For the past two years--and in the midst of a drought--University of Texas water quality researchers have captured runoff from Austin's Loop 1 expressway by regularly spraying both traffic and road surfaces.

Why have they gone to all this trouble?

They're working to protect an environmentally-sensitive aquifer--the Edwards Aquifer. The four-year, $1.4 million project funded by the Texas Department of Transportation (TxDOT) is designed to predict runoff's effects on water quality and quantity in the aquifer before, during, and after construction of Loop 1.

The Edwards is no ordinary aquifer. Not only is it the only underground source for Austin's drinking water (and therefore covered under the 'sole source aquifer" program of the Safe Drinking Water Act). It's also covered by only thin layers of topsoil, so runoff flows virtually unfiltered into the aquifer.

What's more, the Edwards Aquifer is more like a pipeline than a filter, because it's made of cracked, cavernous limestone. "Since limestone is porous, surface water disappears quickly like water flushed through a plumbing system," says Carlos Swonke, Water Quality Coordinator at TxDOT. "In a more conventional, less porous aquifer system, water moves more slowly--slow enough to be filtered."

According to the U.S. Geological Survey, 85% of the water reaching the Edwards originates in creek beds in the recharge zone, so researchers have focused mostly on these areas. To prevent runoff into the creeks, they have tested temporary barriers like geotextile silt "fences" and more permanent water pollution controls like sand filters and sediment ponds.

Their work has been thorough. To capture individual runoff pollutants at predetermined intervals, they have used a rainfall simulator--fifty 4.3 meter-high tripod stands, each mounted with a spray head, extending over a 228.6-meter length of highway. They have taken water samplings above and below new highway construction, in both dry and wet periods, and in varying traffic conditions. They have even conducted an extensive literature search on highway runoff and published their review in a 140-page technical report.

The starting point for the Edwards research team was to discover the kinds and amounts of individual pollutants in the runoff. "the amount of damage to the environment caused by runoff depends on where it ends up," says Lyn Irish, designer of the project's rainfall simulator. "Before we build pollution controls, we've got to find out what's in the water and in what quantities."

The critical "first flush" of runoff which Edwards researchers examined contained nutrients, heavy metals, and suspended solids. Preliminary results showed that sand-only filters didn't work well for these compounds--the filters clogged easily. When the researchers placed an alternative medium below the sand, they got better results.

Once the project's researchers know more about which filters work best in treating runoff, they'll be able to develop a prototype runoff control facility.

By the time the edwards Aquifer research is completed, the Edwards researchers will also have clearer answers to questions like these: What is the relationship between average daily traffic and the amount of pollutants in the runoff? How do the number of dry days preceding a storm affect runoff water quality? How significant are the intensity and the duration of the storm? What structural controls work best for treating storm water runoff? How much runoff do you need to catch to control pollution?

AND...water quality experts across the country will have access to a state-of-the-art watershed computer model which can predict both the type and amount of contaminants for a site-specific location.