The breach of a 50-year-old coal ash storage pond and subsequent ash spill at the Tennessee Valley Authority’s (TVA’s) Kingston Fossil Plant in Roane County, Tenn., last December was caused by a rare and complex combination of conditions, a six-month independent engineering study has found. These included the existence of an unusual bottom layer of ash and silt, the high water content of the wet ash, the increasing height of ash, and the construction of sloping dikes over the wet ash.
The report, “Root Cause Analysis of TVA Kingston Dredge Pond Failure,” was released to the public Thursday by AECOM. The global geo-technical engineering firm had been contracted by the TVA in January to perform a $3 million detailed root cause analysis of the spill.
An estimated 5.4 million cubic yards of material, mostly hydraulic-filled (wet) ash, were released onto some 300 acres on Dec. 22, 2008, at about 1 a.m., following the sudden failure of the north and central portions of a dredge cell at the 1,700-MW plant’s ash disposal site, the report said. The disturbance triggered a floodwater response wave that ran upstream and downstream of surrounding waterways (the TVA’s Kingston Fossil Plant is located on the Emory River close to the confluence of the Clinch and Tennessee Rivers). The combined mass of flowing ash and water pushed one single-family home off its foundation and damaged several other homes. No injuries were reported.
The report listed four main factors as causes of the failure. Undetected over decades, the ash stream in a particular cell of the ash pond eventually became stagnant and its fine suspended solids precipitated into a thin—less than 6 inches thick—“laminated” layer at the base of the dredge cell. The slippery, viscous layer—what the engineers called “slimes”—was unusual in that they were susceptible to “creep.” This caused the overlying collapsible wet ash to liquefy, leading to the movement of the ash mass.
To make matters worse, the dike walls were already under high stress from being repeatedly raised so that the dredge cell footprint was smaller and the ash landfill higher from the saturation of the stored ash and from the construction of sloping dikes over the wet ash. “In AECOM’s opinion, subsurface conditions at the dredge cells were unusual and rarely found,” the report said. “The consequence of failure in the slimes led to the collapse of the dredge cell and loss of the saturated contents of the ash landfill due to the breach of perimeter Dike C.”
The TVA said on Thursday that it had accepted the firm’s findings and retained the services of Stantec Inc., another engineering firm, to assess all of the TVA’s other ash and coal combustion byproduct impoundments to ensure that they are structurally sound.
Associated cleanup and recovery could cost the TVA between $675 million and $975 million, depending on long-term disposal options. Costs incurred through March 2009 have already added up to $77 million. A report (PDF) issued last week by the publicly owned company’s Office of the Inspector General (OIG) said that though the TVA’s financial performance so far was “adequate,” dealing with the spill would be one of several critical financial challenges the company would face.
Other factors that would cause significant financial losses to the company include a downturn in the economy and subsequent declining power sales, a court ruling on a public nuisance climate change lawsuit brought by the state of North Carolina, and losses on accounts established to fund pensions and asset retirements.
Earlier this month, the OIG released another report lambasting the TVA for its emergency response following the spill, as well as for its responsiveness to the media and treatment of victims. On Thursday, the OIG said it would issue a report in early July assessing AECOM’s root cause analysis and the TVA’s management practices that likely contributed to the coal fly ash spill.
In a related story, the Environmental Protection Agency (EPA) on Monday released a list of 44 coal plant impoundments, dams, or other management units at 26 U.S. locations that hold wet-handled coal combustion residues and have a “high hazard potential rating.” The list includes plant impoundments in 10 states, including 12 sites in North Carolina, seven in Kentucky, and a large storage pond in Pennsylvania. It also includes units in Arizona, Indiana, Illinois, West Virginia, Ohio, and Montana.
The EPA said the high-hazard rating at the 44 sites did not mean that impoundments were structurally weak, but rather that a failure could kill people nearby if they failed. The list had been whittled down from 427 slurried coal combustion residue units that had been identified in response to an EPA information request. The agency said that the 44 units would receive high-priority attention as it continued its assessment of impoundment safety.
Sources: AECOM, TVA, POWERnews, EPA