PUBLICATIONS--Release of the report "Modification of a method-of-characteristics solute-transport model In Reply Refer To: June 6, 198 WGS-Mail Stop 411 OFFICE OF GROUND WATER TECHNICAL MEMORANDUM NO. 89.02 Subject: PUBLICATIONS--Release of the report "Modification of a method-of-characteristics solute-transport model to incorporate decay and equilibrium-controlled sorption or ion exchange" by Daniel J. Goode and Leonard F. Konikow This subject report has recently been released as Water-Resources Investigations Report 89-4030. The report documents modifications made to the two-dimensional ground-water solute-transport model of Konikow and Bredehoeft (1978, TWRI Book 7, Chapter C2). These modifications enable simulation of three types of chemical reactions: (1) first-order irreversible rate reaction, such as radioactive decay (2) reversible equilibrium-controlled sorption with linear, Freundlich, or Langmuir isotherms and (3) reversible equilibrium-controlled ion exchange for monovalent or divalent ions. These processes are incorporated in the general solution scheme that uses method-of-characteristics with particle tracking for advection and finite-difference methods for dispersion. The first type of reaction is accounted for by an exponential decay term applied directly to the particle concentration. The second and third types of reactions are incorporated through a retardation factor, which is a function of concentration for nonlinear cases. The model is evaluated and verified by comparison with analytical solutions for linear sorption and decay, and by comparison with other numerical solutions for nonlinear sorption and ion exchange. The report is being sent under separate cover to District, State, and some Subdistrict Offices of the Water Resources Division. The computer program is available on the Software Exchange System (SOFTEX), from the National Water Information System (NWIS) Program Office, or from the authors. Eugene P. Patten Chief, Office of Ground Water WRD Distribution: A, B, S, FO, PO