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Ecosystems Research

Water Quality Analysis Simulation Program (WASP)

WASP is a generalized framework for modeling contaminant fate and transport in surface waters. Its flexible, compartmental approach allows it to address problems in one, two, or three dimensions. It is designed to allow easy substitution of user-written routines into the program structure. WASP has been used to answer questions regarding biochemical oxygen demand, dissolved oxygen dynamics, nutrients and eutrophication, bacterial contamination, and organic chemical and heavy metal contamination. The current WASP software package includes the scientific modules TOXI, EUTRO, and DYNHYD. TOXI models the transport and transformation of chemicals, EUTRO simulates dissolved oxygen and eutrophication processes, and DYNHYD is a hydrodynamic model used for prediction water flow and volume.

All WASP models have the same general data requirements dealing with water body hydrogeometry, advective and dispersive flows, settling and resuspension rates, boundary concentrations, pollutant loadings, and initial conditions. The body of water to be simulated must be divided into a series of computational elements or segments. Segment volumes, connectivity, and type (surface water, subsurface water, surface benthic, subsurface benthic) must be specified.

Structurally, the WASP program includes six mechanisms for describing transport. These fields consist of advection and dispersion in the water column; advection and dispersion in pore water; settling, resuspension, and sedimentation of up to three classes of solids; and evaporation or precipitation. To describe advection within WASP, each inflow or circulation pattern requires specification of the fraction routed through relevant water column segments and the time history of the corresponding flow. Dispersion requires specification of cross-sectional areas between model segments, characteristic mixing lengths, and the time history of the corresponding dispersion coefficient. For each state variable, the user must specify loads, boundary concentrations, and initial concentrations. The dissolved fractions of each variable also must be specified for each segment. Only dissolved concentrations are transported by pore water and only particulate concentrations are transported by solids. A final set of data requirements deal with simulation control. This includes the number of segments and state variables, time step, simulation start and stop time, print interval, and runtime display information.

A body of water is represented in WASP as a series of discrete computational elements or segments. Environmental properties and chemical concentrations are modeled as spatially constant within segments. Each variable is advected and dispersed among water segments, and exchanged with surficial benthic segments by diffusive mixing. Sorbed or particulate fractions may settle through water column segments and deposit to or erode from surficial benthic segments. Within the bed, dissolved variables may migrate downward or upward through net sedimentation or erosion. WASP's explicit one-step Euler solution technique has potential for instability or numerical dispersion in the user-specified computational network. These problems can usually be controlled by manipulating the time step.

WASP has a long history of application to various problems. Some applications have been validated with field data, or verified by model experiments and reviewed by independent experts. Applying WASP requires both modeling sophistication and appropriate scientific and engineering judgement.

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