Numerical Modeling to Evaluate Hydrological Dynamics and Risk Assessment of Bottom Ash Ponds at a Former Coal-Fired Electric Power Plant

摘要

Leaching and transport of ash-related inorganic constituents from bottom ash ponds(BAPs)at a former coal-fired electric power plant(Site)adjacent to a river system in southwest Virginia were modeled to understand subsurface flow dynamics at the Site and to identify an effective management strategy for the BAPs under the recent Coal Combustion Residuals(CCR)legislation.Modeling was conducted assuming baseline(uncapped)and closure(capped)conditions.Subsequently,predicted concentrations of site-related constituents in nearby surface water bodies were evaluated to assess the potential for ecological and human health effects under both conditions.The numerical model was used to examine Site hydrological dynamics and groundwater interaction with nearby surface water bodies.Pathline analyses were performed using MODPATH to evaluate potential surface water receptors under baseline and closure conditions.Dilution attenuation factors(DAFs)were calculated and subsequently used in combination with site-related constituent concentration data as well as ambient upgradient background surface water concentrations to predict mixed surface water concentrations in the water bodies adjacent to the BAPs,which were then used to assess potential risk to human and ecological receptors.Results from the risk analyses indicated that predicted surface water concentrations for ash-related constituents were all below applicable risk based screening levels,indicating that risk to human and ecological health is not expected under either capped or uncapped conditions.This approach of numerical modeling followed by risk evaluation has been found to be an effective management strategy for BAP closure under the CCR legislation.