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 begun in 2010 to better understand the aquifers utilized in east-central Union County. The study covers 650 square miles and extends 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) and the Aquifer Mapping Program of the New Mexico Bureau of Geology and Mineral Resources. 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 flowpaths of groundwater, and to identify the locations and processes of groundwater recharge.