Hydraulic Conductivity of Ash Mixtures and Metal Release upon Leaching1

~. Generation of electricity by coal-f~ed power plants produces large quantities of bottom ash and fly ash. New power plants commonly use fluidized bed combustion (FBC) boilers, which create ashes with high neutralization potential (NP). These ashes, due to their alkaline nature, are often used in surface mine reclamation projects to neutralize acidity and reduce hydraulic conductivity of disturbed overburden materials. Conventional fly ashes from older power plants exhibit a range of pH and NP, with some ashes having neutral or acidic pH and low NP values. This study measured the hydraulic conductivity and Ieachate quality of various mixtures of three fly ashes: 1) a fly ash with low NP (LNP ash) and two FBC ashes (FBC1 and FBC2). Results indicate that water will flow through these materials at a moderate rate (about 104 cm/see), but compaction with machinery and with pressure from overlying fill materials may decrease these hydraulic conductivity rates by an order of magnitude. Several trace elements were found at high levels in the leachates of some ash mixtures after sulfuric acid leaching. LNP fly ash Ieachates had high arsenic and selenium concentrations. Amending LNP conventional ash with FBC ashes reduced arsenic concentrations but not selenium contents. Lead, cadmium, and barium concentrations in fly ash leachates were not high enough to cause water pollution problems. Manganese was released from LNP ash at a constant level, FBC I ash did not release manganese, while FBC2 ash released manganese only after the NP had been exhausted by 60 pore volumes of sulfuric acid leaching.