CCR Soil Liner Evaluation

CCR Soil Liner Evaluation 

Multiple coal combustion residuals (CCR) impoundments in southeast Michigan are documented to be constructed within thick, laterally contiguous glacially compacted natural clay-rich soils. It is likely that the natural soils are providing an equivalent, or a better level of protection from potential migration of contaminants than the composite liner defined in 257.70(b). TRC was retained by two utility clients to evaluate existing site data to assess whether the natural soils at six sites in southeast Michigan are performing equivalently. 

 TRC calculated leakage rates and compared these to the anticipated leakage rates for a single composite liner system. Data showed that anticipated leakage rates between the natural soil barriers and the single composite liners are comparable. 

 Data also showed that site-specific factors such as the thickness of the natural soil barrier, the hydraulic conductivity of the natural soil barrier, and the vertical gradient between the CCR Unit and the underlying aquifer contribute more significantly to the protectiveness of natural soil barriers when compared to single composite liner systems. 

 The travel time results from this study show times that exceed the USEPA’s vulnerability criterion demonstrating that site-specific evaluations can demonstrate protectiveness. 

 TRC concluded that the natural soils below these sites perform better than composite liners. The clients were able to utilize the study to open a dialog with USEPA and other stakeholders for the use of alternative natural soil barriers at sites where equivalent environmental protection is present. These factors are intended to be utilized in development of Federal CCR Rule amendments that allow for the use of natural soil liners at sites where equivalent protectiveness is present relative to the composite liners required by the CCR Rule. 

Client

Confidential Utility Clients

Project Location

Michigan