Both springs and wells draw from groundwater, whether from hundreds of feet down, or issuing forth where recharge levels, pressure, and fissures allow water to surface. Two major aquifers lie beneath us, the Trinity and the Edwards. Though related and interconnected, there are differences in their features. Both are limestone, formed from millennia of seabed deposits when the region was a shallow sea. The many Trinity layers are older and were originally lower, with various layers of impermeability interspersed. The Edwards is newer rock, comprising the layers above the Trinity. Geologic movement shifted the mass of bedrock out of level, uplifting to the west and downsloping to the east, and allowing the different formations to surface and erode in different areas. The Balcones Escarpment, running in an arc past Salado in the northeast to Del Rio in the southwest, shows the faultline that expresses and exposes this tilt, and then plunges the rocks beneath the deep prairie soils of the eastern portions of both Hays County and Texas. Springs, and not coincidentally cities, pop up all along this escarpment. Comal Springs in New Braunfels, San Antonio Springs and San Marcos Springs in their eponymous cities, and Barton Springs in Austin, are four of the five largest spring complexes issuing from the Edwards Aquifer, which are also the largest springs in the entire southwestern United States. Their value and importance cannot be overstated.
These springs supported native populations for millennia, and allowed the earliest anglo settlements in the hill country to take hold. Abundant water was the key; archeological evidence shows continual habitation along these major springs for over 12,000 years. When early European settlers arrived, many of these springs gushed at terrific volumes, some spouting many feet into the air. The geology behind these enormously productive springs is the porous limestone, which both holds water like a sponge and allows it to flow within its gaps, both large and small.
The older Trinity Aquifer formations, surfacing in the western part of the county, are tighter formations; the pores are small and water takes longer to absorb into the rock and move through it. The Edwards, surfacing in the eastern part of the county, is both a younger and softer formation, with much larger gaps within it. Through geologic movement and exposure to somewhat acidic rainwater, large spaces have opened up or dissolved within the rock; these are known as karst formations. As water continues to filter down through the limestone, over the eons and still today, it dissolves and redeposits calcium carbonate. This can create beautiful cave formations, which can be seen throughout the hill country, most notably at Cave Without A Name in Boerne.
These structural differences in the aquifers also mean that water recharges the two aquifers at very different rates. Recharge estimates for the Upper and Middle Trinity are a mere 3-5% per year; that equates to a range of .1” to 2” of recharge per year, well below current rates of consumption. The Lower Trinity is ancient water, and shows no signs of recharge at all. The western areas where the Trinity is the surface formation are also known as the contributing zone to the Edwards Aquifer; the springs that surface here flow eastward to become streams and rivers that eventually drop into the Edwards through the fissures and sinkholes of the karst terrain. That progression into karst terrain distinguishes the boundary of the recharge zone. This pattern of fast recharge via sinkholes means that rainfall can raise water levels in the Edwards by many feet with just one heavy downpour. In some areas along Onion and Barton Creeks, whirlpools will be evident within the stream channels as water drains into the aquifer; the streams often disappear entirely into the ground, leaving stretches of dry riverbed on the other side of the recharge feature. As the stream beds move downhill to the east, springs will sometimes erupt again to refresh the streams, and they will flow again for a stretch. This water moves into and through the Edwards Aquifer with minimal to no filtration, making it the most sensitive aquifer in the state to pollution from land development. Beneath our homes, neighborhoods, towns and fields, water flows through caves and caverns, sometimes forming underground creeks and rivers, sometimes lakes, and emerges at springs in much the same condition as when it went underground. For this reason, locals fight fiercely to protect the land over both the contributing and recharge zones to the Edwards Aquifer, so as to protect the beloved springs all along the escarpment, which bring so much joy come summertime.
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