11.1 Introduction

The EPA's Composite Model for leachate Migration and Transformation Products (EPACMTP) provides estimates of potential human exposure to hazardous chemicals leaching from land disposal facilities [64]. EPACMTP simulates the subsurface fate and transport of contaminants released from land disposal sites, and predicts the associated groundwater exposure in a domestic drinking water receptor well. This model is an improvement over the EPA's Composite Model for Landfills (EPACML) [62]. EPACML accounts for the first-order decay and sorption of chemicals, but disregards the formation and transport of transformation products. In addition, EPACML can describe only uniform, unidirectional groundwater flow. On the other hand, EPACMTP can take into consideration: (i) chain decay reactions and transport of daughter and grand-daughter products, (ii) effects of water-table mounding on groundwater flow and contaminant migration, (iii) finite source as well as continuous source scenarios, and (iv) metals transport.

EPACMTP consists of two modules: an unsaturated zone module called Finite Element and semi-analytical Contaminant Transport in the Unsaturated Zone (FECTUZ), and a saturated zone module called Combined Analytical-Numerical SAturated Zone in 3-Dimensions (CANSAZ-3D). FECTUZ is a one-dimensional model that simulates vertically downward steady-state flow and contaminant transport through the unsaturated zone above an unconfined aquifer. CANSAZ-3D simulates 3-D steady-state groundwater flow and transient or steady state contaminant transport. EPACMTP currently uses a simplified 2-D version of the CANSAZ-3D, and the modules are optimized for computational efficiency.

The following sections describe in detail the formulation and implementation of FECTUZ and CANSAZ-3D. The material presented here is summarized from the EPACMTP background document [64], which contains more detailed descriptions of the modules and the methods described here.