Waste to energy ash monofill mining: An environmental characterization of recovered material.

Samples of combined bottom and fly ash produced at a U.S. waste-to-energy facility were collected from an ash monofill. These samples represented ash monofilled between 1991 and 2008. The ash samples were characterized for total element content and leachability; trends in these parameters were evaluated as a function of sample depth and ash age. Comparison to risk thresholds was used to assess the relative magnitude of the total and leachable mass of elements in the monofilled ash. Natural carbonation was found to have occurred in the monofilled ash, reducing the pH and leachability of Al and Pb. Sb was the element with the highest leachable concentration when compared to risk thresholds, driven primarily by the pH of the ash (9.8). The release of Mo, Sr, Ba, Na and K (all readily soluble elements in ash) was higher (48-122%) when comparing the samples taken from the 0 to 1.5m bore to the 6.1-7.62m bore; total concentration analysis also demonstrated that more of these elements were present in the deeper samples (25-53%). These data support the hypothesis that as infiltrating rainwater moves through an ash monofill leached concentrations are depleted from the upper layers of the ash first.