Duke spokesman Dave Scanzoni declined to comment Thursday about the documents or whether the company implemented the monitoring recommended by its engineers.
The 28-year-old engineering report was among documents subpoenaed last week from the N.C. Utilities Commission by the U.S. Attorney in Raleigh as part of an ongoing grand jury investigation into the spill. The Associated Press filed a public records request with the state agency, which was responsible for regulating Duke's 33 coal ash pits in North Carolina up until 2010.
Starting in the mid-1970s, the commission mandated Duke to submit independent engineering studies every five years affirming the safety of the huge earthen dikes holding back millions of tons of ash and contaminated water from nearby rivers and lakes.
Coal ash is the byproduct left behind when coal is burned to generate energy. It contains numerous toxic substances, including arsenic, selenium, chromium, beryllium, thallium, mercury, cadmium and lead. Some of the chemicals in wet ash can also corrode metal.
Duke hired Law Engineering Testing Co. to perform the required inspections at the Dan River Steam Station in Eden. In its 1986 report, the Charlotte firm noted that part of the pipe was made of metal.
"Part of this culvert is constructed of corrugated metal pipe which would be expected to have less longevity of satisfactory service than the reinforced concrete pipes," the report states.
Among the eight recommendations made at the end of the 45-page report, the engineers specifically suggest Duke check on the pipe under the coal ash dump at least every six months. The report said the inside of the pipes should be checked for leaks anytime there was a significant difference between how much was flowing in and how much was flowing out.
In the days after the Feb. 2 spill, Duke issued public statements expressing surprise that the pipe wasn't made of corrosion-resistant concrete.
"Originally, the media reported and we reported that this was a concrete pipe, the 48-inch one, because what we could see is right here at the river," Duke's lead lobbyist, George Everett, told state lawmakers at a Feb. 17 oversight hearing into the spill. "The rest of this pipe, we discovered, as we worked from the other end, is a metal pipe — asphalt-coated metal corrugated pipe. That's the piece ... that broke."
On Monday, North Carolina regulators cited Duke for leaking pipes under coal ash dumps at five of its plants, some of them constructed from old corrugated metal. Duke said it is working to fix the leaks.
The 1986 study from the Dan River plant also makes numerous references to a prior 1981 review that found problems at the Eden coal ash dump. It is not clear if that earlier report also warned about the metal pipe.
Sam Watson, spokesman for the utility commission, says there is no record that Duke ever submitted that 1981 engineering report to state regulators, as required.
Scanzoni, the Duke spokesman, said Thursday the company still has a copy, though he declined to share it with The Associated Press.