The Geologic History and General Hydrogeology of the Edwards Aquifer

Prepared by: J. Mark Hamilton, P. G. Executive Director, Aquifer Management Services Edwards Aquifer Authority Overview • Background information • What is the Edwards Aquifer. • Difference between porous media, karst, unconfined, and confined aquifers.

• Aquifer Development • How did the Edwards Aquifer form.

• Photographic and Other Examples • Cross sections, faults, fractures and karst features

• Conclusion Background • What is the Edwards Aquifer?

• A karst aquifer system that provides water for:

• About two-million agriculture, municipal, and other direct users.

• Springflow for threatened and endangered species.

• Recreational and other users. The Edwards Aquifer System

Total area within the EAA jurisdictional boundary is over 5,000 square miles. Drainage Area

• Almost 5,500 square miles in size. • Northern portion is Edwards Plateau area. • Southern portion is Texas Hill Country. • Rainfall In this area drains toward the Recharge Zone. Recharge Zone

• Area where Edwards Group is exposed at the surface, such that recharge to the aquifer is possible. • The Edwards Aquifer in this area is an unconfined or water- table aquifer. • Over 1,000 square miles of recharge zone are exposed within the jurisdictional area of the EAA. Artesian Zone

• The area of the aquifer where water is present under pressure (artesian).

• Over 1,900 square miles of Artesian Zone within the jurisdictional area of the EAA. Background Aquifer = a stratum or a zone below the surface of the ground that can hold and yield water in sufficient quantities for use.

Karst = an area of limestone or other soluble rock with sinkholes, caves, and springs. Aquifers and Groundwater (Porous Media Versus Karst)

Porous media Karst aquifers offer aquifers offer some little or no filtration degree of filtration Aquifers and Groundwater (Confined Aquifers)

Karst aquifers have preferential, unpredictable flow paths making removal of contaminants almost impossible. Aquifer Development

• What geologic events occurred that ultimately gave rise to what we now call the Edwards Aquifer?

• The sedimentary rock body we call the Edwards Group was deposited during the Cretaceous Era.

• Various geologic events, post deposition, contributed to the development of the aquifer. Aquifer Development

Deposition of Rock Body During Cretaceous

• Cretaceous Era: 65 -145 million years before present.

• Warm shallow seas repeatedly inundate the region throughout the Era.

• Significant amounts of Cretaceous age sedimentary rock are deposited in the region (2,000 - 2,600 feet). Aquifer Development

Formation Thickness Age

Navarro/Taylor Groups 500’ Upper Cretaceous (65 million years before present) Austin Group 300-500’ Upper Cretaceous

Eagle Ford Group 50’ Upper Cretaceous

Buda Limestone 50’ Upper Cretaceous

Del Rio Clay 50’ Upper Cretaceous

Edwards Group 430-600’ Lower Cretaceous (≈100 million years before present) Glen Rose (Upper/Lower) 500-650’ Lower Cretaceous Cretaceous Deposition Cretaceous Hensell Sand 45’ Lower Cretaceous

Cow Creek 75’ Lower Cretaceous

Total Thickness 2,000-2,600’ 80 Million Years Aquifer Development

Finally,Post Deposition, theInitial Edwards Deposition the Group Edwards ofis Buriedthe Group Edwards by is ExposedYounger Group toCretaceous over the theAtmosphere Glen age RoseSediments, for Formation a Period Post of Exposure Time Aquifer Development – Cretaceous Deposition 2,600 feet 2,600 – 2,000 2,000 Aquifer Development

At the Conclusion of Cretaceous the Edwards Group has:

• Some amount of secondary porosity due to the period of exposure.

• Exposure results in erosion as well as initial infiltration of meteoric waters.

• Subsequent deposition buries the Edwards Group with rocks capable of creating a “confined” system.

• Still, no mechanism for transmitting significant amounts of water through the rock body exists.

• Miocene age faulting will change that. Balcones Faulting – about 20 million yrs. ago

1) Sediment loading in the Gulf

2) Uplift to the west and northwest

3) Resulting extensional forces are compensated for by development of a fault system Aquifer Development – Role of Balcones Faulting on Aquifer Development

Downward forces to the SE in the Gulf

Uplift to the N and NW Sediments “sliding” toward the Gulf Aquifer Development - Role of Balcones Faulting on Aquifer Development Aquifer Development

Why is faulting significant with regard to aquifer development?

• Initiates a new mechanism for entrance of fresh water into the system. • Creates a potential energy difference across the fault scarp. • Initiates downcutting of streams and steam piracy. • Stream downcutting eventually opens drains in the system (and gives rise to our “Texas Hill Country”). • Provides a mechanism for the confined portion of the aquifer to exist. Aquifer Development Role of Balcones Faulting on Aquifer Development Aquifer Development • Examples of faults, fractures, enhanced porosity, and other features of the Edwards Group.

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Aquifer Development Conclusion

• Cretaceous Sedimentary Deposition

• Post Depositional Exposure/Reburial

• Balcones Faulting • Initiates stream “downcutting” and headward erosion • Stream piracy • Opens drains in the system

“And it never failed that during the dry years the people forgot about the rich years, and during the wet years they lost all memory of the dry years. It was always that way.” -John Steinbeck, East of Eden