Urbanization affects water quality no matter where the pavement is laid, as impervious surfaces like roads, roofs, and driveways keep water from seeping into the ground. Instead, the water runs off the land, picking up pollutants like lawn chemicals or car leakage as it goes. It also picks up speed, so that when the runoff reaches lakes and streams, it is moving faster and carrying more sediment than it would in a nonurbanized setting. But the damage isn’t finished when the runoff hits the river. There, high-speed, high-volume runoff erodes the streambed, while increasing pollutant loads that alter water chemistry – fertilizers, for example, can kick off algae blooms. As the pollutant and silt load changes, fish, bugs, and amphibians – all of which help clear flowing streams – have a hard time surviving. Plus, swimmers think it’s yucky.
In places where aquifers recharge slowly, groundwater resources are somewhat buffered from the short-term impact that surface water suffers: Even if your streams are gunked up, your springs may still flow clear. Not so in the Hill Country. The Barton Springs segment of the Edwards Aquifer is composed of hole-filled limestone that resembles Swiss cheese. The aquifer recharge zone is the area where this limestone is open to the surface, allowing rainwater to enter the aquifer with virtually no filtration. Some of what flows into the aquifer flows right back out of springs like the Barton in a matter of hours or days, so increased pollutants can show up quickly. A February 2005 report by the city of Austin found that pollutants in the springs had increased a significant amount over the last decade, and cited the Hill Country development boom as a possible reason.
This isn’t to say that slow-recharging aquifers are immune to the effects of urbanization. When rainwater runs off the land rather than seeping into the aquifer, groundwater reserves are diminished so that wells run dry and springs stop flowing. And they, too, can be polluted over time. The unique geology of the Barton Springs segment of the Edwards, however, makes it a particularly fragile resource, one that requires extra vigilance if it is to be protected. Sidebar
This article appears in June 24 • 2005.