HYDROGEOLOGIC CHARACTERIZATION OF COMAL SPRINGS DURING LOW FLOW PERIODS, EDWARDS AQUIFER, TEXAS- IMPLICATIONS TO SUSTAINING SPRING FLOW THROUGH AQUIFER MANAGEMENT AND SPRING AUGMENTATION

Comal Springs, the largest spring complex in Texas, flows an average of 287 cfs from the Edwards aquifer. Landa Lake, the lake associated with the springs, went “dry” in the summer of 1956 after several years of extended drought. The “drying up” of the spring-fed lake was pivotal in the lawsuit of the Sierra Club vs U.S. Fish and Wildlife Service in 1992 and its subsequent impact on water management in Central Texas. The hydrogeologic setting of the springs and the history of spring flow were investigated to determine detailed spring plumbing, relative impact of regional and local water users on spring flow during low flow periods, and implications to sustainability of spring flow by aquifer management and spring augmentation.

Comal Springs is composed of 8 discrete springs and extensive seepage into the bottom of Landa Lake. The springs are located along the Comal Springs Fault, which separates the Edwards aquifer into up thrown and down thrown fault blocks. Hydrologic studies, dye traces, and hydrochemistry of the springs indicate that the western springs discharge groundwater from the up thrown block and the eastern springs discharge groundwater from the eastern down thrown block.

The sequence of events that caused the springs and lake to go dry was documented by comparing aquifer water levels, lake levels and spring discharge rates to the local historic newspapers articles about the springs. In the 1950’s, the western springs went dry when water levels in the up thrown block dropped below the elevations of the individual spring orifices. The eastern spring orifices and the springs in the lake stopped flowing when water levels in the down thrown block dropped below the lake level. Early in the summer of 1956 the lake stopped flowing to the Comal River; lake levels then dropped six feet in less than three months. The rapid lake level drop occurred because excess flow was no longer leaving the hydrologic system as stream flow and lake levels then mimicked the rapidly declining water levels in the confined aquifer. In addition 10-20 cfs of local ground water pumpage from the Edwards was brought on line to supplement local water supplies.