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I 2. Overview of the Model 2.1 Description of the Program ' The purpose of this section is to describe the overall structure of the program and to present a detailed description of the model's capabilities This section is not intended to discuss the numerical methods and techniques used to solve the flow and transport equations since these methods are discussed in detail in the USGS manual (Konikow and Bredehoeft, 1978) It should be noted, however,that ' most numerical limitations of the USGS code apply to BIOPLUME II and the user needs to be aware of these limitations The user also needs to be aware of the limitations of the method of characteristics for solving the transport equation ' The major steps in the calculation procedure are summarized in Figure 2 1,which presents a simplified flow chart of the overall structure of the computer program The flow chart illustrates that two independent sets of particles, OZ tracer particles and HC tracer particles, are generated Since the tracer ' particles may have to be moved more than once to complete a given time step and the react[on between Oz and HC is assumed instantaneous,the O2 and HC tracer particles are moved independently and their subsequent concentrations are also computed independently The resulting two plumes(HC and O) are combined after every particle move time step to simulate the reaction between OZ and HC in this version of ' the model, it is assumed that three units of oxygen are required to completely mineralize one unit of hydrocarbon (parameter F, equations 7 and 8, section 12 2) More work is necessary before this parameter can be defined on a compound by compound basis This technique, although it probably ' requires more computational time,is extremely beneficial due mainly to the following reasons 1) It provides the capability to simulate retarded HC plumes undergoing biodegradation 2) It allows the simulation of in-situ biorestoration since one can model the injection of oxygenated water 3) It maintains the modular structure of the program which makes future updates relatively simple t For the case of retarded HClumes undergoing biodegradation,the model automatically computes p g g the maximum time increments allowable for the explicit calculations for the retarded HC plume and for the ' non-retarded Oz plume The model then uses the smaller of the two time steps for the explicit solution of the solute transport equation ( e the larger number of particle moves is used to complete the given time step) The flow chart also illustrates that hydraulic gradients are computed once for the aquifer in question ' The flowrates specified for pumping or injection wells are used in the computation However, if one specifies an injection well,then that well can be used to simulate a contaminant source, an oxygen source, or both, by specifying the concentration of HC and/or Oz in the injected water Mass balance computations are performed for both OZ and HC independently at the end of every particle move The mass balance computations are then adjusted to account for the mass loss due to biodegradation after the two plumes have been superimposed The amount of mass loss due to ' biodegradation is printed as part of the chemical mass balance output This is extremely useful since one can correlate the simulated mass loss with the observed mass loss from field data The mass balance computations for 02 are necessary to insure the accuracy of the numerical technique when one is simulating an OZ injection scheme ' In addition to aerobic biodegradation,the model provides two other sources for biodegradation anaerobic decay and reaeration Both are simulated as a first order decay in HC concentrations, and the only input requirement is the coefficient of decay The decay terms are applied at the nodes and not at the tparticles This provides more numerical stability in case the coefficient of decay is much smaller than the ' 2-1 I