(excerpts from the User's Manual)

WASP5 is a dynamic compartment model that can be used to analyze a variety of water qualtiy problems in such diverse water bodies as ponds, streams, lakes, reservoirs, rivers, estuaries and coastal waters.

The model network is a set of expanded control volumes, or "segments," that together represent the physical configuration of the water body. The network may subdivide the water body laterally and vertically as well as longitudinally. Segments is WASP may be one of four types: epilimnion layer, hypolimnion layers, upper benthic layer, and lower benthic layers. Segment volumes and the simulation time step are directly related. As one increases or decreases, the other must do the same to insure stability and numerical accuracy. Segment size can vary dramatically. Characteristic sizes are dictated by the spatial and temporal scale of the problem being analyzed. If the water quality model i s being linked to the hydrodynamic model, then water column segments must correspon d to the hydrodynamic junctions.

Overview of WASP5 Sediment Transport

Simulations may incorporate total solids as a single variable, or alternatively, represent from one to three solids types or fractions. The character of the three solids types is user-defined. They may represent sand, silt, and clay, or organic solids and inorganic solids. The user defines each solid type by specifying its settling and erosion rates, and its organic content.

WASP5 performs a simple mass balance on each solid variable in each compartment based upon specified water column advection and dispersion rates, along with special settling, deposition, erosion, burial, and bed load rates. Mass balance computations are performed in benthic commpartments as well as water column compartments. Bulk densities or benthic volumes are adjusted throughout the simulation.

All solids transport rates can be varied in space and time by the user. There are, however, no special process descriptions programmed as a function of sediment shear strength and water column shear stress. Consequently, the TOXI5 sediment model should be considered descriptive, and must be calibrated to site data.

Overview of WASP5 Dissolved Oxygen

Dissolved oxygen and associated variables are simulated using the EUTRO5 program. Several physical-chemical processes can affect the transport and interaction among the nutrients, phytoplankton, carbonaceous material, and dissolved oxygen in the aquatic environment.

EUTRO5 can be oeprated by the user at various levels of complexity to simulate some or all of these variables and interactions. To simulate only carbonaceous biochemical oxygen demand (BOD) and DO, for example, the user may bypass calculations for the nitrogen, phosphorus, and phytoplankton variables. Simulations may incorporate carbonaceous biochemical oxygen demand (CBOD) and either ammonia (NH3) or nitrogenous biochemical oxygen demand (NBOD) expressed as ammonia. Sediment oxygen demand may be specified, as well as photosynthesis and respiration rates.

Four levels of complexity are identified: 1) Streeter-Phelps, 2) modified Streeter-Phelps, 3) full linear DO balance, and 4) nonlinear DO balance.

Overview of WASP5 Eutrophication

The nutrient enrichment, eutrophication, and DO depletion processes are simulated using the EUTRO5 program. Several physical-chemical processes can affect the transport and interaction among the nutrients, phytoplankton, carbonaceous material, and dissolved oxygen in the aquatic environment. EUTRO5 can be operated by the user at various levels of complexity to simulate some or all of these variables and interactions. Three levels of complexity for simulating eutrophication are identified in the model: 1) simple eutrophication kinetics, 2) intermediate eutrophication kinetics, and 3) intermediate eutrophication kinetics with benthos.

Overview of WASP5 Simple Toxicants

TOXI5 simulates the transport and transformation of one to three chemicals and one to three types of particulate material. The three chemicals may be independent or they may be linked with reaction yields, such as a parent compound-daughter product sequence.

In an aquatic environment, toxic chemicals may be transferred between phases and may be degraded by any of a number of chemical and biological processes. Simplified transfer processes defined in the model include sorption and volatilization. Transformation processes include biodegradation, hydrolysis, photolysis, and oxidation. Sorption is treated as an equilibrium reaction. The implified transformation processes are described by first-order rate equations.

WASP5 uses a mass balance equation to calculate sediment and chemical mass and concentrations for every segment in a specialized network that may include surface water, underlying water, surface bed, and underlying bed. In a simulation, sediment is advected and dispersed among water segments, settled to and eroded from benthic segments, and moved between benthic segments through net sedimentation, erosion, or bed load.

Simulated chemicals undergo several physical or chemical reactions as specified by the user in the input dataset. Chemicals are advected and dispersed among water segments, and exchanged with surficial benthic segments by dispersive mixing. Sorbed chemicals settle through water column segments and deposit to or erode from surficial benthic segments. Within the bed, dissolved chemicals migrate downward or upward through percolation and pore water diffusion. Sorbed chemicals migrate downward or upward through net sedimentation or erosion. Rate constants and equilibrium coefficients must be estimated from field or literature data in simplified toxic chemical studies.

Some limitations should be kept in mind when applying TOXI5. First, chemical concentrations should be near trace levels, i.e., below half the solubility or 10^-5 molar. At higher concentrations, the assumptions of linear partitioning and transformation begin to break down. Chemical density may become important, particularly near the source, such as in a spill. Large concentrations canaffect key environmental characteristics, such as pH or bacterial populations, thus altering transformation rates.

Overview of WASP5 Organic Chemicals

Several environmental processes can affect the transport and fate of organic chemicals in the aquatic environment. The most important include physical processes such as hydrophobic sorption, volatilization, and sedimentation; chemical processes such as ionization, precipitation, dissolution, hydrolysis, photolysis, oxidation and reduction; and biological processes such as biodegradation and bioconcentration. WASP5 explicitly handles most of these, excluding only reduction and precipitation-dissolution. If the kinetics of these reactions are described by the user, they also can be included as an extra reaction.

WASP5 allows the simulation of a variety of processes that may affect toxic chemicals. However, WASP5 makes relatively few assumptions concerning the particular processes affecting the transport, transformations, and kinetics reactions. The model is designed to provide a broad framework applicable to many environmental problems and to allow the user to match the model complexity with the requirements of the problem.

Each organic chemical may exist as a neutral compound and up to four ionic species. The neutral and ionic specias can exist in five phases: dissolved, sorbed to dissolved organic carbon (DOC), and sorbed to each of the up to three types of solids. Local equilibrium is assumed so that the distribution of the chemical between each of the species and phases is defined by distribution or partition coefficients. In this fashion, the concentration of any specie in any phase can be calculated from the total chemical concentration. Therefore, only a single state variable representing total concentration is required for each chemical.

There are often other factors that may influence the transport and transformations of the chemicals simulated. For example, water temperature affects reaction kinetics, sorption may also occur onto dissolved organic carbon, and pH can affect ionization and hydrolysis reactions. These concentrations or properties are included in TOXI5 through the use of model parameters and time functions. They are specified to the model rather than simulated. They may be varied over space and/or over time.

Although it was generally acknowledged that PCBs from soft sediment deposits in the Lower Fox River were sources of PCBs to the water column, little was known about factors controlling transport of PCB-laden bottom sediments from these sites or the magnitude of other sources such as sewage treatment plant effluents, industrial waters, urban runoff, or landfill leaching. Nor was it understood how much PCB was being transported to Green Bay and Lake Michigan, what amount moved to the atmosphere, or what PCB conditions would be like in the next 25 years. The Lower Fox River Study addressed these questions.

This study, conducted in concert with the larger Green Bay Mass Balance Study (GBMBS), was funded by the U.S. Environmental Protection Agency (USEPA), the WDNR, and the U.S. Geological Survey (USGS). Collection of the data necessary to answer the above questions was conducted primarily from May 1989 to April 1990 between Lake Winnebago and the DePere Dam. There were over 130 water column, 1,000 bottom sediment, 175 point-source effluent, 5 landfill wells, and 10 urban stormsewer samples that were analyzed for PCBs.

Using these data as a foundation, a mathematical, physical process-based model was constructed to help answer the PCB transport questions. The four-year modeling effort used the Water Quality Analysis Program (WASP4) (Ambrose and others, 1988) as a framework, and consisted of three major components: water column transport, solids mass balance (both suspended and bottom), and physiochemical processes.

¹ US Geological Survey, Water Resources Division, St. Petersburg, FL
² US Geological Survey, Water Resources Division, Middleton, WI

The full text of this article can be found at the SMIG website.

(Summary from TMDL Case Study, USEPA, 1993, Case Study No. 9, EPA841-F-94-004)

The Appoquinimink River watershed is located in eastern Delaware. The TMDL for phosporus was developed by the Delaware Department of Natural Resources and Environmental Control (DNREC) for the Appoquinimink River using the phased approach to TMDL development. The objectives of the TMDL included characterization of the nonpoint source nutrient loads and their impact on water quality and description of further modeling studies necessary to refine the TMDL. DNREC used available ambient water quality data and existing point and nonpoint source loading data to conduct the initial assessment and characterize the Appoquinimink's water quality problems. In addition, the EUTRO4 version of EPA's Water Qulaity Analysis Simulation Program (WASP4), a water quality model, was used to analyze the dissolved oxygen (DO) and nutrient economy of the river. Phosphorus overenrichment was determined to be the ultimate cause of excursions of applicable DO criteria. A phosphorus TMDL of 18,947 lb/yr was calculated as the sum of the point source allocation (6,862 lb/yr) and the background/nonpoint source allocation (12,085 lb/yr). These allocations reflect a reasonable margin of safety and will prevent further water quality degradation.

This case study describes the specific modeling efforts in more detail. The WASP4 model was used to predict the water quality impacts of various point and nonpoint source loading scenarios. With the additional information about nonpoint source loads collected as part of the initial phase of TMDL development, the modeling study found that even the most aggressive pollution control scenario- which consisted of total removal of point source loads, 50 percent removal of nonpoint source phosphorus and nitrogen loads, and 50 percent removal of the oxygen demand (SOD), ammonia, and phosphorus flux of sediments- provided only a marginal difference in DO levels. These results indicated that the system is driven by SOD. The TMDL has included a schedule for continued monitoring and modeling to address the SOD issue.

Cheng, C., Atkinson, J.F., and DePinto, J.V., 1994: A coupled GIS-water quality modeling study. In Proceedings of the 1994 Hydraulic Engineering Conference, American Society of Civil Engineers, Buffalo, NY, 1994, pp. 247-251.

Cockrum, D.K., and Warwick, J.J., 1994: Assessing the impact of agricultural activities on water quality in a periphyton-dominated stream using the Water Quality Analysis Program (WASP). In Proceedings of the Symposium on the Effects of Human-Induced Changes on Hydrologic Systems, American Water Resources Association, Jackson Hole, WY, June 26-29, 1994, p. 1157.

De Smedt, F., Vuksanovic, V., Van Meerbeeck, S., and Reyns, D., 1998: A time-dependent flow model for heavy metals in the Scheldt estuary, Hydrobiologia, vol. 366, pp. 143-155.

Lang, G.A., and Fontaine, T.D., 1990: Modeling the fate and transport of organic contaminants in Lake St. Clair, Journal of Great Lakes Research, vol. 16(2), pp. 216-232.

Lu, Z., April, G.C., Raney, D.C., and Schroeder, W.W., 1994: DO, BOD, and organic nitrogen transport in Weeks Bay, Alabama, In Proceedings of the National Symposium on Water Quality, American Water Resources Association, Chicago, IL, November 6-10, 1994, pp. 191-200.

Lung, W., and Larson, C.E., 1995: Water quality modeling of the upper Mississippi River and Lake Pepin, Journal of Environmental Engineering, vol. 121(10), pp. 691-699.

Mohr, M., and Sandstroem, S., 1996: Comparison between measurements and simulations with WASP and the MIUU model, Swedish National Board for Industrial and Technical Development, Technical Report NUTEK-VIND-96-5, Stockholm, 53 p (NTIS Order number DE97706031INZ).

Tetra Tech, 1995: Hydrodynamic and water quality mathematical modeling study of Norwalk Harbor, Connecticut: Final Report, Tetra Tech, Inc., Fairfax, VA.

Van Meerbeeck, S., 1994: Simulatie van het sedimenttransport in de Schelde met het waterkwaliteitsmodel WASP5. Thesis Fac. Applied Sciences, Vrije Universiteit Brussel, 108 p.

Vuksanovic, V., 1993: Simulation of the transport of polychlorinated biphenyls (PCB) in the Scheldt Estuary with the water quality model WASP4. Thesis Interuniversity Postgraduate Programme in Hydrology, Vrije Universiteit Brussel, 111 pp.

Vuksanovic, V., De Smelt, R., and Van Meerbeeck, S., 1996: Transport of polychlorinated biphenyls (PCB) in the Scheldt Estuary simulated with the water quality model WASP, Journal of Hydrology, vol. 174, no. 1/2, p. 1.

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