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

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.


Introduction
Groundwater is the largest source of drinking water; its accessibility can be threatened not only by the introduction of pollutants from human activities, but also 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

Sampling
Within the limits of the basins mentioned before, 27 sampling sites were located, mainly in the municipalities of Cuautitlán Izcalli, El Oro, San Felipe del Progreso, Tenango del Valle, Toluca and Zumpango de Ocampo (Figure 1). The sampling was carried out during the rainy season, the sampling for the quantification of: fluoride ions, temperature, pH, electrical conductivity (EC), alkalinity, hardness, chlorides (Cl − ), free chlorine (Cl2) and bicarbonates (HCO3 − ), as well as its preservation, was carried out following the procedures established in the norm NOM-230-SSA1-2002 formation. The aquifer with fractured porosity is build up from andesites (Tpv) and ancient basalts. With these porosity geological properties, the aquifer is integrated as a free upper aquifer and the aquifer with fractured medium as a semiconfined lower aquifer. Even when both layers are divided by impermeable materials, they are considered part of the same aquifer [20,21]. As for the aquifer flow system of the Toluca Valley, it is formed by three main directions, the first originates from the Sierra de las Cruces towards the foothills west-circulation, the second comes from the Nevado de Toluca North direction with exit at Ixtlahuaca, and the third originates from the infiltrations of the Nevado east-northeast direction [20]. This aquifer is characterized by a non-totally impermeable top, formed by aquitards, which allow vertical water infiltration so it can receive recharges or lose water through the top or the base [22,23], the aquifer is located in the physiographic region known as the Trans-Mexican Neovolcanic Belt [22,24].

Sampling
Within the limits of the basins mentioned before, 27 sampling sites were located, mainly in the municipalities of Cuautitlán Izcalli, El Oro, San Felipe del Progreso, Tenango del Valle, Toluca and Zumpango de Ocampo ( Figure 1). The sampling was carried out during the rainy season, the sampling for the quantification of: fluoride ions, temperature, pH, electrical conductivity (EC), alkalinity, hardness, chlorides (Cl − ), free chlorine (Cl 2 ) and bicarbonates (HCO 3 − ), as well as its preservation, was carried out following the procedures established in the norm NOM-230-SSA1-2002 and [25]. For the collection of the samples polyethylene bottles were used previously submerged in an acid solution during an hour and rinsed with deionized water. After collection and transport, the samples were kept refrigerated at 4 • C until their analysis in the laboratory [25]. The physicochemical parameters in situ were determined for each of the samples: pH, temperature (T) and electrical conductivity (EC). The samples were measured in triplicate and consecutive way, an average value of the measurements was obtained [25]. The quantification of fluoride ions (F − ), of each of the samples, was performed three times using the potentiometric method, according to the procedure established in the standard NMX-AA-077-SCFI-200. Likewise, the physicochemical parameters, alkalinity, hardness, Cl − , HCO 3 − and Cl 2 were carried out following the procedures set by [25]. Table 2 shows the results of the physicochemical parameters obtained in situ: temperature, pH and 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 also shows the concentrations of fluoride ions (F − ), chloride ions (Cl − ), alkalinity (CaCO 3 ), hardness (CaCO 3 ) and bicarbonates (HCO 3 − ). The majority of physicochemical in situ parameters showed great spatial variation, except Temperature and pH. The low values of temperature and pH were identified near the recharge zones (Xinanteclatl Volcano, Nevado de Toluca) and increase in the direction of the groundwater flow. According to the determined pH values, the analyzed water is considered neutral to slightly alkaline, the pH values ranged between 6.99 and 7.96, with an average value of 7.51; the values of pH and temperature found in the Valley of México basin (Table 1) ranged between 7.0 and 7.8 and from 21.8 • C to 30.2 • C, respectively. The temperature of the water in the Valle of Mexico basin is higher, by 5 • C, than the temperature reported for the samples from the Lerma-Chapala basin, where the maximum value was 25.5 • C, the minimum of 13.8 • C and the average of 19.15 • C. In relation to the values of electrical conductivity, significant spatial differences were observed ( Figure 2). The highest EC value was found in well 11 (609 µS/cm) located in the municipality of San Felipe del Progreso, a shallow well (72 m), which indicates that the water is more mineralized at this point and that it may be receiving infiltrated surface water. The EC values ranged between 99 and 609 µS/cm, while the mean value was 268.47 µS/cm. The highest values were observed in the municipalities of San Felipe del Progreso, Tenango del Valle, El Oro and Toluca, the values increase where the maximum value was 25.5 °C, the minimum of 13.8 °C and the average of 19.15 °C.

Results and Discussion
In relation to the values of electrical conductivity, significant spatial differences were observed ( Figure 2). The highest EC value was found in well 11 (609 μS/cm) located in the municipality of San Felipe del Progreso, a shallow well (72 m), which indicates that the water is more mineralized at this point and that it may be receiving infiltrated surface water. The EC values ranged between 99 and 609 μS/cm, while the mean value was 268.47 μS/cm. The highest values were observed in the municipalities of San Felipe del Progreso, Tenango del Valle, El Oro and Toluca, the values increase towards the central part of the Lerma-Chapala basin, parallel to the groundwater flow ( Figure 2). Meanwhile, the highest value of EC, observed in the Valley of México basin in the municipality of Zumpango de Ocampo, corresponds to well 5 (1642 μS/cm) and the lowest value in the municipality of Cuautitlán Izcalli, in well 9 (492.3 μS/cm).     The hardness in the water was classified by the World Health Organization [12], based on the concentrations of CaCO 3 (mg/L), such as: soft (0-60), moderately hard (61-120), hard (121-180) and very hard (>180). During the study conducted in the Lerma-Chapala Basin it was found that in 47.37% of the wells, located in the municipalities of Tenango del Valle, Toluca and El Oro, the water is moderately hard; 42.11% of the wells located in the municipalities of San Felipe del Progreso, Tenango del Valle, Toluca and El Oro, the water is hard and in 10.53% of the wells, located in the municipalities of San Felipe del Progreso and Tenango del Valley, the water is very hard. Furthermore, the highest value of alkalinity, well 11 (387.04 mg/L), exceeds the maximum limit allowed by the official Mexican standard (NOM-127-SSA1-2000); the smallest value was found in well 23 (87.08 mg/L), located in the municipality of El Oro.
In the Valley of Mexico Basin, the concentration of hardness showed that in 37.5% of the wells, located in the municipality of Zumpango de Ocampo, the water is hard; in 62.5% of the wells located in the municipalities of Cuautitlán Izcalli and Zumpango de Ocampo, it is very hard. According to the type of water found in this study, it was detected that with respect to soft and moderately hard water, it can be assumed that there is a minimum amount of dissolved salts, the hard and very hard waters contain a high volume of chemical elements such as Calcium and Magnesium [26].
In the Lerma-Chapala basin (  9); that is, 40% and 74% higher, respectively, than those found in the Lerma-Chapala basin ( Figure 3). The presence of bicarbonates in the water can be due to the presence of CO 2 , at a CO 3 2− dilution, or to the alteration of silicates [24,27,28]. The high content of bicarbonate ions, in both basins, can be associated to a great extent with a process of alteration of silicates, coming from the feldspar minerals, this alteration is carried out by the charge of the hydrogen ions towards the minerals releasing the silicates [29,30]. In addition to the silicate weathering, anthropogenic sources may also be responsible for the variation of groundwater chemistry in the study area. Likewise, the source of Cl − ions could have been derived from anthropogenic activities such as agricultural activities, this last because the highest total dissolved solids (TDS) value was found in the Valley of Mexico basin, in well 5 (1050.9 mg/L), these high values of TDS may be related to an ion exchange process and possible agricultural activity [31]. The spatial distribution of the F − ions is shown in Figure 3. The wells that show the highest concentration of these ions are located towards the center of the Valley of Mexico basin, far from the recharge zones and in a well close to the area of recharge of the Lerma-Chapala Basin, one of the activities carried out in the zones is agriculture, which could indicate that the source of fluorine in the water is associated with this activity. The evolution of the presence of the F − ions is observed towards the center of the basins and specifically in the Lerma-Chapala Basin in the direction of the surface water flow (Lerma River). According to the classification established by the WHO, the content of fluoride ions in the wells located in the Lerma-Chapala Basin, mainly the wells from San Juan Zitlaltepec, San José La Loma, Santiaguito, San Juan (municipality of Zumpango de Ocampo) and San Pedrito (Tenango del Valle) is considered medium to high [32]. In 1987, Alanis, in the study conducted in 108 municipalities of the State of Mexico, reported low values of fluorides (0 to 29 mg/L), medium (0.3 to 0.69 mg/L), optimal (0.7 to 0.99 mg/L) and high (1.0 to 1.5 mg/L); that is, they do not exceed the maximum limit allowed by standard NOM-127-SSA1-2000 (1.5 mg/L) ( Table 2). Referring to the classification carried out by the WHO, in this study it was found that 78.95% of the wells have a low content of F − ions, 15.79% a medium content of F − ions and 5.26% a high content of F − ions, the highest value found in this basin was 1.31 mg/L [17,18]. In the Valley of Mexico basin, it can be observed that distribution of F − ions content is as follows: 12.5% of the wells of the is low range, 37.5% in the average range, 25% optimum range and in 25% a high content range (Table 2). It has been mentioned that when the filtration water is rich in calcium ions, water of the Ca-HCO3 type, and comes into contact with a material rich in Na + , the ion exchange process is observed in magnitude, releasing Na + ions and favoring the release of F − , so that the water that reaches the aquifer is of the Na-HCO3 type, and therefore, the concentration of F − ions tends to be greater than 1.0 mg/L. In contrast, when the water in the aquifer is of the Ca-HCO3 type, the concentration of Fions tends to be less than 1.0 mg/L [17,28,[33][34][35].
According to this behavior and the data obtained from samplings (Tables 3 and 4) that the water in the aquifers of the Lerma-Chapala basin is of the Ca-HCO3 type (Figures 4 and 5) [24,28,36]; however, for water from wells in the Valley of México basin, 75% could be of the Na-HCO3 type and 25% of the Ca-HCO3 type. Thus, it is possible to say that the hardness in the water of the Lerma-Chapala basin is a mostly due to calcium and in the Valley of México basin only 25% of the wells have calcium hardness. The Fion presence can be from soil contamination as a result of the use of In the Valley of Mexico basin, it can be observed that distribution of F − ions content is as follows: 12.5% of the wells of the is low range, 37.5% in the average range, 25% optimum range and in 25% a high content range (Table 1). It has been mentioned that when the filtration water is rich in calcium ions, water of the Ca-HCO 3 type, and comes into contact with a material rich in Na + , the ion exchange process is observed in magnitude, releasing Na + ions and favoring the release of F − , so that the water that reaches the aquifer is of the Na-HCO 3 type, and therefore, the concentration of F − ions tends to be greater than 1.0 mg/L. In contrast, when the water in the aquifer is of the Ca-HCO 3 type, the concentration of F − ions tends to be less than 1.0 mg/L [17,28,[33][34][35].
According to this behavior and the data obtained from samplings (Tables 3 and 4) that the water in the aquifers of the Lerma-Chapala basin is of the Ca-HCO 3 type (Figures 4 and 5) [24,28,36]; however, for water from wells in the Valley of México basin, 75% could be of the Na-HCO 3 type and 25% of the Ca-HCO 3 type. Thus, it is possible to say that the hardness in the water of the Lerma-Chapala basin is a mostly due to calcium and in the Valley of México basin only 25% of the wells have calcium hardness.
The F − ion presence can be from soil contamination as a result of the use of phosphate fertilizers and pesticides leached into the aquifer. As of this date, there is no documented or recorded health risk due to exposure to F − in the study area. however, for water from wells in the Valley of México basin, 75% could be of the Na-HCO3 type and 25% of the Ca-HCO3 type. Thus, it is possible to say that the hardness in the water of the Lerma-Chapala basin is a mostly due to calcium and in the Valley of México basin only 25% of the wells have calcium hardness. The Fion presence can be from soil contamination as a result of the use of phosphate fertilizers and pesticides leached into the aquifer. As of this date, there is no documented or recorded health risk due to exposure to Fin the study area.    (Table  5) shows a significant and strong relationship between the fluoride ions and electrical conductivity (R = 0.5933), alkalinity (R = 0.6924), bicarbonates (R = 0.6924) and TDS (R = 0.5933) ( Figure 6). While the relationship between fluoride ions and chloride ions and crude chlorine is shown inverse (R = −0.0208 and R = −0.3520, respectively). In addition, a relationship between electrical conductivity and calcium hardness is established, where it was observed that calcium ions control the electrical conductivity more than other ions. Analyzing the concentrations of F − ions and hardness, found in the aquifers of this basin (Table 5), we observe an inverse relationship (R = −0.69) between both parameters; that is, in the wells where high hardness values are reached (200 to 460 mg/L) the concentration of F − ions is less than 1.0 mg/L; however, wells with hardness values between 120 and 200 mg/L, the concentration of F − ions is greater than 1.0 mg/L (R = −0.88). The correlation analysis of the above-mentioned parameters (Table 5) shows a significant and strong relationship between the fluoride ions and electrical conductivity (R = 0.5933), alkalinity (R = 0.6924), bicarbonates (R = 0.6924) and TDS (R = 0.5933) ( Figure 6). While the relationship between fluoride ions and chloride ions and crude chlorine is shown inverse (R = −0.0208 and R = −0.3520, respectively). In addition, a relationship between electrical conductivity and calcium hardness is established, where it was observed that calcium ions control the electrical conductivity more than other ions. Analyzing the concentrations of F − ions and hardness, found in the aquifers of this basin (Table  5), we observe an inverse relationship (R = −0.69) between both parameters; that is, in the wells where high hardness values are reached (200 to 460 mg/L) the concentration of F − ions is less than 1.0 mg/L; however, wells with hardness values between 120 and 200 mg/L, the concentration of F − ions is greater than 1.0 mg/L (R = −0.88). The correlation analysis of the above-mentioned parameters (Table  5) shows a significant and strong relationship between the fluoride ions and electrical conductivity (R = 0.5933), alkalinity (R = 0.6924), bicarbonates (R = 0.6924) and TDS (R = 0.5933) ( Figure 6). While the relationship between fluoride ions and chloride ions and crude chlorine is shown inverse (R = −0.0208 and R = −0.3520, respectively). In addition, a relationship between electrical conductivity and calcium hardness is established, where it was observed that calcium ions control the electrical conductivity more than other ions.  Table 6 shows the correlation of fluoride ions with respect to temperature, pH, electrical conductivity, chlorides, alkalinity, free chlorine, hardness, bicarbonates and TDS. The correlations between fluoride ions, electrical conductivity, alkalinity, bicarbonates and TDS are positive; this allows us to observe that the content of fluoride ions present in groundwater is associated with high concentrations of bicarbonate ions and possibly the presence of sodium, due to a process of alteration of the silicates at the time of residence of the water. With regard to the presence of alkalinity, a strong   Table 6 shows the correlation of fluoride ions with respect to temperature, pH, electrical conductivity, chlorides, alkalinity, free chlorine, hardness, bicarbonates and TDS. The correlations between fluoride ions, electrical conductivity, alkalinity, bicarbonates and TDS are positive; this allows us to observe that the content of fluoride ions present in groundwater is associated with high concentrations of bicarbonate ions and possibly the presence of sodium, due to a process of alteration of the silicates at the time of residence of the water. With regard to the presence of alkalinity, a strong relationship with fluorides is observed which could be due to the presence of calcium salts and their control by ion exchange [37,38]. In addition to the parameters analyzed in situ, the concentration of residual chlorine was determined before the disinfection process. In 52.6% of the wells, chlorine was observed, with values ranging between 0.3 and 1.3 mg/L; this parameter was called crude chlorine ( Table 2). Crude chlorine may be present in the aquifer due to a reduction process that involves the presence of organic matter. In 2019, in the study conducted by Fonseca, in some wells of the Toluca Valley; evidence was obtained of the presence of crude chlorine, mainly in those wells with anthropogenic organic matter content [28].
In the Alanis study (1987), the reported concentrations of fluoride ions helped to observe if there had been any significant variation in some sampling sites over time [32]. With this study it can be observed that there is no significant difference with respect to time, in some sampling sites the concentration shows a decrease in fluoride ions; this behavior is important because a low content would mean a low contribution of fluorides to humans when consuming this water. CONAGUA facilitated a report made in 1997 by the Institute of Health of the State of Mexico (ISEM), where it proposes to the population of the municipalities of San Felipe del Progreso, Sultepec, Tenango del Valle, Zumpango, Cuautitlan Izcalli, Ecatepec, Texcoco and Tlalnepantla not to consume fluoridated salt in its daily diet, because these areas are considered of high risk for fluorosis; this may be due to the fact that the content of fluoride ions in the groundwater depends on the depth at which the water is extracted, geological characteristics of the place and the time of contact of the water with the ground.
Using a Gibbs diagram, the processes that could be occurring in the aquifer were analyzed; according to the diagram, relationship of TDS and Cl − /(Cl − + HCO 3 − ) (Figure 6a). It is observed that the water from the aquifer of the Lerma-Chapala basin comes from meteoric precipitation (rain and its rapid infiltration), with lateral dispersion associated with water-rock interaction; this suggests that the control mechanism of the groundwater chemistry is due to the high rainfall that reaches 1300 mm/year [23] and the weathering of volcanic rocks, mainly silicates. Samples show HCO 3 − > Cl − ; however, the larger content of Cl − was observed in well 26, this could be associated with infiltration of wastewater, associated with the local recharge and the greater interaction is presented by the sample 22. According to the data found, in the Lerma-Chapala basin, it is possible to consider that the local infiltration water is of the CaHCO 3 type, little evolved. Water from the aquifer of the Valley of Mexico is located in the area of evaporation with lateral dispersion and little evolution. This behavior is observed in the farming areas that receive subsurface runoff by agricultural return waters, raising the sodium and TDS content.

Conclusions
The physicochemical parameters in situ showed great spatial variation, except T and pH. The low values of temperature and pH were identified near the recharge zones and increase in the direction of the groundwater flow. The water analyzed in the Lerma Chapala basin is neutral to slightly alkaline and in the Valley of Mexico basin it is mostly alkaline.
The high content of bicarbonate ions, in both basins, can be associated to a process of alteration of silicates, coming from the feldspar minerals. The source of Cl − ions could have been derived from anthropogenic activities, because the highest TDS value was found in the Valley of Mexico basin. High values of TDS can also be related to an ion exchange process and possible agricultural activity.
The highest EC value was found in the shallowest well, which suggests that the water is more mineralized at this point and that it may be receiving infiltrated surface water. In the Lerma-Chapala basin, the EC increases towards the central part parallel to the flow of groundwater.
The high content of F − ions in the Lerma-Chapala basin reached 1.31 mg/L. In the Valley of México basin, the highest value was 1056 mg/L (25% of the samples). Wells with high hardness values (200 to 460 mg/L) have F − ion concentration less than 1.0 mg/L; however, for wells with hardness values between 120 and 200 mg/L, the concentration of F − ions is greater than 1.0 mg/L.
The aquifer water from the Lerma-Chapala basin is of the Ca-HCO 3 type and the water from the wells of the Valley of México basin, of the Na-HCO 3 and Ca-HCO 3 types. Therefore, the hardness in the water of the Lerma-Chapala basin is mostly calcium, in the Valley of Mexico basin only 25% of the wells have calcium hardness. The source of ions F − can be soil contamination as a result of the use of phosphate fertilizers and pesticides that leach into the aquifer. Up to this date, there is no record of the health risk due to exposure to F − .