During a recent rush hour near the District of Columbia's northeastern corner, a scientist stood along the water's edge trying to be heard above the urban din. Cars merged onto the Baltimore-Washington Parkway, and day laborers flashed past, heading home with lunch coolers strapped to the handlebars of their bikes. In the background, a small cement valley constructed more than 50 years ago burbled with water.

"This is one spot where I leave my clams," Harriette Phelps said, pointing toward the unnamed creek at the bottom of that cement valley in Riverdale. The retired biology professor from the University of the District of Columbia pulled out an industrial-strength shellfish bag, which was soon to be filled with Asiatic clams (Corbicula fluminea).

The clams will act as her messengers. They stay submerged, anchored securely in place, for two weeks. After that time, the scientist returns, fishes them out and rips them apart for results. The clams absorb whatever the river and its feeder streams have to offer. Like all bivalves, they are natural filters, sucking dirty water in, pushing clean water out. Many of the contaminants found in the water remain in their body tissue-data for the taking.

Phelps analyzes the tissue and charts the resulting data on a large map, which she says can function as a map of many active toxic hot spots in the large, urbanized Anacostia watershed. Although she is one of many people using this kind of research method in the United States, hers may be the only work focused on the toxic chemicals in the upstream areas of the Anacostia.

"A lot of the work stops at Bladensburg," she said recently, grimacing. She finds this frustrating because a large amount of pollution enters from the free-flowing portion of the river that lies upstream.

"We don't go into freshwater," acknowledged Ian Hartwell, a toxicologist for the National Oceanic and Atmospheric Administration. Hartwell was the senior author of a report written about the river's contaminant levels in 2007 that detailed contamination in the mainstem of the Chesapeake and the correlation between benthic community impacts, toxicity and contaminant levels. Although the Potomac was assessed, the Anacostia-which feeds into the Potomac and eventually the Bay-was not specifically included in the work.

NOAA's normal work plan, Hartwell explained, only covers tidal areas. All areas north of Bladensburg are outside of the tidal zone, although he agrees that upstream work is important.

"Urban runoff is a ubiquitous problem anywhere in the U.S.," he said. "The more densely populated an area is in a watershed, the more of a problem it will be."

Draining most industrialized sections of the District and Prince George's County, the Anacostia and its tributaries' banks are indeed very built up, with intensive housing and light industrial parks, highways and small manufacturing plants.

It is sometimes hard to even locate the small creeks that feed the river; some have never been given names while others have been long buried in underground pipes where they swell with oily runoff each time it rains. As a result, much of what lives in the river is subject to huge amounts of pollution.

"In the Anacostia, it is clear that exposure to PAHs (polynuclear aromatic hydrocarbons) is associated with liver tumors in brown bullhead catfish," said Fred Pinkney a scientist with the U.S. Fish and Wildlife Service. In a study he conducted in 2000-01, the percentage of the fish with liver tumors ranged from 50-68 percent, a rate equivalent to the highest reported for brown bullheads anywhere in North America. And yet, Pinkney said, "People tend to forget that toxics are still a problem in the Bay."

As with many other Bay tributaries, subsistence fishing continues even though fishing advisories have long been posted by the District Department of the Environment. There is, unfortunately, also a high incidence of cancer and other diseases reported in the local community, which is largely composed of poor and minority residents.

It is this very disconnect that is puzzling and a bit frustrating to Phelps and some other Bay advocates. Discussions about restoration efforts have often focused on nutrient loads, stormwater and other issues but not on reducing or removing PAHs and other toxic chemicals. Even downstream in the Bay's mainstem, little effort is focused on the problem.

"Toxics are really not being addressed as robustly as they should be," remarked Jim Connolly, executive director of the Anacostia Watershed Society.

He has been with the organization for 17 years, and says that in the mid-1990s, more work was done to target the problem of toxics specifically. Since that time, though, priorities seem to have shifted for many of the federal agencies and advocacy groups such as his own.

Problems such as stormwater and nutrient loads are huge almost to the point of being overwhelming, and resources are limited. While he's glad to see the huge push to solve problems associated with stormwater in urban areas of the Anacostia watershed, he agreed that toxic chemicals deserve more attention than they currently receive from almost all of those working to restore the Bay's urban rivers.

"We've had to make some tough decisions and prioritize," said Greg Allen, an environmental scientist with the EPA's Bay Program Office. "Nutrient problems are having the greatest impact on the living resources of the Bay." Targeting those problems get top priority. Problems such as chemical contaminants are extremely important to his agency, Allen added, but the price tag for cleanup and restoration is huge.

Allen further cautioned that the clams alone provide only a partial snapshot. They only detect some of the stressors in any body of water where they are placed.

"We need to use this kind of bio-monitoring in conjunction with other techniques," he said, to get a clear picture of what is happening in the waterways. The conclusions from such studies are valuable but not comprehensive.

Still, one of the most intriguing things about the clams is their ability to uncover certain problems in certain locations in a rather quick manner. The picture they give may be incomplete, but it reveals some startling things. There are, Phelps says, hot spots where long-banned chemicals still enter the water everyday.

One active site is located under a bridge of Interstate 295. Upstream from the bridge, the water is relatively clean. But below the bridge, Phelp's clam research reveals that huge amounts of chlordane are present. This is particularly perplexing, because all domestic and commercial use of the chemical was banned in the United States after 1988.

"It's a legacy site," Phelps explained from the side of the road. "Something from long ago is under there, maybe something that happened during construction."

Indeed, longtime residents recall that the construction companies in the area would often invite people to dump at such places as they sought to make a level ground for the new highways. Anyone could have dumped anything there without any penalty. Perhaps those dump sites are now shifting, moving or leaking, causing fresh problems in the water.

When questioned about how such issues could be resolved, Phelps has sometimes remarked that she is simply presenting the evidence and she wants others to take action, although she sees a sense of urgency in the issue, sometimes to the point of being brusque. She was once described by the Washington Post as having an air of "exasperated purpose."

That sense of purpose may be spreading to others. A soon-to-be-released Anacostia River Restoration plan devotes an entire chapter to toxic contaminants. In July, policy watchdogs were also pleased to see the District ban the use and sale of coal-tar pavement products. Such sealants are common throughout the watershed, and are known to be a source of PAHs. The ban seemed to signal a turn toward serious toxic reduction.

The EPA recently announced plans to step up regulatory programs to stem pollution to the Bay and, in a report responding to an Executive Order issued by President Barack Obama, the agency noted that many of its proposals, including ramped-up stormwater controls, would help to control toxic pollution as well as nutrients, in developed areas. "EPA will provide leadership in the restoration of heavily impacted urban rivers," it stated.

Through it all, Phelps is unapologetic about her style and her energy level. Her map of toxic hot spots in the upper watersheds sometimes seems like a banner she's ready to wave at any moment. Her clams, she thinks, provide a cheap, uncomplicated way to figure out how and where a cleanup could begin upstream. She simply wants people to pay attention to them.