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A.5 Toxic Contaminant Transport and Fate The EFDC code includes two internal submodels for the simulating the transport and fate of toxic contaminants. A simple, single contaminant, submodel can be activated from the master input file. The simple model accounts for water and suspended sediment phase transport with equilibrium partitioning and a lumped first order reaction. Contaminant mass per unit area in the sediment bed is also simulated. The second, more complex, submodel simulates the transport and fate of an arbitrary number of reacting contaminants in the water and sediment phases of both the water column and sediment bed. In this mode, the contaminant transport and fate simulation is functionally similar to the WASP5 TOXIC model (Ambrose, et al., 1993) with the added flexibility of simulating an arbitrary number of contaminants, and the improved accuracy of utilizing more complex three-dimensional physical transport fields in a highly accurate numerical transport scheme. Water -sediment phase interaction may be represented by equilibrium or nonlinear sorption processes. In this mode, the multilayer sediment bed formulation is activated with sediment bed water volume and dissolved contaminant mass balances to allow contaminants to reenter the water column by sediment resuspension, pore water expulsion due to consolidation, and diffusion from the pore water into the water column. The complex contaminant model activates a subroutine describing reaction processes with appropriate reactions parameters provided by toxic reaction processes input file. 12 �901 AA Water Quality and Eutrophication Simulation ® The EFDC code includes two internal eutrophication submodels for water quality simulation (Park, et al., 1995). The simple or reduced eutrophication model is functionally equivalent to the WASP5 EUTRO model (Ambrose, et al., 1993). The complex or full eutrophication model is functionally equivalent to the CE -QUAL -ICM or • Chesapeake Bay Water Quality model (Cerco and Cole, 1993). Both water column eutrophication models are coupled to a functionally equivalent implementation of the CE - QUAL -ICM sediment diagenesis or biogeochemical processes model (DiToro and Fitzpatrick, 1993). The eutrophication models can be executed simultaneously with the 4 hydrodynamic component of EFDC, or EFDC simulated hydrodynamic transport fields may be saved allowing the EFDC code to executed in a water quality only simulation model. The computational scheme used in the internal eutrophication models employs a fractional step extension of the same advective and diffusive algorithms used for salintiy and temperature, which guarantees positive constituent concentrations. A novel ordering of the reaction sequence in the reactive source and sink fractional step allows the linearized reactions to be solved implicitly, further guaranteeing positive concentrations. The eutrophication models accept an arbitrary number of point and non -point source loadings as well as atmospheric and ground water loadings. In addition to the internal eutrophication models, the EFDC model can be externally linked to the WASP5 model. In the external linking mode, the EFDC model generates WASP5 input files describing cell geometries and connectivity as well as advective and diffusive transport fields. For estuary simulation, the transport fields may be intratidally time averaged or intertidally time averaged using the averaging procedure described by Hamrick (1994a). A.5 Toxic Contaminant Transport and Fate The EFDC code includes two internal submodels for the simulating the transport and fate of toxic contaminants. A simple, single contaminant, submodel can be activated from the master input file. The simple model accounts for water and suspended sediment phase transport with equilibrium partitioning and a lumped first order reaction. Contaminant mass per unit area in the sediment bed is also simulated. The second, more complex, submodel simulates the transport and fate of an arbitrary number of reacting contaminants in the water and sediment phases of both the water column and sediment bed. In this mode, the contaminant transport and fate simulation is functionally similar to the WASP5 TOXIC model (Ambrose, et al., 1993) with the added flexibility of simulating an arbitrary number of contaminants, and the improved accuracy of utilizing more complex three-dimensional physical transport fields in a highly accurate numerical transport scheme. Water -sediment phase interaction may be represented by equilibrium or nonlinear sorption processes. In this mode, the multilayer sediment bed formulation is activated with sediment bed water volume and dissolved contaminant mass balances to allow contaminants to reenter the water column by sediment resuspension, pore water expulsion due to consolidation, and diffusion from the pore water into the water column. The complex contaminant model activates a subroutine describing reaction processes with appropriate reactions parameters provided by toxic reaction processes input file.