The purpose of this memo is to transmit for your review a code for the simulation of flow and transport of chemical constituents through an unsaturated zone. The code accounts for the attenuation of chemical constituents in the unsaturated zone and has potential uses in the Toxicity Characteristic (TC) and the Concentration- Based Listing rulemakings. The code was developed to fully explore the comments received on OSW's TC proposal (51 FP 21648, June 13, 1986). As we have indicated to you earlier, the TC proposal would modify the existing toxicity characteristic by: (1) introducing a new leach procedure -- Toxicity Characteristic Leaching Procedure (TCLP); (2) adding 38 organic compounds to the list of TC constituents; and (3) using a ground-water fate and transport model to establish dilution/attenuation factors for the specific organic compounds. The ground-water fate and transport model used in the TC proposal was reviewed by SAB as part of the land disposal restrictions ground-water screening procedure in 1985.