The Wye River on Maryland's Eastern Shore drains a small rural watershed marked by farms and forests. It lacks big industries or large cities. It is not, in other words, where one would suspect to find chemical contamination affecting aquatic life.

Thinking the Wye to be "relatively pristine," scientists selected it several years ago as a "clean" site for a Bay toxics research project. They hoped to compare how a variety of aquatic life fared in the Wye compared with known problem areas of the Bay.

But for two straight years, they were surprised as various test organisms died or showed other biological effects when placed into laboratory tanks containing water or sediment from the Wye.

"I think that's a fairly significant finding," said Lenwood Hall, of the University of Maryland's Wye Research and Education Center. In some of the areas that we suspected to be fairly clean, we are seeing occasional biological effects.

Indeed, the studies headed by Hall and Ray Alden of Virginia's Old Dominion University also found toxic conditions at sites on the Potomac River where problems had never been recorded before. Hall and Alden caution that problems detected at these locations are not nearly so severe as those in several "hot spots" scattered around the Bay.

Findings from their "ambient toxicity testing" project, now entering its fourth year, are helping to screen largely unstudied areas of the Bay and its tidal tributaries to find toxics problems, and to rank how severe those problems are.

Rather than merely testing the water for the presence of chemicals, the project takes a more holistic approach to toxicity testing, which allows the organisms do the talking: Scientists observe what happens to selected species when they are exposed to samples of water and sediment taken from specific sites. Such tests show results not only when there are high levels of a single compound, but also when there are low levels of many chemicals mixing in the environment -- something traditional monitoring programs have difficulty assessing.

"There's no way to look at the literally thousands of contaminants that could be there and could be toxic," Alden said. "That's why we're letting the organisms tell us whether a particular environment is stressful in some ways."

The project is particularly useful at detecting impacts related to nonpoint source pollution in the water, which recent studies have indicated are a major source of toxics to the Bay.

"It's really important that we have some sort of systematic way to go out and look at the Bay instead of just assuming that you don't have a problem because you don't have the data on your desk, which is what people have tended to do," said Jacqueline Savitz, an environmental scientist with the Chesapeake Bay Foundation. "What I think is interesting is that they are finding some sort of an effect almost everywhere."

The ambient toxicity program is, in fact, helping change the way the toxics issue is viewed in the Bay. Rich Batiuk, toxics coordinator with EPA's Chesapeake Bay Program Office in Annapolis which funds the study, said people often have viewed toxics problems as something confined to a few "hot spots" such as Baltimore Harbor and parts of the Elizabeth and Anacostia rivers.

"Before, toxics tended to be more black and white," Batiuk said. "Now we're saying, it's not black and white. It's more of a dark gray to a very light gray problem."

The ambient toxicity program, he said, is "finding the middle gray and light gray areas."

THE ambient toxicity testing program is a sharp departure from the way information about toxics has been gathered in the past. Historically, toxic impact assessments stemmed from programs regulating discharges from industry and other "point sources."

Those programs primarily looked for evidence of toxicity at the end of the discharge pipe and in nearby areas known as "mixing zones." Typically, those programs are chemistry based - that is, water is tested to see whether particular chemicals ex ceeded thresholds deemed "safe" -- rather than biologically based.

Because information from those programs is confined to a few narrow areas, toxics in much of the Bay have gone largely unexamined. As a result, there is little information about impacts from the mostly unregulated nonpoint sources, or effects cau sed by the cumulative impacts of several substances on aquatic life.

Recognizing that information gap, the Bay Program's 1988 Basinwide Toxics Reduction Strategy called for a more comprehensive approach toward assessing the impact of toxics on living resources in the environment.

That led to a 1989 workshop in which scientists from across the nation came together to decide what a more comprehensive monitoring program should look like. Participants concluded that such a program should use a variety of sensitive species as indicators, and that it should look at the impacts in both the water and in the sediments.

"They wanted batteries of toxicity tests because one thing that has been fairly clear in all of environmental toxicology is that there is no one most sensitive organism," Alden said. "Different organisms are sensitive to different things."

As a result, the tests use a number of unheralded but sensitive organisms which many people have never heard of -- sheepshead minnows, grass shrimp larvae, zooplankton, fathead minnows, as well as worms, amphipods, and other species. Because they are all native to the Bay, Alden said, "they should be somewhat pre-adapted to most natural conditions."

In the tests, scientists also attempted to sort out factors such as salinity, dissolved oxygen, pH, and other factors which could affect survival. This proved especially complex for the sediment tests because different kinds of sediment -- mud, silt, or sand -- can affect survival.

Scientists look not only for survival, but changes in reproduction, growth, behavior, and other impacts. Any differences between the species in the tests, and those exposed to uncontaminated conditions, is noted.

The program looks separately for impacts in the sediments and the water because different kinds of organisms can be exposed to toxics in each environment. In addition, because toxics gradually sink to the bottom, sediments provide an idea of the amount of toxics which are accumulating in a particular area. Water tests, by contrast, are more apt to detect toxics in the water from nearby sources, or those that may have been flushed in from a recent storm event.

"If you address both the water column and sediment in terms of toxicity, you're getting a fairly complete picture of toxic conditions in the environment," Hall said. "If you only do one without the other, you aren't.

"Sometimes the data from water column and sediment aren't always similar, and you might see toxicity in one media and not the other," Hall said. "We've seen that during the course of our study."

Samples are also analyzed for the presence of various metals and other chemicals which may be a cause of any observed toxic effect. But the list of chemicals tested is only a small fraction of more than 1,000 that have been detected in the Chesapeake basin.

Instead of providing a cause-and-effect relationship between toxicity observed in the organisms and a particular pollutant, the project only provides an indicator of a problem.

In some tests, such as several conducted in portions of the Potomac River, "significant" effects were seen in cases where no potential chemical contaminants were found. The test organisms were either being affected by chemicals that were not being sampled, by small levels of different chemicals, or by other unknown factors.

"That's what I think the strength of the ambient test is," Batiuk said. "We're not attempting to say it's this chemical or that chemical, but perhaps its the low level concentrations of a mixture of chemicals that in some cases may be the reason for the impacts you're seeing there."

In such cases -- if the amount of toxicity observed is great enough -- Batiuk said the ambient tests provide a signal that managers may want to conduct further testing in a particular area to identify the cause of the observed toxic effect.

THE ambient toxicity testing project is seen to have particular value as the Bay Program moves toward a geographic approach in addressing toxics issue. That approach will target hot spots with known toxic problems such as Baltimore Harbor and the Elizabeth and Anacostia rivers for intensive cleanup activities.

Also, other "warm spots," where problems are suspected, may be targeted for further reduction from any point sources in the area as well as new pollution prevention actions targeted at point and nonpoint sources.

But because most information has been gathered in the hot spots, other areas of potential concern have not been well-defined. The ability of the ambient program to screen and rank those areas is seen as an important tool in defining problem areas.

"I see our program as a way to direct resources, rather than have everybody throw up their hands and say, 'toxics are everywhere,'" Alden said. "Everything is toxic in a certain amount. We're just trying to quantify how bad it might be."

By looking at variables - such as the number of species impacted at a site, the percentage of the population affected, and whether toxicity was seen in all the samples taken at a given location -- scientists and water quality managers can rank the severity of problems at one site versus another.

The testing program is also providing a useful tool for identifying areas impacted by nonpoint source pollution which have historically been unregulated and therefore not been subject to discharge monitoring as have point sources.

As a screening tool, it is well-suited for the job because unlike monitoring industry discharges -- where the chemicals in the effluent are usually known -- a huge range of toxics ranging from auto products to household cleaners to pesticides can be in storm water. "The list of what has regulatory criteria is very modest compared with the list of what you could find out in any partially urbanized area of the Bay," Alden said.

The Bay Program's recently completed Toxics Loading Inventory -- a first attempt to estimate the amount of toxics reaching the Bay from various sources -- indicated that the two largest toxics contributors were nonpoint sources: storm water runoff and atmospheric deposition.

"The emphasis up until now has been on the easy ones - the point sources - where you can kind of turn it off at the pipe," Alden said. "The problems of pollution in general are probably more nonpoint in nature. As human populations grow, it's pro bably going to get even worse."

Because areas may get worse, Hall said it was also important to identify locations with the ambient toxicity testing approach where there are no impacts - particularly if they provide important habitats for natural resources - so special efforts can be made to protect water quality at those sites.

"We want early warning signals that will give us the ability to do something before the resources become too impacted," Hall said.

The lesson from the Wye, which Alden and Hall say most likely was impacted by nonpoint sources, raises questions about other portions of the Bay, according to the researchers. "If we can go into some of these semi-remote areas like the Wye an d see occasional toxic conditions," said Hall, "how many other Wye River scenarios do we have around that we don't even know about?"