A massive coal ash spill is threatening aquatic life in Tennessee’s Emory River and that dredging to clean up the disaster could make things worse, environmental experts said Monday .

The North Carolina-based Appalachian Voices environmental group released a report estimating the Dec. 22 release of 5.4 million cubic yards of ash from the Kington Fossil Plant contained more than 3,800 tons of toxic metals, including arsenic, barium, cadmium and lead.

The group was most concerned about high levels of selenium in fish caught in the Emory River around the plant 18 days after the disaster.

The selenium levels were higher than would have been expected 30 days after the disaster, suggesting the fish were accumulating the toxic metal in their tissue long before the spill, they said.

Appalachian State University biologist Shea Tuberty, the study’s lead author, said fish populations in the Emory River are now at a “tipping point” for survival, their ability to reproduce in jeopardy.

“The ecosystem cannot absorb any more selenium from the ash spill,” said co-author Dennis Lemly, a biologist at Wake Forest University. “If concentrations continue to increase above where they are now, we would expect to see a substantial impact on fish reproduction.”

They said it was too soon to predict how high the selenium levels might go, but did say the Tennessee Valley Authority was allowed under its environmental permits to release the material from the Kingston plant’s ash operation before the spill.

The Tennessee Department of Environment and Conservation said Appalachian State’s selenium sampling results were “significantly higher” than samples taken by the state on the same day less than a 10th of a mile away.

Spokeswoman Tisha Calabrese-Benton said the highest selenium number the department has found is one-10th of the number Appalachian State reported, although it has found exceedances since the spill.

She said the state has followed U.S. Environmental Protection Agency protocols. EPA, which has taken oversight of the Kingston cleanup, also is evaluating the effect selenium, she said.

A public health advisory limiting consumption of fish from the Emory was in effect before the disaster.

Lemly and Tuberty said the danger now isn’t to people, but to everything from the bottom of the food chain up to birds that depend on the fish for their daily diet.

TVA has begun limited dredging in its $1 billion cleanup to remove ash from the river.

“They need to take another approach because dredging, as they are, is just about the best way … to release as much selenium as possible,” said Bryce Payne, an independent soil scientist based in Pennsylvania familiar with the study.

TVA spokesman Gil Francis said the federal utility has “a number of controls as we dredge to minimize stirring up materials and we are monitoring everything upstream and downstream. (If) we see any changes in any of that, we will stop what we are doing and reassess.”