STOMP is a computer model, designed to be a general-purpose tool for simulating subsurface flow and transport, that complements other analytical capabilities developed by Pacific Northwest National Laboratory's Hydrology Group. The simulator was specifically designed to provide scientists and engineers from varied disciplines with multidimensional analysis capabilities for modeling subsurface flow and transport phenomena. STOMP's target capabilities were guided by proposed or applied remediation activities at sites contaminated with volatile organic compounds and/or radioactive material. Developed with the support of the U.S. Department of Energy, Office of Environmental Restoration and Waste Management, the simulator's modeling capabilities address a variety of subsurface environments, including nonisothermal conditions, fractured media, multiple-phase systems, nonwetting fluid entrapment, soil freezing conditions, nonaqueous phase liquids, first-order chemical reactions, radioactive decay, solute transport, dense brines, nonequilibrium dissolution, and surfactant-enhanced dissolution and mobilization of organics.

The STOMP simulator solves the partial-differential equations that describe the conservation of mass or energy quantities by employing integrated-volume finite-difference discretization to the physical domain and backward Euler discretization to the time domain. The resulting equations are nonlinear coupled algebraic equations, which are solved using Newton-Raphson iteration. The simulator has been written with a variable source code that allows the user to choose the solved governing equations (e.g., water mass, air mass, dissolved-oil mass, oil mass, salt mass, thermal energy). Depending on the chosen operational mode, the governing transport equations will be written over one to four phases (e.g., aqueous phase, gas phase, (nonaqueous phase liquid) NAPL phase, ice phase, solid phase). Solute transport, radioactive decay, and first-order chemical reactions are solved using a direct solution technique (e.g., Patankar's power-law formulation, (total variation diminishing) TVD scheme) following the solution of the coupled flow equations. Input is directed through semi-formatted text files and output is available through a variety of user-directed formats. The simulator recognizes a number of boundary condition types and allows their specification both internally and externally to the computational domain.

Written in ANSI FORTRAN 77 and currently being converted to Fortran 90, the simulator has been executed on a variety of platforms at national laboratories, government agencies, private companies, and universities. Full optimization of the simulator has been successful on Convex, Cray, Hewlett Packard, IBM, Silicon Graphics, and Sun workstations and mainframe computers. The theoretical and numerical approaches applied in the simulator have been documented in a published theory guide. The simulator has undergone a rigorous verification procedure against analytical solutions, laboratory-scale experiments, and field-scale demonstrations and currently is maintained under version control procedures. Application and use of the simulator have been documented in two guide manuals. Battelle maintains the copyright to the coding and intellectual property associated with STOMP.