification of ligaments, rheumatic pain, stiffness and the rigidity of the joints (Ozsvath, 2009). Excessive fluoride intake is also reported to adversely affect the gastrointestinal tract, kidneys, and liver, as well as the nervous, reproductive, and immune systems (Ibrahim, 2011). A major source of fluoride ingestion is groundwater, although dentifrices and certain foods are also important sources (EPA, 2010). The present study evaluates groundwater quality in a region that is known for its high incidence of fluorosis and was undertaken to define both the sources of dissolved fluoride and the geochemical processes that influence its occurrence. The goal is to identify geochemical relationships that might aid in understanding the genesis of high fluoride groundwater based on measures of the overall water quality.
The study area occurs within the Umarkot Sub-District of the District Umarkot, Sindh Province, in southeastern Pakistan, near its border with India (Fig. 1). The 3209-km2 Umarkot Sub-District, located between N 24° 54′ to 25° 47′ and E 69° 11′ to 70° 18′, had a population of 312,753, an average annual growth rate of 3.83%, and a population density of 97.5 people per km2 when the 1998 Census was taken (DCR, 2000). There are two distinct physiographic regions within this sub-district; irrigated lowlands (in the west-central area) and a desert marked by stabilized sand dunes (Fig. 1). The groundwater samples collected for this study came from the approximately 2411-km2 desert area, a northwestern extension of the Thar Desert, one of the largest subtropical deserts in the world. This desert, which began to form more than 150 thousand years ago (Singhvi and Kar, 2004), is generally characterized by strong winds, high diurnal temperature variations, scarce rainfall, intense solar radiation, and high rates of evaporation (Rafique et al., 2008; Sinha et al., 1996). The study area is underlain by Archean metamorphic basement rocks and Late Proterozoic (720–745 million years) igneous rocks exposed near Nagar Parkar Town (Naseem et al., 2010). Most of the Nagar Parkar Complex consists of peralkaline to peraluminous A-type granites with some rhyolite plugs and basic dykes (Ahmad and Chaudhry, 2008), and it is part of the Late Proterozoic Malani regional magmatism of Western Rajasthan, India (Ahmad and Chaudhry, 2007). These igneous and metamorphic rocks are covered by Paleocene (the Bara) and Late Pleistocene (the Bartala) sedimentary formations, which are predominantly sandstones and siltstones with minor claystone beds and granitic wash. The wash is composed of quartz and feldspar grains in a kaolinitic matrix with minor amounts of altered ferromagnesian minerals, probably hornblende and biotite. Overlying the entire subsurface sequence is a thick (~ 80 m) layer of dune sand consisting of quartz, feldspars, and a number of accessory minerals, including ferromagnesian minerals (Kar et al., 2001). Groundwater occurs under water table conditions within ephemeral aquifers that are perched on the contact between dune sand and underlying sedimentary deposits. Most of the groundwater samples for this study were collected from wells that are completed within these perched aquifers. Well depths range between 6.67 and 400 m, with mean and median values of 40.67 and 33.33 m, respectively (Table 1).
3. Materials and methods Groundwater samples (n = 152) were collected from dug-wells that were randomly distributed across the Umarkot Sub-District desert (Fig. 1). Stainless steel 5-liter containers tightened with fiber rope were used to collect groundwater samples; temperature, pH, Eh, conductivity, and total dissolved solids (TDS) were measured in the field. Samples taken for laboratory analysis were field-filtered using an ordinary filter paper and placed into 1.5-liter polyethylene bottles that had been pre-washed with dilute (1%) nitric acid according to standard methods (Greenberg et al., 1998). The bottles were first rinsed with groundwater and then completely filled and sealed with caps to avoid the oxidation of redox-sensitive constituents. All sampling information was noted in the field, and the exact sampling coordinates were identified using a GPS unit. Standard methods (APHA, 1995) were followed for analyzing the chemical characteristics of the groundwater samples. Fluoride ion (F−) concentrations were determined by the specific ion electrode method (Greenberg et al., 1998) using a total ionic strength adjusting buffer (TISAB). High levels of precision and accuracy were maintained during all phases of laboratory analysis, which included blank and spiked samples to ensure the reliability of each method. The ionic balance error (IBE) values were within an acceptable limit of ± 5% (Domenico, 1990; Mandel, 1981) for all samples.
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Fig. 1. Isoconcentration map of fluoride distribution in the groundwater of study area.
4. Results and discussion 4.1. Overall groundwater quality Table 1 summarizes the groundwater quality and several useful ion ratios for the data collected in the study area. Using the WHO guideline of 1000 mg/L total dissolved solids (TDS) (WHO, 1996) as a threshold for potable water, it is clear that much of the groundwater in this region is unacceptable for drinking. High Na and Cl concentrations also make the water unacceptable for irrigation purposes. Data were plotted on diagrams developed by Gibbs (1970) for the purpose of inferring the dominant control(s) on water quality
(Fig. 2a, b, Table 1). These plots suggest that groundwater quality in the study area is heavily influenced by evaporation, although some samples appear to reflect only the influence of weathering. This finding is not unexpected for an arid environment, and it suggests that the concentrations of major ions other than Na and Cl are largely controlled by the precipitation of lower solubility minerals (e.g., calcite) as evaporation causes salinity to increase. Hounslow (1995) summarizes the use of various ion ratios to deduce the origin of groundwater quality (Guo and Wang, 2005; Hounslow, 1995). The mean Cl/(sum anions) ratio for samples collected in the study area is 0.615 (Table 1), suggesting that dissolved substances originate from the weathering of non-evaporate minerals (with TDS
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Table 1 Basic statistical parameters of groundwater samples (n = 152) of desert part of Umarkot Sub-District. Ions are in mg/L, except depth (m), EC (μS/cm), temperature (°C), pH and ratios. Parameters
Min.
Max.
Mean
Median
Depth of well pH EC Temp. TDS Na K Ca Mg Cl HCO3 SO4 F− Na/(Na + Ca) Cl/(Cl + HCO3) Mg/(Ca + Mg) Cl/Σanion Na/Ca SAR Ca/(Ca + SO4)
6.67 6.21 654 25.4 518 50 4 2 1 25 61 29 0.06 0.25 0.07 0.004 0.057 0.33 0.29 0.02
400.00 9.54 50,200 42.2 39,067 12,900 147 1396 2000 21,500 2288 3011 44.40 0.99 0.99 0.757 0.895 185.00 66.12 0.75
40.67 7.66 10,422 29.9 6795 1986 36 229 174 3171 441 557 5.22 0.84 0.73 0.426 0.615 14.04 9.80 0.29
33.33 7.61 7935 29.4 5038 1575 28 139 94 2033 345 420 4.09 0.89 0.81 0.472 0.685 8.48 8.70 0.26
values N 1000 mg/L reflecting the added influence of evaporation). The average Mg/(Ca + Mg) ratio of 0.426 (Table 1) is typical of groundwater quality resulting from the weathering of granitic rocks, which are relatively low in ferromagnesian minerals (Cruz and Amaral, 2004; Moghaddam and Fijani, 2008). Chadha (1999) proposed a hydrogeochemical classification scheme for water by plotting the difference between (CO3 + HCO3) and (Cl + SO4) concentrations in meq/L expressed as percent versus (Ca + Mg) and (Na + K) concentrations in meq/L also expressed as percent (Chadha, 1999). This classification is a modified version of the Piper diagram (Piper, 1944) and an expanded version of Durov diagram (Durov, 1948). The resulting diagram has four fields, representing four types of hydrogeochemical processes (Fig. 3). Nearly 84% (N = 127) of the samples collected in the study area are Na–Cl type waters, confirming that evaporation is the dominant process controlling groundwater quality. Of the remaining samples, 8% (N = 12) are Ca–Mg–Cl type waters, 7% (N = 11) are NaHCO3 type water, and only two are classified as Ca–HCO3 type waters (Fig. 3). The latter two samples are presumed to represent the water quality of recharge that is little affected by evaporation. Although carbonate minerals are not prevalent within the study area, calcium-bearing minerals such as anorthite, wollastonite, and diopside are found in the granitic rocks and in some of the sedimentary deposits (Naseem et al., 2010). Water quality produced by either base ion-exchange (Na–HCO3) or reverse ion exchange (Ca–Mg–Cl) type waters would only occur where the aquifer matrix includes the appropriate clay minerals. Ion ratios provide additional insight into the geochemical processes that are occurring within the study area (Table 2). If the Na/Ca ratio for Field 3 samples reflects the effect of evaporation, then it is reasonable to conclude that the relatively low Na/Ca ratio for Field 2 samples reflects a loss of Na and/or increase in Ca through reverse ion exchange and/or the dissolution of calcium-bearing minerals. Conversely, the relatively high Na/Ca ratio for Field 4 samples means that Na increased and/or calcium decreased through base ion exchange and/or calcite precipitation. The relatively low Ca/(Ca + SO4) ratios and high sodium absorption ratios (SAR) for Field 3 and Field 4 samples support the conclusion that ion exchange is influencing groundwater chemistry at the locations where these samples were collected. As has been described in other studies (e.g., Earle and Krogh, 2004; Edmunds and Smedley, 2005; Chae et al., 2006; Guo et al., 2007), the exchange of calcium and magnesium ions in solution for sodium on clays or other secondary minerals can produce a Na–HCO3 type water that is undersaturated with respect to calcite.
Fig. 2. (a, b). Major ion chemistry showing nature of water. Fields after Gibbs, 1970.
4.2. Occurrence of fluoride Fluoride concentrations in groundwater ranged between 0.06 and 44.40 mg/L, with mean and median values of 5.22 and 4.09 mg/L, respectively (Table 1). These results include some of the highest fluoride concentrations ever measured in the groundwater of Thar Desert areas of Pakistan, and a number village wells contained high values: Laplo Kharo #8 (44.40 mg/L), Nehare jo Tar #58 (34.80 mg/L), Lalabah ji Dhani #82 (22.98 mg/L) Koraal #151 (20.40 mg/L), Kerney jo Tar #64 (17.30 mg/L), Meghe jo Tar #60 (16.90 mg/L), Bhujbar #86 (14.30 mg/L), Baroba #74 (14.00 mg/L), Bhojrajio #68,67 (13.95 mg/L, 12.31 mg/L), Kairlo #59 (12.06 mg/L), Kachhbe jo Tar #78 (11.81 mg/L), Dodhario #38 (11.70 mg/L), Dharrasar #81 (11.30 mg/L), Bhojrajio #69 (11.23 mg/L), Mangtor #11 (10.80 mg/L), and Alwaro Shareef #20 (10.68 mg/L) (Fig. 1). The frequency distribution of fluoride in the 152 samples analyzed in this study is shown in Table 3, revealing that roughly 84% of the samples
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Fig. 3. Chadha diagram showing classification of groundwater samples of the study area (after Chadha, 1999). Field 1: Ca–HCO3 type waters, reflecting recharge and weathering. Field 2: Ca–Mg–Cl type waters, reflecting reverse ion-exchange. Field 3: Na–Cl type waters, reflecting evaporation or mixing with seawater. Field 4: Na–HCO3 type waters, reflecting base ion-exchange.
exceeded the WHO prescribed limits of 1.5 mg/L for drinking water (WHO, 1994). Most of these high-fluoride samples also contained greater than 1000 mg/L TDS (Fig. 4). An iso-concentration map of fluoride concentrations in the study area does not reveal any obvious spatial distribution patterns that might reflect the geochemical controls on its occurrence (Fig. 1). Therefore, it is likely that the sources of fluoride are fairly ubiquitous and that variations in concentrations result from factors that influence the overall groundwater quality. Given the geology of the study area, there are three possible sources of fluoride in groundwater: (1) the peralkaline to peraluminous A-type granite bedrock, (2) sediments containing minerals derived from the A-type granites (such as fluorite and fluorapatite), and (3) weathered ferromagnesian minerals contained within sedimentary deposits (e.g., biotite and amphibole). Working in the neighboring Nagar Parkar Sub-District of the Thar Desert, Naseem et al. (2010) demonstrated that a positive relationship exists between fluoride and lithogenic sodium in the groundwater (Naseem et al., 2010), suggesting that fluoride originates from the weathering of granitic rocks. Similar relationships have been found in other high-fluoride regions of the world (Ozsvath, 2006). However, because the groundwater sampled in this study occurs in a perched zone at the contact between dune sand and the underlying sedimentary deposits, it is the minerals contained within these deposits (and not the granitic bedrock itself) that serve as the source of fluoride.
The maximum concentration of fluoride in groundwater is typically controlled by the solubility of fluorite (Edmunds and Smedley, 2005; Ozsvath, 2006), and once the solubility limit for fluorite (CaF2) is reached, an inverse, non-linear relationship will exist between fluoride and calcium concentrations. This relationship is affected by the overall groundwater quality, including pH and ionic strength, which influence the types of geochemical processes that can take place. Fig. 3 shows that higher fluoride concentrations in the study area are associated with both Na–Cl and Na–HCO3 type waters. The ion ratios summarized in Table 2 reveal that the samples plotted in Fields 3 and 4 of Fig. 3 are relatively depleted in calcium and show evidence of base ion exchange (as described in Section 4.1). These associations are used to infer the probable geochemical processes that control the occurrence of fluoride. The prevalence of Na–Cl type groundwater is the result of evaporation, which serves to increase the ionic strength of the water. Initially, this also increases the solubility of minerals within the aquifer matrix, including those that release fluoride into solution. However, as the concentrations of dissolved species continue to rise, groundwater becomes saturated and then supersaturated with respect to the less soluble minerals, which eventually precipitate from solution. From the perspective of fluoride mobility, the solubility of calcite (CaCO3) is of particular importance, because it shares a common ion with fluorite (CaF2), which normally controls the upper limit of dissolved fluoride. Fig. 5 plots the calcite and fluorite saturation indices
Table 2 Ion ratios calculated to classify data on four fields in Fig. 3. Fields
Water type
n (%)
Av. (Na/Ca)
SAR
Av. [Ca/(Ca + SO4)]
Av. F
Field 1 Field 2 Field 3 Field 4
Ca–HCO3 Ca–Mg–Cl Na–Cl Na–HCO
2 (1.31) 12 (7.89) 127 (83.55) 11 (7.24)
1.05 1.20 13.92 31.79
0.84 0.98 10.42 13.96
0.55 0.44 0.29 0.20
2.15 2.15 5.34 7.70
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Table 3 Distribution of fluoride ion in the groundwater of study area (n = 152). Ranges of Fluoride (mg/L)
No. of samples (n)
Distribution (%)
b1.5 1.5–3.0 3.0–5.0 N5.0
24 32 45 51
15.79 21.05 29.61 33.55
for samples collected in this study, which were calculated using PHREEQCI version 2 software (Parkhurst and Appelo, 1999). This plot shows that groundwater in the study area has evolved to the point where all but six of the samples are supersaturated with respect to calcite, which should favor the removal of calcium from solution and allow fluoride concentrations to increase (Kundu et al., 2001). Assuming that Na–HCO3 type waters were produced by base ion exchange (the removal of Ca and Mg from solution in exchange for Na on clay minerals), the net result is the same as described above for Na–Cl type waters. As Ca concentrations are decreased, the groundwater is able to contain higher levels of fluoride before saturation with respect to fluorite is reached. This interpretation is supported by the fact that fluoride concentrations are generally lower in the Ca–HCO3 and Ca–Mg–Cl type waters, where higher Ca levels lead to fluorite precipitation.
Fig. 4. TDS vs. F− relationship on a log scale in groundwater of desert part of Umarkot SubDistrict area.
Fig. 6. pH vs. F− relationship of groundwater of desert part of Umarkot Sub-District area.
The relationship between groundwater pH and fluoride levels also bears mentioning. A number of studies have shown that high fluoride concentrations (up to 30 mg/L) can result from anion exchange (OH− for F−) on clay minerals, weathered micas, and hydroxides (Apambire et al., 1997; Smedley et al., 2002; Guo et al., 2007; Gupta et al., 2006; Nouri et al., 2006; Saxena and Ahmed, 2001, 2003). This mechanism is favored by and/or produces high pH values (usually above 8.0) under conditions that can lead to base-exchange softening and low calcium concentrations. Fig. 6 shows that most of the fluoride concentrations exceeding 10 mg/L within the study area are, in fact, associated with higher pH values. Although high pH would also favor calcite precipitation (thereby increasing fluorite solubility), the presence of weathered biotite and clay minerals within the perched groundwater zone makes it reasonable to infer that anion exchange also plays a role in the occurrence of high fluoride. Fig. 7 presents data from this study in a way that helps to identify the most important groundwater quality measures controlling fluoride concentrations in the study area. HCO3–Ca (i.e., meq/L HCO3 minus meq/L Ca) is plotted along the X axis, with positive values representing excess HCO3 ions left after CaCO3 precipitation. Na–Cl (i.e., meq/L Na minus meq/L Cl) is plotted along the Y axis, where positive values could represent the contribution of Na from non-evaporite mineral sources (Tirumalesh et al., 2007). It is significant that samples plotting in the quadrant with positive values on both axes have higher fluoride concentrations (a mean of 7.40 mg/L) than do those samples plotting
40
F- (av.) = 7.40 mg/L , n = 59 (38.82%)
20
HCO3 - Ca (meq/L)
0 -70
-50
-30
-10 -20
10
-40
F- (av.) = 3.26 mg/L, n = 64 (42.11%)
Na -Cl (meq/L)
-60 -80 -100 -120 -140
Fig. 5. Plot of calcite versus fluorite saturation indices.
Fig. 7. HCO3–Ca vs. Na–Cl relationship of groundwater of Umarkot area.
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in the quadrant with negative values on both axes (a mean of 3.26 mg/L). Other authors have reported similar findings (Gizaw, 1996). 5. Conclusion Much of the groundwater occurring within the desert region of Umarkot Sub-District has been found unsuitable for drinking as a result of high total dissolved solids and the occurrence of elevated fluoride concentrations (84.21% of the samples collected in this study contain fluoride concentrations that exceed the WHO permissible limit of 1.5 mg/L). This study has yielded some of the highest fluoride concentrations ever found in the Thar Desert areas of Pakistan. An evaluation of 152 samples has shown that fluoride originates from the weathering of minerals in the aquifer matrix and possibly the exchange of OH− for F− on clay minerals and weathered micas under high pH conditions. Evaporation is the dominant process affecting groundwater quality, and nearly 83% of the samples collected can be classified as Na–Cl type of water (with TDS values ranging from 518 to 39,067 mg/L). As the ionic strength of groundwater increases through evaporation, aquifer minerals are initially more soluble, but eventually the solubility limits for non-evaporite minerals are reached, causing precipitation. Saturation indices reveal that more than 95% of the samples from the study area are supersaturated with respect to calcite, and it is likely that the removal of calcium from solution through calcite precipitation lowers fluorite saturation indices and permits fluoride concentrations to increase. There is also evidence to suggest that some calcium is removed from solution through base ion exchange, which has the same effect on fluoride concentrations as does calcite precipitation. Consequently, elevated dissolved fluoride levels are associated with certain water types (i.e., Na–Cl and Na–HCO3), and this provides a simple and relatively inexpensive way to identify areas where residents are at risk of developing fluorosis based solely on indicators of the overall groundwater quality. The results of this study reveal that groundwater beneath a majority of Umarkot Sub-District contains fluoride in concentrations that represent a high to very high risk for fluorosis (Fig. 1). This situation warrants the construction of community-based systems by the public authorities to provide a safe water supply for Umarkot residents. Because much of the groundwater is also highly saline, it is recommended that such community systems employ reverse osmosis (RO) technology to produce potable water. In the absence of a safe public water supply, regular groundwater monitoring and integrated database management should be established for the study area. Cases of fluorosis should be studied systematically, and mitigation programs should be launched in the affected areas. Acknowledgment The authors are highly thankful to the Pakistan Science Foundation for financial support of this research under Project No. PSF/RES/SPCSIR/Env(86). References Ahmad, S.M., Chaudhry, M.N., 2007. Geochemical characterization and origin of the Karaigabbro from the Neoproterozoic Nagarparker complex, Pakistan. Geol. Bull. Punjab Univ. 42, 1–14. Ahmad, S.A., Chaudhry, M.N., 2008. A-type granites from the Nagar Parkar complex, Pakistan: geochemistry and origin. Geol. Bull. Punjab Univ. 43, 69–81. Apambire, W.B., Boyle, D.R., Michel, F.A., 1997. Geochemistry, genesis, and health implications of fluoriferous groundwaters in the upper regions of Ghana. Environ. Geol. 33, 13–24. APHA, 1995. Standard Methods for the Examination of Water and Wastewater. 19th ed. American Public Health Association, Washington, DC, USA. Ayoob, S., Gupta, A.K., 2006. Fluoride in drinking water: a review on the status and stress effects. Crit. Rev. Environ. Sci. Technol. 36, 433–487. Carrillo-Rivera, J.J., Cardona, A., Edmunds, W.M., 2002. Use of abstraction regime and knowledge of hydrogeological conditions to control high-fluoride concentration in abstracted groundwater: San Luis Potosi basin, Mexico. J. Hydrol. 261, 24–47.
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Geol. 41, 451–460. Lahermo, P., Backman, B., 2000. The occurrence and geochemistry of fluorides with special reference to natural waters in Finland. Geological Survey of FinlandReport of Investigation 149 Espoo. Mandel, S., 1981. Groundwater Resources: Investigation and Development. Academic Press, New York. Moghaddam, A.A., Fijani, E., 2008. Distribution of fluoride in groundwater of Maku area, northwest of Iran. Environ. Geol. 56, 281–287. Naseem, S., Rafique, T., Bashir, E., Bhanger, M.I., Laghari, A., Usmani, T.H., 2010. Lithological influences on occurrence of high-fluoride groundwater in Nagar Parkar area, Thar Desert, Pakistan. Chemosphere 78, 1313–1321. Nouri, J., Mahvi, A.H., Babaei, A., Ahmadpour, E., 2006. Regional pattern distribution of groundwater fluoride in the Shush aquifer of Khuzestan County, Iran. Fluoride 39, 321. Ozsvath, D.L., 2006. Fluoride concentrations in a crystalline bedrock aquifer Marathon County, Wisconsin. Environ. Geol. 50, 132–138. Ozsvath, D.L., 2009. Fluoride and environmental health: a review. Rev. Environ. Sci. Biotechnol. 8, 59–79. Parkhurst, D.L., Appelo, C.A.J., 1999. User's Guide to PHREEQC (Version 2): A Computer Program for Speciation, Batch-reaction, One-dimensional Transport, and Inverse Geochemical Calculations. Piper, A.M., 1944. A graphic procedure in the geochemical interpretation of wateranalyses. Trans. Am. Geophys. Union 25, 914–928. Rafique, T., Naseem, S., Bhanger, M.I., Usmani, T.H., 2008. Fluoride ion contamination in the groundwater of Mithi sub-district, the Thar Desert, Pakistan. Environ. Geol. 56, 317–326. Rafique, T., Naseem, S., Usmani, T.H., Bashir, E., Khan, F.A., Bhanger, M.I., 2009. Geochemical factors controlling the occurrence of high fluoride groundwater in the Nagar Parkar area, Sindh, Pakistan. J. Hazard. Mater. 171, 424–430. Rafique, T., Naseem, S., Usmani, T.H., Bhanger, M.I., Shirin, K., 2013. Impact of seawater on distribution of fluoride and other ions in groundwater of Diplo Area, Thar Desert Pakistan. Water Environ. Res. 85, 579–586. Saxena, V., Ahmed, S., 2001. Dissolution of fluoride in groundwater: a water–rock interaction study. Environ. Geol. 40, 1084–1087.
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Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Geochemical controls of high fluoride groundwater in Umarkot Sub-District, Thar Desert, Pakistan Tahir Rafique a,⁎, Shahid Naseem b, David Ozsvath c, Riaz Hussain d, Muhammad Iqbal Bhanger e, Tanzil Haider Usmani a a
Applied Chemistry Research Centre, PCSIR Laboratories Complex, Karachi 75280, Pakistan Department of Geology, University of Karachi, Karachi 75270, Pakistan Department of Geography/Geology, University of Wisconsin-Stevens Point, USA d Geophysical Division, Geological Survey of Pakistan, Karachi, Pakistan e HEJ Research Institute of Chemistry, ICCBS, University of Karachi, Karachi, Pakistan b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Fluoride in groundwater ranged from 0.06 to 44.40 mg/L (mean value of 5.22 mg/L). • 84% of samples exceeded 1.5 mg/L (recommended WHO limit for fluoride). • Silicate mineral weathering and evaporation controlled overall groundwater quality. • Weathering of minerals derived from Type-A granite released fluoride. • Water influenced by calcite precipitation and base ion exchange contained high F−.
a r t i c l e
i n f o
Article history: Received 28 December 2014 Received in revised form 10 May 2015 Accepted 10 May 2015 Available online 3 June 2015 Editor: Barcelo D. Keywords: Fluoride ions Mineral weathering Evaporation Calcite precipitation Base ion exchange Umarkot Thar Desert
a b s t r a c t Groundwater samples (n = 152) were collected in the Thar Desert of the Umarkot Sub-District, Pakistan to evaluate the geochemical controls on the occurrence of high fluoride (F−) levels within the study area. Fluoride concentrations range from 0.06 to 44.4 mg/L, with mean and median values of 5.22 and 4.09 mg/L, respectively; and roughly 84% of the samples contain fluoride concentrations that exceed the 1.5 mg/L drinking water standard set by WHO. The overall groundwater quality reflects the influences of silicate mineral weathering and evaporation. Fluoride originates from the weathering of minerals derived from Type-A granite and possibly anion exchange (OH− for F−) on clays and weathered micas under high pH conditions. High fluoride levels are associated with Na–HCO3 type water produced by calcite precipitation and/or base ion exchange. Depleted calcium levels in groundwater allow higher fluoride concentrations to occur before the solubility limit for fluorite is reached. © 2015 Elsevier B.V. All rights reserved.
⁎ Corresponding author. E-mail address: tahirrafi[email protected] (T. Rafique).
http://dx.doi.org/10.1016/j.scitotenv.2015.05.038 0048-9697/© 2015 Elsevier B.V. All rights reserved.
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1. Introduction
2. Study area
Fluorine (F) is the lightest member of the halogen family and the most electronegative and reactive of all elements. Although trace quantities (0.05–0.1% or 500–1000 mg/kg) of fluorine occur in all crustal rocks, it is a strong lithophile and thus concentrated in silicate phases, especially in rocks which crystallize at later stages of magmatic differentiation (Lahermo and Backman, 2000). As a result, fluorine is found most abundantly in granites, alkali granites, and hydrothermal deposits (Apambire et al., 1997; Ayoob and Gupta, 2006; Naseem et al., 2010). Some of the more common fluorine-bearing minerals include fluorite (CaF2), sellaite (MgF2), fluorapatite (Ca5(PO4)3F), cryolite (Na3AlF6), villiaumite (NaF), and topaz (Al2(SiO4)F2). However, micas and amphiboles can also contain F−, which substitutes for OH− in the mineral structures (Edmunds and Smedley, 2005; Westrich, 1982). In natural waters, fluorine occurs as the monovalent fluoride (F−) anion (WHO, 2006), which is capable of forming a number of organic and inorganic complexes (Yamin et al., 2011; Hamzaoui-Azaza et al., 2009). Fluoride concentrations in surface waters are usually low, ranging from 0.01 to 0.3 mg/L (UNICEF, 1999). Concentrations in groundwater are typically less than 1 mg/L but can be much higher where the subsurface mineralogy and geochemical conditions are appropriate (Carrillo-Rivera et al., 2002; Frencken, 1992; Frengstad et al., 2001; Gizaw, 1996). Trace quantities of fluoride originate from the dissolution of amphiboles and micas (Kearns et al., 1980), and higher concentrations usually occur where minerals such as fluorite, fluorapatite, or cryolite are present; however, studies have shown that even biotite weathering can produce concentrations greater than 10 mg/L when groundwater is depleted in calcium (Chae et al., 2007). Higher levels of dissolved fluoride are generally associated with high pH and Na– HCO3 type waters, although the temperature and depth of groundwater and climatic factors can also play important roles (Chae et al., 2007; Gupta et al., 2006). Ion exchange has been found to influence fluoride levels through both base exchange, which lowers calcium concentrations (Apambire et al., 1997; Earle and Krogh, 2004; Edmunds and Smedley, 2005; Chae et al., 2006), and anion exchange, in which OH− in groundwater replaces F− on certain clay minerals or weathered micas (Apambire et al., 1997; Smedley et al., 2002; Guo et al., 2007). Fluorine is one of the unique elements that have negative effects on human health when its ingestion levels are either too high or too low. Fluoride is an essential micronutrient needed in trace quantities to strengthen the hydroxyapatite matrix of the skeletal tissues and teeth, and dental caries are a common problem among populations with insufficient fluoride intake. Prolonged exposure of fluoride at higher levels leads to fluorosis, a silent geogenic disease that presently affects about 200 million people in 28 developed and developing countries, including India, China, Pakistan, Sri Lanka, Turkey, Iran, East Africa's Rift Valley, Scandinavia, Algeria, Libya, Iraq, USA, Canada, Thailand, New Zealand and Japan (WHO, 2006). Areas of Thar Desert in the Sindh Province of Pakistan have recently been reported to have high fluoride concentrations in groundwater resources (Naseem et al., 2010; Rafique et al., 2008, 2009, 2013). There are various manifestations of fluorosis, including dental and skeletal abnormalities, crippled limbs, the calcification of ligaments, rheumatic pain, stiffness and the rigidity of the joints (Ozsvath, 2009). Excessive fluoride intake is also reported to adversely affect the gastrointestinal tract, kidneys, and liver, as well as the nervous, reproductive, and immune systems (Ibrahim, 2011). A major source of fluoride ingestion is groundwater, although dentifrices and certain foods are also important sources (EPA, 2010). The present study evaluates groundwater quality in a region that is known for its high incidence of fluorosis and was undertaken to define both the sources of dissolved fluoride and the geochemical processes that influence its occurrence. The goal is to identify geochemical relationships that might aid in understanding the genesis of high fluoride groundwater based on measures of the overall water quality.
The study area occurs within the Umarkot Sub-District of the District Umarkot, Sindh Province, in southeastern Pakistan, near its border with India (Fig. 1). The 3209-km2 Umarkot Sub-District, located between N 24° 54′ to 25° 47′ and E 69° 11′ to 70° 18′, had a population of 312,753, an average annual growth rate of 3.83%, and a population density of 97.5 people per km2 when the 1998 Census was taken (DCR, 2000). There are two distinct physiographic regions within this sub-district; irrigated lowlands (in the west-central area) and a desert marked by stabilized sand dunes (Fig. 1). The groundwater samples collected for this study came from the approximately 2411-km2 desert area, a northwestern extension of the Thar Desert, one of the largest subtropical deserts in the world. This desert, which began to form more than 150 thousand years ago (Singhvi and Kar, 2004), is generally characterized by strong winds, high diurnal temperature variations, scarce rainfall, intense solar radiation, and high rates of evaporation (Rafique et al., 2008; Sinha et al., 1996). The study area is underlain by Archean metamorphic basement rocks and Late Proterozoic (720–745 million years) igneous rocks exposed near Nagar Parkar Town (Naseem et al., 2010). Most of the Nagar Parkar Complex consists of peralkaline to peraluminous A-type granites with some rhyolite plugs and basic dykes (Ahmad and Chaudhry, 2008), and it is part of the Late Proterozoic Malani regional magmatism of Western Rajasthan, India (Ahmad and Chaudhry, 2007). These igneous and metamorphic rocks are covered by Paleocene (the Bara) and Late Pleistocene (the Bartala) sedimentary formations, which are predominantly sandstones and siltstones with minor claystone beds and granitic wash. The wash is composed of quartz and feldspar grains in a kaolinitic matrix with minor amounts of altered ferromagnesian minerals, probably hornblende and biotite. Overlying the entire subsurface sequence is a thick (~ 80 m) layer of dune sand consisting of quartz, feldspars, and a number of accessory minerals, including ferromagnesian minerals (Kar et al., 2001). Groundwater occurs under water table conditions within ephemeral aquifers that are perched on the contact between dune sand and underlying sedimentary deposits. Most of the groundwater samples for this study were collected from wells that are completed within these perched aquifers. Well depths range between 6.67 and 400 m, with mean and median values of 40.67 and 33.33 m, respectively (Table 1).
3. Materials and methods Groundwater samples (n = 152) were collected from dug-wells that were randomly distributed across the Umarkot Sub-District desert (Fig. 1). Stainless steel 5-liter containers tightened with fiber rope were used to collect groundwater samples; temperature, pH, Eh, conductivity, and total dissolved solids (TDS) were measured in the field. Samples taken for laboratory analysis were field-filtered using an ordinary filter paper and placed into 1.5-liter polyethylene bottles that had been pre-washed with dilute (1%) nitric acid according to standard methods (Greenberg et al., 1998). The bottles were first rinsed with groundwater and then completely filled and sealed with caps to avoid the oxidation of redox-sensitive constituents. All sampling information was noted in the field, and the exact sampling coordinates were identified using a GPS unit. Standard methods (APHA, 1995) were followed for analyzing the chemical characteristics of the groundwater samples. Fluoride ion (F−) concentrations were determined by the specific ion electrode method (Greenberg et al., 1998) using a total ionic strength adjusting buffer (TISAB). High levels of precision and accuracy were maintained during all phases of laboratory analysis, which included blank and spiked samples to ensure the reliability of each method. The ionic balance error (IBE) values were within an acceptable limit of ± 5% (Domenico, 1990; Mandel, 1981) for all samples.
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Fig. 1. Isoconcentration map of fluoride distribution in the groundwater of study area.
4. Results and discussion 4.1. Overall groundwater quality Table 1 summarizes the groundwater quality and several useful ion ratios for the data collected in the study area. Using the WHO guideline of 1000 mg/L total dissolved solids (TDS) (WHO, 1996) as a threshold for potable water, it is clear that much of the groundwater in this region is unacceptable for drinking. High Na and Cl concentrations also make the water unacceptable for irrigation purposes. Data were plotted on diagrams developed by Gibbs (1970) for the purpose of inferring the dominant control(s) on water quality
(Fig. 2a, b, Table 1). These plots suggest that groundwater quality in the study area is heavily influenced by evaporation, although some samples appear to reflect only the influence of weathering. This finding is not unexpected for an arid environment, and it suggests that the concentrations of major ions other than Na and Cl are largely controlled by the precipitation of lower solubility minerals (e.g., calcite) as evaporation causes salinity to increase. Hounslow (1995) summarizes the use of various ion ratios to deduce the origin of groundwater quality (Guo and Wang, 2005; Hounslow, 1995). The mean Cl/(sum anions) ratio for samples collected in the study area is 0.615 (Table 1), suggesting that dissolved substances originate from the weathering of non-evaporate minerals (with TDS
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Table 1 Basic statistical parameters of groundwater samples (n = 152) of desert part of Umarkot Sub-District. Ions are in mg/L, except depth (m), EC (μS/cm), temperature (°C), pH and ratios. Parameters
Min.
Max.
Mean
Median
Depth of well pH EC Temp. TDS Na K Ca Mg Cl HCO3 SO4 F− Na/(Na + Ca) Cl/(Cl + HCO3) Mg/(Ca + Mg) Cl/Σanion Na/Ca SAR Ca/(Ca + SO4)
6.67 6.21 654 25.4 518 50 4 2 1 25 61 29 0.06 0.25 0.07 0.004 0.057 0.33 0.29 0.02
400.00 9.54 50,200 42.2 39,067 12,900 147 1396 2000 21,500 2288 3011 44.40 0.99 0.99 0.757 0.895 185.00 66.12 0.75
40.67 7.66 10,422 29.9 6795 1986 36 229 174 3171 441 557 5.22 0.84 0.73 0.426 0.615 14.04 9.80 0.29
33.33 7.61 7935 29.4 5038 1575 28 139 94 2033 345 420 4.09 0.89 0.81 0.472 0.685 8.48 8.70 0.26
values N 1000 mg/L reflecting the added influence of evaporation). The average Mg/(Ca + Mg) ratio of 0.426 (Table 1) is typical of groundwater quality resulting from the weathering of granitic rocks, which are relatively low in ferromagnesian minerals (Cruz and Amaral, 2004; Moghaddam and Fijani, 2008). Chadha (1999) proposed a hydrogeochemical classification scheme for water by plotting the difference between (CO3 + HCO3) and (Cl + SO4) concentrations in meq/L expressed as percent versus (Ca + Mg) and (Na + K) concentrations in meq/L also expressed as percent (Chadha, 1999). This classification is a modified version of the Piper diagram (Piper, 1944) and an expanded version of Durov diagram (Durov, 1948). The resulting diagram has four fields, representing four types of hydrogeochemical processes (Fig. 3). Nearly 84% (N = 127) of the samples collected in the study area are Na–Cl type waters, confirming that evaporation is the dominant process controlling groundwater quality. Of the remaining samples, 8% (N = 12) are Ca–Mg–Cl type waters, 7% (N = 11) are NaHCO3 type water, and only two are classified as Ca–HCO3 type waters (Fig. 3). The latter two samples are presumed to represent the water quality of recharge that is little affected by evaporation. Although carbonate minerals are not prevalent within the study area, calcium-bearing minerals such as anorthite, wollastonite, and diopside are found in the granitic rocks and in some of the sedimentary deposits (Naseem et al., 2010). Water quality produced by either base ion-exchange (Na–HCO3) or reverse ion exchange (Ca–Mg–Cl) type waters would only occur where the aquifer matrix includes the appropriate clay minerals. Ion ratios provide additional insight into the geochemical processes that are occurring within the study area (Table 2). If the Na/Ca ratio for Field 3 samples reflects the effect of evaporation, then it is reasonable to conclude that the relatively low Na/Ca ratio for Field 2 samples reflects a loss of Na and/or increase in Ca through reverse ion exchange and/or the dissolution of calcium-bearing minerals. Conversely, the relatively high Na/Ca ratio for Field 4 samples means that Na increased and/or calcium decreased through base ion exchange and/or calcite precipitation. The relatively low Ca/(Ca + SO4) ratios and high sodium absorption ratios (SAR) for Field 3 and Field 4 samples support the conclusion that ion exchange is influencing groundwater chemistry at the locations where these samples were collected. As has been described in other studies (e.g., Earle and Krogh, 2004; Edmunds and Smedley, 2005; Chae et al., 2006; Guo et al., 2007), the exchange of calcium and magnesium ions in solution for sodium on clays or other secondary minerals can produce a Na–HCO3 type water that is undersaturated with respect to calcite.
Fig. 2. (a, b). Major ion chemistry showing nature of water. Fields after Gibbs, 1970.
4.2. Occurrence of fluoride Fluoride concentrations in groundwater ranged between 0.06 and 44.40 mg/L, with mean and median values of 5.22 and 4.09 mg/L, respectively (Table 1). These results include some of the highest fluoride concentrations ever measured in the groundwater of Thar Desert areas of Pakistan, and a number village wells contained high values: Laplo Kharo #8 (44.40 mg/L), Nehare jo Tar #58 (34.80 mg/L), Lalabah ji Dhani #82 (22.98 mg/L) Koraal #151 (20.40 mg/L), Kerney jo Tar #64 (17.30 mg/L), Meghe jo Tar #60 (16.90 mg/L), Bhujbar #86 (14.30 mg/L), Baroba #74 (14.00 mg/L), Bhojrajio #68,67 (13.95 mg/L, 12.31 mg/L), Kairlo #59 (12.06 mg/L), Kachhbe jo Tar #78 (11.81 mg/L), Dodhario #38 (11.70 mg/L), Dharrasar #81 (11.30 mg/L), Bhojrajio #69 (11.23 mg/L), Mangtor #11 (10.80 mg/L), and Alwaro Shareef #20 (10.68 mg/L) (Fig. 1). The frequency distribution of fluoride in the 152 samples analyzed in this study is shown in Table 3, revealing that roughly 84% of the samples
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Fig. 3. Chadha diagram showing classification of groundwater samples of the study area (after Chadha, 1999). Field 1: Ca–HCO3 type waters, reflecting recharge and weathering. Field 2: Ca–Mg–Cl type waters, reflecting reverse ion-exchange. Field 3: Na–Cl type waters, reflecting evaporation or mixing with seawater. Field 4: Na–HCO3 type waters, reflecting base ion-exchange.
exceeded the WHO prescribed limits of 1.5 mg/L for drinking water (WHO, 1994). Most of these high-fluoride samples also contained greater than 1000 mg/L TDS (Fig. 4). An iso-concentration map of fluoride concentrations in the study area does not reveal any obvious spatial distribution patterns that might reflect the geochemical controls on its occurrence (Fig. 1). Therefore, it is likely that the sources of fluoride are fairly ubiquitous and that variations in concentrations result from factors that influence the overall groundwater quality. Given the geology of the study area, there are three possible sources of fluoride in groundwater: (1) the peralkaline to peraluminous A-type granite bedrock, (2) sediments containing minerals derived from the A-type granites (such as fluorite and fluorapatite), and (3) weathered ferromagnesian minerals contained within sedimentary deposits (e.g., biotite and amphibole). Working in the neighboring Nagar Parkar Sub-District of the Thar Desert, Naseem et al. (2010) demonstrated that a positive relationship exists between fluoride and lithogenic sodium in the groundwater (Naseem et al., 2010), suggesting that fluoride originates from the weathering of granitic rocks. Similar relationships have been found in other high-fluoride regions of the world (Ozsvath, 2006). However, because the groundwater sampled in this study occurs in a perched zone at the contact between dune sand and the underlying sedimentary deposits, it is the minerals contained within these deposits (and not the granitic bedrock itself) that serve as the source of fluoride.
The maximum concentration of fluoride in groundwater is typically controlled by the solubility of fluorite (Edmunds and Smedley, 2005; Ozsvath, 2006), and once the solubility limit for fluorite (CaF2) is reached, an inverse, non-linear relationship will exist between fluoride and calcium concentrations. This relationship is affected by the overall groundwater quality, including pH and ionic strength, which influence the types of geochemical processes that can take place. Fig. 3 shows that higher fluoride concentrations in the study area are associated with both Na–Cl and Na–HCO3 type waters. The ion ratios summarized in Table 2 reveal that the samples plotted in Fields 3 and 4 of Fig. 3 are relatively depleted in calcium and show evidence of base ion exchange (as described in Section 4.1). These associations are used to infer the probable geochemical processes that control the occurrence of fluoride. The prevalence of Na–Cl type groundwater is the result of evaporation, which serves to increase the ionic strength of the water. Initially, this also increases the solubility of minerals within the aquifer matrix, including those that release fluoride into solution. However, as the concentrations of dissolved species continue to rise, groundwater becomes saturated and then supersaturated with respect to the less soluble minerals, which eventually precipitate from solution. From the perspective of fluoride mobility, the solubility of calcite (CaCO3) is of particular importance, because it shares a common ion with fluorite (CaF2), which normally controls the upper limit of dissolved fluoride. Fig. 5 plots the calcite and fluorite saturation indices
Table 2 Ion ratios calculated to classify data on four fields in Fig. 3. Fields
Water type
n (%)
Av. (Na/Ca)
SAR
Av. [Ca/(Ca + SO4)]
Av. F
Field 1 Field 2 Field 3 Field 4
Ca–HCO3 Ca–Mg–Cl Na–Cl Na–HCO
2 (1.31) 12 (7.89) 127 (83.55) 11 (7.24)
1.05 1.20 13.92 31.79
0.84 0.98 10.42 13.96
0.55 0.44 0.29 0.20
2.15 2.15 5.34 7.70
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Table 3 Distribution of fluoride ion in the groundwater of study area (n = 152). Ranges of Fluoride (mg/L)
No. of samples (n)
Distribution (%)
b1.5 1.5–3.0 3.0–5.0 N5.0
24 32 45 51
15.79 21.05 29.61 33.55
for samples collected in this study, which were calculated using PHREEQCI version 2 software (Parkhurst and Appelo, 1999). This plot shows that groundwater in the study area has evolved to the point where all but six of the samples are supersaturated with respect to calcite, which should favor the removal of calcium from solution and allow fluoride concentrations to increase (Kundu et al., 2001). Assuming that Na–HCO3 type waters were produced by base ion exchange (the removal of Ca and Mg from solution in exchange for Na on clay minerals), the net result is the same as described above for Na–Cl type waters. As Ca concentrations are decreased, the groundwater is able to contain higher levels of fluoride before saturation with respect to fluorite is reached. This interpretation is supported by the fact that fluoride concentrations are generally lower in the Ca–HCO3 and Ca–Mg–Cl type waters, where higher Ca levels lead to fluorite precipitation.
Fig. 4. TDS vs. F− relationship on a log scale in groundwater of desert part of Umarkot SubDistrict area.
Fig. 6. pH vs. F− relationship of groundwater of desert part of Umarkot Sub-District area.
The relationship between groundwater pH and fluoride levels also bears mentioning. A number of studies have shown that high fluoride concentrations (up to 30 mg/L) can result from anion exchange (OH− for F−) on clay minerals, weathered micas, and hydroxides (Apambire et al., 1997; Smedley et al., 2002; Guo et al., 2007; Gupta et al., 2006; Nouri et al., 2006; Saxena and Ahmed, 2001, 2003). This mechanism is favored by and/or produces high pH values (usually above 8.0) under conditions that can lead to base-exchange softening and low calcium concentrations. Fig. 6 shows that most of the fluoride concentrations exceeding 10 mg/L within the study area are, in fact, associated with higher pH values. Although high pH would also favor calcite precipitation (thereby increasing fluorite solubility), the presence of weathered biotite and clay minerals within the perched groundwater zone makes it reasonable to infer that anion exchange also plays a role in the occurrence of high fluoride. Fig. 7 presents data from this study in a way that helps to identify the most important groundwater quality measures controlling fluoride concentrations in the study area. HCO3–Ca (i.e., meq/L HCO3 minus meq/L Ca) is plotted along the X axis, with positive values representing excess HCO3 ions left after CaCO3 precipitation. Na–Cl (i.e., meq/L Na minus meq/L Cl) is plotted along the Y axis, where positive values could represent the contribution of Na from non-evaporite mineral sources (Tirumalesh et al., 2007). It is significant that samples plotting in the quadrant with positive values on both axes have higher fluoride concentrations (a mean of 7.40 mg/L) than do those samples plotting
40
F- (av.) = 7.40 mg/L , n = 59 (38.82%)
20
HCO3 - Ca (meq/L)
0 -70
-50
-30
-10 -20
10
-40
F- (av.) = 3.26 mg/L, n = 64 (42.11%)
Na -Cl (meq/L)
-60 -80 -100 -120 -140
Fig. 5. Plot of calcite versus fluorite saturation indices.
Fig. 7. HCO3–Ca vs. Na–Cl relationship of groundwater of Umarkot area.
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in the quadrant with negative values on both axes (a mean of 3.26 mg/L). Other authors have reported similar findings (Gizaw, 1996). 5. Conclusion Much of the groundwater occurring within the desert region of Umarkot Sub-District has been found unsuitable for drinking as a result of high total dissolved solids and the occurrence of elevated fluoride concentrations (84.21% of the samples collected in this study contain fluoride concentrations that exceed the WHO permissible limit of 1.5 mg/L). This study has yielded some of the highest fluoride concentrations ever found in the Thar Desert areas of Pakistan. An evaluation of 152 samples has shown that fluoride originates from the weathering of minerals in the aquifer matrix and possibly the exchange of OH− for F− on clay minerals and weathered micas under high pH conditions. Evaporation is the dominant process affecting groundwater quality, and nearly 83% of the samples collected can be classified as Na–Cl type of water (with TDS values ranging from 518 to 39,067 mg/L). As the ionic strength of groundwater increases through evaporation, aquifer minerals are initially more soluble, but eventually the solubility limits for non-evaporite minerals are reached, causing precipitation. Saturation indices reveal that more than 95% of the samples from the study area are supersaturated with respect to calcite, and it is likely that the removal of calcium from solution through calcite precipitation lowers fluorite saturation indices and permits fluoride concentrations to increase. There is also evidence to suggest that some calcium is removed from solution through base ion exchange, which has the same effect on fluoride concentrations as does calcite precipitation. Consequently, elevated dissolved fluoride levels are associated with certain water types (i.e., Na–Cl and Na–HCO3), and this provides a simple and relatively inexpensive way to identify areas where residents are at risk of developing fluorosis based solely on indicators of the overall groundwater quality. The results of this study reveal that groundwater beneath a majority of Umarkot Sub-District contains fluoride in concentrations that represent a high to very high risk for fluorosis (Fig. 1). This situation warrants the construction of community-based systems by the public authorities to provide a safe water supply for Umarkot residents. Because much of the groundwater is also highly saline, it is recommended that such community systems employ reverse osmosis (RO) technology to produce potable water. In the absence of a safe public water supply, regular groundwater monitoring and integrated database management should be established for the study area. Cases of fluorosis should be studied systematically, and mitigation programs should be launched in the affected areas. Acknowledgment The authors are highly thankful to the Pakistan Science Foundation for financial support of this research under Project No. PSF/RES/SPCSIR/Env(86). References Ahmad, S.M., Chaudhry, M.N., 2007. Geochemical characterization and origin of the Karaigabbro from the Neoproterozoic Nagarparker complex, Pakistan. Geol. Bull. Punjab Univ. 42, 1–14. Ahmad, S.A., Chaudhry, M.N., 2008. A-type granites from the Nagar Parkar complex, Pakistan: geochemistry and origin. Geol. Bull. Punjab Univ. 43, 69–81. Apambire, W.B., Boyle, D.R., Michel, F.A., 1997. Geochemistry, genesis, and health implications of fluoriferous groundwaters in the upper regions of Ghana. Environ. Geol. 33, 13–24. APHA, 1995. Standard Methods for the Examination of Water and Wastewater. 19th ed. American Public Health Association, Washington, DC, USA. Ayoob, S., Gupta, A.K., 2006. Fluoride in drinking water: a review on the status and stress effects. Crit. Rev. Environ. Sci. Technol. 36, 433–487. Carrillo-Rivera, J.J., Cardona, A., Edmunds, W.M., 2002. Use of abstraction regime and knowledge of hydrogeological conditions to control high-fluoride concentration in abstracted groundwater: San Luis Potosi basin, Mexico. J. Hydrol. 261, 24–47.
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