Hydrogeology of eastern Union County, northeast New Mexico

This paper presents the results of an aquifer characterization study in east-central Union County, New Mexico . The Ogallala Formation and upper Dakota Formation together vary from zero to several hundred feet in thickness and form a complex unconfined aquifer . Confinement increases with depth in the lower Dakota Formation and underlying formations . Shale layers form leaky confining beds . Water level and saturated thickness declines from the mid 1950s to the present have been significant, and large portions of the Ogallala–Dakota aquifer have been dewatered . Water levels in deep wells largely recover after irrigation season ends, but the recoveries are superimposed on a long-term declining water-level trend . Tritium and 14C analyses from groundwater samples indicate that there is no significant recharge occurring to the sampled zones of the aquifer, consistent with the ongoing water level declines . Seepage velocity calculations are consistent with a recharge model in which the groundwater was recharged thousands of years ago, tens of kilometers west of the study area, by rapid infiltration of playa lake waters and of precipitation on porous volcanic features, lava flows, and exposed bedrock of aquifer units .