Groundwater impacts from coal ash in highways

Coal combustion products (CCPs), traditionally treated as wastes, can be successfully used in road construction in a variety of ways. When CCPs are used in roadway construction, however, leaching of trace elements into groundwater can be a concern. The current paper describes a model that can be used to evaluate the impacts of leaching on groundwater for typical roadways in the USA. Three analytical solutions to the advection–dispersion–reaction equation are combined to develop a method for assessing impacts to groundwater caused by leaching of trace elements from CCPs used in highway subgrade, subbase and base layers. The solutions were combined into a computer program (WiscLEACH) with a Windows® graphical user interface. The analytical method was compared to predictions made with HYDRUS-2D, a software package for simulating flow and transport in variably saturated media. Predictions made with the analytical method were also compared to field data from a highway test section where the subgrade was stabilised with coal fly ash. Parametric analyses made with the method indicate that maximum concentrations in groundwater occur near the groundwater table (GWT) and the centreline of the pavement. Peak groundwater concentrations decrease as depth to groundwater increases, the thickness of the byproducts layer decreases, the seepage velocity in the vadose zone decreases or the seepage velocity in groundwater increases. Variables having the greatest influence on maximum concentrations in groundwater are depth to the GWT, thickness of the CCP layer, hydraulic conductivity of the least conductive layer in the vadose zone, hydraulic conductivity of the aquifer material and the initial concentration in the CCP layer.