REFRACTION OF THE REGIONAL WATER TABLE AT THE SOUTHWESTERN EDGE OF THE SOUTHERN HIGH PLAINS

In response to significant reduction in recharge at the southwestern edge of the Southern High Plains, the water table in the High Plains aquifer is refracted into the underlying low-permeability Dockum Group. The refraction occurs where the characteristic playa-focused recharge of the arid High Plains changes to diffuse infiltration in the Holocene sands and dune fields in and adjacent to the Pecos Valley section of the Great Plains physiographic province. Most of the diffuse infiltration is returned to the atmosphere by evapotranspiration where the High Plains aquifer transitions from saturated to unsaturated conditions as the Pecos Valley section is approached. The saturated-to-unsaturated transition in the High Plains aquifer and the potentiometric field in the Dockum in the vicinity of the transition have been investigated with over 150 wells and piezometers in a 15,000 acre area in northwestern Andrews County, Texas. The refracted water table is evidenced by the presence of unconfined ground water in the upper part of the Dockum sediments southwest of the transition. Direct measurement of the water table in the extremely low permeability Dockum claystones is virtually impossible. The potentiometric field in the Dockum has been determined from head measurements in the low permeability siltstones and sandstones in the saturated zone and matric potentials in the claystones in the unsaturated zone. The uppermost siltstone in the Dockum at a depth of 125 feet is unsaturated. The next siltstone at a depth of 180 feet exhibits confined ground water conditions near the saturated-to-unsaturated transition in the High Plains aquifer, then unconfined and eventually unsaturated conditions further to the southwest. The potentiometric field within the saturated part of the Dockum, including the Trujillo and Santa Rosa aquifers at 600 and 1,140 feet depth, indicates relatively large downward gradients across the Dockum Group, with a localized upward gradient near the High Plains aquifer transition zone discharging to the water table in the Dockum at about 180 feet. Ground water discharging to the water table in the Dockum is removed by capillarity and minor vapor transport. Matric potentials in the claystones above the water table indicate upward moisture movement.