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Beyond wild rice: When sulfate goes up, so do mercury, phosphates

By John Myers

Forum Communications

Even if scientists, regulators, politicians and industry leaders find common ground on how much sulfate is too much for wild rice, the issue of sulfate in Minnesota waterways isn’t going away.

Scientists studying wetlands in northern Minnesota recently published the findings of a study that ties elevated sulfate to elevated toxic mercury levels and shows that, when sulfate stops flowing into a wetland, the level of toxic mercury goes back down.

“Sulfate is an issue even if and where wild rice is not,” said Ed Swain, research scientist for the Minnesota Pollution Control Agency and a national authority on mercury in the environment.

Scientists know the same bacteria that convert sulfate to hydrogen sulfide, which might damage wild rice, are the same ones that convert benign mercury that falls from the sky into methyl mercury — the toxic form of mercury that is unsafe for animals and humans in large doses.

That means the more sulfate, the more active the bacteria, and the more bacteria, the more mercury turns toxic, at least in areas of low oxygen, such as underwater in wetlands.

Swain said added sulfate will not stimulate bacterial growth if there is no organic matter such as plants and weeds. Wetlands and lakes that have a lot of plant growth would be much more affected by an increase in sulfate than a fast-moving stream that has a gravel bottom and little organic matter, he added. Added sulfate from human activity also has more impact in waters that had been low in sulfate naturally.

Mercury “concentrations in fish may be the result of increased sulfate” in what had been low-sulfate waters, an earlier study by Minnesota scientists notes. A study published this summer by Minnesota scientists in the journal Environmental Science & Technology confirmed the correlation.

Scientists are also tracking down a link between sulfate in the water and higher levels of phosphate, a key ingredient in algae blooms. That may be why some lakes that have seen increased levels of sulfate are showing signs of rapid aging, with more weeds and algae growth, a process called eutrophication.

When bacteria convert sulfate to sulfide, it releases phosphate from sediment into the water, Swain noted, making it available for algae. That’s because phosphate is usually held in the sediment by iron, but sulfide binds with iron so that it no longer holds onto the phosphate.

Meanwhile, higher water temperatures from a warming climate may be bolstering the bacteria that make all the conversions possible.

The mercury comes from all over the world, going up in smoke from power plants as far away as China, and as close as taconite plants on the Iron Range. The sulfate also can come from all over the world, in the form of acid rain, but also originates locally, in the runoff from mining operations.

Suddenly, sulfate has become a hot topic in Minnesota environmental protection. And officials agree that efforts to reduce the flow of sulfate into the environment are only likely to grow.

“Its time has come,” Mark Tomasek, the PCA’s supervisor of water quality standards, said of sulfate.

In the past, the primary source of human-caused sulfate increases came for acid rain. But rain has been getting less acidic for more than 20 years, thanks to controls on coal-burning power plants and other manufacturing.

Now, the main sources of sulfate to Minnesota streams are municipal and industrial wastewater discharges and diffuse sources such as runoff from waste rock piles on the Iron Range, Swain said.

Nancy Schuldt, water quality coordinator for the Fond du Lac band’s environmental program, said the PCA is late to the game in addressing the sulfate issue. Sulfate should have been a key part of water pollution permits for industry for decades, she said. But the state only recently began raising the issue, prompting vocal resistance from industry and the Legislature.

“We’ve been concerned about sulfate for years,” Schuldt said. “We’re hoping this means the state is going to take it seriously.”

Sulfate naturally leaches from rock and soil. But it is often seen at higher levels in some areas because of human activity, such as mining, mills and treatment plants. Some sulfate comes from the sky during rain and snow. And some runs off the land as water flows into local streams and lakes. On the Iron Range, it’s coming out of wastewater ponds and old stockpiles of mine rubble left behind by operations long since closed.

The St. Louis River, for example, appears to have had a natural sulfate level of about 3 parts per million. But below mining areas, the river runs as high as 50 parts per million sulfate.

Research by Department of Natural Resources scientists, however, notes that the amount of methyl mercury in streams below mining areas is about the same as nearby streams that don’t see runoff from mining, water where any sulfate would come from air pollution or naturally, hinting that sulfate from mining isn’t the reason fish have more toxic mercury.

Another nearly $1 million study under way by the DNR this summer seeks to find a connection, if any, between Iron Range mining, sulfate and high toxic mercury levels in fish, also looking at whether high iron concentrations and other elements in the water may play a role.

In other parts of the state, ethanol plants appear to be contributing sulfate into waterways, Swain said, and humans also pass it on to wastewater treatment plants.