A coalition of environmental groups is calling on the EPA and the Bay Program to severely cut back on the discharges of some of the most toxic chemicals entering the Chesapeake and its tributaries.

In a letter to EPA Administrator Carol Browner, the 12 groups called for a phase out in the Bay watershed of “mixing zones” — areas commonly used to dilute pollution at the end of discharge pipes — for any contaminant that bioaccumulates in the food chain or persists in the environment.

The groups called such a phase out “a critical step in restoring and protecting the Chesapeake Bay.” Their letter follows a proposal from the EPA earlier this fall to phase out mixing zones for six highly toxic chemicals in the Great Lakes.

Mixing zones are an area beyond the end of a discharge pipe where pollutants are allowed to exceed water quality criteria. By mixing with water in the river, lake or stream, the goal of the mixing zone is to allow the contaminant to become diluted enough that it will meet water quality standards outside the mixing zone.

“We’re talking about the worst of the chemicals being ‘treated’ by dilution,” said Kim Coble, a senior scientist with the Chesapeake Bay Foundation, one of the groups signing the letter. “But they end up in the sediment, and they’re there for a very long time, and they continue to wreak havoc.”

Even in low concentrations, some chemicals can accumulate in the soils over time, leaving a legacy of contaminated sediments that linger for years. Also, small amounts of some chemicals can “bioaccumulate” through the food chain until they poison predator species that eat organisms which have consumed small amounts of contaminants.

By eliminating mixing zones for those contaminants, environmentalists say the total discharges would have to be reduced. Those reductions could be made by reducing or eliminating their use; often the chemicals are released in such small concentrations they can’t be measured in the water, even though they build up in the sediment or accumulate up the food chain.

The proposal put forth by the environmental groups is far more sweeping than the 10-year mixing zone phase out that the EPA has called for in the Great Lakes. The Great Lakes action affected only six chemicals, four of which are already banned. Another, mercury, is primarily deposited from air pollution, not discharged into the water. The EPA has estimated that the total cost to dischargers for its Great Lakes proposal would be between $12 million and $35 million a year.

Still, Browner said the efforts could “serve as a model for protecting all of America’s waterways.” She went on to challenge “governors from every state to consider taking the same steps that we are taking to protect and restore the Great Lakes.”

Historically, Bay Program efforts regarding toxics have focused on research and encouraging voluntary reductions rather than actions that would expand regulations. The Bay environmental groups said it was time for the Bay Program to go further.

“If we are serious about being leaders and we’re serious about taking care of the Bay, we should step up to the plate,” Coble said. “I don’t see any reason to wait for a nationwide movement.”

At a recent meeting of the Bay Program’s Principals Staff Committee, officials indicated they did not want the issue addressed in the new “Chesapeake 2000” agreement to be signed next year. That draft document calls for eventually achieving a zero discharge of toxics, but does not set a deadline for the goal.

But the committee — the Bay Program’s second-highest policy-making panel — left open the possibility that the mixing zone issue might be addressed when the Bay Program adopts a new toxics control strategy next year. Ending mixing zones was discussed earlier this year during a series of “stakeholder” roundtable meetings for the new strategy, but no action was recommended until the issue was further reviewed.

“They wanted to have time to digest it,” said Kelly Eisenman, toxics coordinator for the EPA’s Bay Program Office. “But there is no guarantee that it will end up as a strategy.”

No one knows exactly how many contaminants would be affected by such a change. While few chemicals discharged in the Bay watershed are known to bioaccumulate, there are many — especially metals — which persist for decades or longer before they are buried or flushed out of the system.

It’s also unclear how many of the hundreds of dischargers around the Bay would be affected by such an action. For some industries, it could pose a problem.

“It would cause us a great deal of consternation,” said David Johnson, deputy secretary of the Virginia Department of Environmental Quality.

Specifically, Johnson cited the use of tributyltin in Virginia shipyards. The chemical is an additive in paint used on ship hulls to kill barnacles and other organisms that build up on the bottom of ships. TBT is so toxic in small amounts that it could never be used without mixing zones, Johnson said.

Virginia officials have said in the past that ending the use of TBT would put their ship yards at a competitive disadvantage; the state is currently funding research on ways to use TBT without causing a problem.

The environmental groups cited Bay Program reports that show large amounts of chemicals — especially metals — are washing down rivers and into tidal tributaries. Materials accumulate in the sediment in those areas rather than being flushed through the Bay, allowing high levels to build up over time.

They cited Bay Program reports that say 60 percent of the copper and 90 percent of the mercury entering the tributaries is trapped in the sediments where it can harm bottom-dwelling organisms and those that eat them. Further, they said, Bay Program data shows that 10 large areas recently classified as “Areas of Emphasis” — areas with the potential to have adverse impacts — suffer from contamination by persistent metals.

By allowing mixing zones, “what we’re doing is allowing additional loadings of these chemicals that have these long-term or downstream impacts,” said Jackie Savitz, executive director of the Coast Alliance, one of the groups signing the letter. “Those are the kind of chemicals we’re trying to get at here.”

The letter does not call for a set time for the mixing zones to be abolished, but Coble said the groups have discussed a two-step process in which mixing zones were ended in the most contaminated “regions of concern” by 2005, and phased out in the rest of the watershed by 2010.

“What we’re hoping is that the Bay Program will basically incorporate this into its new toxics strategy,” Savitz said. “If they are not willing to start moving forward on this, I don’t know why we are bothering to invest in this effort.”

Besides CBF and the Coast Alliance, the groups signing the letter included Clean Water Action, the Clean Up Coalition, the Maryland Preservation Coalition, the Coastal Creeks Coalition, the Maryland Conservation Council Inc., the Maryland chapter of the Sierra Club, Haztrak, the Maryland Public Interest Research Group, and the Maryland and Virginia chapters of the National Audubon Society.