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Hydrogeology of Union County

Measuring the water level in a windmill well in Union County.
(click for a larger version)

Project Overview

Measuring the water level in a windmill well in Union County.

The agricultural economy of Union County in northeastern New Mexico is highly dependent on groundwater. Ongoing drought, large new groundwater appropriations both within the county and in adjacent parts of Texas, and large water level declines in wells have led to concern amongst county residents over groundwater supplies. This report documents the finding of a hydrogeology study to better understand the aquifers utilized in east-central Union County. The study began in 2010 and covers 650 square miles, from north of Clayton to south of Sedan, and east to the state line. The study was jointly sponsored by Northeastern Soil and Water Conservation District (NESWCD), the Aquifer Mapping Program of the New Mexico Bureau of Geology and Mineral Resources, and Healy Foundation.

The goals of the study were to refine the existing geologic map of the area, describe the geologic framework of the aquifers that are utilized, describe present and historic water levels and trends over time, and utilize these data with geochemistry and age-dating techniques to understand the occurrence, age, and flow-paths of groundwater, and to identify the locations and processes of groundwater recharge.

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 amongst these units. 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. Groundwater extraction from all aquifers in the study area exceeds recharge.

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 large and 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 miles west of the study area. Recharge occurred by rapid infiltration of playa lake waters and of precipitation on porous volcanic features, lava flows, and exposed bedrock of aquifer units. Recharge by these processes continues at present, but in insignificant amounts compared to groundwater extraction rates.

Water Table Map

New and historical water-level measurements were used to define the ground water surface in Union County and the Clayton Underground Water Basin during the winter of 2012 – 2013. Inventoried wells have water levels ranging in depth from a few tens to many hundreds of feet and are completed in rock units ranging from Triassic to Quaternary in age. Groundwater surface topography and gradients are controlled by land surface topography and geology. Very low gradients are present in areas of smooth, gently sloping topography and laterally extensive, transmissive rock units. Water level trends since the 1960s are most reflective of extensive groundwater pumping for irrigation in the eastern part of the study area. Water level declines of up to more than 100 feet have occurred there, while water levels elsewhere in the county have declined slightly or not all. No wells with long-period hydrographs showed a consistent trend of increasing water levels.

Funded by the Aquifer Mapping Program, the Bureau of Geology, Northeastern SWCD and Healy Foundation.

For more information, please contact:
Dr. Geoffrey C. Rawling, Field Geologist


  1. Rawling, Geoffrey C., 2015, Groundwater surface map of Union County and the Clayton Underground Water Basin, northeast New Mexico, New Mexico Bureau Geology Mineral Resources, Open-file Report, v. 0570, pp. 1-16.
  2. Rawling, Geoffrey C., 2013, Hydrogeology of east-central Union County, northeastern New Mexico, New Mexico Bureau Geology Mineral Resources, Open-file Report, v. 0555.
  3. Geoffrey Rawling, 2013, Hydrogeology of east-central Union County, northeastern New Mexico, in: Program with Abstracts, New Mexico Geological Society Annual Spring Meeting, April 12, 2013, Macey Center, New Mexico Tech campus, Socorro, NM, p. 47. (Presentation slides).

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