Bubbling with a life of its own, the cold and crystal-clear waters of Barton Springs give us only a hint of the vast subterranean world no mortal may hope to see. These pristine waters flow to the Springs from the caverns of the Edwards Aquifer, an underground reservoir of billions of gallons of water.
The Edwards Aquifer curves from Bell County in the north to Kinney County in the southwest, and is divided into three sections. The Southern Edwards includes much of south central Texas, and provides drinking water for San Antonio and other cities. The Northern Edwards extends from Austin, through Georgetown, to Salado and supplies drinking water to Round Rock and Georgetown. The Barton Springs section, which encompasses 155 square miles of Travis and Hays counties, delivers water to Barton Springs. In addition, this federally designated Sole Source Aquifer supplies drinking water to over 50,000 rural residents.
Surface boundaries of the Barton Springs section are well defined: Town Lake on the north, the Mount Bonnell Fault on the west, a line paralleling Highway 150 in Hays County on the south and a line roughly paralleling IH 35 on the east. Barton Springs is near the northeast corner of the Aquifer at 435 feet above sea level.
The Aquifer is the legacy of a shallow sea that covered much of Texas during the Cretaceous period, about 100 million years ago. Austin was part of a reef system then, littered with shelled sea creatures whose names often were longer than the animals themselves.
Over millions of years, thick beds of limestone formed from the decomposing shells of these creatures. The Aquifer takes its name from these limestone beds, called the Edwards Formation. The Georgetown Formation also forms part of the Aquifer. Water has dissolved millions of holes in the limestone, from large caves to tiny pockets that give the rock a honeycomb appearance.
Other layers capped the Edwards, until powerful earthquakes sheared the rock, exposing sections of the formation in the hills south and west of town. Through cracks in these exposed layers, water filtered to the bottom of the Edwards, cutting a latticework of caverns that form today’s Edwards Aquifer.
About 300,000 acre-feet of water are stored in this maze of underground chambers – enough to fill the world’s largest building 60 times with plenty to spare.
Of the total recharge water, about 85 percent flows out at Barton Springs and 15 percent is pumped from wells. Most of the well-water is so pure that many people drink it without any chemical treatment.
Water quality at Barton Springs is directly affected by water quality in these creeks. It’s like the old computer programmer’s saying: garbage in, garbage out. Fecal coliform bacteria (found in the digestive tracts of humans and animals), pesticides, nitrogens (found in fertilizers), and other pollutants that wash into creeks in the recharge zone – particularly Barton Creek – quickly find their way into the Barton Springs Pool. This is particularly true after heavy rains, when layers of pollutants that might have accumulated for weeks pour into the Aquifer in large doses. This frequently results in closings of the Pool after a rainfall because bacteria levels are too high, or visibility is too low.
Increased urban development damages the quality of water in the Aquifer. Paved areas reduce the amount of soil and vegetation, which filter some pollutants from runoff water before it enters the Aquifer. Additional houses, apartment complexes and businesses generate more pollutants – from spilled motor oil, antifreeze and gasoline to pesticides, herbicides and fertilizers. Rains wash these compounds into the recharge zone, and hence into the Aquifer – and Barton Springs.
The Aquifer is recharged from the watersheds of six creeks. A watershed is a drainage basin; rain falling in a given watershed is carried by a network of tributaries t0 the parent creek.
As these creeks – Barton, Williamson, Slaughter, Bear, Little Bear and Onion – flow above the Aquifer, water percolates through cracks in the creekbeds to the Aquifer itself. To view the article about the new data of the recharge water, follow this link.
Once underground, the water follows the topography, flowing from the higher southwest corner of the Aquifer (about 850 feet above sea level), to the lowest level at Barton Springs.
Many of the cavities are filled with water. Some of the cavities act as conduits, or tunnels, and feed water to Barton Springs.
But other cavities are relatively dry. They form a network of caves – at least 24 in all – that underlie the Barton Creek watershed. Some of the caves are remnants of ancient conduits, while others were gouged from the sheer cliffs by creeks.
Most of the caves are less than 400 feet long – about the length of a football field, measured from end-line to end-line. But the largest, named Airman’s Cave, is at least 12,000 feet long, making it the eighth longest known cave in Texas.
The caves of Barton Creek are home to many fascinating creatures that have adapted to life in the subterranean world. Airman’s Cave, for example, houses a new species of harvestmen (a relative of the spider) much like the Bee Creek Cave harvestman, which is on the federal endangered species list. Cave X, another major cave in the basin, contains a rare species of blind cave millipede.
Scientists have recently recognized that dozens of rare species of cave animals live in different geological zones near Austin. Barton Creek appears to be a barrier between zones. Many of the rare species found in these caves evolved over hundreds of thousands or millions of years in isolated cave “islands.”
We are still learning to understand these cave ecosystems. One thing is certain though: each depends for survival on unpolluted water from the surface.
Many factors affect water quality
Click the picture for a larger image.