Resumen

HUIZAR ALVAREZ, Rafael; CARRILLO RIVERA, J. Joel  y  JUAREZ, Faustino. Fluoride in groundwater: levels, origin and natural control at the Tenextepango region, Morelos, Mexico. Invest. Geog [online]. 2016, n.90, pp.40-58. ISSN 2448-7279.  https://doi.org/10.14350/rig.47374.

Fluoride in drinking water has been a major concern in many parts of the world as it diminishes the acceptable good quality of groundwater sources. In Mexico this health hazard has been mainly identified along the Sierra Madre Occidental (of felsic nature) and neighbouring territory. Little concern has been given in other parts of the country, as in the State of Morelos in which groundwater is considered by the Ministry of Health fluoride-free. The water supply to the population in the quantity and quality required for human consumption is a public policy issue in Mexico; fluoride (F-) natural content in groundwater varies from 0.001 to 25 mg/L. The highest concentrations are reported in studies carried out in the central, northern, north-western, and areas in the north-eastern part of the country, where a high rate of dental and skeletal fluorosis has been reported. In the southern part of the country, there are very few studies on fluoride; however, there are cases of dental fluorosis, reported in some communities.

This study was carried out in the Tenextepango region in the state of Morelos, located just to the south of Mexico City. Groundwater consumed by the population in this region has a fluoride concentration from 0.5 to 1.9 mg/ L, causing a dental fluorosis condition; infants up to 7 years of age and senior citizens being the most vulnerable. Localities with the highest presence of fluorosis are: Tenextepango, La Longaniza, El Salitre and Xalostoc, where dental fluorosis occurs with a concentration of 0.9 mg/L of fluoride, only.

The aim of this work was to define fluoride levels in groundwater, their distribution in the region of Tenexpenango, its origin, as well as to document its presence and control in the water through the concept of the groundwater flow systems. The hydrogeological conceptual model was established with information on the geological context, water-level monitoring in boreholes, and chemical analyses. Analytical information was obtained from groundwater samples collected on a plain (17 boreholes and five springs) where field (pH, alkalinity, EC and water temperature) parameters were measured at borehole-head. Major cations and anions, as well as trace element content were determined by ICP-MS. These physicochemical data were useful in deducing hydrogeochemical processes, a possible chemical stratification of the water and to comparatively derive the depth travelled by water within the geological environment of reference.

Results suggest that fluoride concentration and groundwater temperature are higher in boreholes pumping continuously, and lower in boreholes taping water only nine hours a day. This response proposes former boreholes to be inducing up-coning of deeper groundwater flows of regional nature Any scheme proposing an increase in water extraction (either in time or in quantity) is to enhance vertical ascending flow to reach the production level permitting such boreholes to capture deeper and warmer regional flow systems with higher fluoride content than that in other boreholes tapping in an intermittent basis.

This hydrodynamic result of extraction time as a continuous decrease in the piezometric surface, is also showing as a decrease of the hydraulic head in the vertical direction just below the extraction level of the borehole, resulting in a deep water ascending flow, whose fluoride content is higher than in the cooler system above (shallow local flow). Also, the calcium content in the warm groundwater is lower than in the cool one, and vice versa. Also a high fluoride concentration corresponds with a high sodium content supporting evidence of a large flow path and long travelling time. Due to affinity of fluoride to calcium in water these ions bind to form fluorite mineral, which precipitates.

Regarding fluoride origin in groundwater, it is meant to be released from rocks of rhyolitic nature primarily by the chemical weathering through hydrolysis processes. The resulting dissolution is from minerals containing fluoride as albite, biotite, amphibole, phlogopite and fluorite, which are widely present in the felsic igneous rocks in the area; once in the groundwater, fluoride will move through hydrodynamic dispersion.

It was concluded that the origin of fluoride in groundwater in this region is result of chemical processes enhanced by travelling time and the high temperature on the regional flows generated beyond the surrounding plain, mainly through felsic rock formations located in the surrounding mountains at the northeast of the study area.

The temperature value and fluoride content in the extracted water in the area vary depending on the extraction time (ie, the longer and/or higher extraction rate). The increase of these parameters show that a continue extraction induces the arrival of water from a deeper regional flow containing fluoride in a significant high content as compared to the shallow water. The control of water extraction with low fluoride content requires conducting control tests on the boreholes allowing to improve the natural quality of the extracted water, allowing a reduction of the fluoride content by setting fluoride vs. temperature at different time intervals after extraction started; and fluoride vs. extraction rate. It becomes important to test different extraction rates (produce different hydraulic gradients in the extraction borehole) to define the extraction rate (or extraction time) with the least fluoride content possible to achieve a fluoride control. The understanding of the groundwater functioning under extraction may avoid additional side-effects of fluoride induction. This could take advantage of the natural system without installing unnecessary and high cost treatment plants which would require costly maintenance, and specialized personnel, which, however, will produce a hazardous sludge that need to be properly managed to avoid additional health risks. It was found advisable that the inhabitants of this region decrease by natural means the fluoride inflow from boreholes containing high fluoride, while a diet with high calcium content should be implemented.

Palabras llave : Groundwater; Fluoride; Fluorosis; Vertical ascending Flow; Water Quality.

        · resumen en Español     · texto en Español     · Español ( pdf )