A geohydrologic analysis of mine dewatering and water development, Tombstone, cochise County, Arizona

Abstract

Tombstone, 73 miles southeast of Tucson, has been a site of extensive dewatering for mining operations. During an 8-year period between 1901 and 1911, a total of 36, 900 acre-feet of water was withdrawn from ground-water storage in the mine rocks with a maximum decline in water level of 440 feet. For the 8-year period of pumping, this volume of water could have supported a city of 30, 000 population. The sedimentary rocks within the mining district have a composite thickness of 8, 600 feet and include the normal stratigraphic sequence of southeastern Arizona from the Precambrian Pinal Schist to the top of the Cretaceous Bisbee Formation0 Siliceous and quartzoseclastic rocks within the mining district have been altered to quartzite, hornfels, and jasperoid and have the best porosity and permeability as a result of innumerable fractures and fissures. Among the siliceous rocks, the Bolsa Quartzite and Bisbee Formation are the best waterbearing units within the district,. The carbonate rocks, predominant in the section, have been fractured and subjected to solutioning0 They have their best porosity and permeability within the zone of maximum circulation near the water table. The mining district is bordered on the west by the impermeable Schieffelin Granodiorite and related intrusives that partly isolate the district from areas of natural ground-water discharge along the San Pedro River. The district is bordered on the south and east by predominantly crystalline carbonate rocks, The igneous core of the Mule and Dragoon Mountains lies farther to the east, An alluvial valley with large ground-water storage lies immediately north and northeast of the mining district A review of the history of previous water development indicates that the magnitude of the yield of the aquifer complex has been exaggerated out of proportion as a result of circumstances connected with dewatering operations. For comparative purposes the coefficient of transmissibility of the aquifer complex is estimated as 10, 000 to 15, 000 gallons per day per foot and the coefficient of storage is estimated as .01. Prediction of the aquifer performance using these two coefficients is greatly modified by anisotropy and boundary conditions within the aquifer complex, and the yield is rapidly reduced with time. As a result, empirical loglog approximations of the time-drawdown curves are proposed for predicting the long-term performance of the aquifer complex0 The geohydrologic evidence indicates that the Tombstone mining district may provide a suitable location for long-term, moderate production of municipal water. With extensive development, the aquifer complex could support a population several times the present size of Tombstone, but this would result in an appreciable drawdown of the water table. On the other hand, the location is moderately good as a site for the consumption of ground-water storage and dewatering that would necessarily accompany mining below the static water level.