Test drilling near two sewage lagoons in The Basin area of the Chisos Mountains, Big Bend National Park, Texas, has shown that the alluvium and colluvium on which the lagoons are located is not saturated in the immediate vicinity of the lagoons. A shallow aquifer, therefore, does not exist in this critical area at and near the lagoons. Should seepage outflow from the lagoons occur, the effluent from the lagoons might eventually be incorporated into shallow ground water moving westward in the direction of Oak Spring. Under these conditions such water could reach the spring. Test borings that bottomed in bedrock below the alluvial and colluvial fill material are dry, indicating that no substantial leakage from the lagoons was detected. Therefore, no contaminant plume was identified. Fill material in The Basin does not contain water everywhere in its extensive outcropping area and supplies only a small quantity of ground water to Window Pouroff, which is the only natural surface outlet of The Basin.

Oak Spring, which is almost 2 miles downgradient from the lagoons, is the sole source of water for The Basin the principal tourist area in Big Bend National Park. Test drilling in the Oak Spring area revealed that the aquifer in the immediate vicinity of Oak Spring is a 5-foot thick sand bed hydraulically confined above and below by relatively thick, compact clay. The sand bed might be bounded locally by faults to the east and west of the spring. The test drilling and seismic surveys in the area also established the existence of a thick, extensive, surficial layer of colluvium consisting of large rhyolite boulders. The colluvial layer, which overlies sedimentary bedrock containing the Oak Spring aquifer, was unsaturated at the borehole sites.

Information from drilling and from hydrogeologic observation indicates that the water from Oak Spring originates as precipitation in the Oak Spring area west of The Basin, with possibly a contribution originating as discharge from The Basin. The rhyolite boulder field in the Oak Spring area, which includes talus from Vernon Bailey Peak, is an effective receptacle for rapid recharge of precipitation. This water could then be efficiently routed into the Oak Spring aquifer in places to the east of Oak Spring where any shallow ground water in the boulder field might enter the subcropping truncated aquifer.

Water-chemistry data, hydrochemical facies, and isotopic data also indicate that water from Oak Spring originates principally from precipitation onto the land surface of the Oak Spring area. Tritium data indicate that Oak Spring water is "modern," with an average age of recharge less than 14 years. The flow rates recorded almost continuously at Oak Spring beginning in December 1986 show a close relation between precipitation and discharge. The highest recorded spring flow of 167 gallons per minute in December 1986 is attributed to record high precipitation in the area during 1986. The lowest recorded flow of 22.4 gallons per minute, in December 1989, followed a period of 20 out of 26 months of below-normal precipitation. Flow at Oak Spring typically lags behind precipitation by about 1 month. This fairly rapid response indicates the spring is fed by a shallow aquifer having good permeability and effective recharge areas with the ability to absorb precipitation rapidly.