Pollution Rising In Tributaries of Bay, Data Show

But then, starting in the late 1990s, the graphs start to show a change.

[an error occurred while processing this directive]

In some rivers, the research shows, the long decline tailed off during that period, and pollution levels flat-lined. Then, in some cases, the contamination started going up again. The researchers looked closely at nine of the biggest rivers that feed the bay. Eight of them showed an uptick in one pollutant or the other, and four showed increases in both.

Some increases were slight: In the Patuxent, for example, the nitrogen level rose just a few percent between 2000 and 2006. But other examples were more troubling: In the James River, near Richmond, all of the decline in phosphorus appeared to be erased.

The Potomac experienced one of the most dramatic reversals. After its 20 percent drop, the level of phosphorus started increasing, until it was nearly 20 percent higher than where it had started in 1984.

Geological Survey scientists said this week that their calculations might have caused some of the upswings to appear more pronounced than they really are.

"We need to be able to look at the last five years in more detail" to determine whether the increases are real, researcher Scott Phillips said.

Also, he noted, the news is not all bad: In many cases, pollutant levels are still far below their 1984 levels.

Still, some bay scientists said the apparent upswings were worrisome.

"If I were making investments, my own personal money, and if I had one that looked like that" -- with a reversal of course in the last few years -- "I'd be pretty curious," said Donald F. Boesch, president of the University of Maryland's Center for Environmental Science. "Rather than looking just at my average return of the 20-year period."

Scientists and environmentalists said the most likely cause of the upswings is urban and suburban development. One recent federal report found that 170,000 people were moving to the Chesapeake watershed every year.

Environmentalists say more people means more concrete, which doesn't allow rainwater to seep naturally through roots and soil. Instead, it becomes polluted runoff, carrying nitrogen, phosphorus and other pollutants into bay tributaries.

"The treadmill is going faster and faster, and we're having to run faster just to stay even," said William C. Baker, the president of the Chesapeake Bay Foundation. On Monday, his group gave the bay's health a score of 28 out of 100, down one point from last year.

Yesterday, the District announced some apparent good news for the bay. Because of an agreement with the EPA, city officials said they would plant 13,500 trees and replace some sidewalks and road medians with green space. Those moves, among others, are intended to provide a natural filter for runoff headed toward the Potomac and Anacostia rivers, both bay tributaries.

"It's a huge turning point for the city's storm-water management efforts," said Jon Capacasa, an official with the EPA's mid-Atlantic regional office.

But the same day also brought bad news: Climate change could soon make the Chesapeake's problems worse. A report from the Pew Center on Global Climate Change found that greenhouse gas pollution might bring more of the rain that washes pollution into the bay and more of the heat that can supercharge algae blooms.