In 2014, the U.S. Geological Survey, in cooperation with Llano Estacado Underground Water Conservation District, Sandy Land Underground Water Conservation District, and South Plains Underground Water Conservation District (hereinafter referred to collectively as the “UWCDs”), began a multiphase study in and near Gaines, Terry, and Yoakum Counties, Texas, to develop a regional conceptual model of the hydrogeologic framework, geochemistry, groundwater-flow system, and hydraulic properties, primarily for the High Plains and Edwards-Trinity aquifer system and to a lesser degree for the Dockum aquifer. The High Plains aquifer system (hereinafter referred to as the “Ogallala aquifer”), contained within the Ogallala Formation in Texas, is the shallowest aquifer in the study area and is the primary source of water for agriculture and municipal supply in the areas managed by the UWCDs. Groundwater withdrawals from deeper aquifers (primarily the Edwards-Trinity [High Plains] aquifer system that is hereinafter referred to as the “Edwards-Trinity [High Plains] aquifer”) augmented by lesser amounts from the Dockum aquifer provide additional water sources in the study area. The Edwards-Trinity (High Plains) aquifer is contained within the Trinity and Fredericksburg Groups. The Dockum aquifer, a relatively minor source of water in the study area, is contained in the Dockum Group, which was evaluated as a single unit. The potential for continual declines of the groundwater in the Ogallala aquifer in the study area and the potential changes in water quality resulting from dewatering and increased vertical groundwater movement between adjacent water-bearing units have raised concerns about the amount and quality of available groundwater.

The developed conceptual model helped in the understanding of the quantity and quality of the groundwater within the Ogallala, the Edwards-Trinity (High Plains), and to a lesser extent, the Dockum aquifers within the study area. The hydrogeologic framework was used to assess the vertical and lateral extents of hydrogeologic units, bed orientations, unit thicknesses, and location and orientation of paleochannels. In general, the Trinity and Fredericksburg Groups and Ogallala Formation exhibit a slight regional dip (dip angle of about 0.14 degrees) to the southeast with dip directions becoming more to the south with each successively overlying unit (105, 110, and 125 degrees for the bases of the Trinity and Fredericksburg Groups and Ogallala Formation, respectively). In general, the Trinity and Fredericksburg Groups thin to the south and are not present in the southern part of Gaines County, whereas the Ogallala Formation becomes thinner from west to east. The combined thickness of the Trinity and Fredericksburg Groups and Ogallala Formation is generally greatest in the north-central part of the study area and thinnest in the southeastern part of the study area. Paleochannel orientation varied over geologic time as formations were deposited and eroded.

Water-quality samples were collected from 51 wells throughout the study area to better understand general water quality and to provide insight into groundwater-flow paths and recharge areas. Groundwater samples were spatially grouped on the basis of similarities found in the physicochemical properties, major ions, trace elements, nutrients, organic compounds, and selected stable isotopes and age tracers. Three groundwater groups were identified in the study area. The first groundwater group (Group 1), represented mostly by groundwater from the Ogallala and Edwards-Trinity (High Plains) aquifers in the northern half of the study area, is considered to be recent recharge, affected by land-use activities, as explained by the younger age, higher concentrations of nitrate plus nitrite, and more frequent detections of organic compounds. Groundwater wells in the second groundwater group (Group 2) are typically in the southwestern and northwestern parts of the study area, and the groundwater in this group is considered to be groundwater recharged during the Pleistocene period, as explained by the relatively old age of the groundwater, high strontium stable isotope ratios, and hydrogen and oxygen stable isotope ratios. The last groundwater group (Group 3) is likely a mixture of groundwater from the first or second groups (or both) with a third, highly mineralized groundwater as explained by having the highest dissolved-solids concentrations in the study area and having some similarities to geochemical characteristics of samples from the first and second groups.

A groundwater-flow system analysis was done to understand the flow of groundwater throughout the aquifer system. Groundwater-level altitudes for the Ogallala, Edwards-Trinity (High Plains), and Dockum aquifers are generally higher in the northwestern part of the study area and lower in the southeastern part of the study area. Groundwater generally flows in a northwest to southeast direction across the study area in each of the aquifers. The groundwater-flow paths closely resemble the mapped paleochannels, indicating that within the study area, the groundwater flows preferentially along the paleochannels, especially within the Ogallala aquifer where dewatering of the aquifer results in a greater effect of the base structure on the flow of groundwater.

The Ogallala aquifer is unsaturated in localized areas in the study area; unsaturated areas are generally near the southern extent of the Edwards-Trinity (High Plains) aquifer, with the largest unsaturated area west of Seminole, Tex. The saturated thickness of the Ogallala aquifer is thickest (more than 125 feet) southeast of Seminole and west of Brownfield, Tex., near the border between Terry and Yoakum Counties. The saturated thickness of the combined Ogallala and Edwards-Trinity (High Plains) aquifers ranges from less than 10 feet along the far southern edge of the study area to more than 350 feet north and east of Brownfield, Tex., and along the border between Terry and Yoakum Counties.

The aquifer hydraulic properties, including hydraulic conductivity and specific yield, were estimated to better understand the ability of groundwater to move through the aquifer system and quantify the volume of available water in storage. The hydraulic-conductivity values varied greatly within the study area (ranging from about 0.03 to about 350 feet per day), and often large variations were found in the same area. Terry County contained the highest and lowest hydraulic conductivity values for the Ogallala aquifer, whereas Yoakum County contained the highest and lowest hydraulic conductivity values for the Edwards-Trinity (High Plains) aquifer. The highest hydraulic-conductivity values for the Dockum aquifer were in Gaines County, whereas the lowest hydraulic-conductivity values were in Terry County. The estimated specific yield values within the study area range from 0.01 to 0.36. Higher specific yield values generally occurred in the western part of the study area except in the Ogallala aquifer where higher specific yield values were in the east. The Ogallala aquifer had the lowest specific yield range and the least specific yield variability among the three aquifers, whereas the Dockum aquifer had the highest specific yield range and the greatest specific yield variability.

Using the estimated saturated thickness and estimated specific yield grids, the water volumes of the Ogallala and Edwards-Trinity (High Plains) aquifers and the combined Ogallala and Edwards-Trinity (High Plains) aquifers were estimated. The available water in the Edwards-Trinity (High Plains) aquifer (16.6 million acre-feet) is almost double the available water in the Ogallala aquifer (8.8 million acre-feet). Although the Edwards-Trinity (High Plains) aquifer contains more available groundwater, pumping is more difficult because of the relatively low hydraulic conductivity and specific yield values compared to the Ogallala aquifer. Overall, the available water within the combined Ogallala and Edwards-Trinity (High Plains) aquifers is about 6.6, 10.2, and 8.6 million acre-feet for Gaines, Terry, and Yoakum Counties, respectively.