3D MODEL OF THE ETLA VALLEY AQUIFER, OAXACA, MEX.: GEOHYDROLOGICAL EVOLUTION AND GEOPHYSYICS

The Etla valley aquifer located to the NW of the City of Oaxaca produces 80% of the total water supplied in the region. Geophysical studies characterized the basin geometry and suggested a maximum sedimentary thickness of 730 m. The Mesozoic Mylonites of the Sierra de Juarez and the metamorphic rocks of the Sierra de Oaxaca underlie the sedimentary infill. The aquifer system is embedded in Tertiary and Quaternary horizons. A three-dimensional ground water flow model, based on the interdisciplinary geophysical, geohydrological and geological studies, was computed using the Visual ModFlow program. The flow model shows a general draining from NW to SE following the axes of the valley. The simulation reveals that the present waterextraction rate produces an important abatement of groundwater level at the City of Oaxaca (SE of the valley). Another important characteristic of the groundwater system is that the flow direction of the aquifer favored the pollution by fertilizers and leakage of the sewage network, dumped to the Atoyac River, the principal drainage. A ten years later model assuming redistribution of the pumping wells, from the City of Oaxaca towards the recharge zones (flanks of the Sierras de Juárez and Oaxaca), allows a slight recovery of the aquifer system.