What's So Bad About Impervious Cover?

Environmentalists say that the real issue that the US290/Loop 1 Task Force should consider is the cumulative effect of impervious cover over the Edwards Aquifer recharge and contributing zones.

In natural, undeveloped parts of Hill Country watersheds, water percolates slowly through the soil for weeks after a rain, gradually emerging into creek beds through many springs, then entering the aquifer through faults and sinkholes in the creekbeds. When impervious cover from roads and buildings blankets an area, more rainwater runs off directly into creeks rather than soaking into the ground. Hence, impervious cover increases peak flows in creeks immediately after rainfall, and decreases base flow in the weeks that follow. Both changes create problems:

* Increased peak flow means that more silt, grease, and other contaminants will be carried directly into creeks from roads, without the filtration that occurs with slow percolation under natural conditions. It also means that more frequent flooding is likely to erode stream banks and carry silt downstream, where it can enter the aquifer through faults, or overflow the dam directly into Barton Pool. Silt that accumulates in the aquifer may be flushed out into Barton Springs during subsequent rains. Eventually the silt could compact within the aquifer, plugging its recharge features and cavities, and thereby decreasing the volume of water transmitted underground.

* Decreased base flow means that creeks and small springs are likely to dry up much sooner after a rainfall. This, combined with increased well pumpage from the aquifer resulting from population growth, could cause sharp declines in aquifer levels during dry seasons. As the aquifer level declines, pollutants become more concentrated, and the odds also increase that bad water will infiltrate into the aquifer from the area east of I-35.

According to scientist Mark Schramm, a member of the Aquatic Biological Advisory Team, "Hundreds of springs in central Texas have dried up, likely due to enormous demands on the aquifer by pumping, the addition of impervious cover, and divergence of surface runoff destined for recharge." -- N.E.