Contamination of F~ in the Asian countries

Groundwater varies from region to region based on temperature, precipitation, rock composition, and terrain. In most of the Asian countries, F' concentration reached 1.5 mg L'1 of groundwater. Several studies in the Province of Afghanistan recorded the highest level of F' up to 79.2 mg L_1. The F“ content in different regions of South Korea ranged from 20 to 79.2 mg L'1 (Chae et al. 2007). The reason behind high F“ content could be due to the geological activities and the presence of F“ containing minerals in groundwater (Msonda et al. 2007). In the Muteh region of Iran, F“ concentration in groundwater ranged from 0.02 to 9.2 mg L'1 (Kesliavarzi et al. 2010), while it was reported up to 14.10 mg L'1 in Yunclieng Basin of China (Li et al. 2015). In many areas, such as Anuradhapura in Sri Lanka (Chandrajith et al. 2012), East Java in Indonesia (Heikens et al. 2005), and some regions of Anatolia in Turkey (Omc 2008), the F" content in groundwater ranged from 10 to 20 mg L_1.

Indian scenario of fluoride and nitrate contamination

India is one of the countries most affected by F“ contamination in groundwater. Nearly 12 million tons of F' deposits on the Earth’s crust are found in our country, which might be the reason of high F“ occurrence in groundwater. Arsenic and F“ are known as two major pollutants in terms of the number of people affected and the distribution area (Sahu 2019). In many areas, such as Birbhum in West Bengal, the F" content in groundwater is ranged from 10 to 20 mg L~l (Batabyal and Gupta 2017, Das et al. 2016a). Nevertheless, for irrigation, this water can be used (Sahu et al. 2017). F" contamination has been recognized as one of the most important natural groundwater quality issues affecting India’s most of arid and semi-arid regions. For example, high F“ concentrations were found in Andhra Pradesh’s Nalgonda area. Several parts of the country were then found with high concentrations of F“ among which Andhra Pradesh, Rajasthan, Haryana, Punjab, Gujarat and Assam were affected severely (Kumar et al. 2016). For areas near river, the groundwater flow rate is higher with lower residence period which provides lesser contact time between water and F“ bearing rocks. This has resulted in lower groundwater F“ concentrations. Groundwater F“ concentrations below 5 mg L_1 were recorded in Indian states of Bihar. Chhattisgarh, Jammu and Kashmir-, Jharkhand, Karnataka, Maharashtra, Tamil Nadu, and Uttar Pradesh. Concentrations of 5 to 10 mg L_1 were registered in Andhra Pradesh, Delhi, Gujarat, Kerala, Orissa, and West Bengal F~. Fluoride concentrations > 10-20 mg L-1 was recorded in areas of Madhya Pradesh, Punjab, Assam and Haryana. It has been confirmed that the maximum F~ concentration exceeded 20 mg L-1 in many parts of Rajasthan (CGWB 2014). Rajasthan and Gujarat (80-100 percent) were stated to be the highest F~ affected regions, followed by Andhra Pradesh and Punjab (60-80 percent). F~ contamination affects about 21-40% of areas in Madhya Pradesh, Maharashtra, Kerala, Tamil Nadu, Jharkhand, West Bengal and Sikkim.

Nitrate is one of the most common groundwater pollutants in rural areas and is recorded from several areas of Tamil Nadu, Orissa, Karnataka, Maharashtra, Bihar, Gujarat, Madhya Pradesh and Rajasthan. The permissible acceptable level of nitrate concentration in groundwater is 45 mg L-1 according to the BIS standard for drinking water. Approximately 37.7 million Indians are expected to be affected by waterborne diseases annually, 1.5 million children reportedly die from diarrhea alone with the loss of 73 million working days each year due to waterborne disease (WHO 2005).

Sources of fluoride contamination

Water has two mam sources of contamination, i.e., geogenic (contamination that occurs naturally) and anthropogenic (human activity related). There are a wide range of rock types and soil-water interactions resulting in a variety of geogenic contamination in India.

4.1 Geogenic sources

Fluoride is naturally present in rocks, soil and water. The chances of fluoride contamination in groundwater increase due to the presence of the fluoride bearing rocks like fluorspar or calcium fluoride (CaF2), apatite or rock phosphate [Ca3F(P04)3], cryolite (Na3AlF6) and magnesium fluoride (MgF2) (Biswas et al. 2009). The groundwater contamination of fluoride may also rise as a result of drainage and gradual dissolution from the weathering of the rocks (Das et al. 1998, Koteswar and Metre 2014). The other causes are chemical waste from different industries, contributing to groundwater contamination. Southern India’s rocks are rich in fluoride, which is the main cause of pollution of fluoride in groundwater (Brindha and Elango 2011) and gr anites in the Andhr a Pradesh district of Nalgonda contain far higher levels of fluoride than the global average of 810 mgkg'1 (Tikki 2014).

Study by Koteswar and Metre (2014) revealed that F~ release in groundwater can occur through fluoride-bearing rocks such as fluorspar and fluorapatite, which further depends on parameters such as mineral solubility, availability, concentration, pH and flowing water velocity. Such rocks’ weathering processes could be one of the reason for water fluoride contamination. The F~ and OH' ions are negatively charged ions and F~ can easily replace OH' ions in the rock during chemical reaction and increase their concentration in rocks and minerals. Whenever carbonate and bicarbonate-rich water passes through such rock types, F" are released as a result of chemical reactions and percolate to groundwater (Saxena and Ahmed 2001).

4.2 Anthropogenic sources

Fluoride can be absorbed into soil and water through many point and diffused sources, i.e., industrial effluents discharged from aluminum factories, phosphate industries, brick kilns, coal power plants and from phosphatic fertilized agricultural fields. The processing industries of aluminum, utilize the fluoride compound cryolite as a catalyst in the process of bauxite mining and the resulting gaseous fluoride is discharged directly into the atmosphere or in the waste streams. The average F~ content in drainage sources for aluminum reduction plants is 107-145 mg L-1. Concentrations of F- ranging from 1000 to 3000 mg L-1 were reported from the production of glass (Sun et al. 1998). The other non-point groundwater sources also lead to fluoride contamination. Modem agricultural activities such as chemical fertilizers (rich in phosphate) and fluoride containing pesticides increase fluoride levels in groundwater. The appreciable amount of fluoride is also reported from some food items (e.g., cereals and pulses), cosmetics and medicines. The use of NaF drugs for osteoporosis and dental earners is very common; different toothpaste brands use excessive amounts of F~ through which the fluoride is introduced into the oral cavity and reaches the body of humans (Meenakshi and Maheshwari 2006).

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