Elsevier

Applied Geochemistry

Volume 98, November 2018, Pages 197-205
Applied Geochemistry

Source apportionment and health risk assessment of fluoride in water resources, south of Fars province, Iran: Stable isotopes (δ18O and δD) and geochemical modeling approaches

https://doi.org/10.1016/j.apgeochem.2018.09.019Get rights and content

Highlights

  • Elevated concentrations of fluoride linked to alkaline water and high δ18O values.

  • Calcite precipitation and ion exchange in groundwater revealed by inverse modeling.

  • Hydrochemical evolution of groundwater and evaporation affected the fluoride levels.

  • High leachable fluorine in dolomitic limestone and marls was found.

  • Approximately four hundred thousand of the inhabitants were at the fluorosis risk.

Abstract

In this study the key factors influencing fluoride enrichment in water resources, south of Fars province, Iran are investigated. For this purpose, hydrogeochemical, statistical and isotopic studies were conducted on water and dominant rock types. Impact on human health is also considered. The results indicated that fluoride concentrations in water resources range from 0.06 to 4.95 mg/L, while 67.5% of the analysed samples displayed concentrations above the world health organization guidelines for drinking water. Fluoride enrichment is associated with Nasingle bondCasingle bondMgsingle bondClsingle bondSO4 water type and the chemical evolution of groundwater along flow path. F/Cl ratio, δ18O and δD results of water samples indicated that evaporation favored F‾ enrichment. Calculated mineral saturation indices and statistical analysis indicated that while the lithological units play a prominent role in fluoride anomalies, anthropogenic sources are negligible. Inverse geochemical modeling and calculated Chloro-Alkaline indices proved the occurrence of reverse ion exchange processes in the groundwater. Conducted leaching tests and X-ray diffraction data on abundant geological formations revealed dolomite and clay minerals as two likely candidates for contributing fluoride to the aquifers. The alkaline nature of water seems to have intensified the F‾ exchange in clay minerals with OH‾ in water. Fluoride exposure risk assessment revealed that children and adults are prone to fluoride-related diseases caused by ingestion of fluoride-rich water. Considering the prevailing climatic conditions and the ongoing geochemical processes, fluoride level in local aquifers will undoubtedly rise with time unless preventive and remediative measures are taken by the responsible authorities.

Introduction

Based on World Health Organization (World Health Organization, 2004) official data, globally about 80% of human diseases are associated with the consumption of low-quality drinking water. It is estimated that more than 200 million people in various parts of the world, particularly northern China, India, Sri Lanka, Mexico, western USA, Argentina, Iran, and many countries in Africa drink water with fluoride levels exceeding the World Health Organization's (World Health Organization, 2017) guideline value of 1.5 mg/L for drinking water (Adimalla and Venkatayogi, 2017; Adimalla et al., 2018a; Amini et al., 2008; Chuah et al., 2016; Edmunds and Smedley, 2013; Li et al., 2016). The US Environmental Protection Agency (EPA) determined the maximum contaminant level (enforceable limit) to be at 4 mg/L for fluoride in drinking water, although the secondary standard (non-enforceable) for United States drinking water is 2 mg/L (USEPA, 2018).

Fluorine is the 13th most abundant element in the Earth's crust (Weinstein and Davison, 2004) and its average crustal abundance is 300 mg/kg (Fawell et al., 2006; Tebutt, 1983). Weathering of F-bearing minerals in rock and soil is responsible for high F‾ concentration in water. Some such minerals include fluorite (CaF2), apatite [Ca5(PO4)3(Cl,F,OH)], topaz [Al2F2(SiO4)], cryolite (Na3AlF6), villiaumite (NaF), micas, clays, amphiboles, and epidotes (Dehbandi et al., 2017a; Keshavarzi et al., 2010). Major anthropogenic sources of fluoride contamination in the environment include excessive use of phosphate fertilizers and pesticides, aluminum smelting, glass and brick industries, industries, coal combustion, semi-conductors, and electroplating (Ali et al., 2016; Li et al., 2016).

Drinking water is typically the largest single contributor to daily fluoride intake (Chidambaram et al., 2013). Fluoride in drinking water has a narrow optimum concentration range in relation to human health. It prevents dental caries in the range of 0.7–1.2 mg/L (Farooqi, 2015). Dental and skeletal fluorosis and other diseases associated with prolonged intake of high-fluoride waters (F‾˃1.5 mg/L), are a potentially serious health problem for human and livestock (Ozsvath, 2009). Fluoride content in groundwater is related to water-rock interaction in the aquifer however, it rarely exceeds 10 mg/L. The highest natural concentration so far was measured 2800 mg/L in Lake Nakuru in Kenya (Davies, 2010).

Fluoride anomalies generally occur in special geochemical conditions such as Fe-rich soil in a humid climate, saline groundwater with high total dissolved solids, and alkaline groundwater in arid and semiarid environments (Sivasankar et al., 2016; Wen et al., 2013). Sivasankar et al. (2016) indicated that the highest fluoride levels tend to take place in semiarid climates with intense surface evaporation (>2000 mm per year), low precipitation (225–400 mm per year), and low groundwater hydraulic conductivity. Stable oxygen (18O/16O) and hydrogen (D/H) isotope ratios are used to identify the effects of evaporation on groundwater systems. A groundwater undergoing evaporation becomes enriched in heavier isotopes (Gonfiantini et al., 2018; Thivya et al., 2016).

High fluoride concentrations in groundwater are already reported form different parts of Iran such as Muteh (Keshavarzi et al., 2010), Dashtestan (Battaleb-Looie et al., 2012a), central Iran (Dehbandi et al., 2018), and Poldasht (Yousefi et al., 2018). Few studies have investigated fluoride contamination in Fars province. A recent work by Rezaei et al. (2017) in Lar area showed that fluoride had a natural source and the equilibrium reaction between calcite and fluorite was important in controlling fluoride levels in groundwater.

The main purpose of this study is to understand the causes of elevated fluoride concentrations in water resources, south of Fars province, Iran. This is essential to define consumption limits and management policies in the study area. To reach this goal, the sources of fluoride and the mechanisms that control its mobility and reactivity in aquatic systems were identified. The main geochemical reactions that influence the chemical composition of groundwater along a flow path are determined by inverse modeling using PHREEQC code. The climatic and geologic aspects of fluoride enrichment in the water resources of the study area have not been studied previously. These aspects are investigated in the present study using stable isotopes (δ18O and δD) data and leaching test, respectively. Finally, the health risk of high fluoride levels intake through drinking water is estimated.

Section snippets

Study area

The study area is located south of the Fars province, Iran (Fig. 1). Total population in the region is 465,795. Almost 350,000 of the population drink groundwater, while the rest consume surface water for drinking. The climate in the area is predominantly arid. Average annual precipitation, evaporation, and temperature in the area are 198.7 mm, 3027.6 mm, and 26.1 °C, respectively (Fars Meteorological Bureau, 2017). The majority of the rainfall occurs in the period December to March. The

Materials and methods

A total of 38 samples were collected from surface, ground, and rain water in the study area during 6th to 12th May 2017 in 1 L polyethylene bottles already washed twice with distilled water according to standard sampling procedure (APHA, 1995). The sampling points were selected considering population density and general direction of the groundwater flow. Lamerd, Lar, Mohr, Gerash, Khonj, Evaz, Alamarvdasht, Juyom, Banarouyeh, and Emad Shahr constitute the main population centers in the region.

Statistical analysis

Descriptive statistics of the chemical analyses of the water samples are presented in Table 1. The overall order of the dominance of cations is Na+˃ Ca2+ ˃Mg2+ ˃ K+ ˃ Si4+ and that of anions is Cl‾ ˃ SO42− ˃ HCO3‾ ˃ NO3‾˃ F‾ ˃ PO43−. Fluoride concentration in water samples varied from 0.06 to 4.95 mg/L averaging 1.8 mg/L. The rainwater and geothermal spring contained 0.09 mg/L and 2.23 mg/L F‾, respectively. Around 67.5% of water resources had fluoride content exceeding the permissible limit

Conclusions

This study indicated that fluoride concentration in water resources, south of Fars province, Iran range from 0.06 to 4.95 mg/L and tend to increase with general groundwater flow path. Overall, 67.57% of the analysed samples exceeded the WHO permissible limit of 1.5 mg/L and indicate a need for de-fluoridation of drinking water in this region. However, 2.7% of the samples also displayed concentrations below 0.5 mg/L which implies that fluoridation is required in these areas to protect the

Acknowledgment

The authors wish to express their gratitude to the research committee and medical geology center of Shiraz University for financial and logistic supports.

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