Millions of tons of manure from Minnesota’s animal feedlots is a risk to consumer health as it threatens to raise nitrate and phosphorus levels in the state’s rivers, lakes and drinking water, a study has found.

Meat and dairy production in the US is dominated by the use of concentrated animal feeding operations (CAFOs) where animals are raised or fattened in close confinement.

Since 1991, the number of CAFOs in Minnesota has trebled, with about 80 million pigs, cows and poultry now held in feedlots. The state has one of the most detailed databases in the US on all of its animal farms.

In “almost all of Minnesota’s farm counties” the combined use of manure plus commercial fertiliser, is “likely to load too much nitrogen or phosphorus or both on to crop fields, threatening drinking water and fouling the state’s iconic lakes and rivers”, found the report by the US NGO, the Environmental Working Group (EWG).

The increasing number of feedlots has come at the same time as a deterioration in nitrate contamination. An earlier EWG investigation found that 63% of Minnesota public water utilities with elevated levels of nitrate saw the contamination worsen between 1995 and 2018.

In an email response, the Minnesota Department of Health (MDH) said “nitrate contamination of groundwater is a growing concern in Minnesota”. It said “high levels” in drinking water “can cause health problems, especially for infants”, and bring extra management costs for community water systems and homeowners.

The planting of corn, often fed to livestock, had risen too, EWG said, up by 1.5m acres since the 1990s. Annual fertiliser sales in the state are up from about 2.3m tons in 1989 to just over 3m tons in 2017, according to a Minnesota Department of Agriculture report. The Minnesota Corn Growers Association did not respond to requests for comment.

Drinking water with high nitrogen levels is linked to cancers and blue baby syndrome, known technically as methaemoglobinaemia. On its water sanitation page, the World Health Organization (WHO) says this condition is caused by “the decreased ability of blood to carry vital oxygen around the body” and it expresses special concern for bottle-fed infants.

The US Environmental Protection Agency says too much nitrogen and phosphorus are also known to support algae blooms in lakes. The algae can produce bacteria toxic to humans.

To manage manure buildup from livestock, some of which collects in giant lagoons, feedlots often spread it on nearby farmland which can already have commercial fertiliser applied, said the EWG study co-author, Sarah Porter. Water running off fields is known to carry nitrates and phosphorus into the state’s rivers, lakes and tap water, she said.

The study has produced a manure map showing every crop field across Minnesota that is likely to receive manure from nearby cattle, hog or poultry feedlots and estimates the amount of manure applied. Using data from county fertiliser sales and other state sources, plus the manure data, the EWG produced two more maps showing their findings for total nitrogen and phosphorus overloads.

A spokesperson for the Minnesota Pollution Control Agency (MPCA), Forrest Peterson, said there are about 18,000 registered feedlots in the state, with the largest CAFOs holding up to 125,000 animals or poultry. Asked for further comment on the study the agency said it needed time to review it.

The MPCA lists more than 3,000 water bodies in the state that fail to meet quality standards, with 85% of those impairments due to non-point pollution from “nitrogen, bacteria, chloride and phosphorus”.

Non-point source pollution is described by the MPCA as pollutants carried by water moving over and through ground into lakes, rivers and streams.

Nitrate is a vital fertiliser component, but as well as a methaemoglobinemia warning, an MDH factsheet says a “growing body of literature indicates potential associations between nitrate/nitrite exposure and other health effects such as increased heart rate, nausea, headaches and abdominal cramps”.

The MDH factsheet adds that some studies “suggest an increased risk of cancer, especially gastric cancer, associated with dietary nitrate/nitrite exposure, but there is not yet scientific consensus on this question”.

The EWG study said feedlot expansion in Minnesota has been concentrated in the southern and central parts of the state, most notably in Martin County. The county, it said, is “home to 15 lakes on Minnesota’s 2020 list of nutrient-impaired water bodies. The list includes Budd Lake, which serves as the drinking water source for the town of Fairmont”.

Phosphorus pollution, the EWG study said, can trigger potentially toxic algae blooms in lakes and rivers.

A July 2015 warning from the MPCA describes the hospitalisation of a child “after being exposed to blue-green algae” while swimming in Lake Henry, about an hour-and-a-half northwest of Minneapolis. In a nearby lake two dogs died from blue-green algae exposure and others were sick.

Porter said animal feedlots can be a source of friction too. “Some people benefit economically, others suffer the side effects.” She said she hoped the manure map and its data will be used by state authorities to make “better decisions about how many more farms they want to provide permits for”.

In its email, the MDH added that the best way to prevent nitrate contamination is at source and said it is working with “city water utilities, non-profits, the agricultural sector” and others on prevention, conservation and systemic change initiatives to protect source water quality.

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