USE OF VERTICAL ELECTRICAL SOUNDINGS IN GROUNDWATER STUDIES: A CASE STUDY IN SOUTH-CENTRAL LAGUNA DE ENCINILLAS, CHIH., MEXICO

The Vertical Electrical Sounding (VES) is a fast and economical technique to perform geophysical exploration in groundwater studies. Thirty five VES were carried out in Laguna de Encinillas, approx. 100 km north of Chihuahua City, in northern Mexico, in order to determine apparent resistivity and thickness of geological units. This study is part of a long term multidisciplinary project trying to elucidate the hydrogeological and hydrogeochemical processes occurring in the El Sauz-Encinillas aquifer. The aquifer is located in alluvial quaternary sediments, with varying granulometry, reaching up to 600 meters of thickness at its center. Despite it being the principal aquifer feeding Chihuahua City, a lack of available geophysical data prevails in its northern part, mostly related to its poor water quality and abundance of clay and evaporitic deposits. The Schlumberger array configuration was used, with AB/2 distance ranging from 250 to 500 meters. Field data obtained was analyzed using the IPI2win computer software. Results show a wide variability in resistivity values throughout the study area. Five geoelectrical units were identified: 1) a hardpan topsoil, with resistivity values ranging from 200-800 Ωm ; 2) an alluvial material mixture (sand/silt) with resistivity values ranging from 25 to 100 Ωm; 3) a lacustrine material (clay/evaporites) with resistivity values ranging from 0.2 to 15 Ωm; 4) a gravel/sand mixture with resistivity values from 100 to 300 Ωm, and 5) a partly fractured rock or conglomeratic material with resistivity values ranging from 400 to 3500 Ωm.