FORENSIC HYDROGEOLOGY: EVALUATING A KARST CRITICAL ZONE ENORMOUSLY ALTERED BY COAL COMBUSTION RESIDUALS (Invited Presentation)

Coal combustion residuals (CCR) are one of the country’s largest sources of industrial waste with 1.2x10¹¹ kg generated in 2014. Leachate can contain hazardous substances including arsenic, chromium, and selenium. Unlined CCR storage in karst systems can dramatically alter aquifer structure and function and can allow transport of CCR leachate into groundwater and surface water. The US EPA’s 2014 CCR Rule requires new evaluation of CCR management and monitoring of groundwater.

This research evaluates the hydrogeology of Odom’s Bend, Tennessee, where about 140 ha of CCR have been deposited over 45 years from the Tennessee Valley Authority (TVA) Gallatin Steam Plant into the unlined valley of Sinking Creek, a highly developed karst flow system. While the task would have been straightforward prior to anthropogenic disturbance, flooding of the aquifer and adjacent Cumberland River by Old Hickory Lake as well as the presence of the ash itself have required a creative strategy. Potentiometric surface maps show directions of groundwater flow, and pre-CCR topographic maps with sinkholes, blind valleys and sinking streams reflect the geometry of the underlying karst flow system. Decades-old TVA memos provide key data.

After construction of an outfall dam in Sinking Creek Valley and filling began in 1970, the ash pond level rose about to about 1.3 m above the river, but the pond then dropped by about 60 cm. A “boil” in the river described in a TVA memo occurred around the same time, presumably from a major spring now under the lake, but unfortunately TVA has been unable to locate any records of its location. The pond level then stayed below the outfall level for the next 8.6 years, so that essentially all CCR input left via karst leakage and evapotranspiration. Memos describing a CCR inflow rate of 22,700 l/min suggest that more than 100 billion liters of ash related liquids were lost, most through the karst system into the Cumberland River. In 1978 repair of some sinkholes raised the water to the outfall level, though CCR may have still been leaking through the karst system. While access points for traditional dye tracing are now covered by the lake and ash, CCR constituents including boron can serve as groundwater tracers. These data are informing decisions for the most appropriate closure method and can be shared for other similar karst sites.