When 400,000 people in Ohio were told by authorities to stop drinking their tap water for two days this August, the warning centered attention on something most people assumed only troubled creatures lower down the food chain.

But the waters of Lake Erie, near Toledo, had become so ripe with toxin-producing algae — potentially dangerous to humans — that residents, regulators and lawmakers had to take notice.

In the warm days of summer, the relatively shallow waters of the lake had become a giant Petri dish for blue-green algae, or cyanobacteria. That flourishing cyanobacteria, in turn, produced potentially deadly cyanotoxins, particularly one called microcystin. When farmers in the region heard about it, they knew critics would soon would be looking in their direction.

Agricultural runoff isn’t the only cause of cyanobacteria, but it is a major one. So, in the days after the water order, lawmakers in Ohio began discussing legislation that would limit when farmers could apply fertilizer and manure. That legislation is pending.

Meanwhile, members of Congress whose districts touch the lake renewed calls for the Environmental Protection Agency to regulate cyanotoxins, and provide guidance to water utilities on how and when to test for the potentially deadly toxins.

Farm groups bristled. For years, nitrogen- and phosphorous-laden agricultural runoff has been blamed for a growing “dead zone” in the Gulf of Mexico and algal blooms in bodies of water around the country. But polluted drinking water, they figured rightly, would raise the level of attention.

“What Toledo brought forward is that this is a major public health issue that’s not going to get better,” said Mae Wu, an attorney with the Natural Resources Defense Council. “We’re finally waking up to the fact that it’s not just affecting recreational use of water, it’s the water coming out of our tap.”

Now members of Congress are responding. In the week before Thanksgiving, Ohio Republican Rep. Bob Latta introduced a bill (HR 5753) that would amend the Safe Drinking Water Act (PL 93-523), ordering Congress to develop a strategic plan for “assessing and managing risks associated with cyanotoxins in drinking water provided by public water systems.” That legislation came on the heels of a House Energy and Commerce subcommittee hearing on cyanotoxins.

At the hearing, John Donahue, president of the American Water Works Association, which represents water plant operators and utilities, noted in his testimony that there remains uncertainty around the dynamics behind algal blooms, and even uncertainty around the possible effect on human health.

“However, there is no uncertainty about one critical aspect of the problem: It is always associated with amounts of nitrogen and phosphorus in the water,” Donahue said. “Although each watershed is unique and has its own mix of nutrient sources, across the nation the most prominent uncontrolled sources of nitrogen and phosphorus are non-point sources — that is, runoff. These sources are at the same time both the hardest to manage and the furthest from being subject to meaningful federal regulatory authority.”

Sources of Pollution

Under the Clean Water Act, larger, concentrated feeding operations are required to have discharge permits. But other agricultural runoff, considered “non-point” sources of pollution, aren’t regulated.

Farm groups have long argued that farmers are taking meaningful steps toward curbing runoff through voluntary conservation measures. Under the 2014 Farm Bill (PL 113-79) for example, new federal spending is available for farmers to limit runoff from their operations. (The Regional Conservation Partnership Program will make $200 million available each year, and 35 percent of that can be used in those designated as “Critical Conservation Areas.”)

After the recent hearing, the Ohio Farm Bureau stressed in a letter that farmers “are not sitting idly by.” They’ve been reducing phosphorus application in the Lake Erie watershed by 180,000 pounds, the bureau said, and “investing tens of million of dollars of their own money” implementing conservation practices on their farms.

Rep. John Shimkus, R-Ill., who chairs the subcommittee on the environment and the economy, which held the recent hearing on the subject, echoed the bureau’s position and stressed that new technologies, including fertilizers applied directly to seed, promised to reduce polluted runoff even further.

Water treatment operators, along with advocacy groups, are urging Congress to consider regulating non-point sources of pollution under the Clean Water Act, noting that farm conservation programs are largely voluntary. But, given the nature of runoff, that will be next to impossible, some critics and farm groups note.

Donahue said in his testimony that public water facilities, which provide the majority of drinking water to Americans, will increasingly bear the burden for upstream pollution — and the costs will ultimately be passed on to consumers. “It would not be fair to put the entire burden of addressing this problem on municipal wastewater and drinking utilities,” he said.

Currently the EPA doesn’t regulate cyanotoxins under the Safe Drinking Water Act, but is considering doing so. The agency is also working on developing health advisories that establish a level of contaminants under which health effects are not expected to occur for two common cyanotoxins, including microcystins, and intends to complete those in 2015.

Critics note, however, that the health advisories aren’t regulatory, and that the EPA has not set standards for most of the pollutants on its Contaminant Candidate List, which ultimately leads to regulations.

The agency has said that “given the urgency,” it is working to get more data on cyanotoxins by collaborating with state and federal agencies, including the U.S. Geological Survey and the National Oceanic and Atmospheric Administration. “The EPA is taking aggressive action,” said Peter Grevatt, who directs the agency’s office of groundwater and drinking water.

Lynn Thorp, the national campaign director for Clean Water Action, notes that state-run nutrient reduction programs, and efforts such as the Source Water Collaborative, made up of regulators, utilities and environmental groups, among others, play a critical role in reducing runoff.

But, ultimately, federal water laws will have to be integrated so that upstream contamination doesn’t burden the utilities that have to maintain drinking water standards. “Regulating cyanotoxins in drinking water is not sufficient to prevent this shift of burden,” she said in her recent testimony.

That could mean yet more attention on more farms if algal blooms proliferate, as expected, in coming years.