Risk to Regional Drinking Water Supply

Overview

138,000 homes depend on the Texas Water Company (TWC) for drinking water. TWC’s Park Shores Surface Water Treatment Plant draws water directly from Canyon Lake, using intake screens located in the Guadalupe River arm of the lake. Because Devils Hollow flows into this same arm, any change in water quality upstream — including nutrient loading, algae growth, zebra mussel activity, or emerging contaminants — can affect the water entering TWC’s treatment system.

The Canyon Ranch wastewater permit allows treated effluent to enter the Devils Hollow watershed. During storm events, this effluent can be carried into Canyon Lake and toward the Park Shores intake screens, creating a direct pathway for contaminants to reach the regional drinking‑water supply.

How Canyon Ranch’s Effluent Reaches the Park Shores Intake Screens

The Canyon Ranch permit authorizes treated effluent to be discharged through an "unnamed tributary" (Mystic Shores backyards) into a dry creek that flows through Devils Hollow. Devils Hollow empties into the Guadalupe River arm of Canyon Lake — the same arm where TWC’s intake valves are located.

Canyon Lake flows from west to east toward the dam. This means the effluent pathway is: 

Canyon Ranch → Mystic Shores Backyards → Devils Hollow (dry creek) → Guadalupe River arm → Northern Shoreline → Park Shores Surface Water Treatment Plant

Unlike other wastewater plants around Canyon Lake:

• Canyon Ranch’s discharge is upstream of the intake

• The flow direction is aimed directly at the intake valves

• Other plants discharge farther down the lake, where distance and circulation reduce risk

• Even Simmons Valley’s future discharge will not be oriented toward the intake

This makes Canyon Ranch’s effluent uniquely positioned to affect the drinking‑water supply.

Flow Path from Devils Hollow to the Intake Valves

Phosphorus and Hydrilla Growth

The Canyon Ranch permit allows effluent containing 1.0 mg/L of phosphorus. Canyon Lake’s baseline phosphorus level is approximately 0.01 mg/L — a 100‑fold difference.

Elevated phosphorus can:

• stimulate algae growth

• increase cloudiness of the water

• fuel aquatic vegetation expansion

• degrade water clarity

• accelerate eutrophication (excess nutrients, particularly nitrogen and phosphorus, accumulating in the lake, leading to an overgrowth of algae and depletion of oxygen)

Zebra Mussels and Intake Fouling

In July of 2019, the Park Shores intake screens were severely fouled by zebra mussels. Zebra mussels thrive in nutrient‑rich water and attach to hard surfaces, including:

• intake screens

• pipes

• pumps

• treatment infrastructure

The fouling event caused:

• water‑supply interruptions

• emergency restrictions

• reduced treatment capacity

• costly mechanical cleaning and maintenance

Any increase in nutrients — including phosphorus — can accelerate zebra mussel growth and increase the frequency of fouling events.

Stormwater + Wastewater = Higher Transport Toward the Intake

Under normal conditions, effluent may not reach the intake in large quantities. But during storm events:

• stormwater accelerates transport

• dry creeks become losing streams (water infiltrates the ground rather than flowing downstream)

• Devils Hollow carries higher volumes

• contaminants move rapidly through karst

• lake inflow spikes

• the intake draws from a more contaminated water column

Stormwater is the delivery mechanism that can carry nutrients, pharmaceuticals, and other contaminants farther and faster than expected.

Pharmaceuticals and Emerging Contaminants

Wastewater contains substances not found in natural lake water, including:

• pharmaceuticals

• personal‑care products

• endocrine‑disrupting compounds

• household chemicals

• microplastics

• synthetic organics

Most drinking‑water treatment plants — including TWC’s — are not designed to remove these emerging contaminants completely.

Even low concentrations can:

• affect aquatic life

• alter algae growth patterns

• accumulate in sediments

• enter drinking‑water treatment systems during storm events

This is a long‑term concern for Canyon Lake and the regional drinking‑water supply.

Why Canyon Ranch’s Discharge Is Different

Other wastewater plants around Canyon Lake:

• discharge farther down the lake

• are not oriented toward the intake

• have longer dilution pathways

• have slower transport routes

Canyon Ranch’s discharge is:

• closer

• upstream

• aligned with Devils Hollow’s flow

• aimed directly at the intake valves

This makes its impact more immediate and more direct.

Regulatory Gap

Texas wastewater permits do not require:

• evaluation of drinking‑water intakes

• analysis of nutrient impacts on zebra mussels

• assessment of emerging contaminants

• modeling of storm‑event transport

• cumulative impact analysis for lake‑fed water systems

This gap leaves Canyon Lake and the regional drinking‑water supply vulnerable to long‑term water‑quality degradation.

What We Are Asking For

We are asking TCEQ, GBRA, and regional water providers to:

• evaluate nutrient loading impacts on Canyon Lake

• assess zebra mussel fouling risk at the intake

• consider emerging contaminants in effluent

• review storm‑event transport pathways

• strengthen watershed protections similar to the Highland Lakes

Take Action

Residents can:

• stay informed about Canyon Lake water‑quality trends

• report algae blooms or zebra mussel sightings

• support watershed‑protection initiatives

• attend public meetings on regional water supply

• encourage TWC and GBRA to evaluate intake vulnerability