Edwards Aquifer 101: City protects its zone for Barton Springs

Edwards Aquifer 101: City protects its zone for Barton Springs

By Christi Covington Friday, 13 June 2008

 

When the Green Water Treatment Plant that feeds off Town Lake closes next year, Austin will no longer take any of its drinking water from the Barton Springs.

However, that does not mean the city will ease water quality ordinances that regulate the Barton Springs Recharge Zone in Southwest Austin.

“The crown jewel of Austin’s park and recreation system is the Barton Springs,” said Matt Hollon, an environmental planner with the city. “It is really important. Right now we are not chlorinating that thing and we certainly don’t want to have to. In fact there is an endangered species, the Barton Springs salamander, that lives there that would probably get killed, so that is not an option.”

The recharge zone, designated for its sensitivity by green signs along MoPac, is part of the Edwards Aquifer. When water hits the zone, it seeps into the ground at a fast rate and enters the stream system eventually supplying the Barton Springs in downtown Austin.

Because of water’s ability to penetrate ground quickly in this area and possibly carry pollutants with it, development regulations are strict—by voter approval.

The people’s wish

Hollon said the Save Our Springs Ordinance passed by the entire city in 1992 is more protective than any similar standard he knows of in Texas and is also one of the most strenuous in the nation.

It allows little impervious cover on individual sites in Southwest Austin. Depending on where a property is located, the
ordinance gets increasingly strict. In the recharge zone, landowners cannot develop or put impervious cover, which is anything from rooftops to parking lots, over more than 15 percent of the lot. The ordinance also requires large water controls, such as ponds or catchment systems, so the water can be redirected into the soil.

It is an ordinance that makes sense to Nico Hauwert, a hydrogeologist with the city, for he has seen nationwide studies indicate that once an area has 10 to 15 percent impervious cover, it starts to impact an aquifer. At around 40 percent impervious cover, “the effect starts to be undeniable and at 80 percent, it is very obvious,” he said.

The concern with impervious cover is when water hits, it cannot enter the soil and feed the springs. It also can gather pollutants and wash them into streams.

Water works

Although Austin is even now getting very little water that comes from the Barton Springs Zone, it is estimated that another 40,000 to 50,000 people do drink from it in communities such as Sunset Valley.

“The interest in the aquifer from a utilitarian perspective is that it can provide water to a community, even during a drought season, because it stores groundwater that fills the springs,” Hauwert said.

The Barton Springs Zone is part of the Edwards Aquifer, which stretches from south Texas to north of Austin. An aquifer is any rock or sand body that holds water, a bit like a sponge.

In the case of the Edwards Aquifer, it is marked by a karstic limestone, which is a made of calcite that dissolves more quickly than other minerals, making it particularly porous like Swiss cheese.

Hauwert said the aquifer is believed to have formed when a major pressure, caused by an earthquake, pulled the rocks apart. Central Texas and the Edwards Aquifer sit on a part of what Hauwert calls the “scar” of this catastrophic event, the roughly 15-mile wide Balcones Fault line. It is partially because of the fault that Central Texas has so many caves. Because of the caves, again, water can seep into the ground quickly.

Within the Edwards Aquifer is the Barton Springs Zone, the area feeding the Barton Springs in downtown Austin. As for how much water the springs can provide, Hauwert said that is best gauged in a drought season. During the 1950s, when the region experienced a serious drought season, the flow dropped to 10 cubic feet per second. Austin is currently on the edge of a drought and the spring moves around 40 cubic feet per second. The average is 50 cubic feet per second.

Within that zone are several areas defined by their different levels of sensitivity. The most sensitive portion that Austin protects is the recharge zone where the porous rocks close or at the surface allow the water to enter quickly.

West of the recharge zone is the contributing zone. Here, the limestone is under layers of other rock and soil in the Hill Country. When rain hits the surface, it is more likely to flow down in shallow streams to the recharge zone. An analogy would be water poured onto a countertop.

“It does not seep into the surface, but flows toward the sink,” Hauwert said.

Mall upheaval

The SOS Ordinance may be the strictest guidelines the city has against development, but a series of regulations paved its way beginning in the early 1980s. Hollon points to the construction of the Barton Creek Square Mall as the “big igniting event” for environmental concerns in Austin.

“What they did is they took kind of a normal hilltop and they just leveled it,” he said. “They flattened it and basically moved the ground around, so it was this big plateau. Apparently there were quite a few sediment plumes.”

Sediment plumes can occur when soil not covered by trees or other ground cover washes away and lands in the creek. It was clogging the creeks, caves and other karst features. The council passed in 1980 and 1981 the Barton Creek and Williamson Creek watershed ordinances, which demanded certain setbacks for development.

The year before, in 1979, the city council passed the Austin Tomorrow Comprehensive Plan. It reflected concern for the Barton Springs Zone and the Hill Country, noting that development should not be encouraged in these areas.

Why protect?

Hollon said it is difficult to determine how the aquifer is doing overall, but he thinks some evidence suggests it is “trending in the wrong direction.”

Studies have shown small traces of contamination in the Barton Springs that can range from herbicides typically used for lawn care to increasing amounts of nitrate, which encourages algae blooms that eat oxygen needed for other water life.

None of it is enough to harm anyone swimming, but he would rather be safe than sorry when it comes to protecting the Barton Springs Zone. Because the groundwater travels rapidly and influences the springs, it has not been recommended for drinking water since the 1920s.

“This science of watershed protection and water quality protection has only been in existence for around 30 years,” he said. “One of the things I like to implore people to consider is we are making decisions that might permanently harm invaluable resources, things we cherish. So we need a little bit of room, a little bit of grace period, margin for error frankly, to get this done so we don’t make major blunders.”

Each feature in the Barton Springs Zone, which feeds the popular springs in downtown Austin, plays a different role in supplying both Southwest Austin’s groundwater and streamflow.

1. Barton Springs Zone: It is the groundwater portion of the Edwards Aquifer that feeds Barton Springs.

2. Watersheds: These land surface areas drain into the streams that run largely east-west on the Edwards Aquifer, which goes south-north. Barton Springs Zone watersheds include the Barton Creek, Onion Creek, Bear Creek, Little Bear Creek, Slaughter Creek and Williamson Creek watersheds.

3. Recharge Zone: It sits within the Barton Springs Zone of the Edwards Aquifer and is the most sensitive area because it is porous at the surface. Water that hits it enters the ground quickly.

4. Contributing Zone: It sits on the west side of the Barton Springs Zone of the Edwards Aquifer in the Hill Country. Water often flows downhill and feeds the recharge zone.

5. Karstic Limestone: It is the rock feature in the Barton Springs Zone that makes the ground porous. Often likened to Swiss cheese, it is filled with holes where water can flow.

6. Runoff: It is the water that does not enter the ground but flows toward streams. It is often associated with carrying pollutants after it hits impervious cover, which is anything from rooftops to parking lots.

7. Balcones Fault Zone: It is believed to have formed during an earthquake. Its unusual vertical layers allow water to enter the ground quickly. It is also responsible for numerous Central Texas caves.

In a series of dye trace studies in the 1990s, the City of Austin and the Barton Springs/Edwards Aquifer Conservation District determined where water goes once it enters the Barton Springs Recharge Zone, meaning they could guess where pollutants would go if the water was contaminated. The groundwater can flow four miles a day or more.

According to city environmental planner Matt Hollon, other dye trace studies have indicated that what happens in Austin’s Barton Springs Recharge Zone does not impact the portion of the Edwards Aquifer used by the City of San Antonio for drinking water. Vice versa, the San Antonio portion of the aquifer does not affect Austin. A high point separates the direction of the water drainage.

Source: Barton Springs Zone Dye Trace Study

Regulated protections

The Save Our Springs Ordinance in Austin is the strictest regulation the City of Austin has to protect the areas considered particularly sensitive in the Edwards Aquifer. Passed in 1992, it requires certain creek setbacks, as well as catchment systems, to reirrigate water into the soil. It also limits impervious cover, which is anything from rooftops to parking lots. The limitations are determined by property location.

For properties in the...

• Williamson Creek, Slaughter Creek, Onion Creek, Bear Creek and Little Bear Creek watersheds
  • (All creeks in the area other than Barton Creek)
  • Impervious cover must not be greater than 25 percent
• Barton Springs Recharge Zone
  • Impervious cover must not be greater than 15 percent
• Barton Springs Contributing Zone
  • Impervious cover must not be greater than 20 percent

Water in Sunset Valley

Unlike Austin, the small, incorporated community of Sunset Valley gets all of its drinking water from groundwater through a well system. To protect the recharge zone, the city requires a maximum of 18 percent impervious cover on each lot, which must be a minimum of an acre. Commercial projects must also have no more than 18 percent impervious cover.

Source: City of Sunset Valley