The Toxicity Characteristic Leaching Procedure (TCLP) tests whether a waste is hazardous or nonhazardous; most solid residuals pass the TCLP. However, the TCLP poorly simulates the alkaline pH, low redox potential, biological activity, long retention time,andorganic composition of mature landfills. These same conditions are likely to favor mobilization of arsenic from metal oxide sorbents.