Along Potomac, an Ecological Mystery Surfaces

So the trail goes cold at McCutcheon's.

"We can say where they're coming from" in this instance, Merrifield said. "But that doesn't solve the problem of how they're getting into the Potomac."

Several weeks ago, WTOP radio reporter Mark Segraves made an attention-getting discovery. He said he opened a barrel near Great Falls and found a thick liquid inside. The industrial smell "knocked us back when we opened it," Segraves said.

Lab tests showed that the liquid was found to contain five "volatile organic" compounds, Segraves reported, including some that could cause ill health effects in humans.

After that, Rep. Chris Van Hollen (D-Md.) sent a letter to the Environmental Protection Agency, requesting an investigation of the barrels.

The matter has been referred to the Maryland Department of the Environment. Department spokeswoman Julie Oberg said that emergency response personnel would begin looking this morning for the drums and "finding out the origin, if possible."

Good luck, environmentalists have said.

After months of research, their best guess is that there is no one source for the barrels. Instead, they said, the barrels wash down from houses, farms, and docks across the watershed -- not dumped on purpose, but lost through carelessness and high water.

Many other things get into the river the same way. The Alice Ferguson Foundation's most recent cleanup found eight refrigerators, 23 bicycles and 2,507 sports balls of all kinds. There were 207.8 tons of trash, picked up across 297 cleanup sites.

Mac Thornton got a good view of that kind of flotsam when he turned away from the barrels on Olmsted Island and surveyed everything else that had washed up.

"There's a picnic bench here, water coolers, shoes," he said, talking from the island by cell phone. "Tons and tons of bottles and cans."

Thornton said this chapter of trying to track the barrels' origins might wind up being just one thread in a larger, even more disheartening story about the Potomac's annual tide of trash.

"The barrels are sort of the tip of the iceberg," he said. "They're just the most obvious thing."