Groundwater Modeling of Playa-Focused Recharge at the Southwestern Edge of the High Plains Aquifer in West Texas

Ongoing hydrogeologic investigations at a licensed low-level radioactive waste disposal site in Andrews County, Texas include monitoring of a groundwater system in Ogallala (Neogene), Antlers (Cretaceous), and Gatuna (Neogene/Quaternary) Formation sediments on the southwestern edge of the Southern High Plains physiographic province. The disposal site is underlain by unsaturated sands and gravels of these laterally contiguous sediments, informally termed the OAG. North of the site these sediments develop into the continuously saturated High Plains or Ogallala aquifer. Episodes of focused recharge to the OAG sediments have been observed during the last 9 years of groundwater level monitoring near small playas and topographic depressions that collect water after large precipitation events. During smaller precipitation events water is stored in the unsaturated zone and evapotranspired, predominately by mesquite, creosote and grasses. The climate is semi-arid with average evaporation rates far exceeding the estimated average annual precipitation of 15-16 inches/year. Monitoring at more than 250 wells in the OAG and environmental tracer studies confirm a conceptual model of playa-focused natural recharge to the groundwater system with little or no recharge occurring in inter-playa areas. The thickness of the OAG saturated sediments in the vicinity of the disposal sites ranges from 3 feet to 10 feet, with areas of discontinuous saturation away from areas of focused recharge. Environmental tracer studies show the groundwater in the OAG is poorly mixed and disconnected from regional groundwater flow further to the northeast. The disposal facilities are located over a structural high on the erosional surface of the Dockum Group, which immediately underlies the OAG sediments. This feature, referred to as the red bed ridge, is roughly coincident with the topographic divide between the High Plains and the Pecos Valley physiographic provinces and serves as a divide for groundwater in the OAG unit, resulting in largely unsaturated OAG sediments over the crest and south/southwest of the ridge. The spatial distribution of OAG groundwater is controlled by proximity to local recharge areas and the paleo-topographic surface eroded onto the top of the Dockum, as the gradient of the top of Dockum is greater than the water table gradient. Most of the recharged water is directed northward away from the waste disposal sites. Recharge events beneath the largest surface depression north of the waste disposal sites resulted in an increase in groundwater level elevation of about 6 feet with corresponding smaller increases in groundwater levels at the southern limit of saturation, to the north of the disposal sites. Local reversals in groundwater gradient between recharge events, however, have been observed between the areas of focused recharge and the waste disposal sites. The episodic nature of recharge events to the groundwater system and the need to predict maximum groundwater levels on the order of tens of thousands of years has led to development of steady state and transient MODFLOW-SURFACT groundwater models.