Vulnerability to contamination of the Zaachila aquifer, Oaxaca, Mexico

The aquifer of Zaachila was evaluated following conventional methods (DRASTIC, AVI, and GOD). Depth to the water table was established from a set of wells. Accordingly, the groundwater flows from N to S. Hydraulic conductivity ranges between 1.81647E-05 and 1.70411E-04 m/s, while transmissivity varies between 22.01 and 220.85 m²/day. The net mean annual recharge was estimated at 98 mm/year. The aquifer is constituted mainly by sand, gravel and clay. The soil cover is up to 1.5 m thick and is constituted by lime, clay and sandy material. According to GOD, some zones of this aquifer have a medium to high vulnerability. DRASTIC assigns a high vulnerability to large part of the area. The highest values are observed in the southern and middle sections, from Oaxaca city to the south including San Bartolo Coyotepec, as well as Zaachila town and airport. The AVI method assigns to the valley a high to very high vulnerability. A sensitivity analysis suggests that depth to water table is the key factor determining vulnerability, followed by impact to the vadose zone and soil type.