Geology and Ground -Water Resources of Comal County,Texas

The purpose of this report on the geology and ground-water resources of Comal County in central Texas is to determine the sources of the waters that supply Comal Springs, the largest springs in the Southwest, and other springs and wells. Comal County has an area of about 559 square miles and in 1950 had a population of 16,325. Comal Springs discharge within the city limits of New Braunfels, the county seat of Comal County. With the exception of a small outcrop of basaltic rock near the western boundary of the county, all the rocks in the county are sedimentary in origin and range in age from Cretaceous to Recent. The main water-bearing formations, the Edwards and Glen Rose limestones, are a part of the Comanche series whichhas a maximum thickness of about 1,900 feet in Comal County. The Gulf series which is about 500 feet thick yields very little water. The Uvalde gravel of Pliocene (?) age is found only on hilltops and is too thin to retain water. Small yields for domestic and stock use are obtained from the Leona formation of Pleistocene age, which occurs as terraces along the main streams and has a maximum thickness of about 50 feet. Extensive faulting has exposed almost all the Cretaceous rocks. Seven main faults which are a part of the Balcones fault zone in central Texas cross the county in a northeasterly direction. They are normal faults with the downthrow to the south or southeast, are roughly parallel, and have a combined displacement of about 1,500 feet. The direction of movement of ground water is largely controlled by these faults. Studies of hydraulic gradients; chemical analyses; correlation among water levels, rainfall, and discharge measurements of Comal Springs; and relative runoff of streams within the county prove rather conclusively that more than half of the water discharged by Comal Springs is supplied by a large underground reservoir which also supplies many artesian wells in the San Antonio area. The data show that a relatively large proportion of the water comes from recharge areas wes