Tuesday, October 12, 1999

How low can you go--if you're a drop of water in an aquifer?

A drop of water that falls in Texas faces many challenges that other drops do not. In Central Texas a drop of water's biggest challenge is to make it to the Edwards Aquifer. This is no easy task. Getting to the Edwards Aquifer all depends on how low you can go.

If you were a drop of water trying to get to the Edwards Aquifer, you would probably start your journey as an h2o water molecule sunbathing on the surface of theocean. As the sun heats up the ocean, you change from a liquid to a gaseous water vapor. The nice thing about evaporating is that you leave behind all of your impurities: water vapor contains only pure water. (Evaporating water is an important way of recycling water.) You are formed into a drop of water as you rise into the cool atmosphere and condense. Now you join up with your buddies to form a cloud. While you hang out in the clouds you fatten up a little and become an even larger drop of water. When you are too heavy, you fall thousands of feet to the ground-hopefully within the Edwards Aquifer recharge zone.

The entire Edwards Aquifer extends along the narrow belt of Balcones Fault Zone from north of Georgetown through Austin, San Marcos, New Braunfels, San Antonio, Hondo, Sabinal and Uvalde to Brackettville. Most drops of water that fall to the ground in the Balcones Fault Zone fall as raindrops. Surviving the thousand-foot fall is the easy part. Now you have to get to the Edwards Aquifer. An aquifer is a natural underground reservoir of water. The reservoir of water is actually a soaked area of gravel, sand, or limestone, in the case of the Edwards Aquifer. No matter where you land, you are in a watershed. If you are one of the lucky ones, you land in a creek or river or lake and join your fellow drops of water. Or, you might fall to the ground instead. While traveling along the land, you often flow over parking lots and pick up motor oil or across an over-fertilized lawn and pick up excess nutrients that you carry with you. This is how many drops of water become polluted and then pollute other bodies of water.

If your creek is connected to the Edwards Aquifer, then portions of the white, limestone creekbed contain secret passages leading to an underground river of the aquifer. You flow through a small crack or a large cave opening in the creekbed that takes you into a thick layer of a very special, hole-filled rock called Edwards limestone. How low you can go depends on where you are in the limestone aquifer and whether you want to be drinkable water. The Edwards Aquifer is the sole drinking-water supply for the 1.5 million people of San Antonio and the surrounding areas. Under Interstate Highway 35 at the Travis/Hays County Line, the limestone aquifer that contains drinkable water is about 700 feet below the surface. East of Interstate Highway 35, the aquifer continues deeper into the ground, but as you percolate through the ground you dissolve so much limestone that you are no longer drinkable.

The lowest you can go in the Edwards Aquifer is the bad water line. The bad water line is the line that separates freshwater from salty water. If you, as a drop of water, plan to get back to the ocean and start the journey again, you might as well sit back and relax. It might take thousands of years to find yourself sunbathing in the oceans waiting to evaporate once again.

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