A numerical leaching model is described which considers interactions among four components. One is calcium, which is associated with the alkalinity of the treated waste. One is the hydrogen ion and one is lead, which represents a contaminant that can precipitate as a hydroxide. The other is acetate, which is associated with the acid used in the leaching solution. A chemical equilibrium sub-model calculates concentrations of eight species as a function of the total concentration of the four components. A modified Crank-Nicholson algorithm is used to solve the dynamic material balances for each component in the solid. The results of the numerical model agree well with predictions of a simple analytical model for infinite bath. The model is applied to simulate leaching conditions such as found during an acidic leaching test such as the TCLP. The simulated behavior of calcium and pH is simple, but lead is found to initially leach from the solid and then to leach back into the solid. This behavior is described in terms of the interaction between lead chemistry and calcium chemistry. The model shows that lower concentrations of contaminants in the TCLP test can be achieved in some cases by increasing the leachability characteristics of the treated solid. Therefore, the TCLP test is not always a good method for measuring leachability of contaminants from solidified wastes.