The mercury leaves the stack in one of three forms — as an elemental gas, a divalent gas, or as a sort of varnish coating dust particles. Elemental mercury rises high into the atmosphere, merging with an ethereal "global pool" of mercury. It can remain aloft for a year or more before falling out of the atmosphere. The divalent and dust-bound mercury is heavier and tends to precipitate nearby. Canadian studies suggest that recently deposited divalent mercury is more "reactive" than elemental mercury and therefore more readily transformed to methylmercury. 

This heavier, more reactive mercury shows up in high concentrations in the Burnt River, which roils blue and chalky past the plant. Twenty-five miles downstream, it merges with the Snake River at Brownlee Reservoir, on the Oregon-Idaho border. Mercury advisories are listed for all species of sport fish caught there. But the undisputed hotspot along Brownlee's 58-mile length is the Burnt River inlet, where the river slows and the mercury carried from the Durkee plant settles out. Tissue samples collected from smallmouth bass and catfish here contained twice as much mercury as the next most contaminated Brownlee sample, exceeding the Food and Drug Administration's mercury "action level" of one part per million by 40 percent.

The Powder River Watershed, 20 miles to the north, has also received heavy doses of Durkee mercury. The EPA estimates that of the 231 pounds of mercury deposited annually in the watershed, a full 150 pounds comes from Durkee. Most of the rest comes from the global pool, generated largely by Asian factories. 

Since the passage of the Clean Air Act in 1970, the nation's cement plants have functioned in a regulatory blind spot. In 1990, the Clean Air Act was amended to require emitters — including cement kilns — to meet standards for a host of nearly 200 pollutants, including mercury. Rules were issued for coal-fired power plants, hazardous waste incinerators and other emitters, but the EPA failed to set rules for cement kilns by the 1997 deadline.

In 1999, the U.S. District Court for the District of Columbia found the EPA in violation of the Clean Air Act and gave the agency two years to issue new rules for cement kilns. It was a deadline the EPA failed — and would repeatedly fail — to meet. "Over the last 12 years, we sued EPA over and over, and we won over and over," says James Pew, an attorney with Earthjustice, the environmental law firm that headed the campaign for regulation. "There's a long string of court decisions saying, 'EPA, you're doing this wrong. …' They weren't terribly interested in what the courts said or what the law said."

Then, last April, the EPA reversed course, announcing its determination to make cement plants comply with the Clean Air Act. (Many attribute that decision to a pro-regulation shift at the agency after the 2008 presidential election.) The proposed new rules — expected to be finalized this year — are part of the EPA's National Emissions Standards for Hazardous Air Pollutants, or NESHAP, program and cover an array of pollutants including sulfur dioxide, particulates, hydrocarbons, hydrochloric acid and mercury. Separate EPA reports estimate that curbing emissions from cement plants nationwide, including an 81 percent cut in mercury, will prevent between 620 and 1,600 deaths a year and reduce national health costs by between $4.4 billion and $11 billion.

But it's difficult to assess the ecological and health impacts of past emissions from the nation's cement plants. Until 2000, the EPA did not even require the plants to report their hazardous emissions to its Toxics Release Inventory database. Because of the lack of reliable data, estimates of the amount of mercury vary wildly. In 2006, for example, the EPA doubled an estimate from earlier that year — from 12,000 to nearly 23,000 pounds annually — after it found evidence of widespread underreporting. As of 2007, according to the agency, 8,500 waterways in 43 states were listed as "impaired" with mercury.

Even if the laws could be tightened tomorrow — and mercury emissions could be "turned off" like water from a tap — the problems would persist for decades, says Don Essig, a water quality specialist with the Idaho Department of Environmental Quality. Like carbon dioxide in the atmosphere, mercury remains in aquatic ecosystems for a long time.

And mercury is not the only dangerous pollutant coming from the nation's cement plants. Worldwide, cement production contributes significantly to climate change, with cement plants accounting for roughly 5 percent of manmade carbon dioxide emissions. Coal and petroleum coke are the most common fuel sources, but a number of plants are permitted to burn "alternative" fuels, including slaughterhouse waste, old tires and railroad ties. (Cement kilns permitted to burn "hazardous wastes," including ink solvents and petroleum residues, have been regulated under NESHAP since 1999.) Industry representatives say that these materials would otherwise clog landfills, and the high heat within the kilns prevents the formation of potentially harmful constituents. But critics argue that plants that burn such fuels release particulates, dioxins, furans and heavy metals, and should be regulated as waste incinerators. (None of those pollutants would be controlled under the new NESHAP rules.) In addition, fly ash from coal-fired power plants and slag from iron blast furnaces are often mixed into the cement as strengtheners and can significantly increase the metal content of emissions.

The Durkee plant's new mercury-control system is supposed to be completed in July 2010, according to the company. A scaled-down prototype of the filtration system — which uses powdered carbon to trap mercury in the exhaust stream –– cut mercury by 70 to 95 percent in test runs.

The mercury reduction agreement reached between the company and state in 2008 requires an 85 percent cut in emissions. However, that will be overridden if the EPA rules go into effect next year, says Douglas Welch, an engineer with the Oregon State Department of Environmental Quality. The new federal rules would require the Durkee plant to cut mercury by about 98 percent by 2013, a goal Welch doubts is attainable. "At a certain point, you inject more and more carbon and you get diminishing returns on mercury," Welch says.

He is also unsure whether the reductions achieved in the scaled-down tests can be duplicated when applied to the entire system. "In the worst case," says Welch, "it's conceivable that they'd have to close up shop for good."

Not surprisingly, cement industry representatives fiercely oppose the new standards. Andy O'Hare of the Portland Cement Association, a Skokie, Ill.-based industry group, has even questioned their legality, saying that new emissions standards must be "demonstrable and achievable." The ability of any one facility to simultaneously reduce mercury, sulfur dioxide, particulate matter, hydrocarbons and hydrochloric acid to the levels specified under the new standards, he says, has not been demonstrated.

The potential loss of Durkee foreshadows an ominous trend, O'Hare warns: the mass outsourcing of U.S. cement production. Compounding the economic loss would be an increase in overall global emissions because cement would be made in countries with lax environmental standards, such as Venezuela, Indonesia and China. "We're working closely with EPA to ensure that whatever rules are passed next year allow us to keep these high-paying jobs in the U.S.," says O'Hare.

Ash Grove is pushing for a regulatory "subcategory" at Durkee — a special designation that would allow the plant to emit more mercury than stipulated under the new rules. "We have strong community support to create a subcategory for our Durkee plant based on the high level of naturally-occurring mercury in the limestone," Ash Grove spokeswoman Jacqueline Clark wrote in an e-mail. "We have also garnered the support of area community elected officials, state elected officials, and federal elected officials." 

Justin Hayes, however, prefers to frame the issue of mercury pollution in moral terms — as an affront to his young children as well as to future generations of anglers. "No matter what anyone says, I'm not against industry," says Hayes. "But if Ash Grove can't make their cement without putting poison into the air and into the water and into the fish, then they should absolutely go out of business."

Jeremy Miller's last feature article for the magazine, "Rebooting Urban Watersheds," appeared in the June 1 issue.