INVESTIGATION AND SUCCESSFUL DEVELOPMENT OF THE N-AQUIFER NEAR BLANDING, UTAH

A multi-disciplinary, multi-phase assessment of the local groundwater regime resulted in successful targeting and completion of three potential municipal wells for the City of Blanding in southeastern Utah. Geologic and hydrogeologic data, including new geologic mapping, were compiled in a GIS database. Structural data guided a ground geophysical survey. The three wells were completed to depths of about 1900 feet in the N aquifer (Navajo Sandstone and associated formations).

Initial review of hydrogeologic and drill hole data, including data from oil and gas wells, led to the following significant findings: 1) A gently southward-dipping potentiometric surface for the confined N aquifer lies about 700 feet beneath the City; 2) NE-SW, NW-SE, and E-W joint/fault systems are common in the region; 3) A N-S fault (Lems Draw fault) having about 1000 feet of east-side-down offset at a depth of about 5000 feet had been identified by oil and gas seismic exploration immediately east and southeast of the City. A photo-lineament appeared to be the surface expression of the Lems Draw fault, but any near-surface offset (obscured by mesa-capping loess) appeared limited to less than 100 feet.

Seeking zones of secondary permeability in fault and fracture systems, we conducted a very deep resistivity survey (CSAMT/AMT). No promising anomalies were found in surveyed joint systems. Pronounced low-resistivity anomalies were found in several parallel zones over the Lems Draw fault. These zones, presumably reflecting the presence of faults, dip steeply to moderately eastward.

The amount of displacement apparently decreases upward on the Lems Draw fault and near-surface displacement is distributed on parallel splays. It appears also that a gentle syncline coincident with and parallel to the splays has formed as a result of sag on the down-thrown side of the fault zone.

Holes were bored to intersect three low-resistivity zones within the Navajo Sandstone. Step-drawdown and constant rate aquifer pumping tests were performed at rates that ranged from 300 to 525 gallons per minute. Calculated transmissivity ranged from 125 to 209 feet squared per day. Two of the three wells are being equipped and will provide good quality water to the City of Blanding. The third well is in reserve.