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Over 122 million Americans were served water with toxic byproducts.
Manure runoff plays a major role in disinfection-related contamination.
Trihalomethanes, linked to cancer, exceeded safe levels in 5,919 systems.
Six of the seven Colorado River Basin states had widespread exceedances.
EWG’s Tap Water Database lets users check their own water quality by ZIP code.

Wednesday, April 16, 2025 — A new report released April 10 by the nonprofit Environmental Working Group (EWG) draws a troubling link between industrial livestock production and dangerous levels of cancer-causing chemicals in drinking water across the United States—including in six of the seven Colorado River Basin states.

According to EWG’s analysis, disinfection byproducts known as trihalomethanes (TTHMs)—chemicals formed when disinfectants like chlorine interact with organic matter such as animal waste—were found at unsafe levels in the drinking water of more than 122 million people between 2019 and 2023.

These findings stem from EWG’s Tap Water Database, a national resource that allows users to enter their ZIP code and view a detailed breakdown of contaminants in their drinking water. The database is built using state-supplied records and public testing data.

The Manure Connection.

Industrial-scale livestock operations, or animal feeding operations (AFOs), produce massive volumes of manure. To dispose of this waste, it is commonly spread across nearby farm fields as fertilizer. When it rains, that manure runs off into rivers, lakes, and reservoirs—many of which serve as sources for drinking water systems.

Water utilities treat that surface water with disinfectants to protect against diseases like cholera and typhoid. However, this necessary treatment process also triggers chemical reactions with organic material, creating TTHMs.

While the Environmental Protection Agency (EPA) sets a legal cap of 80 parts per billion (ppb) for TTHMs in drinking water, EWG’s own health guideline—based on cancer risk—is just 0.15 ppb. “Our peer-reviewed research shows the lower level represents a one-in-one-million lifetime cancer risk because of consuming TTHMs in drinking water,” the report states.

The EWG identified 5,919 community water systems that tested at or above the EPA’s limit at least once between 2019 and 2023—about 15 percent of all systems tested nationwide. Of those, 73 percent rely on surface water.

Colorado River Basin: State-by-State Breakdown.

Six of the seven Colorado River Basin states appear prominently in EWG’s findings.

Arizona

The City of Phoenix, serving 1.7 million residents, had four tests exceeding the EPA limit.
Statewide data showed multiple systems with TTHM levels at or above 80 ppb.

California

281 water systems had at least one TTHM test over the EPA limit.
Affected population: over 15.7 million.
Los Angeles and San Jose systems both recorded exceedances.
California also sets lower public health goals for TTHMs, although they are non-enforceable.

Colorado

Included among the 49 states with exceedances, although not in the top ten.
Specific system counts were not detailed but are covered in the Tap Water Database.

Nevada

Las Vegas Valley Water District, serving 1.54 million people, had two exceedances.
Statewide data shows additional smaller systems with high TTHM levels.

New Mexico

Identified as a state with manure-applied farmland and water systems above EPA limits.
Specific test data indicates smaller systems breached 80 ppb.

Utah

Included in the report as having affected systems, although not named in the top ten.
Manure-applied acreage and livestock concentrations suggest potential ongoing risk.

Wyoming

Not highlighted for significant TTHM exceedances.
Limited population and fewer large water systems may account for lower detection.

Texas, not one of the Colorado River Basin states, stood out as the worst-affected state nationwide:

707 water systems had TTHM levels over the legal limit.
8.68 million Texans were served by these systems.
San Antonio’s water system, serving 2 million, exceeded the limit three times.

Health Risks and Public Impact.

Research cited in the EWG report links long-term TTHM exposure to increased risks of bladder and colorectal cancer, especially in men. Additional risks include low birth weight, stillbirth, craniofacial defects, and congenital heart conditions.

The EWG also flagged a concerning pattern: many of the states with the most contaminated systems are also among the nation’s top livestock producers. This includes Texas, Oklahoma, California, and North Carolina.

What Can Be Done?

The report calls for tighter manure management practices, such as streamside buffers, timing rules to avoid spreading manure before rain, and filter strips to block runoff. Yet, the funding for these solutions remains low.

In 2024, the USDA’s Environmental Quality Incentives Program allocated over $1.3 billion to conservation—but just $35,345 went to filter strips. Meanwhile, more than $50 million went toward expanding manure storage.

Water utilities can implement advanced treatments like granular activated carbon or nanofiltration, but these options are costly. The report emphasizes the need to restore funding from the Inflation Reduction Act, which was recently frozen under the Trump administration.

Know What’s in Your Water.

To learn what’s in local drinking water and how to filter contaminants like TTHMs at home, visit EWG’s Tap Water Database. By entering a ZIP code and selecting a water provider, users can see contaminant levels and receive personalized filter recommendations.

As EWG concludes: “The way food is produced in this country should not pose such serious potential damage to public health.”

Know what you’re drinking? Livestock manure linked to toxic tap water

The Manure Connection.