Manure is one source triggering cancer-causing chemicals in drinking water for 122 million

The way U.S. livestock are raised can have negative consequences for human health. Manure contaminates water, which requires disinfection, and the process can trigger the formation of cancer-causing byproducts. 

These disinfection byproduct chemicals are a problem facing more than 122 million people served by almost 6,000 community water systems throughout the country. At some point between 2019 and 2023, their water had unsafe levels of these disinfection byproducts, according to EWG’s latest Tap Water Database.

The database lets anyone enter their ZIP code and find out whether these contaminants and others are in their drinking water.

The way food is produced in this country should not pose such serious potential damage to public health. Adjustments to how meat is produced are feasible. Farmers can take steps to reduce how much manure ends up in water, and regulations can advance this goal. In turn, these changes would curb the amount of manure that gets into water, which could lead to the creation of fewer toxic byproducts.

This report looks at the presence of trihalomethanes, a type of disinfection byproduct, in U.S. water systems, and the role manure from food production plays in the generation of disinfection byproducts.

A problem in many states

Animal feeding operations raise hundreds to thousands of cows, pigs, chickens and turkeys in concentrated barns or feedlots, creating massive amounts of manure. It’s regularly applied to nearby farm fields, and runs off fields after rain, ending up downstream in bodies of water used for drinking water. 

To prevent the spread of illness from cholera, typhoid and other serious diseases linked to drinking water, utilities treat polluted water with disinfectants like chlorine. Disinfection is essential to making water safe to drink. But the disinfectants interact with the manure, creating disinfection byproduct chemicals. 

How disinfection byproducts are triggered by manure

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Many types of organic matter can trigger the formation of disinfection byproducts, like plant debris and human waste. But manure runoff from farm fields is a major source. Soil erosion, crop plant material and other fertilizers are additional sources of agricultural organic matter that can help to create byproducts in water during the disinfection process.

One of these byproducts is a type known as trihalomethanes, which the Environmental Protection Agency regulates as total trihalomethanes, or TTHMs. Water systems must keep TTHM levels at or below the EPA’s maximum contaminant level, or MCL, of 80 parts per billion, or ppb. But many drinking water sources throughout the U.S. exceed this limit.

MCLs are designed to protect public health and to factor in the cost and feasibility of water treatment. If a community water system tests at or above 80 ppb for TTHMs, the EPA requires the system to reduce the contamination. 

For many chemicals, EWG develops its own health guidelines in drinking water. At 0.15 ppb, EWG’s health guideline for TTHMs is much lower than the EPA’s, since our peer-reviewed research shows the lower level represents a one-in-one-million lifetime cancer risk because of consuming TTHMs in drinking water. 

EWG’s Tap Water Database shows 5,919 water systems serving over 122 million people all had at least one drinking water test for TTHMs at or above the EPA MCL between 2019 and 2023. 

This represents 15 percent of the over 40,000 community water systems in the country that tested for TTHMs. Most of these systems, 73 percent, rely on surface water like reservoirs for their drinking water. 

That’s a concern for people living in these communities, because research shows that exposure to TTHMs even below the MCL can harm human health. Several studies have reported that consumption of TTHMs in drinking water increases the risk of bladder cancer, as well as colorectal cancer in men. 

Research also shows that problems during pregnancy, including low birth weight, cardiovascular defects, craniofacial defects and stillbirth, can be found in babies whose mothers drank water containing TTHMs during pregnancy.

Health risks associated with consuming drinking water contaminated with TTHMs

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Areas with highest TTHM levels

Water systems provide drinking water to communities of all sizes. The EPA requires systems to test drinking water for a list of contaminants and to keep levels of regulated contaminants below designated amounts. 

Systems in 49 states as well as the District of Columbia tested at or above the MCL. New Hampshire was the only state not included, and that’s because it did not provide EWG with drinking water contamination data, despite a public records request.

Although TTHM contamination is a problem throughout the country, over half of the systems were located in just 10 states, many of which are the biggest states in the U.S. (See Figure 1.) 

The 10 states with the most systems testing at or above the MCL at least once represent 54 percent of those systems, serving almost 64.6 million people. Most of the systems with unsafe levels of TTHMs by far were located in Texas – over 700 systems at or above the MCL, which was 12 percent of all systems in the state. 

Figure 1. The 10 states with the most water systems testing at or above 80 ppb for TTHMs at least once between 2019 and 2023.

^{Source: EWG, data from EWG’s Tap Water Database}

Unsafe levels of TTHMs were found in systems of all sizes, from those serving only a very small number of people to those serving millions. 

Of the systems at or above the MCL, 57 percent were very small or small, serving 3,300 people or fewer. Forty-three percent were medium, large or very large, each serving over 3,300 people. Sixteen of the systems whose water tested at or above the MCL each serve over a million people, including the systems of the two biggest cities, New York City and Los Angeles. (See Figure 2.) 

Unsafe levels of TTHMs contaminated some of the largest systems many times over the years, as with the Washington Suburban Sanitary Commission, which tested at or above the MCL 218 times in the five years covered by this report.

Figure 2. The 16 biggest systems that tested at or above 80 ppb between 2019 and 2023.

^{Source: EWG, data from EWG’s Tap Water Database}

There were also many systems whose drinking water contained TTHMs at levels even higher than the MCL. There were 3,336 systems serving over 47.6 million people that tested at or above 100 ppb for TTHMs at least once between 2019 and 2023, 229 systems that tested at or above 200 ppb, and 69 systems that tested at or above 300 ppb.

Research shows that drinking water with TTHMs at or above 80 ppb increases the risks to public health. But considerable evidence also shows that even drinking water with a smaller amount of TTHMs – at levels below the MCL – can increase the risk of cancer or birth defects. The MCL does not fully protect against the risk of cancer caused by exposure to trihalomethanes.

Most U.S. water systems tested above EWG’s health guideline of 0.15 ppb at least once over the five years. And many water systems were approaching the EPA MCL but did not reach it – almost 3,500 systems tested at or above 60 ppb but below 80 ppb. 

California regulators have assigned non-enforceable health guidelines to TTHMs that are much lower than the EPA’s MCL. The state set separate public health goals, which protect against cancer effects for four different trihalomethanes, ranging from bromoform’s public health goal of 0.5 ppb to bromodichloromethane’s goal of 0.06 ppb. These goals are not regulatory limits. They are “based exclusively on public health considerations.” So they provide a better idea of what TTHM levels pose minimal risk. 

The role of food production and manure

To produce the large amounts of meat and animal products that Americans consume, 99 percent of farm animals are raised in animal feeding operations or feedlots. 

Animal feeding operations housing hundreds or thousands of animals produce enormous amounts of manure – so much that it can be hard to get rid of. So while manure can fertilize crops, the main reason it is applied to farm fields is to dispose of it. 

Rain and other types of precipitation cause more than the usual amounts of manure to get into water. Emerging research shows that the time between manure application to fields and precipitation may affect disinfection byproduct levels in drinking water for systems downstream.

The map below shows all community water systems that serve more than 3,300 people with at least one test at or above the MCL for TTHMs between 2019 and 2023. The map also includes the number of farm acres in each state receiving manure, according to the 2022 Census of Agriculture. There are many systems with TTHMs at or above the MCL in states with considerable manure application, which are the darker states on the map. 

The case of Texas is particularly noteworthy. It had more medium, large and very large water systems with unsafe TTHM levels than other state. And it’s the state with the third most farm acres where manure was applied. 

Interactive map

Community water systems with total trihalomethanes above federal limits (2019-2023)

This map shows all community water systems in the U.S. that provide drinking water for more than 3,300 people where at least one test between 2019 and 2023 contained TTHMs at or above the MCL of 80 ppb. 

View here

There is also overlap between the states with the most systems with high levels of TTHMs and states with the most livestock. 

Of the 10 states with the most systems that tested at or above the MCL at least once, six are also listed among the states with the most livestock. Six of the states are among the 10 states with the most cattle, chickens and turkeys, or hogs, according to the Census of Agriculture. Three top states – North Carolina, Oklahoma and Texas – are among the 10 states with the most livestock in two categories for two different animal types.

States with the most water systems that tested at or above 80 ppb for TTHMs at least once between 2019-2023, and were in the top 10 cattle-, poultry- or hog-producing states.

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What should be done

Food production should not damage public health so seriously. To prevent TTHMs from being produced, more must be done to reduce the amount of livestock manure in animal feeding operations that gets into drinking water sources. 

Research shows the ways farmers commonly manage animal waste do not protect water sources from the waste’s pollution. Current regulations that require the largest animal facilities to discharge and manage manure in certain ways do not adequately protect water.

Particularly in rural areas, efforts to prevent manure from getting into sources of drinking water – and in turn, reducing the amount of disinfection byproduct levels – must be intensified.

To lower the amount of manure that ends up in drinking water sources downstream, farmers need to adopt more conservation practices on their farms and plan to manage the manure from their farms more transparently. Currently, the EPA requires manure management plans only from the largest farms, which make up just 2 percent of all animal feeding operations. 

Certain practices are vital to fixing the problem – stream buffers and filter strips, or strategies like avoiding manure application on frozen fields or on fields a few days before or after rain. 

One example is Minnesota’s Buffer Law, which requires grasses or shrubs to be grown in a permanent buffer up to 50 feet wide between a farm field and body of water, which reduces runoff. 

While federal conservation programs pay farmers to implement practices like these, many of the most beneficial practices have received very little funding in the past. 

For example, the Department of Agriculture’s Environmental Quality Incentives Program, one of the largest federal conservation programs, sent over $1.3 billion to farmers in 2024. Only $35,345 of that total was spent on filter strips, small bands of grass that can reduce runoff from crop fields. Much more money goes to practices that encourage the growth of animal feeding operations, including more than $50.5 million to farmers building waste storage facilities like manure pits. 

The Inflation Reduction Act, enacted in 2022, began to send significantly more money to farmers for conservation practices. But that was before the Trump administration froze most IRA conservation funding. Billions of dollars have been cut off – funds for farmers to implement conservation practices, some of which would have reduced manure runoff. 

This money needs to be released as soon as possible, so farmers can implement urgently needed practices. 

Water systems can also use treatments like enhanced coagulation, granular activated carbon or nanofiltration to remove organic matter from water before it is disinfected, to reduce the formation of disinfection byproducts, including TTHMs. 

But depending on the size of the water system, installing a water treatment system that can remove organic matter, like granular activated carbon, could cost thousands or millions of dollars up front, and additional money to operate and maintain. 

At home, people can use EWG’s updated Tap Water Database to find out what contaminants are in their drinking water. It describes which home water filters are best to use for removing disinfection byproducts, based on the contaminants found in a particular system. Activated carbon and reverse osmosis home water filtration systems are usually the most successful.

Find out what’s in your tap water—
and how to protect yourself:

• Enter your zip code in EWG’s Tap Water Database for your results.
   
• Use EWG’s Water Filter Guide to find the best filter for your area.
   
• Tell your local official that drinking water safety is important to you.

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