For years, the Department of Energy's Oak Ridge Operations kept a secret about its Y-12 nuclear munitions plant: over the course of 13 years, the plant's operators had managed to lose 2.4 million pounds of mercury into the surrounding earth, air and water.

The startling information was contained in a June, 1977, report by Union Carbide Corp., the DOE's primary contractor at Oak Ridge. It was released last May, under pressure from Tennessee officials and a newspaper's Freedom of Information Act request.

Mercury, converted into organic methylmercury by organisms and sent up through the food chain, can cause nerve, kidney and chromosomal damage in humans. It is potentially lethal.

DOE officials knew, even before it got the classified report, that mercury was in the food chain.

But despite evidence that fish from the rivers and streams around Oak Ridge contain methylmercury at levels greater than the Food and Drug Administration's safety guideline, the DOE failed to share the Union Carbide report with city or state health officials.

The DOE didn't even tell the Tennessee Valley Authority in 1977 when asking for advice on monitoring mercury in fish.

"I'm sure a qualified investigator could search in the stream and could find mercury in the sediment of Poplar Creek," said J.F. Wing, who until recently was Oak Ridge's chief environmental officer. "He could find it in the Clinch River for some little distance. He could find fish and turtles with mercury in them. I don't think it represents a significant exposure . . . . Why get all exercised about this?"

State officials, however, are plenty exercised, even though they agree with Wing that the mercury does not appear to represent a health threat to Oak Ridge's 30,000 inhabitants now.

They said that the episode illustrates the dangers of relying on the DOE to sound the alert about potential environmental hazards.

Most of the mercury was lost in the early years of Y-12's operation, when it used a processing technique that relied heavily on that element. The process was phased out in 1963.

But in the early 1970s mercury started showing up in sediment samples from stream beds on and off the Oak Ridge reservation.

Still, it wasn't until DOE officials found elevated mercury levels in the fish from the Clinch River and the east fork of Poplar Creek, which runs through the city of Oak Ridge, that they took some serious steps to investigate the problem.

"Most people don't eat dirt," Wing said. "The focus was on fish. That was the pathway to man. It did not appear to represent a danger."

In 1977, DOE officials took their preliminary findings to the TVA, which was monitoring mercury in fish. In a memo after the first meeting, Wing reported that TVA officials were "interested . . . but not alarmed in any way."

In the meantime, the memo said, Oak Ridge should keep a lid on the information, lest it "reach the local public in an aberrant perspective and cause undue concern over nonexistent health hazards or relatively insignificant environmental matters."

TVA officials said that Wing never bothered to tell them about the 2.4 million missing pounds of mercury.

"That alarming fact was withheld from us," TVA Chairman S. David Freeman said at a congressional hearing in Oak Ridge last month. "We had a grossly inadequate basis for making a judgment."

A month after it had the Union Carbide report, Oak Ridge sent the TVA an update. The document didn't mention the missing mercury. Instead, it speculated that "a potential source of the elevated mercury levels in Poplar Creek and the Clinch River is the city of Oak Ridge's sewage treatment plant."

In 1981, still unaware of the missing mercury, Oak Ridge city officials began dredging Poplar Creek. The sediment was used to cover new sewer lines in front of a junior high school. Recent samples of that soil show mercury at levels up to 358 parts per million.

The federal government considers .5 parts per million of mercury a hazard when found in fish. However, it has not established what is a dangerous level in soil.

Should the DOE have warned the city? "That requires the application of hindsight," Wing said. "I still don't think it represents a threat to public health and safety."