Bedrock aquifers of eastern San Juan County, Utah

Abstract

This study is one of a series of studies appraising the waterbearing properties of the Navajo Sandstone and associated formations in southern Utah. The stu<¥ area is al:x>dy area is about 4,600 square miles, extending from the Utah-Arizona State line northward to the San Juan-Grand County line and westward from the Utah-Colorado State line to the longitude of about 109°50'. Some of the water-yielding formations are grouped into aquifer systems. The C aquifer is comprised of the DeChelly Sandstone Member of the Cutler Formation. The P aquifer is comprised of the Cedar Mesa Member of the Cutler Formation and the undifferentiated Cutler Formation. The N aquifer is comprised of the sedimentary section that includes the Wingate Sandstone, Kayenta Formation, Navajo Sandstone, Carmel Formation, and Entrada sandstone. The M aquifer is comprised of the Bluff Sandstone Member and other sandstone units of the Morrison Formation. The D aquifer is comprised of the Burro Canyon Formation and Dakota Sandstone. Discharge from the ground-water reservoir to the San Juan River between gaging stations at Four Corners and Mexican Hat is about 66 cubic feet per second. The N aquifer is the main aquifer in the study area. Recharge by infiltration of precipitation is estimated to be 25,000 acre-feet per year. A major ground-water divide exists under the broad area east of Monticello. The thickness of the N aquifer, where the sedimentary section is fully preserved and saturated, generally is 750 to 1,250 feet. Hydraulic conductivity values obtained from aquifer tests range from 0.02 to 0.34 foot per day. The total volume of water in transient storage is about 11 million acre-feet. Well discharge somewhat exceeded 2,340 acre-feet during 1981. Discharge to the San Juan River from the N aquifer is estimated to be 6.9 cubic feet per second. Water quality ranges from a calcium bicarbonate to sodium chloride type water