A fly-over video of our 3D model of the ash pond at Duke Energy's Dan River Steam Plant. The video shows the ash pond after a storm water drain pipe under it collapsed, releasing tens of millions of gallons of toxic water and ash into the Dan River. The model shown in the video was created u…

RALEIGH, N.C. • A team of academic researchers equipped with a drone estimates that up to 35 million gallons of coal ash and contaminated wastewater spilled into the Dan River earlier this month.

Researchers at Wake Forest University's Center for Energy, Environment and Sustainability released results Tuesday from a study using photos collected by the drone that flew over the Duke Energy coal ash dump that ruptured Feb. 2 in Eden. The scientists used images captured by the drone to create a digital three-dimensional model of the pit, allowing them to calculate the volume of toxic ash that flowed out when a pipe collapsed.

The reported amount equals about 53 Olympic-sized swimming pools, making it the third-largest coal ash spill in the nation's history.

"We think we have a precise estimate of the ash and water slurry that flowed out of the pond based on our methodology," said Miles Silman, a biology professor at Wake involved with the project. "The uncertainty comes from the amount of water that continued to drain from the ash, which is a question for hydrologists. Our work provides an independent estimate of the spill."

The researchers' calculations put the size of the spill within the broad range that Duke's engineers had initially estimated.