City Water Series: The Hidden Threat — How Zebra Mussels Are Affecting Your Water Quality and Water Bill

If you live in a Great Lakes city, you've probably heard of zebra mussels. Those small, striped invaders that cling to rocks and boat hulls. But what you might not realize is that these tiny shellfish are quietly attacking something you never see: your city's water supply inlet.

As they clog and coat the intake structures and tunnels that feed your treatment plant, they drive up operating costs, strain equipment, and ultimately show up on your monthly water bill. This problem goes far beyond just annoying boaters — it affects every single glass of water you pour at home.

Why the Great Lakes Are the Perfect Breeding Ground for Zebra Mussels

Zebra mussels didn't evolve in the Great Lakes. They arrived in ballast water from ships in the 1980s and found a nearly perfect environment:

• Plenty of calcium in the water to build their shells.
• Few natural predators are capable of controlling their population.
• Massive hard surfaces on rocks, docks, boat hulls, and infrastructure to latch onto.
• Explosive reproduction — a single female can release tens of thousands of eggs in a season.

Once established, they become extremely difficult to remove. Even when utilities scrape or flush them away, new larvae in the water column settle and restart the cycle.

The Hidden Threat: How Zebra Mussels Invade the City Water Supply Inlets

Water Supply Inlet Building Up With Zebra Mussel Colonies

Water Pipe Plugged With Zebra Mussels

Imagine your city's water supply inlet, a large intake crib or structure offshore, connected to massive pipes buried beneath the lakebed. These intakes draw millions of gallons of Great Lakes water to your treatment plant every day.

Here's the hidden threat: those intake structures and pipes are an ideal home for zebra mussels. They are filter feeders, and the constant water flow provides the perfect habitat for feeding and reproduction.

From larvae to latch-on: Microscopic larvae (veligers) drift in the lake and are pulled into the city water supply inlet. Inside, they seek hard surfaces — crib screens, tunnel walls, supports, and even measuring instruments then anchor themselves and grow.

A living concrete layer inside the inlet: Over time, they form thick colonies that coat the inside of pipes. A 10-foot tunnel can function like a 7–8-foot tunnel as mussel layers build up.

The impact on your water bill: As the inlet narrows, pumps must work harder. Licensed divers must clean the intake more often. Energy use increases. Over time, these costs show up in your water rates.

The Real Cost: How Zebra Mussels Turn Into Higher Utility Bills

When zebra mussels obstruct the city water supply inlet, utilities face:

• Higher electricity costs as pumps run longer and under more load.
• More frequent cleaning of intake tunnels, screens, and crib structures.
• Increased diver hours for inspection and maintenance.
• Expensive capital upgrades for coatings, screens, and mussel-control systems.
• Investment in new technologies to combat the growing problem.

These expenses eventually become part of your household water bill.

Zebra vs. Quagga Mussels: A Double Threat to City Intakes

Zebra mussels may get the headlines, but quagga mussels now dominate deeper parts of the Great Lakes. Together, they attack almost every surface of a city's water supply inlet system:

• Zebra mussels: Prefer hard surfaces such as intake cribs, tunnel walls, steel, and concrete.
• Quagga mussels: Colonize deep, cold water and softer sediments.

The result is a double-layer invasion, from the intake crib to the deep tunnels.

Scrubbing the City Water Supply Inlet: How Utilities Fight Back

Divers Cleaning Zebra Mussel Colonies Off Underwater City Water Inlet Tunnels 

Water utilities rely on dangerous, expensive, and labor-intensive methods:

Diver teams: Divers enter dark, cold structures offshore with high-pressure jets and scraping tools.

"Pigging" the tunnels: Large pipe-cleaning tools scrape off mussel colonies, sometimes removing tons of material.

Mechanical screens & traveling filters: These advanced systems clean continuously but still require maintenance.

Old Water Intakes, New Problems

Many Great Lakes intake tunnels were built decades ago and were never designed for invasive mussels:

• Rough concrete provides ideal surfaces for attachment.
• Long tunnels are difficult and dangerous to clean.
• Retrofitting anti-fouling coatings is extremely expensive.

Utilities are now forced to retrofit aging infrastructure at significant cost — and customers pay for it.

How Zebra Mussels Change the Raw Water Before Treatment

Zebra and quagga mussels alter raw water in ways that affect treatment:

• Increased clarity — but not necessarily purity.
• Deeper sunlight penetration encourages algae growth.
• Waste and shells accumulate near the inlet.
• Chlorine demand at the plant can increase.

These changes increase treatment complexity and the potential for DBP formation.

The Chemical Conundrum: Controlling Mussels With Chlorine

Many utilities use low-dose chlorine at the inlet to keep mussels from settling. But this comes with tradeoffs:

• DBP formation: Early chlorine exposure increases the potential for THMs and HAAs.
• Taste and odor issues: Improper dosing may affect water flavor.
• Ecosystem considerations: Even regulated dosing has environmental implications.

What Your Water Report Won't Tell You

Your annual Consumer Confidence Report (CCR) does not include:

• Raw water conditions at the city water supply inlet.
• DBP variations throughout the distribution system.
• Costs and challenges of mussel control at the inlet.

What’s Being Done to Protect City Water Inlets

Utilities are experimenting with a range of technologies:

• UV treatment at the inlet to damage larvae.
• Controlled chemical dosing with chlorine or oxidants.
• Anti-fouling coatings on intake surfaces.
• Improved tunnel-cleaning and pigging equipment.

What This Hidden Threat Means for Your Home

Zebra mussels directly influence your utility costs and water quality:

• Higher pump energy use.
• More diver operations and maintenance.
• More chemical control at the inlet.
• More complex treatment steps and higher operating costs.

How Home Filtration Helps You Take Control at the Tap

Smart home filtration helps protect your water quality regardless of inlet conditions:

• Improves taste and odor.
• Reduces chlorine and chloramines.
• Reduces many DBPs formed during treatment.

A combination of a whole-house carbon filter and a point-of-use filter (such as an under-sink carbon block or RO system) is the most effective.

Learn More About Zebra and Quagga Mussels

• Cary Institute Fact Sheet
• Illinois Extension: Invasive Mussels
• James Howard — Mussels & Great Lakes
• Great Lakes Now — Mussel Legacy

For more information on water treatment options for your home and family, check out our City Water Series posts.

🏙️ City Water Series — Protecting Your Water

Zebra mussels are far more than a nuisance on dock pilings and boat hulls. They are a persistent, costly threat to the municipal water inlet that supplies your drinking water system. They clog critical infrastructure, drive up energy and maintenance costs, and complicate the chemistry needed to keep your water safe.

You may not see their impact on the lakebed or intake tunnels, but you feel it in higher utility costs and the extra treatment steps required to keep your water within strict standards. By understanding this hidden threat and adding smart home filtration, you can protect your family's water quality, even as your utility battles zebra mussels far offshore.

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