Test Well Drilling Investigation to Delineate the Downdip Limits of UsableQuality Ground Water in the Edwards Aquifer in the Austin Region, Texas

G. E. Kretzschmar


The Edwards aquifer is the principal source of ground water for the Georgetown and Round Rock areas. In addition, Barton Springs is an important natuTsl feature of the Edwards aquifer that provides 8 significant recreational facility for Austin area residents. As the need for further water development becomes pressing, it is useful to know in more detail the quantitative aspect ofground-water resources in the Edwards. Only a very limited amount of water-quality data is available to define the eastern downdip boundary of the Edwards aquifer within this area. To better delineate the downdip limits of usable-quality ground water in the Edwards aquifer in the Austin region, a test well drilling investigation was initiated in December, 1985. The Texas Water Development Board's modified Failing 1500 drill rig, two water trucks, and drilling crew drilled eight test holes in Travis, Williamson, and Bell Counties. Additional well data was gathered on existing water wells. The field investigation included the following: (a) a total of 6,613 feet was drilled; (b) 2,232 feet was drilled in the Edwards aquifer; (c) 417 feet of the Edwards Limestone was cored with approximately 90 percent core recovery; (d) 938 feet of surface casing and 432 feet of liner pipe was sel Chemical analyses offormation waters from lhe test holes and other selected wells indicate that the "bad-water" line, where lhe aquifer contains water of 3,000 milligrams per liter or more dissolved solids, of the Edwards aquifer is generally further west than indicated by previous information. Additionally, core study and testing, geophysical logging, and hydrogeologic tests indicate the following: (a) where present, the Regional Dense Member of the Person Formation represents a hydrogeologic boundary dividing the aquifer into upper and lower units which contributes to variation in the chemical quality of the aquifer waters; and (b) the porosity of the lower unit is more consistent and unifonn than the upper unit.