MT3D-USGS version 1: A U.S. Geological Survey release of MT3DMS updated with new and expanded transport capabilities for use with MODFLOW

This report describes an updated version of the MT3DMS computer program, hereafter referred to as "MT3D-USGS."MT3D-USGS can be used with many of the new features recently developed for MODFLOW.The overarching goals for developing MT3D-USGS are two fold-to keep pace with advancements in MODFLOW and to provide users of the MT3DMS solute transport simulator with expanded functionality for tackling increasingly complex water-quality issues.MT3D-USGS has been tested with applications distributed with MT3DMS (obtained from http://hydro.geo.ua.edu/mt3d/), as well as with new applications described herein.As the number of MODFLOW packages has grown, so too has the possibility for various inter-package flow exchanges (for example, water discharging to land surface can be amended to streamflow).Benchmark simulations have been designed to ensure that flow exchanges and associated solute transport work properly.However, as MT3D-USGS is applied to transport problems, previously undetected errors may be identified.In such instances, users are requested to send notifications of suspected errors either in this documentation or the model it describes to the contact listed on the MT3D-USGS webpage.Users are encouraged to check for model updates on the MT3D-USGS webpage (http://dx.doi.org/10.5066/F75T3HKD).MT3D-USGS is not a replacement for MT3DMS.Separate updates to both codes will likely continue in the future.Therefore, MT3D users should check appropriate webpages for updates and releases of MT3D-USGS and MT3DMS separately.Hill Plateau Remediation Company (CHPRC); and on behalf of the Zone 7 Water Agency, California.The kinetic reaction capability was developed under a contract to the U.S. Environmental Protection Agency (EPA), led by Dr. Jim Weaver.MT3D, in all of its variations, would not exist without Professor Chunmiao Zheng.MT3D was first developed by Professor Zheng in 1990 while working at S.S. Papadopulos & Associates, Inc. (SSP&A) with partial support from the U.S. Environmental Protection Agency (USEPA).From the outset, MT3D was released as a pubic domain code, although commercial versions with enhanced capabilities were also developed.MT3DMS-the second generation of MT3D-was developed by Professor Zheng with funding from the U