Presence and Distribution of Fluoride Ions in Groundwater for Human in a Semiconfined Volcanic Aquifer

Cecilia Judith Valdez-Alegría,Reyna María Guadalupe Fonseca-Montes De Oca,José Luis García-Rivas,Rosa María Fuentes-Rivas,Beatriz García-Gaitán

doi:10.3390/resources8020116

Abstract

Dental and emaciated fluorosis is derived from the chronic intake of fluoride ions (F−) by consumption of food, tooth products and drinking water from groundwater. Recent reports indicate that in some regions of Mexico, the incidence of fluorosis in temporary and permanent dentitions have increased in recent years. The purpose of the present investigation was to determine the presence and distribution of F− ions in semi-confined aquifers, located in the basins of Lerma-Chapala and Valley of México. Temperature (T), pH, electrical conductivity (EC) and alkalinity were determined in situ, in 27 groundwater wells. The hardness, chloride ions (Cl−), free chlorine (Cl2), total dissolved solids (TDS) and bicarbonates (HCO3−), were determined in the laboratory. The high content of bicarbonate ions (HCO3−) is associated with the alteration of silicates and the presence of chloride ions (Cl−) due to the anthropogenic activity. The highest value of fluoride ions (F−) (1.31 mg/L) was identified in a well of the Lerma-Chapala basin and in 25% of the samples (1.056 mg/L) of the Valley of México basin. The groundwater in the Lerma-Chapala basin is of the Ca-HCO3 type and the water in the Valley of Mexico basin is Na-HCO3 and Ca-HCO3. According to the Gibbs diagram, the water of the Toluca Valley aquifer is recharged by local meteoric precipitation and consequent the water-rock interaction occurs. The correlation analysis shows the significant relationship between F− and CE (R = 0.5933), F− and alkalinity (R = 0.6924), F− and HCO3− (R = 0.6924) and F− and TDS (R = 0.5933). The correlations confirm that the content of fluoride ions present in groundwater is associated with high concentrations of bicarbonate ions and the presence of sodium, due to a process of alteration of the silicates.

Highlights

Groundwater is the largest source of drinking water; its accessibility can be threatened by the introduction of pollutants from human activities, and by natural processes [1]
Fluorite is found in sedimentary rocks and cryolite is found in igneous rocks. These minerals are almost insoluble in water; they can only be present in groundwater when conditions favor their dissolution or when industrial effluents with high concentrations of F− ions are discharged into water bodies [2]
Electrical conductivity, of the 19 water samples collected in the Valley of Toluca aquifer and the Tenango del Valle aquifer, both located within the limits of the Lerma-Chapala basin; it shows the concentrations of fluoride ions (F− ), chloride ions (Cl− ), alkalinity (CaCO3 ), hardness (CaCO3 )

Summary

Introduction

Groundwater is the largest source of drinking water; its accessibility can be threatened by the introduction of pollutants from human activities, and by natural processes [1]. The natural contamination of groundwater due to geological environments can be an important factor that limits its usage for human consumption. The high or low concentration of certain ions in the water is a Resources 2019, 8, 116; doi:10.3390/resources8020116 www.mdpi.com/journal/resources. Fluoride is one of the ions that cause health problems. Cryolite (Na3 AlF6 ) and the infiltration of rainwater through them, increases the concentration of fluoride in the groundwater causing natural contamination. Fluorite is found in sedimentary rocks and cryolite is found in igneous rocks. These minerals are almost insoluble in water; they can only be present in groundwater when conditions favor their dissolution or when industrial effluents with high concentrations of F− ions are discharged into water bodies [2]

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