Degradation of Groundwater due to Fluoride Contamination in India

Introduction: 

India is the largest consumer of groundwater in the whole world. It uses around 230 cubic kilometers of groundwater per year, which is over a quarter of the global total. More than 60 percentage of groundwater in India is used for irrigation, whereas almost 85% of groundwater is used for drinking purposes. 

Even though groundwater is a renewable water resource, the world’s groundwater supply, especially India’s, is steadily decreasing. With the growing population, changing weather patterns, and the decrease in water availability in surface water bodies as well as an increase in pollution of surface water bodies, people are relying more and more on finite groundwater reserves built up over centuries, for household, agricultural, and industrial needs. However, the groundwater is at risk due to overexploitation and the degradation of groundwater quality. Climate change, land use, and population growth are posing a great threat to the groundwater and thereby impacting both, the water quality and quantity. 

Groundwater quality degradation and supply of safe drinking water now has become a major concern in India. And one of the main contaminants contributing to groundwater pollution throughout the nation is fluoride, especially in the southern Indian states. Fluoride contamination in groundwater in India is the most severe and well-known. There are 19 fluoride-affected states in India with fluoride contamination being widespread, alarming, and intense. Endemic fluorosis is prevalent in India since 1937 and was first reported in the state of Andhra Pradesh. 

Fluoride and its extent

Fluorine is considered to be the lightest halogen and one of the most reactive among all chemical elements. The occurrence of fluoride in groundwater is due to the weathering and leaching of fluoride-bearing minerals from rocks and sediments. Fluorine typically occurs as a negatively charged ion in water, either in trace amounts or as a major ion with a high concentration.

Furthermore, 50% of the groundwater sources in India have been contaminated by fluoride and more than 90% of the villages use groundwater for drinking purposes. In fact, more than 40 million people in India are affected due to the prevalence of dental fluorosis. 

The causes of fluoride in groundwater vary depending upon several factors, such as pH, temperature, the presence or absence of complexing or precipitating ions, the size and type of geological formations traversed by water, and the solubility of fluorine bearing minerals. High occurrence of fluoride in the groundwater is expected from sodium bicarbonate-type water which has a calcium deficiency. 

Fluoride and Human Health 

The beneficial and harmful effects of fluoride ingestion are divided by a narrow margin. Fluoride ingestion through potable water with concentrations of about 1.0 mg/l is considered to strengthen the teeth and skeleton (bone); however, fluoride concentrations beyond 1.5 mg/l are undesirable because prolonged consumption of such water causes various health hazards. 

 Fluorosis is a hazardous disease that affects millions of people every year across the world. More than 200 million people over the globe suffer from endemic fluorosis, caused primarily due to excess fluoride in drinking water. A high concentration of fluoride, which is often above 1.5 mg L-1, composes a severe problem in a large part of India. About 80% of the diseases in the world are due to the poor quality of drinking water, and the fluoride contamination in groundwater is responsible for 65% of endemic fluorosis around the globe. 

But at the same time water without the presence of optimal fluoride concentration can cause several health issues. The teeth may form with weaker enamel and cannot remineralize early signs of decay. And for the same reason, many kinds of toothpaste and handwashes contain some amount of fluoride. 

Some of the major health problems caused by fluoride problems in drinking water are dental fluorosis which is caused due to the prolonged consumption of water with fluoride concentrations between 1.5 and 40 mg/l, and skeletal fluorosis which occurs due to the consumption of water with fluoride between 4.0 and 10 mg/l, and deformation of bones both in adults and children. Fluorosis has the greatest impact on growing teeth, and children less than 7 years old are particularly more vulnerable. 

Health is just one of the many areas impacted by fluoride contamination. People are also witnessing a social impact. Dental fluorosis results in discoloration or physical damage in teeth due to prolonged consumption of water with high fluoride concentrations.

To better understand the fluoride contamination let us study the groundwater quality of Alleppy, Southern India. 

Fluoride affecting the groundwater quality of Alleppy, Southern India

Allapuzha commonly known as Alleppey is one of the densely populated coastal towns of Kerala state in southern India. Almost 2,40,991 people living in this region use groundwater as the primary source of drinking. The groundwater is being extracted from a multi-layer aquifer system of unconsolidated to semi-consolidated sedimentary formations, which range in age from Recent to Tertiary. This has resulted in the decline of groundwater levels up to 35 mbgl (meters below ground level). 

But there is an even more serious problem with the groundwater in Alleppey. The groundwater in the Alleppey region shows high fluoride content up to 2.88 mg/l and high salinity. And this number is not going to stay static for a long time. The source of fluoride concentration in the groundwater could be due to the dissolution of fluorapatite, which is a common mineral in the 

tertiary sediments of the area. Suppose there is a rock with fluorapatite and if there is an imbalance in the calcium content then the leaching of the fluorine to the aquifers is possible. So a thorough study of aquifers is important to study the leaching behavior of fluoride. Long residence time and sediment groundwater interaction are some of the major factors responsible for the high Fluoride contamination in groundwater in India. The water quality index revealed that 62% of the groundwater is of poor quality and is not suitable for domestic purposes. Since the groundwater is the primary source of drinking water in the area, proper treatment strategies and regulation the groundwater extraction is required as the quality deterioration poses a serious threat to human health.

Defluoridation 

Defluoridation is a technique that involves the removal of fluoride ions in drinking water. Defluoridating methods can be classified into two parts: additive and adsorptive. Certain chemicals are added to the water in additive methods, and optimal defluoridation conditions are maintained. In turn, the fluoride ion in the water reacts with these reagents to generate an insoluble compound that is removed as ad flocs. Adsorptive methods, on the other hand, choose a bed with higher surface activity and run water across it. Fluoride ions are adsorbed on the bed surface as a result of surface activity, resulting in a reduction of fluoride ions in the exit stream.

The various methods used for the removal of excess fluoride from water are given below: 

Nalgonda Technique:

Nalgonda process is one of the many defluoridation methods that has been developed by Nalgonda people later NEERi made it. After evaluating a variety of materials, the National Environment Engineering Research Institute (NEERI) in Nagpur developed the Nalagonda process. The addition of aluminium salts, lime, and bleaching powder, followed by quick mixing, flocculation, sedimentation, filtering, and disinfection, is used in this technique. The dose of aluminium salt increases as the fluoride and alkalinity of the raw water increase. Lime aids in the formation of dense flocks, allowing insoluble fluoride salts to settle quickly. This method is preferable at all levels since it is cost-effective and can be handled easily. 

Adsorption Technique:

Several adsorbent materials have been tried to find out an economical and efficient defluoridating agent. Activated carbon, activated alumina, calcite, activated sawdust, activated alumina coated silica gel, activated fly ash, groundnut shell, rice husk, coffee husk, magnesia, tricalcium phosphate, and many more. These are some of the various adsorbent materials that have been reported in the literature. The most commonly used agents for defluordation are activated alumina and activated carbon. The adsorption method is typically used in wastewater treatment plants to remove excess fluoride content in the water. 

Ion Exchange Method:

Ion Exchange is a reversible process in which there is an interchange of one kind of ion present in an insoluble solid with another like-charged ion that is present in a solution surrounding the solid. This reaction is used for softening or for demineralizing the water, the purification of chemicals, and the separating of substances. 

For defluoridation of water, various types of cation and anion exchange resins have been used such as polyanion (NCL), Tulsion A-27, Deacedite FF (IP), Amberlite XE-75, and many more. These resins are used in the form of chloride and hydroxy

Ion exchange removes fluoride up to 90-95% while retaining the taste and colour of the water. 
 

Conclusion: 

When fluoride levels in drinking water are high enough to cause fluorosis, a variety of defluoridation methods and materials can be used. The decision on fluoride limitations in drinking water for any place, however, must be determined by average annual daily temperatures, dietary habits, nature, and levels of activity in that area.

Because communities differ in socioeconomic, economic, and traditional status and customs, the adoption of existing technology would require some local research. As a result, a basic study is needed to determine the appropriateness of a pre-selected technology in order to ensure the intervention's long-term viability.