GROUND-WATER AVAILABILITY ANALYSIS FOR THE STARS

By Bill Stein, C.P.G., Senior Hydrogeologist LBG-Guyton Associates, Austin, Texas Office

LBG-Guyton Associates recently investigated the ground-water resources of the McDonald Observatory to estimate the current water supply and the ability of their system to meet future demands. The McDonald Observatory is located on top of Mount Locke at an elevation of about 6,800 feet in the middle of the Davis Mountains of West Texas. Over 100,000 people visit the observatory each year, which is located approximately 16 miles northwest of the City of Fort Davis and 450 mile west of Austin, Texas.
    The McDonald Observatory was constructed in the 1930's when W.J. McDonald, a wealthy banker, donated the bulk of his fortune to the University of Texas "to erect and equip an astronomical observatory for the promotion and study of astronomy." A partnership began shortly after between the University of Texas and a number of other universities around the country to construct and operate the facility.
    The observatory is currently equipped with six optical telescopes with a wide range of state-of-the-art instrumentation for imaging and spectroscopy, and one of the first and most productive lunar ranging stations. A giant new telescope, the Hobby Eberly Telescope (HET), is one of the largest telescopes in the world. The HET is an innovative departure from classical single-reflector telescope design.

Increasing Water Demand
    In recent years, water use for the facility has leveled off at approximately 5 million gallons per year. The number of staff and other employees at the observatory are not expected to increase in the future; however, plans for an expanded visitors' center to accommodate the ever growing crowds of visitors to the observatory are being developed. The increase in visitors may result in larger transient demands for water. Currently, the observatory is supplied water from two wells -located approximately 2 miles directly south of the observatory at an elevation of about 5,310 feet. Two large Gardner Denver Triplex booster pumps move water at approximately 680 psi from storage tanks near the wells up about 1,470 feet to two tanks at the top of Mount Locke at an elevation of about 6,780 feet (see photograph).
    The wells draw water from a fractured volcanic aquifer composed of Tertiary volcanic rocks and associated sediments. LBG-Guyton Associates performed pumping tests on the two wells. Even though a larger horsepower pump capable of yielding more water could be installed in one of the wells, this would not necessarily give the current system the ability to supply more water to the observatory. Currently the capacity of the two wells exceeds the ability of the booster pumps to move the water up to the top of mountain.
    An original well used for supplying the observatory has been abandoned since the two current wells were brought on line in the 1970's. The well was drilled to 1,330 feet but had filled in to about 870 to 900 feet with sediment or debris. The water level and pumping capacity had declined over the years prior to abandonment, which facilitated the need for constructing the new wells. LBG-Guyton has recommended investigating the condition of the old well by use of downhole video. During this pro-ce-dure, the nature of the blockage in the well can be determined and the water level can be measured to determine if the water level has risen over the past 25 years since the well was last used. After the survey, a determination can be made of the feasibility of rehabilitating this well and what would be needed to bring this well back on line.

Lower Costs
    This approach may be less expensive than drilling a new well and could result in a more proven capability. Additionally, this well could probably be "grandfathered in" with respect to permitting the well through the Jeff Davis County Underground Water Conservation District and the Texas Natural Resource Conservation Commission, which also may be more cost effective than drilling a new well. Additional production in the old well may be possible if the well is deepened. Based on the difference in land-surface elevation between the old well and the two existing wells, additional productive zones may be encountered by deepening the well.
    Another alternative to rehabilitating the old well is drilling a new well in order to diversify the observatory's water resource. A new site might be selected at a lower elevation than the old well, which would result in less needed drill depth. Use of fracture trace analyses has also been suggested to try to determine those areas that might have greater fracture density and corresponding larger aquifer transmissivities. A site might be possible near the new visitors' center.
    A means of solving the booster pump limitation has been developed whether the old well is rehabilitated or a new well is drilled -- keeping the additional well independent of the two current wells and booster system that pushes water to the top of the mountain. New tanks would be installed along with a system that would supply only the new visitor's center and possibly the lower mountain facilities. This system would operate independently from the top of the mountain and therefore would reduce or eliminate the need to flow water back down the mountain from the top pressure plane.