2017 State of Our Environment Report

Aquifers

Importance

The Edwards and Trinity aquifers are integral parts of the unique landscape of the western portions of our city. They consistently provide large volumes of clean water to well-owners, springs, swimmers, and rare and endangered species. About 75 percent of all known springs and seeps in the Austin area occur in the Edwards and Trinity aquifers (Figure 1). Understanding our natural resources is a critical step toward protecting the resources that we cherish and utilize.



Figure 1. Percentage of spring occurrence for major and minor aquifers in the Austin area
In East Austin, near-surface water-bearing units of gravel, sand, and clay deposited on top of clay-rich limestone contain groundwater that is perched above the impermeable layer. Springs and seeps discharging from these geologic units supply the baseflow for our eastern creeks (Figure 2). These old river alluvial layers were deposited during the ice age when the river known now as the Colorado River flowed at a higher elevation. Now, these units form isolated ridges or hillside terraces and contain about 16 percent of the Austin area’s springs and seeps.


Figure 2. Alluvial Terrace Springs along Terry Creek, a small tributary to the Colorado River, at the Upper Mill Dam Waterfall

Status and Trends

Staff biologists and geologists provide technical assistance during the City review process for the identification, evaluation, and protection of critical environmental features (CEFs) such as karst recharge features (caves and sinkholes), springs, wetlands, rimrocks, and bluffs (Figure 3). The biologists also monitor populations of threatened and endangered salamanders.





Figure 3. CEFs identified by Watershed Protection Department review staff, fiscal year 2017
Accomplishments this year include creating a comprehensive database of more than 700 karst features and conducting a water tracer study to fine tune our knowledge of surface water recharging the Edwards Aquifer in Onion and Little Bear creeks (Figures 4 and 5). Tracers arrived at Barton Springs in five to six days (Figure 6), demonstrating high migration velocities common for water in karst aquifers and underscoring the sensitivity of these systems to stormwater runoff.

Figure 4. Adding tracer and water to recharge sinkhole. Photo by: Saj Zappitello.
Figure 5.  Adding tracer to quarry pond. Photo by: Sylvia Pope.
Figure 6. Preliminary results of 2017 tracer test from Onion Creek and Little Bear Creek. Data sources: Grayscale basemap from ESRI and partners (2017), Edwards Aquifer boundary from Texas Commission on Environmental Quality (2005).
Did you know that Barton Springs salamanders also occur outside of Barton Springs? This year, staff biologists documented the presence of this endangered species at four springs along Barton and Onion creeks where they had never been seen before. DNA sequence data are being collected from salamanders to determine whether they migrate between different springs and watersheds. These species serve as an indicator of water quality for the springs and aquifer. During surveys, biologists captured, photographed, and released the salamanders. These amphibians’ unique markings allow us to track individuals over time as they are recaptured (Figure 7). From these data, we can estimate the population size (Figure 8) and assess whether migration is occurring between the springs. Results of this work will provide new insights about ecosystem health and integrity in the Barton Springs segment of the Edwards Aquifer.
Figure 7. Photo of Barton Springs salamander recently documented at a spring on Barton Creek, noting the unique marking used to identify individual salamanders
Figure 8. Estimated abundance (with 95 percent confidence intervals) of Barton Springs salamanders at Eliza Spring from October 2014 to September 2017

Annual Focus

Eliza Spring is one of four springs in Zilker Park where endangered Barton Springs salamanders live. Prior to human disturbances, trees and wetland plants shaded Eliza Spring and its stream (Figure 9). In 1903, Andrew Zilker built a concrete amphitheater around Eliza Spring for the Elks Club. The original amphitheater allowed spring water to flow through a “keyhole” to the stream (Figure 10). Limestone masonry filled in the keyhole in the 1920s (Figure 11). The stream habitat was eliminated by re-routing the water underground through a metal pipe more than 80 years old and was at the risk of failing, threatening the salamander habitat in the amphitheater.
Figure 9. The natural state of Eliza Spring before the amphitheater was built in 1903


Figure 10. The original configuration of the Eliza amphitheater in 1903. The “keyhole” where water exited between the steps was filled in the late 1920s.
Figure 11. The Eliza Spring amphitheater after the keyhole was filled and stream buried into a pipe
The Eliza Stream Daylighting Project removed the failing pipe (Figure 12) and restored approximately 250 square feet of salamander stream habitat, allowing more endangered salamanders to live here and thereby improving the resiliency of the species. The stream design included considerations that ensure suitable salamander habitat conditions, including ideal water speed and depth, type and size of rocks, and native stream vegetation. Water was released into the new stream on September 2, 2017 (Figure 13). Following the water release, the salamander habitat in the amphitheater saw immediate benefits.  The water depth decreased in the amphitheater, which reduced the amount of sediment and increased the area that salamanders can inhabit. By mid-September, WPD biologists found aquatic moss and several species of invertebrates in the new stream. Finding the invertebrates is important because they comprise the salamanders’ food source and are necessary for the salamanders to survive in the stream. The stream is protected for salamanders with fencing that enables pool users to view the stream, including an overlook area that provides an educational opportunity for the public to see more natural Barton Springs salamander habitat. This project is included in the City’s U.S. Fish and Wildlife permit, which allows Barton Springs Pool to remain open for recreation. It is also part of the Barton Springs Pool Master Plan and was sponsored by the City’s Parks and Recreation Department.
Figure 12. Eliza stream construction showing wall foundations, rock walls for the slope and stream channel, and stream channel being built
Figure 13. The re-created Eliza stream shortly after opening in 2017, showing the stream from the pool sidewalk
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