Manure-Linked Contamination Affects 6,000 US Water Systems

Livestock manure and organic waste from large-scale farms, which house thousands of cows, pigs, chickens, and turkeys in barns or feedlots across the United States, present significant health risks to millions of Americans.

Many farms utilise livestock manure as fertiliser for their fields. However, given the large quantities generated, excess manure is frequently moved to neighbouring farms for disposal. This waste can contaminate nearby waterways through leaching or runoff.

When these affected freshwater sources are treated with chlorine and other chemicals, they can produce harmful by-products known as trihalomethanes (TTHMs). Research has linked exposure to TTHMs with serious health issues, including cancer and birth defects.

The “Manure is one source triggering cancer-causing chemicals in drinking water for 122 million” report from the Environmental Working Group (EWG), released on April 10, 2025, examined the presence of trihalomethanes (TTHMs), a type of disinfection byproduct, in US water systems and investigated how manure used in food production generates TTHMs in drinking water.

The findings reveal that unsafe levels of TTHMs were detected in at least one in 6,000 community water systems across 49 states and Washington, DC, impacting approximately 122 million people between 2019 and 2023.

The Environmental Protection Agency (EPA) establishes a safe limit for TTHMs at 80 parts per billion; however, research suggests this limit should be much stricter, at just 0.15 parts per billion or even lower.

The report also identifies that the states with the highest number of water systems exceeding 80 parts per billion for TTHMs are predominantly major cattle, poultry, or hog-producing states, including Texas, Oklahoma, California, Illinois, North Carolina, and Pennsylvania.

The figure below is from EWG’s research. It shows the top 10 states with the highest water systems testing above the EPA’s safe TTHM levels.

Unsafe levels of trihalomethanes (TTHMs) have been detected in water systems of all sizes, from those serving only a small number of people to those supplying millions. 

The Environmental Working Group (EWG) provides a Tap Water Database that allows users to enter their zip code to check for the presence of TTHMs and other disinfection byproducts in their drinking water. Additionally, the EWG offers an interactive map highlighting US water systems where TTHMs are at or above 80 parts per billion, based on tests conducted between 2019 and 2023. The map also indicates the number of farm acres in each state that received manure according to the 2022 Census of Agriculture.

Ingesting trihalomethanes (TTHMs), even for a short period, is associated with various birth problems, including birth defects, low birth weights, and stillbirths. Longer exposure has been linked to colorectal and bladder cancers.

What should be done to reduce the impacts of manure on drinking water?

The impact of food production needs to be minimised, particularly by reducing the amount of livestock manure that contaminates drinking water sources. Research indicates that current farming practices do not effectively safeguard water sources from pollution, and the existing regulations permitting large animal feeding operations to discharge their manure are inadequate to protect water bodies.

According to Anne Schechinger, an analyst for the EWG and author of a new report, the Environmental Protection Agency (EPA) only mandates manure management for the largest farms—specifically, cattle facilities with more than 1,000 animals or hog facilities with over 2,500 animals. However, these large facilities account for just 2% of all animal feeding operations in the country. This means smaller facilities with 995 animals or fewer still producing significant amounts of manure fall under the radar.

To lower the amount of manure entering water bodies, farmers need to adopt more conservation practices on their farms and be more transparent about their manure management plans. “More conservation programs on the farm fields could help with runoff, too,” Schechinger stated.

“Those conservation practices that are good for the climate are also good at reducing runoff of manure and nitrogen from farm fields,” she added. “That’s a no-brainer.”