Evaluating long-term waste leaching

There are many types of waste materials that are under consideration for widespread application in beneficial uses out of the federally regulated landfill environment. Examples of materials and applications include blast furnace slag used as an aggregate in pavements, and coal and municipal solid waste ashes used as road base course materials or non-structural fill material. In these cases, it is important to consider an assessment of long term release and exposure potential as well as changes in the leaching behavior over time. It is fairly well established that the leaching properties of many types of waste materials change dramatically over time due to a number of chemical weathering phenomena. Two aspects related to the evaluation of long-term leaching will be presented. First, the results of research focused on the relationship between long-term mineralogical changes in waste materials and changes in the material's leaching behavior will be presented. Coal fly ash and MSW bottom ash were the materials focused on in this work and mineralogical changes were characterized by FTIR, XRD and SEM. Leaching changes were assessed using a pH- dependent leaching protocol. The second aspect germane to the evaluation of long-term leaching is the hydraulic environment in which these waste materials are used. Field data and model results of a finite element unsaturated flow model analyzing flow through pavement base systems will be presented, which provide a rationale for the choice of appropriate leach testing and the calculation of cumulative release potential from use of a contaminated recycled material in a typical high-volume application.