The Saturated Zone Model Subcommittee of the Environmental Engineering Committee of the EPA Science Advisory Board (SAB) has prepared a report on the Agency's CANSAZ (Combined Analytical-Numerical Saturated Zone) flow and transport model for use in EPACMS (the EPA Composite Model for Surface Impoundments). The Subcommittee examined the appropriateness of the assumptions underlying CANSAZ for use in a nationwide Monte Carlo assessment, as well as the adequacy of the code for simulating conditions beneath surface impoundments. Specifically, the Subcommittee evaluated five topics which were (1) proper mathematical formulation, (2) adequacy of physical assumptions, (3) adequacy of the Monte Carlo approach, (4) adequacy of field testing of the model, and (5) the overall modeling approach. While the mathematical equations and the numerical solution methods are properly formulated and solved, the Subcommittee suggests improvements to the physical assumptions, improvements to the current input data-set documentation and validation, and improvements needed to validate the EPACMS model for use in either a site-specific evaluation or a nationwide assessment. The Subcommittee highly prefers site-specific evaluation, but recommendations are made to improve the assessment should the Agency choose to utilize the nationwide Monte Carlo approach. These suggestions are to incorporate regional variations in the assessment, explicitly ban the disposal of delisted wastes in extreme hydrogeologic settings, and provide a mechanism for all constituents in a waste to be evaluated in a listing or delisting decision.

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