Urban and rural areas rely on groundwater for drinking and around 90% of the rural people depend on groundwater for domestic purposes. Groundwater pollution is associated with many sources like geogenic or anthropogenic source, nitrogen pollutants, agricultural pesticides, dissolution or weathering of the rocks and soil.
Contamination due to toxic metals and metalloids is a huge health problem noticed in semi-arid area of Anantapur district, Andhra Pradesh. Owing to natural and anthropogenic activities like agricultural and industrial activities, groundwater quality is slowly deteriorating, leading to a water shortage crisis and numerous environmental health implications. As public health linked with the quality of drinking water, its systematic monitoring is very essential.
The paper ‘Geochemical and health risk assessment of fluoride and nitrate toxicity in semi-arid region of Anantapur District, South India’ by Muralidhara Reddy B and Sunitha V deals with the fluoride and nitrate concentration in groundwater resources of the area, and its related potential health risk assessment. For this purpose, 50 groundwater samples were analysed for testing hydrochemical parameters including major ions, fluoride and nitrate.
Toxic elements like fluoride and nitrate are released into groundwater and can be accumulated in crops and may cause a significant impact on human health. Considering this factor, the important objective of the present study is to assess the health risk of fluoride and nitrate toxicity in groundwater, which further aids in taking proper suitable measures in preventing health risk in and around parts of Anantapur.
The study area of Mudigubba, Nallamada, Kadiri mandals covering an area of 1136.3 sqkm falls in the south-eastern part of Anantapur district. Important geological formations observed in this area are peninsular gneisses of Archean age comprising of pink granites, schists, composite gneisses of Dharwar age, intruded by a few pegmatite dykes. Denudational hills, dissected pediments, pediplain, alluvium are few geomorphic features noted in this region. The study area is drained by the Chitravathi River and its tributaries.
The results published in the Journal of Environmental Chemistry and Ecotoxicology, Volume 2, 2020 revealed that most of the groundwater samples are alkaline in nature. Hydrochemical type of groundwater in the study area are mainly Na+-K+-Cl−-SO42− and few samples belong to Na+-K+-HCO3−. Non-carcinogenic health risks due to fluoride and nitrate exposure through consumption of groundwater were assessed using the USEPA method. In the study area, groundwater samples, fluoride and nitrate concentration range from 1.2 to 5.9 mg/L and 36 to 92 mg/L.
Based on these results, 86% of fluoride (>1.5 mg/L) and 94% of the nitrate (>45 mg/L) in groundwater samples exceed the permissible limit. The Total Hazard Index (THI) quotient for adults, children and infants had health hazards (THI > 1) in 98%, 88%, and 80% of samples, respectively. Hence, proper precautionary measures have to be taken to control health risk in this area.
Fluoride occurrence in the groundwater
Fluoride concentration in the study area ranges from 1.2–5.9 mg/L with a mean of 3.23 mg/L. Based on the results, 86% of the groundwater samples exceeded the permissible limits. Groundwater is the only source of drinking water in the study area.
The study area was majorly occupied by peninsular gneisses of archean age rocks, which have an abundant source of fluoride-rich minerals. A fractured hard rock zone with pegmatite veins are major sources of fluoride in groundwater. High sodium, potassium, bicarbonate and an alkaline pH increase the ion exchange of fluoride by hydroxyl ions (OH−).
The Na+ gained by groundwater from the chemical weathering of rock-forming minerals is used as an index of the weathering of minerals. This is responsible for leaching fluoride from the minerals present in the soils and rocks.
In this way, geogenic (fluoride bearing minerals: fluorite, apatite, biotite, hornblende, cryolite, villiaumite, topaz, amphibole and micas) sources are the principal key factors for fluoride content under the alkaline condition in the groundwater of the study area.
Moreover, other than the geogenic sources, a significant quantity of fluorine could be contributed by intensive application of phosphate fertilizers, which also contain fluorine as an impurity and are leached down to the saturated zone by irrigation return flows.
Nitrate occurrence in the groundwater
Nitrate concentrations in the study area range from 36 to 92 mg/L in groundwater samples with a mean of 61.5 mg/L. Based on the results, 94% of the groundwater samples exceed the permissible limit of nitrate (45 mg/L). Nitrate contamination is primarily due to extensive use of N-fertilizer and manures, which was the principal cause for higher nitrate concentration in the study area groundwater.
Also, this contamination can potentially originate from agricultural fertilizer leaching, septic tank leakage, inappropriate removal of domestic wastes and effluent of organic matter. Consequently, nitrate has the highest penetration depth into soil horizons and finally into the groundwater resources. In this way, increased nitrate concentration in groundwater supplies can potentially cause human health problems, for example, low oxygen levels in the blood of infants, and it is well known as methemoglobinemia.
Human health risk assessment of fluoride and nitrate in groundwater
The study used the human health risk assessment methodology proposed by USEPA (United States Environment Protection Agency) to assess the non-carcinogenic health risk of fluoride and nitrate to adults, children and infants. The evolution of non-carcinogenic risks due to fluoride and nitrate in groundwater is of greater concern in many countries. Continuous consumption of contaminated drinking water affects human health.
In order to estimate the non-carcinogenic health risk to human health due to the intake of fluoride and nitrate in the groundwater, the total hazard index quotient (THI) was estimated. It indicates a higher concentration of flouride and nitrate ions in the semi-arid region of Anantapur.
From the results revealed that hazard quotient (HQ) of fluoride in the study area ranges between 0.01 and 3.78 (adults), 0.01 to 3.84 (children), 0.01 to 2.92 (infants) and nitrate ranges from 0.87 to 2.21 (adults), 0.21 to 2.24 (children), 0.16 to 1.71 (infants). According to USEPA guidelines, HQ values of fluoride concentration were 68% (adults), 68% (children), 64% (infants) and nitrate concentration were 98% (adults), 88% (children), 78% (infants) of the groundwater samples are exceeding the permissible limit (>1).
From the results, Total Hazard Index (THI) values in this region range from 0.88 to 5.46 (adults), 0.22 to 5.89 (children) and 0.18 to 4.22 (infants). According to USEPA guidelines, THI quotients for adults, children and infants in 98%, 88%, and 80% of samples have exceeded the permissible limit (>1) and may cause adverse health risks. The THI value in each of the three groups was >1, which has to be taken care of.
Conclusion
Groundwater is alkaline and hard in nature. Fluoride concentration in this region varies from 1.2–5.9 mg/L. A possible source of fluoride in groundwater is the weathering and leaching of fluoride bearing minerals from the basement granitic rocks of this region under an alkaline environment. Around 86% of the groundwater samples exceed the permissible limit (1.5 mg/L) of fluoride indicating a high health risk to residents and appropriate defluoridation methods is important to purify this water.
The study area nitrate concentration in groundwater varies from 36 to 92 mg/L. 94% of the samples are exceeding the permissible limit (>45 mg/L). Anthropogenic activity is the main reason for the increase of groundwater nitrate concentration, and continuous intake of the higher nitrate water may cause health problems. The high nitrate concentration in the groundwater could be a result of excessive utilization of chemical fertilizers and manure that could be released into groundwater by weathering host rock fractures.
The groundwater of the study area is mainly controlled by rock weathering/rock water interaction dominance, which shows that continuous and extended rock water interaction has led to the enrichment of fluoride concentration in the groundwater of the study area. Groundwater chemistry depends on silicate weathering and partially by carbonate dissolution.
The non-carcinogenic risks of fluoride and nitrate are computed for the study area. According to USEPA guidelines, the recommended THI value must be <1. 98% of samples exceeding the standard limit of THI for adults, 88% of groundwater are above the recommended standards of THI for children and 80% of groundwater is higher than the recommended limit of THI for infants. Adults are more prone to the risk of fluoride and nitrate concentrations in the groundwater.
The study results indicate the potential health outcomes from fluoride and nitrate exposure and recommend the security and the executive measures for maintainable improvement of groundwater resources to decrease the health disorders in the study area.