Keith Eshleman, forefront, made a surprising discovery several years ago: nitrogen concentrations in forested streams were dropping by large amounts, and that power plant standards Congress passed in 1990 were the main reason why. Photograph, Cheryl Namazie, UMCES

They found that nitrogen in forested streams declined by nearly half (from 1986 to 2009) in nine predominantly forested Appalachian watersheds around the Potomac basin. A sewage treatment upgrade or an agriculture best-management practice will improve nutrient concentrations in the small waterway where the discharge occurs. But this change was much broader ­— across three states and different topographies. It had come from the sky.

Eshleman credited the improvement to Congress and President George H.W. Bush for the 1990 Clean Air Act amendments, which closed some loopholes that had allowed power plants and other industrial facilities to emit nitrogen oxide and sulfur dioxide that produce smog and soot that pose serious health hazards. At least one third of the Chesapeake’s pollution comes from atmospheric deposition: gases and fine particles deposited in the water by rain, snow, winds, and settling.

A pollution reduction was good news for a struggling estuary. But much of the federal-state Chesapeake Bay Program’s resources were being spent on reducing pollution from agriculture. Then, as now, the federal government paid for farmers to plant cover crops, install manure pads, erect riparian buffers and grass waterways, and build stream-bank fences — all in hopes of keeping nitrogen and phosphorus out of streams. Now, it seemed that legislative action two decades before, largely to protect human health, had also improved stream health.

Eshleman and Sabo followed up with a 2016 paper that said that Chesapeake Bay regulators had no direct evidence that the money farmers spent on pollution-reduction practices was improving water quality — not yet, anyway. This study examined 18 sites in forested and agricultural watersheds and found that less atmospheric deposition led to less nitrogen in all of their waterways. Eshleman didn’t say the practices that farmers were putting in place had failed to work; he simply said that research had yet to prove they were the main driver of the improvements to date.

“I’m providing an alternative explanation for the improvements,” Eshleman said. “We’re asking, which one is the big driver for recent improvements in the Bay, atmospheric deposition or agriculture? If you had one knob that has been turned, which one was it? I’m arguing atmospheric deposition is the big knob that has been turned effectively so far. The [Bay Program] thinks it’s agriculture.”

Appalachian Lab director Eric Davidson said the main take-away is that curbs on emissions from power plants, factories, and vehicles are helping humans, streams, and the animals that depend on them. Any rollback of the Clean Air Act Amendments of 1990 would bring harm.

“No one’s saying we should let up and stop trying to improve agricultural practices that reduce nitrogen and phosphorus runoff to the Bay. Those are still wise things to do, and some of them can actually save farmers money too,” Davidson said. “But let’s give credit where credit is due, which, for the moment, is that the scientific evidence says that air pollution controls are reaping huge benefits.”

—kobell@mdsg.umd.edu