SummaryIn 1999, the U.S. Department of Energy initiated the development of an assessment tool that enables users to model the movement of contaminants from all waste sites at Hanford through the vadose zone, groundwater, and Columbia River and estimate the impact of contaminants on human health, ecology, and the local cultures and economy. This tool was named the System Assessment Capability (SAC) and is an integrated system of computer models and databases used to assess the impact of waste remaining on the Hanford Site.This manual describes the atmospheric model and computer code for the Atmospheric Transport Module within SAC. The Atmospheric Transport Module, called RATCHET2, calculates the timeintegrated air concentration and surface deposition of airborne contaminants to the soil. The RATCHET2 code is an adaptation of the Regional Atmospheric Transport Code for Hanford Emissions Tracking (RATCHET), as described by Ramsdell et al. (1994). The original RATCHET code was developed to perform the atmospheric transport for the Hanford Environmental Dose Reconstruction Project.Fundamentally, the two sets of codes are identical; no capabilities have been deleted from the original version of RATCHET. Most modifications are generally limited to revision of the run-specification file to streamline the simulation process for SAC. In this regard, many model parameters that were set within the RATCHET run-specification file are now set internally to the RATCHET2 code. New variables have also been added to the RATCHET2 run-specification file to allow for flexibility in implementing the code into SAC. For example, the center of the model domain is now specified in the run-specification file rather than set internally within the RATCHET code.Other notable changes from RATCHET to the RATCHET2 code include:• Meteorological data input has been changed from direct access files to sequential files, and the file format has been modified to accommodate the meteorological data available from the current meteorological data acquisition system at Hanford.• The format of the meteorological station file has been modified so that station locations are specified in latitude and longitude.• The portions of RATCHET that dealt with determining mixing-layer thickness have been changed in RATCHET2. The mixing-layer thickness is determined for each station and then interpolated to nodes following the procedure described for RASCAL Version 3.• For SAC, the RATCHET code has been revised to estimate annual, time-integrated concentrations normalized to the annual release rate for each analyte class (noble gas, iodine, and particle).• The code is run for a single-source with a unit release; SAC scales the results from RATCHET2 to the appropriate emission rate.• Decay calculations have been disabled within RATCHET2; SAC calculates radionuclide decay where appropriate.iv This manual has three major sections: a description of the model, a user's guide, and a programmer's guide. These sections discuss RATCHET2 from three different perspectives. The fi...
