Updates of the computer code associated with "Computer model of two-dimensional solute transport and dispersion in ground water" UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY RESTON, VA. 22092 In Reply Refer To: March 4, 1986 WGS-Mail Stop 411 OFFICE OF GROUND WATER TECHNICAL MEMORANDUM NO. 86.02 Subject: GROUND WATER--Updates of the computer code associated with "Computer model of two-dimensional solute transport and dispersion in ground water" by L. F. Konikow and J. D. Bredehoeft (1978): U. S. Geological Survey Techniques of Water-Resources Investigat- ions (TWRI), Book 7, Chapter C2 Attached are "update notes" by Leonard F. Konikow that document revisions made and verified since the previous update of October 12, 1983, to the subject solute-transport model. Following are brief descriptions of the updates: June 10, 1985--Changes to the computer code are given that allow the user to specify 16 particles per node. This option can lead to increased numerical accuracy in some cases. The original program allowed for the placement of 4, 5, 8, or 9 particles per node. July 26, 1985--Changes to the code correct a programming error in the routine to calculate changes in concentration caused by dispersion. For most problems, the difference in computed results will be negligible. However, if the problem being solved involves dispersive fluxes large relative to the convective (advective) fluxes, then the original program could have generated negative mass-balance errors of up to several percent. July 31, 1985--The solute-transport equations presented in the TWRI include two terms that should have canceled out during the derivation and expansion of the basic governing equation. The five equations that are affected are given in their correct form. The corresponding changes to the computer code were given in the August 26, 1981, update. August 2, 1985--Changes to the computer code allow the model to simulate a first-order irreversible-rate reaction and (or) equilibrium-controlled sorption-desorption for a linear isotherm. August 8, 1985--Changes to the computer code allow the user to solve the transport equation within a secondary subgrid of the primary finite-difference grid for the flow model. This can result in a great savings in computer calculation time and storage requirements for problems in which the hydraulic gradients within the area of interest for transport are influenced by hydraulic stresses and (or) boundary conditions outside of the area in which solute transport is occurring. August 12, 1985--Changes to the computer code allow the user to specify only one particle per node. This will significantly reduce the computer time required to solve a given problem, but may adversely affect the numerical accuracy of the solution to the solute-transport equation. Hence, this option is recommended only for test runs in which high numerical accuracy is not needed, such as when the main objectives of the run are to check input data, boundary conditions, or overall problem formulation. Collectively, these changes improve the accuracy, efficiency, and usability of the model. Revisions can be implemented by following the instructions in the updates. Mr. Konikow advises that the modified computer code will be available through SOFTEX (entry ID=VAMWM00007) for U.S. Geological Survey users. (s) Charles A. Appel for Eugene P. Patten Chief, Office of Ground Water Attachments WRD Distribution: A, B, S, FO, PO