Aquifer vulnerability assessment in the oaxacan complex: integrating the DRASTIC methodology, TEM, and water quality analysis

This research explores the relationship between aquifer vulnerability, water quality and agricultural and mining activities carried out in a southern portion of the Central Valleys of Oaxaca. This site is located on a shallow aquifer with an average thickness of 30 – 40 m, composed mainly of gravel, sand, clay and silt. These materials constitute the alluvial fill that is considered the main hydrogeological unit, which overlies the metamorphic basement of the Oaxacan complex, which acts as a lower limit restricting the flow of water towards deep infiltration. The study applies a comprehensive methodology, which combines several approaches: the thematic weighting of layers through the DRASTIC method, the analysis of physical–chemical parameters of 145 water samples to determine quality and contamination indices, the interpretation of geological-geophysical data obtained from 11 Transient Electromagnetic Surveys (TEM) that allowed the identification of isoresistive units associated with the geology of the area, these data were complemented with geological field work. The results show that the area dedicated to agriculture has a medium to high vulnerability, with water quality varying from slightly contaminated to highly contaminated, which translates into a moderate to high level of contamination. According to the geological-geophysical interpretation, this area is located in U2b, which presents resistivities that range between 200 and 300 Ohm.m related to alluvial deposits and conglomerates. In the mining activity area, a medium vulnerability was identified, with water quality indices varying from very contaminated to contaminated and contamination levels from moderate to high. The isoresistive unit at this site corresponds to U3, with values greater than 350 Ohm.m, associated with andesitic rocks. At the recharge sites, a low vulnerability was determined, with acceptable water quality indices and low levels of contamination.