Solute Leaching from Fly Ash Amended Soil Under Varying Degrees of Saturation

Most of the fly ash produced in the US is stockpiled or disposed of in settling ponds where solute leaching can pose an environmental and health concern. Therefore, a series of saturated and unsaturated column experiments were conducted to evaluate the impact of saturation and pore solution residence time (i.e., equilibration time) on the leaching of solutes from fly ash when incorporated within surface horizon material from a loamy sand soil. Repacked soil columns were leached at various moisture contents using an Unsaturated Flow Apparatus (UFA), a modified centrifuge for conducting steady-state leaching experiments. Additional column experiments were conducted under saturated conditions to isolate the effects of residence time from that of water content. Addition of 10 percent fly ash (by weight) significantly increased water-holding capacity in the loamy sand. Leachate concentration of As and Se increased as residence time increased. Most Se leaching took place in the first few pore volumes, indicating that weathering the fly ash may help alleviate some of its phytotoxic effects. Since saturation was related to residence time in these experiments, the effect of water content could not be isolated, though there appeared to be a delay in the “leaching front” as the soil became desaturated. However, comparisons of unsaturated leaching studies to saturated leaching studies are difficult due to the variation in water content under unsaturated conditions.