New Mexico Geological Society Annual Spring Meeting — Abstracts

This study incorporates new and historical geologic and hydrologic data to understand the aquifer system in east-central Union County. The Ogallala formation and upper Dakota group both vary from zero to several hundred feet in thickness and form a complex unconfined aquifer. Deeper levels of the Dakota group, the Morrison formation, and the Entrada sandstone are confined to varying degrees. Shale layers form leaky confining beds amongst these units.

Water level and saturated thickness declines from the mid -1950s to the present have been extensive. Large portions of the Ogallala-Dakota aquifer have been dewatered. There is much variability in well hydrographs spatially and with formation and depth, however, it clear that ground water extraction from all aquifers in the study area is outpacing recharge. Water levels in deep wells recover after irrigation season (generally March to October) ends, but the recoveries are superimposed on long-term declining water-level trends.

With one exception, sampled groundwaters are largely Ca-Mg-HCO₃-SO₄ waters. Based on recalculation of historical analyses, the water chemistry has not changed since the 1950s. Waters from the Morrison formation in the southwest corner of the study area have distinct Na-HCO₃ chemistry that can be explained by cation-exchange processes occurring on clay mineral surfaces. These waters appear to be isolated from the other aquifers by effective confining layers and probably discharge to surface drainages. Stable isotopic compositions of sampled groundwaters have a weak evaporative signature.

Although recharge mechanisms invoked for the southern High Plains are probably operative in the study area, analyses of tritium and ¹⁴C in well water samples indicate that there is no significant recharge occurring to the sampled water-bearing zones of the aquifer. This is consistent with the large and ongoing water level declines. Simple seepage velocity calculations support a recharge model in which these waters have traveled tens of miles from recharge areas west of the present study area. The sampled groundwaters were recharged thousands of years ago by rapid infiltration of playa lake waters and precipitation on porous volcanic features, lava flows, and exposed bedrock of the aquifer units. Both the physical mechanism and proposed geographic region of recharge are hypotheses worthy of future study.