ST. LOUIS (AP) — The nation's rivers and streams remain stubbornly polluted with nutrients that contaminate drinking water and fuel a gigantic dead zone for aquatic life in the Gulf of Mexico, according to a recently released Environmental Protection Agency assessment.

It's a difficult problem that's concentrated in agricultural regions that drain into the Mississippi River. More than half of the basin's miles of rivers and streams were in poor condition for nitrogen and phosphorus from fertilizer that drains into waterways, the agency found. For decades, federal and state officials have struggled to control farm runoff, the biggest source of nutrient pollution that is not typically federally regulated.

It's a problem only expected to get harder to control as climate change produces more intense storms that dump rain on the Midwest and South. Those heavy rains flood farm fields, pick up commercial fertilizers and carry them into nearby rivers.

"It's really worrying that we are clearly not meeting the goals that we've set for ourselves," said Olivia Dorothy, director of river restoration with the conservation group American Rivers.

The assessment is based on samples collected in 2018 and 2019 and it allows experts to compare river conditions from previous rounds of sampling, although different sampling sites were used. It takes years for the agency to compile the results and release the report, which is the most comprehensive assessment of the nation's river and stream health. Phosphorus levels dipped slightly while nitrogen levels remained almost exactly the same.

About half of all river miles were found to be in poor condition for snails, worms, beetles and other bottom dwelling species that are an important indicator of biological health of the river. About a third were also rated as having poor conditions for fish based on species diversity.

"Controlling pollution is a big job. It is hard work," said Tom Wall, director of watershed restoration, assessment and protection division at EPA. "Things are not getting worse, despite the tremendous pressures on our waterways. And we would like to see more progress."

Water pollution from factories and industry is typically federally regulated. The Biden administration recently proposed toughening regulations on meat and poultry processing plants to reduce pollution, Wall said.

When nutrient pollution flows into the Gulf of Mexico, it spurs growth of bacteria that consume oxygen. That creates a so-called "dead zone," a vast area where it's difficult or impossible for marine animals to survive, fluctuating from about the size of Rhode Island to the size of New Jersey, according to Nancy Rabalais, professor of oceanography and wetland studies at Louisiana State University.

That affects the productivity of commercial fisheries and marine life in general, but nutrient pollution is also damaging upstream. Too much nitrate in drinking water can affect how blood carries oxygen, causing human health problems like headaches, nausea and abdominal cramps. It can especially affect infants, sometimes inducing "blue baby syndrome," which causes the skin to take on a bluish hue.

The EPA established the hypoxia task force in the late 1990s to reduce nutrient pollution and shrink the dead zone, but it relies on voluntary efforts to reduce farm runoff and hasn't significantly reduced the dead zone.

Anne Schechinger, Midwest director with the Environmental Working Group, said new regulations are needed, not voluntary efforts. She said the Biden administration has done a lot to improve drinking water, but not enough to reduce agricultural runoff.

Methods to prevent runoff include building buffers between farmland and waterways, creating new wetlands to filter pollutants and applying less fertilizer.

It's a politically fraught issue, especially in major Midwest farming states that significantly contribute to the problem. Many of those states cite their voluntary conservation programs as evidence they're taking on the problem, yet the new EPA data shows little progress.

Minnesota is one of the few states that has a so-called "buffer law" that requires vegetation to be planted along rivers, streams and public drainage ditches. But because groundwater and surface water are closely connected in much of the Upper Midwest, nutrient pollution can end up leaching underground through farm fields and eventually bypass those buffers, ending up in streams anyway, said Gregory Klinger, who works for the Olmsted County, Minnesota soil and water conservation district.

There should also be a focus on preventing over-fertilizing – about 30% of farmers are still using more than the recommended amounts of fertilizer on their fields, said Brad Carlson, an extension educator with the University of Minnesota who communicates with farmers about nutrient pollution issues.

Martin Larsen, a farmer and conservation technician in southeast Minnesota, said he and other farmers are interested in practices that reduce their nutrient pollution. He's broken up his typical corn and soybean rotation with oats and medium red clover, the latter a kind of plant that can increase nitrogen levels in the soil naturally. He's been able to get by with about half as much fertilizer for a corn crop that follows a clover planting as compared to a corn-corn rotation.

Growing oats and red clover as cover crops improves soil, too. But Larsen said it's difficult for many farmers to plant them when they often rely on an immediate payback for anything they grow. Cover crops are planted on just 5.1% of harvested farmland, according to 2017 data from the U.S. Department of Agriculture.

Larsen said since regulations are so unpopular, more should be done to incentivize better practices. For example, he said that could include companies shifting the makeup of feed they use for animals, giving farmers an opening to plant some crops that use less fertilizer. Or government programs that do more to subsidize things like cover crops.

He said that many farmers in his community acknowledge the need to do things differently. "But we also feel very trapped in the system," he said.

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Walling reported from Chicago.

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Follow Melina Walling on X: @MelinaWalling.

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