1. Central Texas Hydrogeology
EVS 311. Field Seminar on Environmental Science
and Sustainability

2. Central Texas Hydrogeology

3. Learning Objectives
• Environmental Science - hydrogeology of Edwards Aquifer
• Orientation in time and space
• Aquifer basics
• Where it flows and why
• Sustainability - why Edwards Aquifer is important to Texas
• Use of the aquifer
• Surface contamination
• Urban development impacts

4.  Semi-arid environment subject to frequent droughts
 Intensification of arid conditions projected with climate change
 Heavily dependent on groundwater resources
 Population growth and urban sprawl
 Groundwater Rule of Capture makes management challenging
Why Should You Care?

5. Who uses Edwards Aquifer…and for
what purpose?
• San Antonio
• Drinking water
• 1.8 million people
• Largest city solely
dependent on
drinking water
• Austin
• Barton Springs
• Cultural icon &
“jewel”
• Over 700,000
visitors / yr

6. Nearly 4 million people rely on Edwards
Aquifer from Mexico to Central Texas
Salado
Austin
San Marcos
Del Rio New
Braunfels
Uvalde
Brackettville
Georgetown
San Antonio
And that’s just the humans!

7. And now, David Attenborough

8. What is an aquifer?

9. Types of aquifers
• Sedimentary - better aquifers
• Sandstone - isotropic (can fracture, but no dissolution)
• Karst - anisotropic (fracturing augmented over time by
dissolution); soluble terrain, often limestone bedrock,
characterized by caves and sink holes
• Metamorphic & Igneous - rarely good aquifers
• Fractured granite - Enchanted Rock; compare to Inner Space
Caverns or Natural Bridge Caverns
• Metamorphic - all rocks can fracture…
• Volcanic rock - including lava tubes!

10. http://www.circleofblue.org/waternews/
In Texas, a karst aquifer is the sole source
water supply for > 2 million people
Karst aquifers supply 25% of the
world’s drinking water

12. Karst rocks – enhanced porosity
• Certain horizontal strata create
preferential pathways
• Remember the draperies from
Loop 360 trip
• As organisms burrow through,
they can change the porosity
• Voids can be the burrows or the
surrounding matrix

13. Scale can increase over time
Hang En Cave (Vietnam)

14. Matrix flow v. Conduit flow

15. Karst aquifer
(conduit flow)
Flow: cm/day vs. km/day
Sandstone aquifer
(matrix flow)
After Mahler (2004)
Karst aquifer have matrix and conduit flow
Karst aquifer
(conduit and matrix flow)
What does this mean for water quality?
Matrix flow

16. Architecture aside…a mental model

18. Barton Springs
After Musgrove et al. (2010)
Barton Springs salamanderBarton Springs salamander

19. Edwards Aquifer cross section

20. Project setting
What makes up the boundaries?
Groundwater divide

21. Watershed

22. Austin area watersheds

23. Conceptual model of groundwater flow
Garner & Mahler (2007)
How do we know this?

24. Hauwert et al. (2004)
Water table elevations
Groundwater flow to NE
Identify conduit flow paths
Velocities up to 12 km/day
What geologic feature(s) is the groundwater flow
direction consistent with?

25. Dye Tracing

26. Dye Tracing

27. Balcones Fault Zone

28. Ry Tracing
1981
1994
2005
2008

29. Conceptual model of groundwater flow
Garner & Mahler (2007)

30. Why is groundwater in urban karst
aquifers vulnerable?

31. • Runoff generator
Impervious cover

34. Sustaining karst aquifers
• Land use regulation
• Save Our Springs – 15% Impervious Cover Recharge &
25% Impervious Cover Contributing
• Water quality protection districts
• Land control – buy land and hold it
• Balcones Canyonlands Preserve
• Hill Country Conservancy