PRELIMINARY HYDROLOGIC SURVEY OF THE SIERRA TARAHUMARA, CHIHUAHUA, MEXICO: IDENTIFICATION OF AQUIFERS

The objective of this study was to carry out a hydrologic survey of 185 km² of the Sierra Tarahumara volcanic tuff region near the village of Creel, Chihuahua, Mexico. This work was done in cooperation with a Jesuit mission that drills wells for the Tarahumara Indians and has been drilling an excessive number of dry wells at a cost of $US 3500 per well. Precise GPS locations were determined for eleven producing wells, seven dry wells, six springs and five streams, using the RTK phase differential method. Corrections to the GPS data were made with NGS Opus to find exact locations of base stations. Relative elevations were measured with a precision of 22 cm. The hydraulic heads of wells were measured with a Durham Geo Slope Indicator Water Level Indicator. In order to create consistent flow paths for groundwater, it was necessary to assign wells and springs to a minimum of four separate aquifers. The four aquifers were Aquifer A with average hydraulic head = (2382 ± 13) m a.s.l., Aquifer B with average hydraulic head = (2333 ± 3) m a.s.l., Aquifer C with average hydraulic head = (2307 ± 5) m a.s.l., and Aquifer D with hydraulic head = 2275 m a.s.l. Aquifer D was represented by a single well. Comparison of stream levels with hydraulic heads of wells and springs showed that streams on the Atlantic – Pacific topographic divide were gaining streams, while streams on the slope off the divide were losing streams. Since Aquifers B and C have the same lateral extent and related work has shown Aquifer C to have higher hydraulic conductivity, future work will focus on determining the depth to Aquifer C.