Hydrogeology & Groundwater Risk

Understanding Why This Region Is Uniquely Vulnerable

The geology of northwestern Comal County - one of the most karst‑intensive regions in the Hill Country - determines how wastewater behaves once it leaves the pipe. This region is defined by karst terrain, faults, fractures, and highly permeable limestone formations — a combination that creates rapid infiltration pathways and direct connections between surface water and groundwater (into the Cow Creek formation of the middle Trinity aquifer).

Surface water (both wastewater and storm water) have the potential to flow into the groundwater quickly due to numerous faults and fractures. 

This page summarizes the key geological factors that make the Canyon Ranch discharge location one of the most sensitive and vulnerable areas in the region.

The Karst Foundation

Karst forms when water dissolves limestone over long periods of time. This process creates:

• • Conduits

  • Voids

  • Dissolution features

  • Unpredictable underground pathways

In karst terrain, water does not move slowly through soil. It moves:

• Fast

• Far

• With little natural filtration

This is why wastewater discharged into a dry creek in a karst region behaves very differently than wastewater discharged into a perennial stream.

Rapid Infiltration Pathways

Karst terrain creates direct routes for water to enter the subsurface. These include:

• Enlarged fractures

• Vertical conduits

• Cavities formed by dissolution

Ron Harris’s field observations in Devils Hollow show this clearly: “Fractures have a high recharge efficiency… water dissolves limestone rock, making them bigger and creating caves.”

These features allow wastewater to infiltrate quickly — often within minutes or hours — bypassing the slow, predictable movement assumed in the permit modeling.

Faults & Fractures

Faults and fractures are abundant between the Canyon Ranch outfall and the Guadalupe River.

Faults

Faults create vertical displacement in rock layers, opening pathways for water to move downward.

Fractures

Fractures are cracks that open without vertical movement. They act as high‑efficiency conduits for water to percolate downward.

Ron’s presentation documents:

• Numerous fracture zones in Devils Hollow

• Dissolution‑enlarged fractures forming caves

• Parallel fracture sets visible in the dry Guadalupe River bed

• Three major faults along the flow path, including the Spring Branch Fault

“Fractures are many, and they’re three‑dimensional in nature.”

These features dramatically accelerate groundwater movement.

Alignment With Devils Hollow

Several mapped fracture zones align directly with Devils Hollow — the channel that will carry Canyon Ranch effluent.

This alignment matters because:

• Devils Hollow is dry most of the year

• Dry channels promote pooling, spreading, and infiltration

• Fractures beneath the channel provide direct access to the Cow Creek Formation

• The Cow Creek is exposed at the surface inside the Nature Preserve

Ron explains: “Once it’s in Devils Hollow… it will be in direct contact with the Cow Creek Formation.”

The Cow Creek Formation supplies drinking water to thousands of wells in northwestern Comal County. While most Mystic Shores residents receive treated surface water from Texas Water Company, many surrounding neighborhoods rely on Cow Creek groundwater. Cypress Cove — served by Cypress Cove Water Supply Corporation has wells that lie near the Canyon Ranch outfall and along the flow path, making that community particularly vulnerable. Additional private wells exist in Cross Canyon Ranch, Canyon Lake Estates, and other nearby developments, making groundwater protection a regional concern. 

For nearly two miles, wastewater will interact with exposed Cow Creek limestone — a highly permeable drinking‑water aquifer.

Recharge Zone Reality

While the Canyon Ranch outfall itself sits in the Trinity Aquifer Contributing Zone, the flow path immediately downstream enters the Trinity Aquifer Recharge Zone. In this part of northwestern Comal County, the Cow Creek Formation — a major drinking‑water aquifer — is exposed at the surface and actively recharges through faults, fractures, and dissolution‑enhanced pathways. Unlike the Edwards Aquifer Recharge Zone, the Trinity recharge zone has no special protections, even though its geology is equally vulnerable. Groundwater in the Middle Trinity ultimately migrates into the Edwards aquifer along fault‑controlled pathways, meaning infiltration along this flow path can affect both aquifers. This makes the Canyon Ranch discharge location uniquely sensitive and significantly under‑protected. 

Why This Region Is Uniquely Vulnerable

Several factors combine to create extreme vulnerability:

1. Karst terrain with high permeability

Water moves rapidly and unpredictably.

2. Exposed Cow Creek Formation

Direct contact between surface flow and drinking‑water aquifer.

3. Numerous mapped faults and fractures

Documented in Devils Hollow and the Guadalupe River bed.

4. Dry‑creek discharge location

Daily effluent in a dry channel infiltrates far more than it flows.

5. High density of private wells

More than 2,500 wells rely on the Cow Creek Formation. Approximately 150 wells lie near the discharge point.

6. Permit modeling based on incorrect assumptions

The permit assumes slow, predictable movement in a perennial stream — not the reality of karst infiltration.

Why This Matters

• Effluent can enter the aquifer quickly

Karst + fractures + dry channel = rapid infiltration.

• Wells lie directly in the flow path

Many wells are shallow, older, or located near fracture zones.

• TCEQ’s review did not account for this complexity

No environmental impact study was required. No fracture analysis. No karst vulnerability assessment. No hydrogeologic modeling.

Ron summarizes the core issue:

“The geology of this region is complex, sensitive, and highly vulnerable to infiltration. The permit does not reflect that reality.”

The Takeaway

Wastewater discharged into Devils Hollow will not behave as modeled in the permit. It will:

• • Infiltrate

  • Move through fractures

  • Travel underground

  • Interact with the Cow Creek Formation

  • Reach wells

  • Reach the aquifer

  • Potentially reach Canyon Lake

Any long‑term wastewater plan in this region must begin with an accurate understanding of the karst foundation and the faults and fractures that define it.

The geology tells us how wastewater moves.

Supporting Materials

Detailed Geology Video: A full, detailed explanation by geophysicist Ron Harris is available here.

Geology Video Slide Deck: PDF of slides

This hydrogeologic analysis was developed by Ron Harris, a retired geophysicist with 38 years of experience solving complex geoscience problems across North America. Ron holds a Bachelor of Science in Geophysical Engineering from the Colorado School of Mines and has published with the Society of Exploration Geophysicists and the American Association of Petroleum Geologists. He has presented technical findings at professional meetings across the United States and internationally. Ron currently serves as Vice President of the Comal Trinity Groundwater Conservation District Board of Directors, representing Precinct 4.