PRELIMINARY HYDROLOGIC SURVEY OF THE SIERRA TARAHUMARA, CHIHUAHUA, MEXICO: WELL PRODUCTIVITY

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. Related work on measurement of hydraulic heads showed that four aquifers were present: 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. Although numerous wells had broken pumps, baildown tests were performed on seven wells by purging the well and measuring the recovery in hydraulic head with time using the Durham Geo Slope Indicator Water Level Indicator. Using the Hvorslev Method, the geometric means of the hydraulic conductivities K were calculated as K = 3.0 × 10^-7 m/s for Aquifer A, K = 9.0 × 10^-9 m/s for Aquifer B, and K = 1.6 × 10^-7 m/s for Aquifer C. Geometric means for well yields Y were calculated as Y = 2400 L/hr for Aquifer A, Y = 18 L/hr for Aquifer B, and Y = 57 L/hr for Aquifer C. The hydraulic conductivities are typical for fractured volcanic tuff. No functioning wells were present in Aquifer D. Comparison of locations of producing wells and seven additional dry wells with distances from fracture traces as determined from aerial photographs did not show a simple relationship between well yield and distance from a fracture trace. However, well yield did appear to be correlated with fracture density as measured on the ground in the vicinity of a well. Since related work has shown Aquifers B and C to have the same lateral extent, future work will focus on determining the depth to Aquifer C.