Geophysical characterization of the Etla Valley aquifer, Oaxaca, Mexico

The water supply needs of the city of Oaxaca have increased considerably over the last twenty years. The aquifer that produces 80% of the total water in the region is located in the Valley of Etla, to the NW of the city. Intensive exploitation of the aquifer has reduced the water quality. Basin geometry was estimated from 2.5-D models along four profiles, three W-E and one N-W. The sedimentary thickness reaches up to 730 m. Gravity and geologic interpretations suggest that the Mesozoic mylonites of the Sierra de Juárez and the metamorphic rocks of the Sierra de Oaxaca underlie the sedimentary infill. The fault pattern controls the tectonic behavior of the region and groundwater circulation. The Valley of Etla is a steep graben bounded by the Etla and Oaxaca faults. The main aquifers are in the Tertiary and Quaternary horizons. Twenty-three Vertical Electrical Soundings (VES) across the central part of the valley yield a thickness of the first aquifer of 20 m to 50 m. Wells in the area were used to control the resistive horizons and the stratigraphy. Electric tomography studies in the southern portion of the valley showed the base of the first aquifer at 50 m depth. The water-bearing sedimentary layers increase in thickness to the north of the valley, following the basement. Electromagnetic measurements reveal a vadose zone of at least 20 m thickness towards the center of the Valley. Zones of low resistivity were found in the northern and southern sections of the Valley. A possible contamination is suggested towards the northern portion of the City of Oaxaca.