FIMAR: A new Fluoride Index to mitigate geogenic contamination by Managed Aquifer Recharge
Graphical abstract
Introduction
Major health concern due to high fluoride (>1.5 mg/L) in groundwater is well known and drinking this groundwater has affected millions of people in different parts of the world. An estimated 260 million people consume water with fluoride concentration above 1 mg/L (WHO, 1994). Prolonged intake of fluoride above 4 mg/L is known to cause skeletal fluorosis (Ayoob and Gupta, 2006; Mohammadi et al., 2017). Children are the most affected by high fluoride in drinking water leading to bone deformities and learning disabilities (Raju, 2017; Yu et al., 2018).
Of the total 85 million tons of fluoride deposits on the earth's crust, 12 million tons are found in the rocks and sediments of India (Teotia and Teotia, 1994). Hence, fluoride problem in groundwater is intensive and alarming in the country (Brindha and Elango, 2011). Nineteen administrative states in India contain groundwater with high fluoride concentration (>1.5 mg/L) (CGWB, 2010). Understanding the pathways and mechanisms that drive the fluoride enrichment in groundwater is vital to formulate scientific methods to overcome this geogenic contamination. Ex-situ methods to remove excess fluoride are more practical in areas where pre-treatment of water is carried out prior to water supply. But, these ex-situ treatment methods are seldom used due to the lack of skills in operation and maintenance and due to the problem associated with the disposal of fluoride-rich sludge, which is of environmental concern. Hence, in rural areas with insufficient or limited water supply, people have to depend on the untreated high fluoride containing groundwater for drinking and domestic use.
In recent decades, managed aquifer recharge (MAR) through percolation ponds, check dams, riverbank filtration etc. are more and more used towards increasing groundwater storage, but not focused towards improving the groundwater quality. Very few studies exist on the impact of MAR on lowering fluoride concentration in groundwater, but with mixed results (Bhagavan and Raghu, 2005; Andrade, 2012; Gowrisankar et al., 2017). Few studies have also shown an increase in fluoride after implementing MAR (Bhagavan and Raghu, 2005; Pettenati et al., 2014). Brindha et al. (2016) indicated that an understanding of the temporal variation in groundwater level and fluoride concentration, measured at least on bi-monthly basis is necessary to identify suitable locations for MAR to mitigate this problem. Such studies carried out in fluoride endemic areas helped in identifying locations for recharge and in successfully reducing the fluoride concentration in groundwater below 1.5 mg/L (Brindha et al., 2016).
In order to cope up with wide-scale depletion in groundwater resources, various steps are taken by the Indian Government as well as other agencies. Huge investment is made for sustainable groundwater resources development and management (Ministry of Water Resources, 2016). Nevertheless, such MAR measures have to be taken carefully and judiciously in regions with fluoride-rich groundwater. Hence, in this study, a new tool is proposed which can be used by different agencies and the society to identify appropriate locations for improving groundwater storage and overcome the fluoride problem. This index is applied to a fluoride endemic area in the Pambar River basin and the results are evaluated. The three key objectives of this study are: (1) to identify the spatial and temporal evolution of high fluoride groundwater from the geogenic sources and the mechanisms of fluoride enrichment in groundwater in a part of the Pambar River basin, (2) to develop a new index to map areas where MAR can be adopted to mitigate high fluoride groundwater and (3) to determine the feasibility of MAR to reduce high fluoride in the identified locations through numerical and geochemical models.
Section snippets
Description of the study area
The study area (about 600 km2) is located at about 180 km SW of Chennai, the capital of the southern state of Tamil Nadu and forms a part of the Pambar River basin (Fig. 1a). It is drained by the Pambar River and the streams form dendritic to sub-dendritic drainage pattern (Fig. S1a). The altitude varies from 340 m msl to 1021 m msl and the slope varies from 0° to 41.3° (Fig. S1b). Highest elevation (1021 m msl) is at the summit of the Yelagiri hill, in the north. Except for the hills and
Materials and methods
A field survey was carried out in January 2011 and preliminary observations were made in more than 100 wells located in this area. Lithology, thickness of the weathered rock, depth of the well, groundwater level, electrical conductivity (EC) and pH were recorded. EC and pH were measured using digital portable meters (YSI 556MPS). Elevation of the points from where depth to groundwater level was observed was determined using TRIMBLE explorer 3 Global Positioning System. After this primary
Results and discussion
Totally 280 groundwater samples were collected and analysed during this study. Detailed chemical parameters and composition of groundwater samples are summarized in Table S2. Groundwater chemistry indicates 85% of the area with alkaline pH (up to 9.5) conditions. Most of the analysed groundwater samples indicate fresh to brackish conditions. Geochemical evolution of groundwater shows Ca-Na-HCO3, Na-Cl, Ca-Mg-Cl and Ca-HCO3 types (Fig. S4). General dominance of cations was in the order of
Conclusion
An index (FIMAR) based on geospatial analysis is developed to identify suitable locations for MAR structures to mitigate high fluoride in groundwater. This was applied to a fluoride endemic region forming a part of the Pambar River basin, southern India. The dissolution of fluoride from fluoride-bearing minerals like fluorite, fluorapatite, biotite and hornblende and the reverse ion exchange process in the weathered and highly fractured aquifer system is responsible for the elevated amount of
Acknowledgements
The present work is part of the Doctoral work of the first author. Funding by the Centre with Potential for Excellence in Environmental Science scheme of the University Grants Commission (Grant no. F.No.1–9/2004(NS/PE)) is gratefully acknowledged.
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