Aquifer tests of the Navajo sandstone near Caineville, Wayne County, Utah

Abstract

Ground water in the Navajo Sandstone near Caineville4was studied during 1975-77 as part of an investigation of water in bedrock in the lower Dirty Devil River basin area. The purpose of the study near Caineville was to determine the water-bearing properties of the Navajo by utilizing data obtained mainly during test drilling and aquifer testing by the Intermountain Power Project. Consolidated rocks that crop out in the area range in age from Cretaceous to Jurassic. The area is underlain by older Mesozoic and Paleozoic sedimentary rocks that extend to a depth of at least 7,160 feet (2,182 meters). The Navajo Sandstone is the major known aquifer in the area, yielding 2,800 and 3,110 gallons per minute (177 and 196 liters per second) to two test wells near Caineville. The Navajo Sandstone is massive, crossbedded, very fine to fine-grained and approximately 900 feet (274 meters) thick. For the practical purpose of aquifer analysis, it is hydraulically isotropic in unfractured parts. It has an average hydraulic conductivity of 0.5 foot per day(0.15 meter per day), and an inferred transmissivity of 450 feet squared per day (42 meters squared per day). Folding of the sandstone has produced fracturing that probablypermits some interformational leakage. The leakage under natural conditionsis upward, under high artesian head; and it probably contributesto the salinity of the water in the sandstone. Fracturing of the sandstone has enhanced the permeability andmade the formation heterogeneous and probably anisotropic on a regionalscale. Values for transmissivity, obtained from tests of variouslengths, range from 1,330 to 4,250 feet squared per day (124 to 395meters squared per day). This indicates that conventional aquifer-testanalysis does not yield the uniform results that might be expected ofthe relatively homogeneous sandstone. For the purpose of calculating long-term pumping effects, a generalized selected value for transmissivity of the Navajo Sandstone is 1,500 feet squared per day (139 meters squared per day); and a generalized selected value for the storage coefficient under artesian conditions is 0.001. For water-table conditions, the value for the specific yield is estimated to be between 5 and 10 percent.