The effort to rid Dover lake of contaminants the largest to date in the country.

Dover’s Mirror Lake is a contradiction unto itself: attractive and inviting on the surface, but potentially noxious beneath.

Over the years, mankind has laid a mixture of toxic chemicals onto the lakebed, polluting it to the point where state officials have warned it is unsafe to eat fish caught in the lake’s waters.

Meter-long core samples of sediments under the lake traced many of the pollutants − mercury, dioxins, polychlorinated biphenyls and numerous others − as far back as the early 20th century.

State environmental officials had recognized the problem years ago, but until recently there seemed to be no economically feasible way to deal with it.

Beginning Nov. 4, the state Department of Natural Resources and Environmental Control started coating the lakebed with pellets of activated charcoal, a substance that not only will contain the decades of pollution, but also may make the lake once again safe for fishing.

The project was the idea of Dr. Richard Greene of DNREC’s Division of Watershed Stewardship. He’d been searching for a way of mitigating the pollution when he came across a procedure developed by Dr. Upal Ghosh of the University of Maryland, Baltimore County.

Ghosh, who researches the effects of toxic pollutants on the environment, had developed a way of using the charcoal, the same kind found in home water filters, to effectively bottle up the contamination and keep it out of the ecosystem.

Ghosh’s idea had been tried successfully before, but only to a small degree, Greene said. Ghosh did some reconnaissance of Mirror Lake and agreed the procedure could work on a larger scale.

“We went to our supervisors, who went to their supervisors, up to the secretary of DNREC,” Greene said. “They all loved the idea.”

Greene and his staff secured almost $1 million in material and contractor funding through the state Hazardous Substance Cleanup Act, which levies a tax on petroleum projects to be used for waste remediation projects. Costs are kept down with volunteer help whenever possible.

Construction crews laid the groundwork for the project in mid-October, setting up orange netting around the lake and a yellow turbidity curtain across the water. The curtain reaches to the lake bottom approximately 5 to 6 feet below the surface, and contains any silt and contaminants stirred up by the work.

The project not only covers the 3.5 acres of the lake and its immediate environs, but stretches down the St. Jones River to just past the Court Street Bridge.

The heart of the project is the roughly 88 tons of activated charcoal that for the time being are contained in huge bags along the lake shore. Approximately the size of a pencil eraser, the jet black charcoal pellets are literally blown onto the lakebed using air pressure.

Once the bottom is coated with the charcoal, nature will do the rest, said John G. Cargill IV, of DNREC’s Site Investigation and Restoration Branch.

“We have bioactivity, organisms that live in the bottom of the lake,” he said. “They’ll mix up the sediment and the carbon and that will help create a barrier.”

That carbon barrier will keep the contaminants contained below the lakebed, Cargill said, and over time new sediment brought in by tidal action and other forces will cover it with cleaner deposits. Eventually the contamination leaking into the food chain will decrease, making the water cleaner and the lake’s fish safe to eat.

Tests have shown the charcoal infusion can contain up to 90 percent of PCBs and up to 84 percent of mercury contamination. The result is that a process that could have taken between four and seven decades, if allowed to proceed naturally, could mean a safer Mirror Lake within the decade.

In addition to work lining the lakebed, work crews are placing stone and 16-inch diameter logs made of machined coconut fibers to stabilize the shoreline. The lake’s famously unattractive sandbar, which appears at times of low tide, will be transformed into an intertidal wetland, populated by plants native to Delaware.

The project also will leave the lake’s crop of spatterdock pond lilies undisturbed; although they now are dying off, the plants should return when the weather warms up next spring.

Both Cargill and Greene are enthusiastic about the project and its expected results.

“This is real applied research,” Greene said. “This is the largest project of its kind in the country to date.

“This was born out of a desire to solve a contamination problem. This puts us on the map.”