Central Texas Hydrogeology

Central Texas HydrogeologyEVS 311. Field Seminar on Environmental Science and Sustainability

Central Texas Hydrogeology

Learning ObjectivesEnvironmental Science - hydrogeology of Edwards AquiferOrientation in time and spaceAquifer basicsWhere it flows and why

Sustainability - why Edwards Aquifer is important to TexasUse of the aquiferSurface contaminationUrban development impacts

Semi-arid environment subject to frequent droughtsIntensification of arid conditions projected with climate changeHeavily dependent on groundwater resourcesPopulation growth and urban sprawlGroundwater Rule of Capture makes management challenging

Why Should You Care?

Who uses Edwards Aquiferand for what purpose?San AntonioDrinking water1.8 million peopleLargest city solely dependent on drinking water

AustinBarton SpringsCultural icon & jewelOver 700,000 visitors / yr

Nearly 4 million people rely on Edwards Aquifer from Mexico to Central Texas

And thats just the humans!

And now, David Attenborough

What is an aquifer?

A body of rock, sediment, or soil that contains drinkable water and can transmit this water to wells or springs in economically usable quantities

Unsaturated VadoseSaturated Phreatic 8

Types of aquifersSedimentary - better aquifersSandstone - 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 aquifersFractured granite - Enchanted Rock; compare to Inner Space Caverns or Natural Bridge CavernsMetamorphic - all rocks can fractureVolcanic rock - including lava tubes!

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 worlds drinking water

Karst rocks enhanced porosityCertain horizontal strata create preferential pathwaysRemember the draperies from Loop 360 trip

As organisms burrow through, they can change the porosityVoids can be the burrows or the surrounding matrix

Scale can increase over timeHang En Cave (Vietnam)

Matrix flow v. Conduit flow

Karst aquifer(conduit flow)

Flow: cm/day vs. km/daySandstone aquifer(matrix flow)After Mahler (2004)Karst aquifer have matrix and conduit flowKarst aquifer(conduit and matrix flow)

What does this mean for water quality?

Matrix flow

15To better visualize the difference between a porous media aquifer (like sandstone) and a karst aquifer, imagine that you had a big box of sand, and a next to it a big block of concrete. Take a sledgehammer and smack the concrete block to create some big cracks running through the concrete. Now, pour some acid through the cracks to dissolve away some space in between them. Your model is ready.

Let some water rain onto the sand, and some rain onto the home-made limestone. What happens? The rain will seep slowly through the sand and may not even make it out the bottom of the box, but will pour through the enlarged fractures in the limestone. The speed with which water travels through karst can be thousands of times faster than through sand.

Notice that the rain moves through the sand similarly regardless of location. Thats because the sand has similar properties (grain size, space between the grains) everywhere in the sandboxits homogeneous. However, the rain does not move the same way regardless of location in the karst. It moves very quickly through the enlarged fractures, and may not even infiltrate into the unfractured part. The rocks does not have similar properties regardless of location, some parts are fractured and some arentits inhomogeneous.

Now, turn both boxes on their sides and rain on them again. The water flows through the sandbox in about the same way. In other words, the direction of flow through the sand does not depend on which way the box is oriented, it just flows in the direction of gravity. Thats called isotropic. But if you turn the concrete box on its side and rain on it, that wont be true. The direction of flow is controlled by the location of the fractures, and you can imagine that you might even get sideways flow instead of downward flow. Thats called anisotropic.

The fact that karst is inhomogeneous and anisotropic means that it is very difficult to predict with mathematical models how the flow will behave. So we need other tools, like tracer tests, to determine how water flows through karst aquifers.

http://www.bseacd.org/aquifer.html

http://www.bseacd.org/graphics/Groundwater_Tracing_Study_R.pdf

Architecture asidea mental model

Barton SpringsAfter Musgrove et al. (2010)Barton Springs salamander

Barton Springs salamander

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Edwards Aquifer cross section

Project setting

What makes up the boundaries?Groundwater divide

Watershed

The geographic area of land that drains water to a shared destination

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Austin area watersheds

Conceptual model of groundwater flow

Garner & Mahler (2007)

How do we know this?

CW did a Think-pair-share with this with the prompt, How do we know this?23

Hauwert et al. (2004)

Water table elevationsGroundwater flow to NEIdentify conduit flow pathsVelocities up to 12 km/dayWhat geologic feature(s) is the groundwater flow direction consistent with?

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Dye Tracing

Dye Tracing

Balcones Fault Zone

Ry Tracing

1981199420052008

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Conceptual model of groundwater flow

Garner & Mahler (2007)

CW did a Think-pair-share with this with the prompt, How do we know this?29

Why is groundwater in urban karst aquifers vulnerable?

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Runoff generator

Impervious cover

Sustaining karst aquifersLand use regulationSave Our Springs 15% Impervious Cover Recharge & 25% Impervious Cover ContributingWater quality protection districts

Land control buy land and hold itBalcones Canyonlands PreserveHill Country Conservancy

Here's the same vault a day later! Note the whirlpool above the vault.http://droughtmonitor.blogspot.com/2009_10_01_archive.htmlAntioch Cave is the largest-capacity recharge feature in the District. The District has recently completed a renovation of the recharge facility. The vault that overlies the entrance to Antioch Cave has two valves that can open, allowing recharge to the aquifer. The idea is that the valves are closed during the first part of a flood event, and then will open automatically as the water becomes cleaner34