GEOLOGIC FACTORS CONTROLLING THE RESPONSE OF FRACTURED BEDROCK, BASIN-FILL, AND PERCHED AQUIFERS TO RECENT GROUND-WATER DEVELOPMENT IN THE CEDAR PASS AREA, EAGLE MOUNTAIN CITY, NORTH-CENTRAL UTAH

Recent drilling, potentiometric monitoring, aquifer testing, and water-chemistry sampling in the Cedar Pass area have helped the Utah Geological Survey (UGS) understand the importance of structure and stratigraphy to ground-water sources, which have been heavily developed for domestic and municipal use over the past two decades. Cedar Pass is located in Eagle Mountain City between the northern end of the Lake Mountains and the Traverse Mountains, north-central Utah. Ground-water flow must exit the northern part of the principal Cedar Valley basin-fill aquifer by moving into a transmissive fractured sedimentary bedrock aquifer. I have identified two small perched aquifers in addition to the large basin-fill and bedrock aquifers using information gathered during UGS-supervised monitoring-well drilling, water sampling, and well-log analysis of recently drilled domestic wells. One perched aquifer is contained in Tertiary volcanic bedrock between Paleozoic bedrock and basin-fill sediments. The other is in unconsolidated sediments in a small graben. Based on potentiometric data collected in Cedar Valley and Cedar Pass, the volcanic bedrock, in conjunction with a normal range-bounding fault, impedes ground-water flow to the east from the principal basin-fill aquifer in Cedar Valley.

The UGS conducted two aquifer tests, one lasting 5 months, and the other lasting 46 days, on two municipal supply wells completed in different Paleozoic carbonate formations in the Cedar Pass area. Aquifer response in both wells was characteristic of a dual-porosity system. Transmissivity of the fractured bedrock aquifers ranged from 6,000 to 12,600 ft²/day (560 – 1170 m²/d).

Water-level monitoring from 2004 to 2009 shows that the bedrock aquifers may not be fully recovering from pumping during the irrigation season. Dominant ion water-chemistry type is influenced by the lithology surrounding the sampled wells and corroborates with potentiometric evidence that the perched aquifers are not affected by pumping in the bedrock aquifers. Radiogenic and stable isotope chemistry results indicate some mixing of modern recharge with old water, possibly induced by the recent increases in pumping.