University of Texas Geology Dept.
Playas and Recharge of the Southern High Plains; Ogallala Aquifer
Report on investigation #242

Playas and Recharge Zones - Southern High Plains of Texas fig 1 - 142k GIF

Pecos river basin and alluvial aquifer - 19k GIF

The Rio Conchos/ Rio Grande/ Pecos River basin - 102k GIF

One of the most obvious features of the Southern High Plains of Texas and New Mexico (fig. 1) is the abundance of small, mostly seasonal lakes commonly known as playas (fig 2). Almost all surface water drainage in the region is captured by the approximately 19,250 playas (Texas Department of Water Resources, 1980). Playas, therefore, are especially important hydrologic features, and they perform a multitude of often interlinked environmental and economic roles. These include providing aquatic habitat, supplying water for range cattle, capturing tailwater from irrigated cropland for reuse, and serving as disposal sites for municipal, industrial, and agricultural wastewater.

Yet another role of playas, often misunderstood or overlooked (see app.), may be the most important of all: that playas function as the focal points for recharce of surface waters to the underlying Ogallala (High Plains) aquifer. Recent studies utilizing stratigraphic evidence (Gustavson and others, 1993) and chemical and isotopic methods (Scanlon and others, 1993, 1994; Wood and Sanford, 1995) have suggested that in the High Plains, recharge of the Ogallala aquifer occurs predominantly through the playas and negligible recharge takes place in interplaya areas. As focal points of recharge, playas take on a new significance because of their capture of most surface water drainage and the consequent potential for introduction of contaminants to the Ogallala aquifer.

The first goal of this study, therefore, was to evaluate the plausibility, from a hydrologic point of view, that playas are the principal source of recharge to the Ogallala aquifer on the Southern High Plains. This theory was evaluated by utilizing a numerical ground-water flow model of a part of the aquifer underlying 11 Texas counties (fig. 1 and fig. 2).

This playa-focused conceptualization of recharge is markedly different from that used in previous numerical models of the Ogallala aquifer, the region's principal water resource. Previous studies, designed to evaluate long-term-use trends and future availability on a broad regional scale, conceptualized recharge as a spatially uniform surface process. These studies used either a single regional recharge rate or recharge rates that were varied over very large geographic areas.

Although the spatially uniform recharge approach was well suited to these water resource evaluations, it is not suitable for quantifying rates and controls on the transport of surface-introduced contaminants if recharge is highly focused. In particular, because of the relatively small playas (approximately 3 percent of total area), comparatively high recharge rates would have to occur through the playas to achieve recharge volumes similar to those of the spatially uniform approach. Such highly concentrated recharge could, conceptually, result in highly localized heterogeneous directions and rates of ground-water movement in the Ogallala aquifer.

Therefore, the second goal of this study was to use a particle-tracking program to evaluate potential contaminant movement in the Ogallala aquifer under the playa-focused recharge scenario. As a case study, a smaller part of the Ogallala aquifer and several playas at the U.S. Department of Energy's Pantex Plant, near the center of the study area, were simulated with the numerical model to track potential contaminant movement within the aquifer.

Nomenclature
Regional investigations that include this study area have also used the term High Plains aquifer instead of Ogallala aquifer (Gutentag and others, 1984). The term High Plains aquifer was used in those studies because stratigraphic units other than the Ogallala Formation, such as the Cretaceous Trinity, Fredericksburg, and Washita Groups and the Triassic Dockum Group, locally contain significant volumes of ground water in hydrologic continuity with the Ogallala aquifer (Knowles and others, 1984; Luckey and others, 1986; Nativ and Gutierrez, 1988).

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