Playas and Recharge of the Ogallala Aquifer on the Southern High Plains of Texas--An Examination Using Numerical Techniques

This study used a numerical model to examine the playa-focused recharge theory by comparing it with the spatially uniform approach and with a zonal approach. Much recent hydrologic, geologic, and chemical evidence suggests that the approximately 19,250 seasonal lakes on the Southern High Plains of Texas and New Mexico, commonly known as playas, are the principal focal points of recharge to the Ogallala aquifer. As focal points of recharge, playas take on a new significance because of their capture of essentially all surface water drainage in the region and the consequent potential for introduction of contaminants to the Ogallala aquifer. In this study, we evaluate the hydrologic plausibility of this playa-focused recharge theory through construction of a finite-difference ground-water flow model that simulates a part of the Ogallala aquifer underlying 11 Texas counties on the Southern High Plains.This study differs in scale and purpose from previous ground-water models of the Ogallala aquifer, which were typically regional-scale water resource evaluations. Because of their broad scale, earlier models adequately conceptualized recharge as a spatially uniform surface process applied at a single regional rate or varied only over very large geographic areas. However, because playas are small features, comparatively high recharge fluxes would have to occur through them to achieve recharge volumes typical of the spatially uniform approach. This study used a numerical model to examine the plausibility of the playa-focused recharge theory by comparing it with the spatially uniform approach and with a zonal approach wherein recharge rates were set on the basis of the geologic unit present at the surface.Model development was initiated under the assumption of steady-state conditions before the significant ground-water withdrawals that started about 195%0 in the study area. Among the most important data to characterize this part of the Ogallala aquifer were the hydraulic conductivity data, which were derived from the available records of well tests. Initial steady-state simulations were used to calibrate boundary conditions, and the appropriate regional rate of recharge, and to adjust the hydraulic conductivity distribution. A geographic information system (GIS) was used to determine that playas constitute 2.74 percent of the surface area in the Blackwater Draw part of the study area and to translate individual playa locations and areas to the appropriate finite-difference cells of the model. Further steady-state modeling was then used to compare recharge scenarios.The playa-focused recharge theory was further tested by using the calibrated model to perform transient simulations to evaluate the significant declines in water levels from 1960 to 1990. However, this effort was hampered by a lack of data regarding the number, placement, and withdrawal rates of the abundant uncataloged wells in the study area. Finally, a smaller part of the Ogallala aquifer and five playas at the U.S. Department of Energy's Pantex Plant, near Amarillo and near the center of the study area, were simulated to evaluate potential contaminant movement within the aquifer utilizing a particle-tracking procedure.The results of the steady-state simulations established that the playa-focused recharge theory is entirelyplausible hydrologically. Quantitative comparisons of discrepancy between the model-simulated and the measured 1959-60 water-table surfaces under the playa-focused recharge scenario are as good as or better than those of the spatially uniform or zonal approaches. A recharge rate of 219 mmyr-l (8.62 inches yr-l) focused through playas across the Blackwater Draw part of the Southern High Plains and 9 rnrn yr-l (0.354 inch yr-l) for the Ogallala outcrop area yielded the best result. The playa-focused recharge rate of 219 mm yr-l (8.62 inches yr-1) is volumetrically equivalent to the zonal recharge rate of 6 rnrn yr-l (0.236 inch yr-l) for the entire Blackwater Draw Formation outcrop area. Furthermore, the transient simulations verified the playa-focused recharge theory by satisfactorily matching the significant declines in water levels over the 1960-90 period, especially those in the area of the City of Amarillo's Carson County Well Field (ACCWF). Finally, the 1990 simulated water-table surface for the Pantex Plant area was used to evaluate potential contaminant movement within the Ogallala aquifer. The results of the particle-tracking procedure indicate that a particle entering the Ogallala aquifer would reach wells to the north and northeast of the Pantex Plant with a traveltime of several tens to hundreds of years.