Water Quality

Analysis Simulation

Program (WASP)

Version 6.0

DRAFT: User’s Manual

By

Tim A. Wool

Robert B. Ambrose1

James L. Martin2

Edward A. Comer3

US Environmental Protection Agency – Region 4

Atlanta, GA

1Environmental Research Laboratory

Athens, GA

2USACE – Waterways Experiment Station

Vicksburg, MS

3Tetra Tech, Inc.

Atlanta, GA
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

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Table of Contents

1. Forward _________________________________________________________1-1

2. Acknowledgements_________________________________________________2-2

3. Introduction ______________________________________________________3-3

3.1. Overview of the WASP6 Modeling System ___________________________ 3-3

3.2. Installation____________________________________________________ 3-5

3.3. Technical Support ______________________________________________ 3-5

3.4. Tool Bar Definition _____________________________________________ 3-5

3.5. File Menu_____________________________________________________ 3-7

3.5.1. Importing Old WASP Input Files __________________________________________ 3-7

3.5.2. Exporting Old WASP Input Files __________________________________________ 3-8

3.5.3. User Preferences ______________________________________________________ 3-8

3.6. Project Files___________________________________________________ 3-9

3.6.1. New ________________________________________________________________ 3-9

3.6.2. Open ______________________________________________________________ 3-10

3.6.3. Edit _______________________________________________________________ 3-10

3.6.4. Save_______________________________________________________________ 3-10

3.6.5. Save-as_____________________________________________________________ 3-10

3.7. Input Parameterization _________________________________________ 3-11

3.7.1. Data Set Description___________________________________________________ 3-12

3.7.2. Model Type _________________________________________________________ 3-12

3.7.3. Comments __________________________________________________________ 3-12

3.7.4. Restart Options_______________________________________________________ 3-13

3.7.5. Date and Times ______________________________________________________ 3-13

3.7.6. Non-Point Source File _________________________________________________ 3-13

3.7.7. Hydrodynamics ______________________________________________________ 3-13

3.8. Systems _____________________________________________________ 3-14

3.8.1. System Options ______________________________________________________ 3-15

3.8.2. Dispersion/Flow Bypass________________________________________________ 3-16

3.8.3. Density_____________________________________________________________ 3-16

3.8.4. Maximum Concentration _______________________________________________ 3-16

3.8.5. Boundary/Load Scale & Conversion Factor _________________________________ 3-16

3.9. Segmentation Screen___________________________________________ 3-17

3.9.1. Segment Definition ___________________________________________________ 3-17

3.9.2. Segment Environmental Parameters _______________________________________ 3-19

3.9.3. Initial Concentrations__________________________________________________ 3-20

3.9.4. Fraction Dissolved ____________________________________________________ 3-21

3.10. Segment Parameter Scale Factors _________________________________ 3-22

3.11. Dispersion ___________________________________________________ 3-23

3.11.1. Exchange Fields ___________________________________________________ 3-24

3.11.2. Dispersion Function_________________________________________________ 3-24

3.11.3. Dispersion Time Function ____________________________________________ 3-25

3.12. Flows _______________________________________________________ 3-25

3.12.1. Flow Function _____________________________________________________ 3-27

3.12.2. Flow Time Function ________________________________________________ 3-27
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3.13. Boundaries___________________________________________________ 3-27

3.13.1. Boundary Time Function _____________________________________________ 3-28

3.14. Loads _______________________________________________________ 3-28

3.14.1. Load Time Function ________________________________________________ 3-29

3.15. Loads Scale and Conversion _____________________________________ 3-29

3.15.1. Time Step ________________________________________________________ 3-30

3.16. Print Interval_________________________________________________ 3-31

3.17. Time Functions _______________________________________________ 3-32

3.18. Constants____________________________________________________ 3-33

3.19. Fill/Calculate & Graphing _______________________________________ 3-35

3.19.1. Toolbar Definition __________________________________________________ 3-38

3.20. Validity Check________________________________________________ 3-38

3.21. Model Execution ______________________________________________ 3-39

4. Visual Graphic Post-Processor ______________________________________4-42

4.1. Main Toolbar_________________________________________________ 4-42

4.2. Model Output Selection_________________________________________ 4-43

4.2.1. Opening Model Output_________________________________________________ 4-43

4.3. Spatial Graphical Analysis_______________________________________ 4-45

4.3.1. Overview___________________________________________________________ 4-45

4.3.2. Spatial Grid Toolbar___________________________________________________ 4-46

4.3.3. Geographical Information System Interface _________________________________ 4-47

4.3.4. BMG File Creation with Digitize _________________________________________ 4-48

4.3.5. Controlling Spatial Analysis _____________________________________________ 4-49

4.3.6. Selecting Dataset _____________________________________________________ 4-51

4.3.7. Selecting Slice/Geometry _______________________________________________ 4-52

4.3.8. Selecting Variable ____________________________________________________ 4-52

4.3.9. Selecting Time _______________________________________________________ 4-53

4.3.10. Palette ___________________________________________________________ 4-53

4.3.11. Animation ________________________________________________________ 4-53

4.3.12. Plot Mode ________________________________________________________ 4-53

4.3.13. GIS Configuration __________________________________________________ 4-54

4.3.14. Layers ___________________________________________________________ 4-54

4.3.15. GIS Toolbar_______________________________________________________ 4-55

4.4. x/y Plot______________________________________________________ 4-56

4.4.1. Overview___________________________________________________________ 4-56

4.4.2. Toolbar ____________________________________________________________ 4-56

4.4.3. Creating x/y Plot _____________________________________________________ 4-57

4.4.4. OK/Cancel __________________________________________________________ 4-67

4.4.5. Zooming the Axes ____________________________________________________ 4-68

4.4.6. Adding an Additional Curve_____________________________________________ 4-71

4.4.7. Color/Black & White View _____________________________________________ 4-72

4.4.8. Observed/Measured Data _______________________________________________ 4-72

4.4.9. Printing Results ______________________________________________________ 4-75

4.4.10. Creating Tabled Data ________________________________________________ 4-76

4.4.11. Curve Calculations _________________________________________________ 4-77

5. The Basic Water Quality Model ______________________________________5-1
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5.1. General Mass Balance Equation ___________________________________ 5-1

5.2. The Model Network _____________________________________________ 5-3

5.3. The Model Transport Scheme _____________________________________ 5-7

5.4. Application of the Model_________________________________________ 5-8

6. Chemical Tracer Transport __________________________________________6-1

6.1. Overview of WASP6 Tracer Transport ______________________________ 6-1

6.2. Transport Processes_____________________________________________ 6-1

6.2.1. Hydrodynamic Linkage _________________________________________________ 6-2

6.2.2. Hydraulic Geometry ____________________________________________________ 6-4

6.2.3. Pore Water Advection __________________________________________________ 6-7

6.2.4. Water Column Dispersion _______________________________________________ 6-7

6.2.5. Pore Water Diffusion ___________________________________________________ 6-8

6.2.6. Boundary Processes ____________________________________________________ 6-9

6.2.7. Loading Processes ____________________________________________________ 6-10

6.2.8. Nonpoint Source Linkage _______________________________________________ 6-11

6.2.9. Initial Conditions _____________________________________________________ 6-12

6.3. Model Implementation__________________________________________ 6-13

6.4. Model Input Parameters ________________________________________ 6-13

6.4.1. Environment Parameters _______________________________________________ 6-13

6.4.2. Transport Parameters __________________________________________________ 6-17

6.4.3. Boundary Parameters __________________________________________________ 6-18

6.4.4. Transformation Parameters ______________________________________________ 6-19

6.4.5. External Input Files ___________________________________________________ 6-19

7. Sediment Transport ________________________________________________7-1

7.1. Overview of WASP Sediment Transport _____________________________ 7-1

7.1.1. Sediment Transport Processes ____________________________________________ 7-1

7.1.2. Sediment Loading _____________________________________________________ 7-4

7.1.3. The Sediment Bed _____________________________________________________ 7-5

7.2. Model Implementation___________________________________________ 7-9

7.2.1. Model Input Parameters _________________________________________________ 7-9

7.2.2. Environment Parameters_________________________________________________ 7-9

7.2.3. Boundary Parameters __________________________________________________ 7-10

7.2.4. Transformation Parameters ______________________________________________ 7-11

8. Dissolved Oxygen__________________________________________________8-1

8.1. Overview of WASP6 Dissolved Oxygen______________________________ 8-1

8.2. Dissolved Oxygen Processes_______________________________________ 8-3

8.2.1. Reaeration ___________________________________________________________ 8-4

8.2.2. Carbonaceous Oxidation_________________________________________________ 8-7

8.2.3. Nitrification __________________________________________________________ 8-9

8.2.4. Denitrification _______________________________________________________ 8-10

8.2.5. Settling ____________________________________________________________ 8-10

8.2.6. Phytoplankton Growth _________________________________________________ 8-10

8.2.7. Phytoplankton Death __________________________________________________ 8-11

8.2.8. Sediment Oxygen Demand ______________________________________________ 8-11

8.3. Model Implementation__________________________________________ 8-14

8.3.1. Streeter-Phelps_______________________________________________________ 8-14
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8.3.2. Environment Parameters________________________________________________ 8-15

8.3.3. Transport Parame ters __________________________________________________ 8-15

8.3.4. Boundary Parameters __________________________________________________ 8-16

8.3.5. Transformation Parameters ______________________________________________ 8-16

8.4. Modified Streeter-Phelps ________________________________________ 8-18

8.4.1. Environment Parameters________________________________________________ 8-20

8.4.2. Transport Parameters __________________________________________________ 8-20

8.4.3. Boundary Parameters __________________________________________________ 8-21

8.4.4. Transformation Parameters ______________________________________________ 8-22

8.5. Full Linear DO Balance_________________________________________ 8-22

8.5.1. Environment Parameters________________________________________________ 8-24

8.5.2. Transport Parameters __________________________________________________ 8-24

8.5.3. Boundary Parameters __________________________________________________ 8-25

8.5.4. Transformation Parameters ______________________________________________ 8-26

8.6. Nonlinear DO Balance __________________________________________ 8-26

9. Eutrophication____________________________________________________9-1

9.1. Overview of WASP6 Eutrophication________________________________ 9-1

9.1.1. Phosphorus Cycle ______________________________________________________ 9-3

9.1.2. Nitrogen Cycle ________________________________________________________ 9-3

9.1.3. Dissolved Oxygen _____________________________________________________ 9-3

9.1.4. Phytoplankton Kinetics _________________________________________________ 9-3

9.1.5. Phytoplankton Growth __________________________________________________ 9-5

9.1.6. Phytoplankton Death __________________________________________________ 9-12

9.1.7. Phytoplankton Settling _________________________________________________ 9-14

9.1.8. Summary ___________________________________________________________ 9-14

9.1.9. Stoichiometry and Uptake Kinetics________________________________________ 9-15

9.2. The Phosphorus Cycle __________________________________________ 9-16

9.2.1. Phytoplankton Growth _________________________________________________ 9-18

9.2.2. Phytoplankton Death __________________________________________________ 9-18

9.2.3. Mineralization _______________________________________________________ 9-18

9.2.4. Sorption ____________________________________________________________ 9-18

9.2.5. Settling ____________________________________________________________ 9-21

9.3. The Nitrogen Cycle ____________________________________________ 9-21

9.3.1. Phytoplankton Growth _________________________________________________ 9-23

9.3.2. Phytoplankton Death __________________________________________________ 9-24

9.3.3. Mineralization _______________________________________________________ 9-25

9.3.4. Settling ____________________________________________________________ 9-25

9.3.5. Nitrification _________________________________________________________ 9-25

9.3.6. Denitrification _______________________________________________________ 9-26

9.4. The Dissolved Oxygen Balance ___________________________________ 9-26

9.4.1. Benthic - Water Column Interactions ______________________________________ 9-26

9.4.2. Benthic Simulation____________________________________________________ 9-28

9.5. Model Implementation__________________________________________ 9-32

9.6. Simple Eutrophication Kinetics ___________________________________ 9-33

9.6.1. Environment Parameters________________________________________________ 9-34

9.6.2. Transport Parameters __________________________________________________ 9-34

9.6.3. Boundary Parameters __________________________________________________ 9-35

9.6.4. Transformation Parameters ______________________________________________ 9-36

9.7. Intermediate Eutrophication Kinetics ______________________________ 9-38
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9.7.1. Environment Parameters________________________________________________ 9-39

9.7.2. Transport Parameters __________________________________________________ 9-39

9.7.3. Boundary Parameters __________________________________________________ 9-39

9.7.4. Transformation Parameters ______________________________________________ 9-40

9.8. Intermediate Eutrophication Kinetics with Benthos ___________________ 9-43

10. Simple Toxicants _______________________________________________10-1

10.1. Simple Transformation Kinetics __________________________________ 10-3

10.1.1. Option 1: Total Lumped First Order Decay _______________________________ 10-3

10.1.2. Option 2: Individual First Order Transformation ___________________________ 10-4

10.2. Equilibrium Sorption___________________________________________ 10-4

10.3. Transformations and Daughter Products____________________________ 10-6

10.4. Model Implementation__________________________________________ 10-7

10.4.1. Model Input Parameters ______________________________________________ 10-7

10.4.2. Environment Parameters _____________________________________________ 10-7

10.4.3. Transport Parameters ________________________________________________ 10-8

10.4.4. Boundary Parameters ________________________________________________ 10-9

10.4.5. Transformation Parameters __________________________________________ 10-10

11. Organic Chemicals______________________________________________11-1

11.1. TOXI Reactions and Transformations ______________________________ 11-3

11.2. Model Implementation__________________________________________ 11-4

11.2.1. Model Input Parameters ______________________________________________ 11-4

11.2.2. Transformation Parameters ___________________________________________ 11-4

11.3. Ionization____________________________________________________ 11-6

11.3.1. Overview of TOXI Ionization Reactions _________________________________ 11-6

11.4. Implementation______________________________________________ 11-10

11.5. Equilibrium Sorption__________________________________________ 11-11

11.5.1. Overview of TOXI Sorption Reactions__________________________________ 11-12

11.5.2. Computation of Partition Coefficients __________________________________ 11-15

11.5.3. Option 1. Measured Partition Coefficients. ______________________________ 11-15

11.5.4. Option 2. Input of Organic Carbon Partition Coefficient.____________________ 11-15

11.5.5. Option 4. Computation of Solids Dependant Partitioning. ___________________ 11-16

11.5.6. Option 1: Measured Partition Coefficients. ______________________________ 11-18

11.5.7. Option 2: Input of Organic Carbon Partition Coefficient.____________________ 11-18

11.5.8. Option 3: Computation of the Organic Carbon Partition Coefficient. ___________ 11-18

11.5.9. Option 4: Solids Dependant Partitioning. _______________________________ 11-19

11.6. Volatilization ________________________________________________ 11-19

11.6.1. Overview of TOXI Volatilization______________________________________ 11-20

11.6.2. Volatilization Option 1. _____________________________________________ 11-22

11.6.3. Volatilization Option 2. _____________________________________________ 11-22

11.6.4. Volatilization Option 3. _____________________________________________ 11-23

11.6.5. Volatilization Option 4. _____________________________________________ 11-23

11.6.6. Volatilization Option 5. _____________________________________________ 11-25

11.6.7. Volatilization Option 1 _____________________________________________ 11-28

11.6.8. Volatilization Option 2 _____________________________________________ 11-28

11.6.9. Volatilization Option 3 _____________________________________________ 11-29

11.6.10. Volatilization Option 4 _____________________________________________ 11-29

11.6.11. Volatilization Option 5 _____________________________________________ 11-30
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11.7. Hydrolysis __________________________________________________ 11-30

11.7.1. Overview of TOXI Hydrolysis Reactions________________________________ 11-30

11.7.2. Option 1. First Order Hydrolysis. _____________________________________ 11-30

11.7.3. Option 2. Second Order Hydrolysis. ___________________________________ 11-30

11.8. Photolysis___________________________________________________ 11-34

11.8.1. Overview of TOXI Photolysis Reactions ________________________________ 11-34

11.8.2. Photolysis Option 1.________________________________________________ 11-35

11.8.3. Photolysis Option 2.________________________________________________ 11-38

11.8.4. Photolysis Option 1 ________________________________________________ 11-39

11.8.5. Photolysis Option 2 ________________________________________________ 11-41

11.9. Oxidation___________________________________________________ 11-43

11.9.1. Overview of TOXI Oxidation Reactions ________________________________ 11-43

11.10. Biodegradation ____________________________________________ 11-45

11.10.1. Overview of TOXI Biodegradation Reactions ____________________________ 11-46

11.11. Extra Reaction_____________________________________________ 11-49

11.11.1. Overview of TOXI Extra Reaction_____________________________________ 11-49

12. REFERENCES ________________________________________________12-1
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1. Forward

WASP6 is an enhanced Windows version of the USEPA Water Quality Analysis

Simulation Program (WASP). WASP6 has been developed to aid modelers in the

implementation of WASP. WASP6 has features including a pre-processor, a rapid data 

processor, and a graphical post-processor that enable the modeler to run WASP more

quickly and easily and evaluate model results both numerically and graphically. With

WASP6, model execution can be performed up to ten times faster than the previous

USEPA DOS version of WASP. Nonetheless, WASP6 uses the same algorithms to solve 

water quality problems as those used in the DOS version of WASP.

WASP6 contains 1) a user-friendly Windows-based interface, 2) a pre-processor to assist 

modelers in the processing of data into a format that can be used in WASP, 3) high-speed 

WASP eutrophication and organic chemical model processors, and 4) a graphical postprocessor for the viewing of WASP results and comparison to observed field data. 

Because of the architecture utilized in the design of WASP6 it is going to be relatively

easy to develop other kinetic modules for WASP. Currently, we are planning on the

development of an enhanced eutrophication model that will include the addition of the

following state variables: 2 additional algal groups, salinity, full heat balance, coliforms,

second BOD group, sediment digenesis model. 
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2. Acknowledgements

The US EPA would like to acknowledge the generous donation that ASci Corporation

has made by releasing the Windows of WASP to EPA and the public domain. Their

realization of the environmental good that will come from the use of this model and their 

unselfish attitude should be commended.

Furthermore, the authors would like to recognize Mr. Jim Greenfield, EPA Region 4 for 

his support and encouragement in the development and enhancement of WASP6. The

authors would like to express gratitude to Mr. Mohammed Lahlomo for his support and 

efforts in bringing WASP6 to the public domain.
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3. Introduction

The Water Quality Analysis Simulation Program— (WASP6), an enhancement of the

original WASP (Di Toro et al., 1983; Connolly and Winfield, 1984; Ambrose, R.B. et al., 

1988). This model helps users interpret and predict water quality responses to natural

phenomena and man-made pollution for various pollution management decisions. 

WASP6 is a dynamic compartment-modeling program for aquatic systems, including

both the water column and the underlying benthos. The time-varying processes of

advection, dispersion, point and diffuse mass loading, and boundary exchange are

represented in the basic program.

Water quality processes are represented in special kinetic subroutines that are either

chosen from a library or written by the user. WASP is structured to permit easy

substitution of kinetic subroutines into the overall package to form problem-specific 

models. WASP6 comes with two such models -- TOXI for toxicants and EUTRO for 

conventional water quality. Earlier versions of WASP have been used to examine

eutrophication of Tampa Bay; phosphorus loading to Lake Okeechobee; eutrophication of

the Neuse River and estuary; eutrophication and PCB pollution of the Great Lakes

(Thomann, 1975; Thomann et al., 1976; Thomann et al, 1979; Di Toro and Connolly, 

1980), eutrophication of the Potomac Estuary (Thomann and Fitzpatrick, 1982), kepone

pollution of the James River Estuary (O'Connor et al., 1983), volatile organic pollution of

the Delaware Estuary (Ambrose, 1987), and heavy metal pollution of the Deep River,

North Carolina (JRB, 1984). In addition to these, numerous applications are listed in Di

Toro et al., 1983.

The flexibility afforded by the Water Quality Analysis Simulation Program is unique. 

WASP6 permits the modeler to structure one, two, and three dimensional models; allows 

the specification of time-variable exchange coefficients, advective flows, waste loads and

water quality boundary conditions; and permits tailored structuring of the kinetic

processes, all within the larger modeling framework without having to write or rewrite

large sections of computer code. The two operational WASP6 models, TOXI and

EUTRO, are reasonably general. In addition, users may develop new kinetic or reactive 

structures. This however requires an additional measure of judgment, insight, and

programming experience on the part of the modeler. The kinetic subroutine in WASP

(denoted "WASPB"), is kept as a separate section of code, with its own subroutines if

desired.

3.1. Overview of the WASP6 Modeling System 

The WASP6 system consists of two stand-alone computer programs, DYNHYD5 and

WASP6, which can be run in conjunction or separately. The hydrodynamics program,

DYNHYD5, simulates the movement of water while the water quality program, WASP6, 

simulates the movement and interaction of pollutants within the water. While

DYNHYD5 is delivered with WASP6, other hydrodynamic programs have also been

linked with WASP. RIVMOD handles unsteady flow in one-dimensional rivers, while
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SED3D handles unsteady, three-dimensional flow in lakes and estuaries (contact CEAM

for availability).

WASP6 is supplied with two kinetic sub-models to simulate two of the major classes of

water quality problems: conventional pollution (involving dissolved oxygen,

biochemical oxygen demand, nutrients and eutrophication) and toxic pollution (involving

organic chemicals, metals, and sediment). The linkage of either sub-model with the

WASP6 program gives the models EUTRO and TOXI, respectively. This is illustrated in 

Figure 3-1 with blocks to be substituted into the incomplete WASP6 model. The tracer 

block can be a dummy sub-model for substances with no kinetic interactions. In most 

instances, TOXI is used for tracers by specifying no decay.

Figure 3-1 Basic WASP Structure and Kinetic Systems

The basic principle of both the hydrodynamics and water-quality program is the

conservation of mass. The water volume and water-quality constituent masses being

studied are tracked and accounted for over time and space using a series of mass

balancing equations. The hydrodynamics program also conserves momentum, or energy,

throughout time and space.

WASP Version 6.0 represents a complete re-design in the functionality and look and feel 

of the US EPA Water Quality Analysis Simulation Program (WASP). WASP uses the

US EPA model source code as the basic engine for the model. A new Windows based 

preprocessor was developed and incorporated into the modeling framework. Now there 

is no distinction between the model and the preprocessor. In fact, the eutrophication
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model is a dynamic link library (DLL) that is executed by the preprocessor. WASP no 

longer requires input files, the data needed to execute the model is passed to the model

DLL using dynamic data exchange. The model input dataset reading routines have been

removed from the model. This was done to make a more efficient means of storing the 

model-input dataset and not worrying about all of the formatting issues associated with

the DOS based model.

3.2. Installation

The WASP6 installation is accomplished much like any other Windows software installation. 

To initiate the installation:

1. Place the WASP6 CD in your CD-ROM drive.

2. Select Start/Run from the Windows menu.

3. Enter d:/setup (If your CD-ROM drive is not drive D, type the appropriate 

letter instead).

4. Choose OK.

5. Follow the instructions on the screen prompts to complete the installation.

3.3. Technical Support

3.4. Tool Bar Definition

When the user first loads WASP6 a toolbar is displayed. This toolbar allows the user to 

navigate the different options and data entry forms of the program. Depending upon the 

settings in the User Preferences (Figure 3-3) some or all the toolbar icons are visible. If a 

toolbar icon is visible but not colored, this indicates that the function is not yet available. 

This typically means that some prerequisite was not met yet.

This icon instructs the program to initiate a new file.

This icon allows the retrieval of a previously created model input

file or project file.

This saves the active file to disk. Note that Save-as is available 

from the File Menu structure.

This toggles the input definition icons on/off. 
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This executes the appropriate model based upon the currently

loaded input file.

This icon is only available when the model is actually running. 

The user can abort the model simulation by pressing this icon. 

NOTE: It may take several minutes for the model to abort.

This loads the graphical post processor. If the user has a project or 

model input file selected this information is passed to the post

processor.

Model Parameterization

Model Time Step Definition Screen (Page 3-31) 

Model Simulation Result Interval (Page 3-32)

Model Segment Definition Screen (Page 3-17)

Model System Definition (Page 3-15)

Segment Parameter Scale Factors (Page 3-23)

Model Kinetic Constant Definition (Page 3-35)

Waste Load Time Series Definition (Page 3-29)

Environmental Time Series Definition (Page 3-33)

Dispersion Data Entry (Page 3-24)

Flow Data Entry (Page 3-26)

Boundary Condition Time Series (Page 3-28)

Input Dataset Validity Check (Page 3-39)
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3.5. File Menu

Because WASP has changed the methods in which model input data is stored the user

may have to import old datasets in the new framework. Old WASP input files had an

extension of INP, which stood for input file. These old style input files were ASCII

formatted files that could be read by most word processors and utility text editors. 

WASP still stores the model-input data in individual files, but now they have the

extension WIF, WASP input file. The new style input file is binary which allows for

rapid saving/retrieving of information. The preprocessor can only view this file in a

meaningful manner. WASP6 also supports a Project File format where the user can

provide other WASP6 related files. Project files are edited from the project menu item.

Figure 3-2 File Dialog Menu

3.5.1. Importing Old WASP Input Files

If you have previous version of WASP input files you can import them into the new file 

structure. For an old file to be successfully imported into the new structure the file must 

be a valid WASP input file (one that is read by the DOS version of WASP and produces 

reasonably results). If the file you are trying to import is incomplete or can't be read

successfully by the DOS version of WASP, the import may only be partially successful.
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To import a file the user should open the old file with the preprocessor. This will initiate 

the import of the data. The user will see a description of activities as the import

progresses.

3.5.2. Exporting Old WASP Input Files

WASP6 can export a WIF file format to the previous WASP file format. This would be 

useful for sharing input files with other people who do not use WASP6. The Export

function is available from the file menu; you will be required to provide a filename in

which to export the information.

3.5.3. User Preferences

The user has the ability to set several options within WASP6. The first option is whether 

to display a condensed version of the toolbar or the complete toolbar. The user also has 

the ability to enable logging. This option is used for debugging purposes only. The

logging function will generate a logging of all communications between the WASP6

program and the model DLL’s. The last option allows the user to specify that the model 

runtime grid remains visible whether the model is running or not. This is a good way to 

look at the final values predicted by the model(s). 

Figure 3-3 User Preferences
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3.6. Project Files

The user can develop WASP input datasets without ever using the project file option. 

The Project file allows the user to specify in one place all of the files that are used for a 

given input/output file. The user can create a project file by selecting New Project from

the Project Menu.

Figure 3-4 WASP6 Project Menu

 There are three types of files that can be added to the project menu: 1) *.WIF – WASP6 

input files, 2) *.DB – database files containing observed data, 3) *.SHP –

ArcInfo/ARCView shape files. Once a project has been created the user can modify and 

change whenever needed. When the user opens a project file, the WIF file is loaded by 

WASP6. When the post-processor is loaded the associated result file for the given WIF, 

any DB or SHP files are automatically read in. 

3.6.1. New

The new project menu item initiates the creation of a new project file. The user can add 

as many of the three accepted file types given above to the project file. Once the file has 

been created and files added the user should use the save function to write the project file 

to disk.
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3.6.2. Open

This menu item allows the user to open a previously created project file. Once open is 

selected the user is given a standard Windows file dialog box. Note that project files

have the extension of *.WWP.

3.6.3. Edit

The edit menu item allows the user to modify the contents of the opened project file. The 

users can remove/add files to the project.

3.6.4. Save

The save function writes the project file information to disk. When this option is selected 

the file is written without user intervention.

3.6.5. Save-as

The Save-as function allows the user to save the previously loaded project file to another 

filename. This is useful when conducting sensitivity analysis and do not want to lose the 

initial project. When the user selects the Save-as function they are presented with a

standard Windows file dialog box.
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Figure 3-5 Project File Definition

3.7. Input Parameterization

When creating a new input dataset the input parameterization data entry form is the first 

one that needs to be completed. This form provides basic information that is needed by 

the program to parameterize the other data entry forms that follow. This screen informs 

the program what type of WASP6 file you are going to be creating.
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Figure 3-6 Dataset Parameterization

3.7.1. Data Set Description

This field provides a one-line descriptor for the defined input data file. This descriptor is 

displayed on the caption line of the main WASP6 window.

3.7.2. Model Type

The model type dialog box allows the user to specify which WASP6 model type

(EUTRO/TOXI) the input dataset is being created. Setting the model type parameterizes 

WASP6 for that particular model type. Note that if you define a model as one type and 

change types later all model type specific data will be re-initialized (Time Functions,

Kinetics, Parameters, Boundaries, Initial Conditions, Loads). 

3.7.3. Comments

The dialog box provides space for the user to annotate important information about the

dataset. The model does not need this information. 
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3.7.4. Restart Options

WASP6 provides the user with the ability to use restart files between simulation runs. A

restart file is a “snap-shot” of the model conditions at the end of the simulation. This

“snap-shot” can be used as the initial conditions for a future model run. Note that the 

future model run must be of the same model type and segmentation scheme. There are 

three options for Restart:

1. No Restart File – WASP6 does not create a restart file (default).

2. Create Restart File – WASP6 creates a restart file that contains the final volumes and 

concentrations for each of the segments and systems.

3. Create/Read Restart File – WASP6 creates a file as described above, but reads initial 

volumes and concentrations from a previously created restart file.

3.7.5. Date and Times

The previous versions of WASP did not require that the model time functions be

represented in Gregorian date format. The Version 6.0of WASP requires all time

functions be represented in Gregorian fashion (mm/dd/yr hh:mm:ss). The user in the

Start Time dialog box must specify the starting date and time. This date and time

correspond to time zero in the old version of the model.

3.7.6. Non-Point Source File

The non-point source file is an external file that contains a time-series of loads (kg/day) 

for a given segment and system. This file is typically created either by the user manually

or using other software like the Stormwater Management Model (SWMM) in conjunction

with the Linked Watershed/Waterbody Model. This file can be used to provide loading 

information to WASP6 on virtually any time scale, from timestep to timestep, to year

average loads. 

3.7.7. Hydrodynamics

There are currently three surface flow options available for WASP. The first two options 

pertain to how WASP will calculate the exchange of mass between adjoining segments

with flow in both directions across a segment interface. The three flow options available 

for surface water flow are:

1. WASP will calculate net transport across a segment interface that has 

opposing flow. WASP will net the flows and move the mass from the 

segment that has the higher flow leaving. If the opposed flows are equal no 

mass is moved. 

2. Pertains to mass and water being moved without regard to net flow. 
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3. This option is used when linking WASP to a hydrodynamic model. When 

option 3 is selected the user cannot provide any additional surface flow 

information. Upon execution of a WASP input dataset using option 3 the 

hydrodynamic linkage file must already be created and exist in the directory 

that the input dataset resides. The file must have the extension of *.HYD.

The hydrodynamic linkage dialog box allows the user to select a hydrodynamic linkage

file. The hydrodynamic linkage file provides flows, volumes, depths, and velocities to

the WASP6 model during execution. There are several hydrodynamic models that have 

been linked with WASP6. The models include: DYNHYD5, RIVMOD, EFDC and

SWMM's transport module.

When linking to a hydrodynamic interface file, the user is restrained from entering

additional surface flow information.

3.8. Systems

The system data entry form allows the user to define system specific information. A

system in WASP6 is a state variable within the model. The state variables in WASP6

change from one model type to another. The user controls, which state variables, will be 

considered in their model input dataset from within this screen. 
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Figure 3-7 WASP6 System Bypass and Global Scale Factors

3.8.1. System Options

There are three options for this field: Simulated, Constant, and Bypassed. The user can

select which of the system options by selecting the option from the drop down dialog box 

for each individual system.

• Simulated: indicates to WASP that the user wants the model to calculate all 

equations associated with that state variable every time step. This is the most 

common selection.

• Constant: indicates to WASP that the user wants to hold the mass of this system 

constant and not allow the equations pertaining to this system to be calculated 

but allow its mass to influence the rates and fate of the other system's that can 

be affected by the presence of this systems mass. An example would be to 

include the influence of algae on dissolved oxygen without simulating the 

dynamics of algae. The user would provide initial concentrations for algae (that 

would never change), and enter rate constants for respiration and oxygen 

production. This would simulate a steady state effect of algal influences on 
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dissolved oxygen without providing all the information needed to simulate 

algae.

• Bypassed: indicates to WASP that NO calculations should be done for the 

particular system. When a system is bypassed in WASP the user does not have 

to provide boundary concentrations or initial conditions. When bypassing 

systems in WASP make sure that you are not removing an integral part of the 

problem you are trying to solve.

For both the advective and dispersive transport functions in WASP, the user has the

ability to bypass the effect of the particular transport phenomenon on the particular state 

variable in WASP. If the user would like to see the effect of algae on the system when it 

is not allowed to transport, the user sets the bypass flag for Chlorophyll-a to "Y" in either 

advection or dispersion (possibly both)

3.8.2. Dispersion/Flow Bypass

The dispersion/flow bypass option allows the user to specify whether a state variable will

transport by either one of these processes. If the user did not want a state variable to be 

affected by dispersion or flow they should check the appropriate box.

3.8.3. Density

The density of each constituent must be specified under initial conditions as well (g/cm3).

3.8.4. Maximum Concentration

The maximum concentration column allows the user to specify what would be the

expected maximum concentration (mg/l) of any of the given state variables. If WASP6

predicted a concentration greater than the supplied value here the model simulation

would be terminated.

3.8.5. Boundary/Load Scale & Conversion Factor

The boundary scale and conversion factors are specified for each individual system. The 

conversion factor can be used for converting the boundary time series information to the 

appropriate concentration units used by WASP6. The scale factor can be used to

attenuate the boundary concentrations without re-entering the time series data. An

example would be if the user wanted to know what the effects of doubling the loads

would be on water quality. Instead of re-entering the time series data, setting the scale 

factor to 2 would cause WASP6 to multiple the times series by 2.
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3.9. Segmentation Screen

This data entry form allows the user to define the number of segments that will be

considered in the simulation. Segments are the spatial component in which WASP6

solves it’s set of equations. Segments have volume, environmental and constituent

concentrations associated with them. The segment data entry form has four tables

associated with them: 1) Segment Definition, 2) Environmental Parameters, 3) Initial

Conditions, 4) Fraction Dissolved.

3.9.1. Segment Definition

The segment definition screen is where the user provides segment specific geometry

information. It is import that the user has a good understanding in how their water body

will be segmented prior to entering the information on this screen.

Figure 3-8 Segment Definitions

Inserting/Deleting Segments

Before the user can define a segment, the user needs to insert a segment by clicking on

the insert button. This will cause a segment to be inserted at the active row in the table. 
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If this is the first segment to be inserted it will initiate the table and insert a row at the 

top.

To delete a segment, highlight the row in which you want to delete and click on the delete 

button.

Segment Naming Convention

WASP6 automatically names the segments by numbers 1 through the number of

segments. WASP6 also allows the user to give an alphanumeric name to individual

segments. This alphanumeric name is there for the convenience of the user and will

appear on the other screens (Dispersion, Flow) as well as in the post processor so that the 

user does not need to keep track of the segments by number alone. When you initially

insert a segment it is automatically given the name WASP Segment. To name segments 

highlight the cell and type the name for each segment.

Volumes

This column represents the volume of the segment that is being defined. The units for

volume are cubic meters. Note that WASP6 does not assume a cubic formation for a 

segment, the shape is arbitrary. 

Water Velocity/Depth

There are several options for specifying water velocity and depth to WASP6. Depth and 

velocity can be held constant by entering their values in the Depth and Velocity

multiplier field and setting the exponent to zero. The user may also allow depth and

velocity to vary as a function of flow. To do this the user must provide a depth and

velocity multiplier and exponent. The velocity (m/s) is computed from the formulation

aQb while the depth (m) is computed from cQd, where a & d are coefficients and Q is the 

flow (m3/sec).

Segment Type

WASP6 supports four different segment types. The user must provide a segment type for 

each of the segments being defined. The segment type dialog box is used to define the 

segment type for the segment being defined.

1. Surface Water Segment – any segment that has an interface with the atmosphere. 

Only segment type 1’s have reaereation. 

2. Sub-Surface Water Segment – water segment without atmospheric interface.

3. Surface Benthic Segment – surficial benthic segment.
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4. Sub-Surface Benthic Segment – all benthic segments below surface benthic segments.

Bottom Segment

The bottom segment is used to define which segment is below the currently being defined 

segment. If the segment does not have a segment below it, the bottom segment should be 

set to none or zero. The bottom segment definition is used to define the optical light

path; it is not used in transport calculations.

3.9.2. Segment Environmental Parameters

This table contains segment specific environmental parameters. These parameters are

different for the various WASP6 model types. The segment parameter information

interacts directly with the Parameter Scale Factor screen. 

The user only needs to provide information for the environmental parameters that are

going to be considered in the simulation. Some parameters are used to directly define

segment specific information (i.e. SOD), others are used to point to environmental time

functions (i.e. Temperature). The pointers to environmental time functions allow the

user to define spatial and temporal variation for segment parameters such as: temperature, 

water velocity, pH, and bacteria concentration.
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Figure 3-9 Environmental Parameters

3.9.3. Initial Concentrations

Because WASP6 is a dynamic model, the user must specify initial conditions for each

variable in each segment. Initial conditions include the constituent concentrations at the

beginning of the simulation. The products of the initial concentrations and the initial

volumes give the initial constituent masses in each segment. For steady simulations,

where flows and loadings are held constant and the steady-state concentration response is 

desired, the user may specify initial concentrations that approximate expected final

concentrations. For dynamic simulations where the transient concentration response is

desired, initial concentrations should reflect measured values at the beginning of the

simulation. 
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Figure 3-10 Segment Initial Concentrations

3.9.4. Fraction Dissolved

In addition to chemical concentrations, the dissolved fractions at the beginning of the

simulation must be specified for each segment. For tracers, the dissolved fractions will

normally be set to 1.0. For tracers, as well as dissolved oxygen, eutrophication, and

sediment transport, the initial dissolved fractions remain constant throughout the

simulation. For contaminants, the fraction dissolved is recomputed based upon user

specified partitioning relationships.
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Figure 3-11 Fraction Dissolved for Constituents

3.10. Segment Parameter Scale Factors

This screen defines which parameters will be considered in the simulation as well as

specifying a parameter scale factor. By default the scale factor is 1.0. Before an

environmental segment parameter will be considered by WASP6 the used box must be

checked. Un-checking this box will remove the parameter from the simulation, but all

entered information is not lost. An example of using this feature is looking at the

influence of SOD on dissolved oxygen. Make the first simulation with the SOD

parameter checked; make the next run with it un-checked. The differences between the 

two runs are the influence of SOD. The user can also change the scale factors for each

parameter. For example, if you wanted to double SOD set the scale factor to 2.0
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Figure 3-12 Segment Parameter Scale Factors

3.11. Dispersion 

The dispersion-input screen is a complex screen that contains four tables. Under

dispersion, the user has a choice of up to two exchange fields. To simulate surface water 

toxicant and solids dispersion, the user selects water column dispersion in the

preprocessor or sets the number of exchange fields to one. To simulate exchange of

dissolved toxicants within the bed, the user should also select pore water diffusion in the 

preprocessor or set the number of exchange fields to two. 
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Figure 3-13 Dispersion Entry Forms

3.11.1. Exchange Fields

This table in the upper left portion of the screen allows the user to define dispersion for 

two types of exchanges. To use one of these exchange fields you must check the Use box 

and enter a scale and conversion factor. When the use box is unchecked the information 

for the particular exchange field is not passed to the model during execution.

1. Surface Water Exchange - The exchange of both dissolved and particulate fraction.

2. Pore Water Exchange - This exchange field moves only the dissolved portion of a

constituent. 

3.11.2. Dispersion Function

For each of the exchange fields the user can define up to 10 exchange functions. Each

exchange function can have its own set of exchange segment pairs and a corresponding

dispersion time function. WASP6 allows the user to provide names for each of the

exchange functions. To add an exchange function click on the insert button. To delete a 

function, select the function by highlighting the row and click on the delete button. This 
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will delete the corresponding segment pairs (lower left table) and the dispersion time function 

(lower right table).

To insert exchange functions for surface dispersion, highlight the Surface dispersion

exchange field (upper left table) go over to the exchange function table (upper right table) 

and press insert. The bottom tables are a function of the selection in the upper tables.

Segment Pairs

The segment pairs define the between which an exchange will occur. It does not matter 

in which order they are defined. Neither the preprocessor, nor the model makes any

checks to make sure the segments are connected in any manner. Connectivity is the

responsibility of the user.

Cross Sectional Area

Cross-sectional areas are specified for each dispersion coefficient, reflecting the area

through which mixing occurs. These can be surface areas for vertical exchange, such as 

in lakes or in the benthos. Areas are not modified during the simulation to reflect flow

changes.

Characteristic Mixing Length

Mixing lengths or distance are specified for each dispersion coefficient, reflecting the

characteristic length over which mixing occurs. These are typically the lengths between

the center points of adjoining segments. A single segment may have three or more

mixing lengths for segments adjoining longitudinally, laterally, and vertically. For

surficial benthic segments connecting water column segments, the depth of the benthic

layer is a more realistic mixing length than half the water depth.

3.11.3. Dispersion Time Function

Dispersive mixing coefficients can be specified between adjoining segments, or across

open water boundaries. These coefficients may represent pore water diffusion in benthic 

segments, vertical diffusion in lakes, and lateral and longitudinal dispersion in large water

bodies. Values can range from 1x10-10 m2/sec for molecular diffusion to 5x102 m2/sec 

for longitudinal mixing in some estuaries. Values are entered as a time function series of

dispersion and time, in days. 

3.12. Flows

The flow groups works exactly the same way as the exchange group. The only difference is 

that the advective group has 6 transport processes that can be defined by the user.
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1. Surface Water Flow – This group transports both the particulate and dissolved fractions of 

a constituent. If the user has selected the hydrodynamic linkage option they will not be 

able to enter information here.

2. Pore Water – This group only moves the dissolved fraction of a constituent.

3. Solids Transport 1 – This group moves solids field 1

4. Solids Transport 2 – This group moves solids field 2

5. Solids Transport 3 – This group moves solids field 3

6. Evaporation/Precipitation – This field adds/subtracts water only from the model network. 

No constituent mass is altered.

Figure 3-14 Flow Entry Forms
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3.12.1. Flow Function

The user has the ability to define 10 flow functions for each of the six flow fields. Each 

flow function would have its own flow continuity input (lower left table) and time

variable flow input (lower right table). The user must highlight the flow field and flow

function in which to enter information. WASP6 allows the user to provide names for

each of the flow functions. To insert an exchange function click on the insert button. To 

delete a function, select the function by highlighting the row and click on the delete

button. Note: this will delete the corresponding segment pairs (lower left table) and the 

flow time function (lower right table).

To insert flow functions for surface flow, highlight the Surface Flow field (upper left

table) go over to the flow function table (upper right table) and press insert. The bottom 

tables are a function of the selection in the upper tables.

Segment Pairs

The segment pairs define the segments from/to, which flow, occurs. The order in which

the segment is defined should be the path of positive flow. In other words, if segment 1 

flows to segment 2, when a negative flow is entered in the time function the flow will be 

from 2 to 1. Note: Neither preprocessor, nor the model makes any checks to make sure 

the segments are connected in any manner. Connectivity is the responsibility of the user.

Fraction of Flow

The fraction of flow column allows the user to specify the fraction of the flow that

transports from one to segment to the other. This field is used to split flows (diverge) for 

various reasons.

3.12.2. Flow Time Function

The time function table allows the user to enter time variable flow information. The user 

must provide the date, time and flow (cms). 

3.13. Boundaries

Boundary concentrations must be specified for any segment receiving flow inputs,

outputs, or exchanges from outside the model network. The boundary segments are

automatically determined by WASP6 when the user defined the transport patterns. 

Therefore, the user cannot enter boundary information until the transport information has

been entered. WASP6 requires that a boundary concentration be specified for every

system that is being simulated for every boundary segment. To specify a boundary for a 

system, move the cursor to the system that a boundary needs to be specified and right

click on the system.
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Figure 3-15 Boundary Concentration Definitions

3.13.1. Boundary Time Function

The time function table allows the user to enter time variable boundary concentrations

(mg/l). The user must provide the date, time and concentration. 

Note: For chlorophyll-a boundary conditions the units are ög/l

3.14. Loads

Waste loads may be entered into WASP6 for each of the systems for a given segment. 

To add a load right mouse click on the system, select add load and check the segments 

that will be receiving a load for the selected system. Once this is done, the user will be 

able to select the segment to define the load. There will be an entry for every segment in 

which the user wants to define a load. The user can delete a load by selecting the system, 

right mouse click and select delete. 
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Figure 3-16 Waste Load Definition Screen

3.14.1. Load Time Function

The time function table allows the user to enter time variable loadings (kg/day). The user 

must provide the date, time and concentration. 

3.15. Loads Scale and Conversion

The user has the ability to provide scale and conversion factors that can be used to

attenuate or convert loading mass. The conversion factor for a given system can be used 

to convert loads measured and reported in one unit to convert to WASP6 required units of 

kg/day. The scale factor column can be used to attenuate the loads without re-entering 

the time function information. If the user wanted to see the impacts of doubling the

loads, a scale factor of 2 would be entered for the desired system. 
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Figure 3-17 Waste Load Scale and Conve rsion Factors

3.15.1. Time Step

The user is provided two options for setting the model timestep. WASP6 has the ability

to calculate its own timestep. If this option is desired the user should set the appropriate 

flag. Regardless of which timestep option is used, the user must provide a time series 

here. The last date in the time series determines the simulation end time. If the user

elects to provide the timestep to the model, the user specifies time and time step pairs. 

When WASP is simulating, it will plot the information internally and will change the

time step based on the time function entered by the user.
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Figure 3-18 Model Time Step Definition Screen

3.16. Print Interval

The print interval is the user specified time function in which simulation results will be

written to the simulation result file. The WASP model does not have to write information 

at every time step but can be controlled by the user. Depending on the size of the

network and time frame being simulated by WASP, the simulation result files may be

rather large. The user has full control over the time frame in which the information is

written to the simulation result file. This function works like all other time functions in

WASP. The user must provide the desired time step and simulation time that this interval

is used. The user must provide at least two pairs of data.
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Figure 3-19 WASP6 Print Interval Definitions

3.17. Time Functions

The time function data entry forms allow the user to enter time variable environmental

information. WASP6 offers a selection of all the environmental time functions for a

given model type.
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Figure 3-20 WASP6 Environmental Time Function Definitions

The user may provide information for all the time functions or toggle on/off any of the

functions by clicking the Use dialog box. To enter information for a time function, place 

the cursor on the desired function. The time series data form for the given time function

is displayed in the lower table. The user should enter time/date and value for the time

function. 

3.18. Constants

This data entry group includes constants and kinetics for the water quality constituents

being simulated by the particular WASP model. Specified values for constants apply

over the entire network for the whole simulation. The user selects which constant group 

they would like to define kinetic constants. To select a Constant Group the user should 

click on the drop-down menu for a complete list. 
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Figure 3-21 Kinetic Constant Group Selections

Once a constant group has been selected, the user is given the opportunity to enter

constant data. WASP6 allows the user to activate constants by checking the Use dialog

box and then entering a kinetic constant value. When a constant is un-checked the

information is not passed onto the model, but the users constant value is preserved.
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Figure 3-22 Kinetic Constant Definitions

3.19. Fill/Calculate & Graphing

Most of the data entry screens have the ability to automatically fill and make calculations 

on the fields of the table. To accomplish this marks the fields using standard Windows 

functions and then press the fill/calculate button.
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Figure 3-23 Column Fill/Calculate Option

WASP6 also allows the user to plot time series data from any of the appropriate tables. 

To plot a time series press the plot button.
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Figure 3-24 Time Series Graphing Option

The user can zoom the x and y-axis. To zoom the x-axis the user should place the cursor 

at the starting point of the zoom, hold down the left mouse button and drag the box to the 

right to select the full area to zoom. Zooming the y-axis is done the same way except 

using the right mouse and dragging down. 
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Figure 3-25 Graphing Zoom Option

3.19.1. Toolbar Definition

The user is provided a toolbar at the bottom left hand corner of the graph window. This 

toolbar provides basic control over the graph.

3.20. Validity Check

The validly check makes a check of the user provided input data to make sure there are 

no troubles. This is quick way to make sure all your data is correct and within the

dimensioned capabilities of the selected model type.
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Figure 3-26 Dataset Validity Check

If a problem occurs during the validity check the information is passed to the user. If no 

problems are found the user should press the Okay button.

3.21. Model Execution

To execute the loaded input dataset the user should press the Model Execution icon on

the main toolbar. WASP6 loads the appropriate model DLL (TOXI/EUTRO) based upon 

the model type set by the user in the Model Parameterization entry form.

Note: Before you can run the model you must have an input dataset open in 

WASP6
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Figure 3-27 Model Data Retrieval

Once the model is executed WASP6 provides information back to the user on where it is 

in the simulation. The first set of information is the status of the data retrieval from the 

preprocessor. WASP6 does not read the conventional input files from the previous

versions of WASP6 and WASP, it makes requests to the preprocessor for the information 

as it is needed. Depending upon the size of your model network and amount of time

variable data this set can take some time. Once the model data has been retrieved it will 

begin the simulation. Once the simulation has started a grid will appear on the screen,

this grid contains intermediate results for each of the state variables for each of the

segments. The user can scroll this grid to look at the results. The user can shrink or 

stretch a column by dragging the column boundary in/out.
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Figure 3-28 WASP6 Runtime Grid
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4. Visual Graphic Post-Processor

The Post-Processor was developed as an efficient means of processing the vast amount of 

data produced by the execution of the WASP6 models. It has the ability to display results 

from all the models (EUTRO and TOXI) included in the WASP6 modeling package. The 

Post-Processor reads the output files created by the models and displays the results in two 

graphical formats:

1) Spatial Grid – a two dimensional rendition of the model network is

displayed in a window where the model network is color shaded based 

upon the predicted concentration.

2) x/y Plots -- generates an x/y line plot of predicted and/or observed model 

results in a window.

There is no limit on the number of x/y plots, spatial grids or even model result files the user 

can utilize in a session. Separate windows are created for each spatial grid or x/y plot created 

by the user. 

The Graphical Post-Processor is routinely executed from WASP6. Also, the user can use 

the Windows Explorer or Run button to execute the program. If executed from within

WASP + with an input file selected, the corresponding model output files will be loaded. 

If executed from within WASP6 without an input file selected or by some other means, 

the user will need to use the file options for opening the files they want to display.

4.1. Main Toolbar

There are several toolbars and speed menus available. The main tool bar is available

below the pull down menus provide the following functionality to the user. Depending

upon the current status, some icons may not be available to perform a task, thus are not 

active.

Open File Icon. This initiates the open file dialog box that allows the user to 

open a model result file (*.BMD), geometry backdrop file (*.BMG) or observed 

data database (*.DB).

Create Spatial Animation Grid Window. This opens a spatial analysis grid only

after a backdrop file (*.BMG) file has been selected. The user can open as many 

of these windows using the same backdrop file or any others that are loaded.

Creates a Spatial Animation Window using GIS coverage’s. This option is only

available when GIS coverage’s have been opened. One of the GIS coverage’s
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required is model network coverage.

Creates x/y plot Window. This opens an x/y plot window only after model data 

(*.BMD) or observed database data (*.DB) have been loaded. The user can open 

as many of these windows as desired to review any data that is loaded.

Edits the load observed data database

4.2. Model Output Selection

The Graphical Post-Processor was designed to allow the user to rapidly evaluate the

results of the WASP model simulations and its support programs. Observed data can also 

be stored in a database format.

Four types of data are recognized:

The first data type is created from the execution of the WASP models (*.BMD). 

The output from WASP is written in a binary file format. The model results 

cannot be read directly by any other program. 

The second file type that can be read is a Paradox table file (*.DB). The Paradox 

table file is used to provide observed/field data to be plotted against model

predictions. 

The third file type is an ArcView shape file. These files can be used in the spatial 

analysis mode to aid the user in displaying the model network with respect to its 

geography and surrounding characteristics.

The last file type that is used is the binary model geometry file. This file is used to 

provide the spatial grid geometry information so that the model results can be 

depicted within the model grid.

4.2.1. Opening Model Output

Prior to working with any model data or observed data, the files must be selected by the 

user. There is no limit to the number of files that can be opened. If the user would like to 

open additional files, the procedure given below will illustrate how to load each of the

different file types.

To open a file, the user can use the menu system and select open file or press the open 

file icon. This will display a file dialog box as illustrated in Figure 4-1. From this file 

dialog box the user can navigate to any drive and directory to which their computer is

attached. By pressing the down arrow on the file type dialog, a list of valid file

extensions is displayed for the user. Selecting an extension will result in the display of a 

picklist of the available files in the current drive and directory.
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Figure 4-1 File Dialog Box

BMD Format

To open a WASP simulation result file, select binary model data from the file type box. 

This will cause the file dialog box to display only those files that have the extension

*.BMD. The user has the ability to move around between drives and directories to select 

a file to review. The user can either double click the mouse on the desired file or

highlight the file and press the open button. Once the file is open the x/y plot icon will

become available. 

Note: The user must load a binary model geometry file (*.BMG) before the 

spatial grid analysis icon is available.

BMG File

BMG files are used to provide spatial analysis grid geometry information. These files are 

specific to model input datasets (i.e. BMG files must correspond to information in the
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BMD files). BMG files are created using a utility program called Digitize. The file must 

be created prior to execution. The BMG file is selected in the same manner as the other 

files by setting the file type to BMG and selecting the file. 

Note: The user must load a binary model geometry file (*.BMG) before the 

spatial grid analysis icon is available.

Observed Data

The observed data is in a Paradox 4.5 or higher format (*.DB). In order to display

observed data versus the predicted model results, the database must be in a specific form. 

To load an observed data database follow the procedures described above, change the file 

type to *.DB. Select the database and press open.

Note: The "observed data" database is expected to be in a certain file format 

with pre-defined field names.

4.3. Spatial Graphical Analysis

The spatial graphical analysis allows the user to review model results for the whole

network for a given constituent and time. This mode of graphical representation of the

model results is very effective in illustrating model predictions to non-technical 

audiences.

4.3.1. Overview

The spatial graphical analysis function allows the user to illustrate the model results on a 

spatial grid using shading to represent predicted values. There are two options for

creating spatial analysis grids. The first option allows the user to develop a binary model

geometry file (*.BMG) to illustrate a portion of the modeling network or the complete

network. The BMG is developed specifically for a particular model dataset with

corresponding assignments for each of the model computational elements that are to be

displayed. In other words, the BMG has polygons that are to be shaded based upon the 

predicted concentration of the model computational elements assigned to the polygons by

the user at the time the BMG file is created. 

The spatial grid analysis provides three modes for looking at the model results: shaded,

wired frame, and violation/criteria shading. These various modes allows the spatial

graphical analysis mode to illustrate information from model simulations in such a

manner to make easier for the non-technical person to understand the results. The
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following section provides information on how to use and configure the spatial graphical

option.

To initiate a spatial grid, the user should press the spatial grid analysis icon: this will

generate a spatial grid window as illustrated in Figure 4-2. The user can create as many 

of these windows as desired. Each window can be configured to show different model

results as well as different modes.

Figure 4-2 Spatial Analysis View

4.3.2. Spatial Grid Toolbar

The second option for the spatial analysis tool is to allow the user to develop the model

network using ArcView and combine the model network with other GIS coverage’s. To 

use this method the user must have a copy of ArcView and good working knowledge of 

how to use it.

Each spatial grid that is generated by the user has its own set of controls that allow the 

user to manipulate the contents. A description for each of the icons on the spatial grid 

toolbar are given below:
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Configure Animation Grid. This button allows the user to access the spatial grid 

configuration screen where the user can select what backdrop file and model data 

will be displayed.

Step Animation Grid Forward One Time Interval. This button causes the active

spatial grid to step forward one time interval in the model result file.

Step Animation Grid Back One Time Interval. This button causes the active

spatial grid to step backward one time interval in the model result file.

Plays animation sequence forward in time. 

Plays animation sequence backward in time.

4.3.3. Geographical Information System Interface

The graphical post processor has the ability to display ArcView shape files with the

model network being one of the layers. To use this option the user must have access to 

GIS coverage’s and a copy of ArcView. The post processor can display any number of 

coverage’s, the shape files must be loaded using the file open option or be contained in

the project file. Before a model network can be displayed the user must develop a

coverage and related database using the ArcView program.

Creating Coverages

To create a coverage the user will need to rely on a good understanding of ESRI’s

ArcView/ArcInfo programs. The model network will need to be added as a layer

(Theme) in ArcView. The best method for creating this layer is to create a base map

layer out of ArcView. This layer should contain the waterbody being modeled my

WASP6. Draw the segmentation on the printed map. If the segmentation includes subsurface segments, you may need to create yet another layer. Once the segmentation has 

been drawn on the base map, place the map in your digitizer and register the map

following the directions provided in the online help. Once the map has been registered, 

the user must digitize the segment polygons. To do this the user must add a new theme, 

with a separate polygon for each segment. The user will need to create a database table 

of attributes for this layer. This database contains basic information that the post

processor uses to align the model predicted results with the correct segment. When

creating this table the user is required to have at least one field, SEGID. This field must 

be called SEGID and be a string field. An optional field that can be added is LABEL. 

LABEL is a string field as well. The LABEL field can be used to put an alphanumeric 
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description next to the segment on the spatial plot. The user is referred to the ArcView

documentation on how to create, edit and modify tables. 

Figure 4-3 Model Database Definitions

4.3.4. BMG File Creation with Digitize

Before a spatial analysis grid can be displayed a BMG file needs to be created. This file 

is created using a utility program called Digitize. Digitize allows the user to draw the

model network or the portion of the model network on the screen and assign cross section 

numbers to polygons that they represent. There is a one to one correspondence to

polygons that are drawn with a cross section in the model BMD file.

Routine Execution of Digitize

The Digitize program is a DOS based program. Because Digitize needs access to the

computers communication ports and uses its own communication drivers the program

cannot be executed within the Windows environment. Digitize needs to be executed

from the Windows Command Prompt option. To start Windows in the command prompt 

mode, restart your Windows system. When the Starting Windows string appears on the 

screen press the F8 key and select command prompt. To execute Digitize change

directory to where the WASP6 modeling environment has been installed.

Digitize [parameters] digfile

The last step in creating the BMG file is linking all of the various slices into one BMG

file. The BMG file is created using a program called Digilink.
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Developing a Spatial Grid

The spatial grid file (BMG) is a specially created file that provides geometric

information. This geometric information is generated using the Digitize program. The

Digitize program has two types of interfaces in which the user can enter the information:

mouse and digitizer tablet. If the user has access to a digitizer this is probably the most 

advantageous mode for generating BMG files. 

 With Mouse

The mouse interface allows the user to develop the model grid by drawing the model

network using the mouse. Each computational element being defined in the model

network needs to be represented by an individual polygon. To use Digitize with the

mouse interface the program needs to be loaded with the -m command line parameter. 

This instructs Digitize to get the drawing information from the mouse.

 With Digitizer

The digitizer interface allows the user to develop the model grid by actually rendering the 

model network using a digitizer tablet. This is probably a more exact way of developing 

the model spatial grid. The digitizer interface allows the user to take engineering

drawings of the waterbody and lay the model network right on top. When using the

digitizer interface the user needs to instruct Digitize as to the communication port from

which to read the information. This is done by using a command line parameter -COM2, 

which instructs Digitize to get the information from the digitizer tablet from

communication port 2.

4.3.5. Controlling Spatial Analysis

Each of the spatial grid analysis windows can be configured several different ways to

display the model results. The user can elect to display results in one of three animation

modes:

1) Time – this mode animates the results for a single variable forward/backward 

in time. This is the preferred mode of the spatial analysis grid.

2) Variable – this displays the shaded results for each of the variables in the 

model output file. Pressing the forward/backward icon will cause the spatial 

animation grid to display the results for each of the variables. 

3) Slice – this option displays the results for a given variable over the range of 

slices views found in the BMG file.

Figure 4-4 illustrates the configuration screen for the spatial analysis grid non-GIS.
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Figure 4-4 Spatial Analysis Configuration (Non-GIS)

The GIS option works much like the other, with the exception of the ability to display

individual GIS layers.
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Figure 4-5 Spatial Analysis View GIS

4.3.6. Selecting Dataset

Because more than one model simulation result file can be loaded at a time, the user has 

the ability to select which of the model results files to display. Each spatial analysis grid 

can display only one model result file.

To select a currently loaded dataset to display in the current spatial analysis grid, press

the Configuration Button and select the drop down picklist for Data Set. This will make 

the results from the selected file appear in the spatial grid.

Note: The spatial grid analysis can only display information from one model 

result file at a time. The user has the option of creating as many spatial analysis 

grid windows as needed.
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4.3.7. Selecting Slice/Geometry

The Binary Model Geometry (BMG) file is selected from the geometry drop down

picklist. A BMG file can contain more than one view (slice) of the model network. A

slice is simply a set of polygons that are assigned to a set of cross sections. A BMG file 

can contain more than one slice and is created during the Digitize development. To select 

a slice that is contained within the BMG file, select the slice name from the drop down

selection from the configuration screen.

4.3.8. Selecting Variable

The user has the ability to select any one of the simulation result variables that is written

to the output file by the execution of the WASP models. The model result file (EUTRO 

or TOXI) will determine the variables that are available for display. To select a variable 

for a selected output file use the variable drop down picklist and select the variable you

want the spatial analysis grid to display. 

Figure 4-6 Selecting Variable to Display

Note: The spatial analysis grid can only display one result variable at a time.
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4.3.9. Selecting Time

The spatial analysis grid displays results for a given variable at a given time. Not only

does the user have the ability to control the display of variables, but can control the

display time as well. The user can select what simulation time to display the results for a 

given variable. This pull down picklist allows the user to move the spatial analysis grid 

to specific points in time without stepping through each time interval.

4.3.10. Palette

The user has the ability to modify the color palette that is used to shade the plot. The 

most common modification to the color palette would be to invert the currently loaded

palette. Clicking on the Invert radio button will cause the color palette to be inverted (i.e. 

the colors for the lower values will become the colors for the higher values). 

The user also has the ability to select a different color scheme by selecting a new palette. 

To select a new palette the user should press the browse button and select a palette.

4.3.11. Animation

The most common use for the spatial analysis grid is to animate the model predictions. 

When the forward/backward buttons on the toolbar are pressed, the spatial analysis grid is 

updated with the next time or variable depending upon the varying parameter. By

continually pressing the forward button the user can create “a movie” of the modelpredicted results. Alternatively, the user may press the movie icon. The speed of the 

animation is controlled by the options under the spatial plot parameters. 

4.3.12. Plot Mode

Three modes are provided by which data can be displayed in the spatial grid analysis

window: 

Shaded

This mode displays the simulation results by shading the model computational element

based upon the predicted concentration. A color legend is displayed on the right hand

side of the spatial analysis window, and the computational elements are shaded in

accordance to the legend based upon the concentrations predicted by the model.

Wire Frame

The mode displays the simulation results in a wire frame. This mode differs from the

shade mode in that the model computation elements are not filled with a color based upon 
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predicted concentrations. The wire frame mode outlines the outside perimeter of the

computational element in a color corresponding to the predicted concentration. 

4.3.13. GIS Configuration

If the user creates a GIS spatial view the controls and options for configuring the

windows are little different. Figure 4-7 illustrates the configuration screen for the GIS

option. 

Figure 4-7 GIS Spatial Plot Configuration

Note: GIS layer files must be loaded prior to this option being available.

The dataset, variable, delay options work exactly the same way as described above.

4.3.14. Layers

The user can select and order that GIS layers will be displayed. The user is given a

dialog box that contains a list of all the loaded layers; the user can select which one to 
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display by using standards Windows selection methods. Once a layer has been added to 

the selected dialog window, the user can control the order in which the layer is drawn by 

moving the layer up and down in the window. The model network should be the last 

layers to be drawn to make sure that another layer does not over write it.

Select All

This will select all of the loaded layers. Once they are selected the user should press the 

Add Button.

Select None

De-selects any selected layers.

Move Up/Down

The move/up down button will move the selected (highlighted) layer up in down in the

list. The layers at the top of the list are drawn first.

Layer Color

The user has the ability select a specific color for a given layer. Because the GIS layers 

do not have a color associated, the post processor assigns a color. This assignment may 

not be desirable, to change the layer color select the layer and press the layer color

button. The user will be presented a color selection dialog box. Select the color and 

press Okay.

4.3.15. GIS Toolbar

The GIS toolbar has a couple of additional buttons that are not found on non-GIS spatial 

analysis view.

This button will save the GIS view to a bitmap file. The user is presented a conventional file dialog box.

This copies the GIS view to the Windows Clipboard. Once in the clipboard it can be pasted into virtually another 

Windows program.

This prints the GIS view to any printer connected to the system.
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4.4. x/y Plot

4.4.1. Overview

The x/y plot mode represents the conventional method by which scientific data is

displayed. While x/y plots are the conventional mode, the flexibility and control the user 

has over the way the x/y plots are configured is not. The user is provided as much

flexibility as possible when developing x/y plots. The user can plot different model

results files simultaneously, multiple variables and observed versus model predictions.

Figure 4-8 Example Graph

4.4.2. Toolbar

The x/y plot window has its own set of controls that allows the user to perform the

various options that are available. The user can access these options either via the tool

bar or the speed menu. The following is a description of the x/y plot toolbar:

Animate x/y Plot. This option is available when there is more than one

line on the graph. It will toggle the display one line on the graph at time 

for visual inspection. It is used to “declutter” the graph to see differences.
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Configure x/y Plot. This icon is used to configure what data is used and 

how it is displayed in the x/y plot.

Un-Zoom One Level. This option will un-zoom the graph one user

performed zoom level at a time. It undoes a zoom one step at a time.

Un-Zoom x/y Plot. This icon will un-zoom the graph to its original axes 

dimension at the time of creation of the graph. 

Print x/y Plot to Printer. This option allows the user to print the x/y plot 

to a printer. A normal Windows printer dialog box will appear to allow

the user to control the appearance of the graph.

Copy Graphic Image to Clipboard. This option makes a copy of the x/y

plot onto the Windows clipboard. This clipboard image can then be

pasted into programs like a word processor for publication.

Save Graph to Disk. This option creates a Windows bitmap file (*.BMP) 

of the x/y graph window. The saved BMP can be imported into programs 

like a word processor for publication.

Export Graph to ASCII Table. This option creates an ASCII file

containing the values for each of the lines from the graph window. This 

table can be imported into a spreadsheet or other programs.

Curve Calculations. This icon brings up the curve calculation dialog box. 

The user can perform data transformation on the data or use one of the

predefined functions.

Create Data Table from Graph. This creates a table of data represented by 

the lines in the graph window. The user will be able to review the data 

and copy data from the table to programs like spreadsheets.

Toggle graph window between color and black & white. Because some 

printers do not transpose the color to black and white effectively, this

option allows the user to force black & white or color.

Causes re-draw of the plot window.

4.4.3. Creating x/y Plot

The first time that the x/y plot icon button requesting a x/y plot is pressed, the graph

configuration menu appears. It is from this menu that the user has control over the content of 
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the x/y plot window. The user can plot data within this x/y window for any or all of the 

currently loaded files (simulation result files or observed data). The user has full control over 

the appearance of graph (axis, title and legend labeling).

Figure 4-9 Graph Curve Attribute Screen

Graph Characteristics

The user has the ability to define the style of the x/y plot window. The user can define 

line types for the grid, colors for the various portion of the graph and control the fonts for 

the various text components. Once the user develops a style they have the ability to save 

this style to disk. Each individual x/y plot window can recall the style. The user can

define a default style that is automatically recalled every time an x/y plot window is

created.

The user specifies the title for the graph in this dialog box and can control the animation

speed.
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Figure 4-10 x/y Plot Characteristics

Domain Label

This dialog box is used to describe the label that will be displayed below the x-axis on the 

graph. Typically, the x-axis is used for either time or distance.
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Figure 4-11 Graph Domain Labeling

Primary Range Label

This dialog box is used to describe the label that will be displayed above the y1 axis on 

the graph. The y1 axis is the one on the left-hand side of the graph and is typically used 

for concentration.
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Figure 4-12 Y Axis Labeling

Secondary Range Label

This dialog box is used to describe the label that will be displayed above the y2 axis on 

the graph. The y

2

 axis is the one on the right hand side of the graph and is typically is 

used for concentration.
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Figure 4-13 Secondary Y Axis Labeling

Time/Segment

This radio button option is used to inform that the x-axis domain is going to be used for 

time or distance. If the user selects distance, another option is available for the x-axis 

domain definition that is described below.

Map Segment to Distance

If the user selects the x domain to be of type segment, the user has the option of mapping 

the segment number to that of a river mile or distance. If this option is selected the user 

will need to create a database file that has the segment number with its corresponding

distance. When this option is properly configured the distance and its corresponding

concentration will appear on the x/y plot.

Restrict Domain

By default, the x-axis is automatically scaled using built-in heuristics based upon the

range of the data. For the most part these heuristics provide very meaningful graphs. 

However the user has the ability to set the range of the x-axis manually. To manually
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control the range of x-axis, the user should select the "restrict x-axis range" from the

graph configuration window. Once this is selected, the user may enter values for the

minimum and maximum values for the x-axis. 

Note: There are more sophisticated methods for manipulating the range of the xaxis (see Zooming X-Axis). 

Restrict y1/y2 Range

By default the y-axis is automatically scaled using built-in heuristics. For the most part 

these heuristics provide very meaningful graphs. However the user has the ability to set 

the scale of the y-axis manually. To enter a value for the y scale the user should select to 

restrict y-axis range from the graph configuration window. Once this is selected, the user 

may enter values for the minimum and maximum values for the y-axis being defined. 

Note: The user can control the range for both the Y1 and Y2 axes.

Graph Title

This dialog box is used to describe the title that will be displayed at the top of the graph. 

The user is limited to 25 characters within the title line.

Animation Delay

The animation delay dialog box allows the user to define the delay when the user presses 

the animation button from the x/y toolbar. 

Color/Black & White

This radio button selection works the same as if the user pressed the Color/Black &

White icon from the toolbar. This will toggle the x/y plot between color and black &

white.

Curve Definition

The curves dialog window indicates how many lines are currently defined for the given

graph. To add a curve to the graph the user should press the Add Curve button. This will 

bring up the curve definition window. If the user wants to edit the attributes of a

previously created curve the user should select the curve to be edited in the curve window 
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by clicking the mouse pointer on the curve and press edit curve. There is no limit to the 

number of curves that can be defined for a given x/y plot.

Okay/Cancel

The Okay/Cancel button determines what state the user wants to leave the form. If

anything was modified in the plot parameter screen and the user selects Okay, the x/y plot 

window will be updated with the newly entered/selected information. If the user selects

Cancel, any entered information would be lost and the x/y plot window would remained 

unchanged.

Curve Attributes/Adding a Curve

If the Add Curve button is pressed in the graph configuration menu, a second dialog box

appears to define the attributes of the curve. Defining those attributes will result is a

single curve being added to the x/y plot.

Figure 4-14 Input File Selection
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Data Source

Data are obtained from several sources and are available for plotting within the x/y plot 

window. For the data to be available for a given plot window it must either be read into 

memory (read from disk: simulation result files or observed database) or created from a 

previous calculation.

The radio buttons located within the Data Source dialog window will be available for

selection if the particular type is available for plotting. The model simulation results file

and the observed data databases are loaded using the "Open File" dialog from the main

menu. The calculated data sources are created from within the x/y plot window. A

calculated data source created in one x/y plot window is available to other x/y plot

windows as well.

Predicted

The predicted data type is assigned to the files that contain model simulation results.

Observed

The observed type is assigned to the observed data database. The observed data database 

must have a specific file format containing specifically named data fields. 

Calculated

The user created the “calculated” field using built-in functions. One such function is the 

model partition function, where the user can calculate the difference between one curve 

and another. The results of this calculation are stored in a user defined Calculated data 

structure.

Selecting Data File

The selection of the data source radio buttons will determine the content of the data file 

dialog window. If a model simulation result file is loaded and the Predicted radio button

is selected the filenames will appear in the Predicted Data window. The user selects the 

file to obtain plotting information from by pressing the left mouse button on the filename. 

The selected file will become highlighted. Once the file has been selected the user needs 

to select the variable and segment from which to retrieve information for plotting. 

Selecting Variable

The variable list that is displayed in the variables dialog window is taken directly from

the selected file. If it is a model simulation result file it will contain the output variables 

of either the hydrodynamic or water quality model. 
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If the selected file is an "observed data" database, the variables are the names of stored 

constituents in the database. If the selected file was a calculated data structure, the name 

of the variable will be that assigned by the user when the calculated field was created.

To select and view the variables in list the user can use the scroll bar to move through the 

list. To select a variable to plot, the user should click the left mouse button on the desired 

variable to select. The selected variable will become highlighted.

Selecting Segment

The last item that has to be selected before an x/y plot can be created is what segment or 

computational element to plot. Once a Predicted Data file is selected, a list of the

available segments to plot is displayed in the Segment dialog window. To select and view

the segments in the list the user can use the scroll bar to move through the list. To select 

a segment to plot, the user should click the left mouse button on the desired segment. 

The selected variable will become highlighted. If the data source is an observed data

database the segment is typically a station identification number.

Representation

The representation radio button dialog box allows the user to assign a specific

characteristic for the defined curve. The user has the option of drawing a line, symbol, 

line & symbol or force solid for the defined curve.

Note: A different line style, color and symbol is automatically created for each 

line defined in a particular x/y plot window.

Second Y

The user may assign any number of the defined curves to the second y

2

 axis (scale

located on the right hand side of graph). When a second y

2 is requested, the curve

assigned to that y2 axis will be scaled and plotted independently of any curve assigned to 

y

1

 axis.

Observed Data

Observed data can be added to any x/y plot window. Before observed data is available 

for plotting the observed data database must have been opened for use by the user. 

Observed data is opened using the open file dialog box. Once the observed data database 

has been opened the data will be available for plotting in the x/y plot window. To select 

observe data to plot, the user should press the Observed radio button. If there is more 
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than one database file loaded, the user will be presented a choice of file in the predicted 

data window. The user should select the database file, select the variable from the

variable window and then select the Station ID from the segment window. 

Note: Observed data can be plotted exclusively if the user desires.

Legend Description

The user can control the information that is automatically placed in the x/y plot legend

that is used to illustrate the contents of the graph. The user has four options for the

legend:

1) Data Set – setting this radio button will cause the use of the dataset name 

selected in the Predicted Data dialog window for the legend of the defined 

line.

2) Variable -- setting this radio button will cause the use of the variable name 

selected in the Variable dialog window for the legend of the defined line.

3) Segment -- setting this radio button will cause the use of the segment name 

selected in the Segment dialog window for the legend of the defined line.

4) User-Defined -- setting this radio button will cause the use of the userdefined item to be placed in the User-Defined Dialog box. 

Note: If the user does not want a legend, the user should select “user define” 

and not type anything into the user defined dialog box.

4.4.4. OK/Cancel

Once this information was been selected along with any other user definable parameters

the user should press Okay to generate the graph. The observed data will be plotted on 

the x/y graph along with any predicted or user selected data. If they press cancel, all the 

information entered will be lost.
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4.4.5. Zooming the Axes

The user can zoom any of the axes within the x/y plot window. The user can zoom the xaxis, y

1

-axis and y

2

-axis exclusively or in combination. Zooming the axis allows the user 

to view data in smaller time or concentration scales to visualize subtle changes in the

model results.

Zooming X Axis

The user has several options available performing a zoom function. The quickest and

most efficient way to zoom the axis is to place the mouse cursor close to the x-axis line 

(within the plot area) at the beginning time of the area to zoom. Then press and hold the 

left mouse and paint the area to zoom by dragging a box to the end time. The zoomed 

area will be painted.

Figure 4-15 Zooming the x Axis

Upon releasing the left mouse button, after the user has defined the area to zoom, the x/y 

plot will be redrawn for the given time period selected by the user. Figure 4-16 illustrates 

a zoomed x-axis, from the example given above.
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Figure 4-16 Zoomed x Axis

Note: The user can zoom the axis in steps (i.e. zoom look at the graph and then 

zoom the zoomed graph some more). The user also has two other functions 

available from the toolbar: un-zoom x/y plot, or return to previous zoomed level.

Zooming the Y

The y-axes may also be zoomed. The user has several options available to indicate the

desire to perform a zoom function. If only one y-axis has been defined, the axis can be 

zoomed just like the X. To zoom the y-axis, place the mouse cursor close to the y-axis 

line (within the plot area) at the beginning concentration of the area. To zoom, press and 

hold the right mouse and paint the area to zoom by dragging a box to the end

concentration (Figure 4-17). The area that will be zoomed will be painted. 
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Figure 4-17 Zooming the y Axis

Upon releasing the right mouse button, after the user has defined the area to zoom, the 

x/y plot will be redrawn for the given y concentration range. Figure 4-18 illustrates a

zoomed y-axis, from the example given above.

If the user has defined more than one y-axis, the zooming function becomes a little

trickier to perform. If the zooming function on the speed menu is set to Auto-Detect (the 

default), the Graphical Post-Processor will determine which axes (x, y1, or y2) the user is 

trying to zoom based upon the position of the mouse at the time the left mouse button is 

pressed. If the user is closer to the Y1 axis and presses the right mouse button and drags a 

box vertically, it assumes the user wants to zoom the Y1 axis. The converse is true for the 

y

2

 axis. If the user is closer to the x-axis and drags a box horizontally, it is assumed the 

user wants to zoom the x-axis. 
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Figure 4-18 Zoomed y Axis

Note: If the user is having trouble controlling which axis to zoom, the speed 

menu allows the user to select which axis to zoom.

4.4.6. Adding an Additional Curve

The user has the option of adding as many curves as desired to any given x/y plot

window. There is no limit to the number of curves that can be defined. It is

recommended that no more than five curves be defined for a given x/y plot as resolution 

and comprehension will be diminished.

Note: The user can create as many curves per x/y plot and x/y plot windows as 

desired.
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4.4.7. Color/Black & White View

The x/y plots can either be displayed in black & white or color depending upon user 

preference or intentions. To toggle between these two modes the user can press the black 

and white icon from the toolbar, toggle the radio box in the configuration menu or use the 

speed menu. An example of a black & white plot is given in Figure 4-19. When the user 

requests that a black & white graph be printed or copied to the clipboard, a black & white 

image is generated.

Figure 4-19 Example of Black & White Graph

4.4.8. Observed/Measured Data

Overview

The user may plot observed/measured data against that predicted by the model. Observed 

data have to be stored in a particular file format to be available for plotting. The file

formats are Paradox 4.5 or higher database tables. Also, the Paradox database must

contain at least four requisite field names. These field names are used to align the data 

from the database to the dialog boxes of the x/y plot curve parameters.
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Creating a Database

The user has several options available for creating the observed data database. If the user 

has Paradox they can use it to create the observed data database. If the user does not 

have Paradox, the user is provided the option of creating an observed data database. If 

the user elects to create the observed data database in Paradox there are four important 

fields that the database must contain:

1) DateTime – this field is of type Timestamp. It is used to store the date 

& time of the observed data point.

2) PCODE – this field is of type alphanumeric. It is used to describe the 

type of measurement being stored (i.e. Dissolved Oxygen).

3) STATION_ID -- this field is of type alphanumeric. It is used to 

describe the sample station identification.

4) RESULT -- this field is of type numeric. It is used to describe the 

measured value of PCODE at time DATETIME at STATION_ID.

The four fields above have to be defined exactly as described to be useful. Any variance 

from what is given above and the file will not be recognized. The observed data database 

can contain more than the four fields described above, but at a minimum must contain

these fields.

To create a new observed data database the user should press the Create New Database 

icon on the main toolbar. This will cause a new observed data database table to be 

generated that the user can populate with their own data. Figure 4-20 illustrates a newly 

created observed data database table. The user must populate the database with their own 

data by using the insert record function (pressing the + sign). The user can paste data into 

this table from other applications as well. To do this the user needs to insert as many

records that they will be pasting into the table.
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Figure 4-20 Creating Observed Data Database

Loading a Database

Once an observed data database has been created it must be loaded like any other file 

before it is available for plotting. To load an observed data database file, the user should 

select the open file icon or select it from the file menu. Upon doing this a file dialog box 

will appear, the user should set the file type to that of *.DB. The user should navigate to 

the drive and directory where the observed data database is stored and select the file and 

press open. Once the contents of the observed data database are read into memory they 

are available for plotting.

Selecting Data

Selecting data from the observed data database file is done in the same manner as

selecting data from the model simulation results files. The user should select Add Curve 

from the x/y plot configuration menu, when the curve attribute menu appears the user

should select the Observed Data radio button. Once the radio button has been activated 

the previously loaded DB file name should appear in the data file dialog box. The user 

should highlight the DB filename with the mouse, select the database PCODE and
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Station_ID to plot. Once these items have been selected the user should press the Okay 

button and the graph will be re-drawn with observed data included.

Note: The user has the same control over the observed data’s appearance as 

the simulation results.

4.4.9. Printing Results

The user has the option of printing the currently active x/y plot to any installed Windows 

printer device (this could be a laser printer, color printer, fax, or even e-mail). The user 

has the option of either printing the x/y plots in full color or changing them to black and 

white before printing. Depending upon the printer and the users Window’s 95 setup, the 

printer may automatically convert the colors to gray scale before printing. This may or

may not be advantageous, if the graphs appear “muddy” the user should convert the

graphs to black and white before printing to a printer, saving to a file or copying to a 

clipboard.

To change a plot to black and white the user should either select the Black and White 

radio button from the configuration menu or press the Black & White/Color toggle icon

from the toolbar.

To print the currently active x/y plot the user should press the print icon from the toolbar. 

A standard windows print dialog box will appear. The user can select the appropriate

output device to print the figure.

To File

The user has the option of saving a bitmap file of the currently active x/y plot to a file. 

This is useful for saving simulation results for comparison or inclusion into a presentation

or publication. 

To save the currently active x/y plot to a file as a bitmap, the user should press the save 

graph to bitmap icon from the toolbar. The user will be given a standard windows file 

dialog box that allows the user to designate the drive, directory and filename.

 To Clipboard

The user also has the ability to copy a graphic image of the currently active x/y plot to the 

Windows clipboard. Once an image is copied to the Windows clipboard it can be pasted 

into virtually any Windows program that has graphics support. 
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To copy the graphics image to the clipboard the user should press the copy to clipboard 

icon from the toolbar. Once the image has been copied it is available for pasting.

4.4.10. Creating Tabled Data

Internal data structures are utilized to store the actual numbers that are used to generate 

the x/y plots. A method is provided for the user to create a table of the data that is 

represented in the current x/y plot. Pressing the data table icon on the x/y plot toolbar 

creates this table. This will cause the creation of a table of data as illustrated in Figure 

4-21. The data in this table is “read-only”. This basically means the user can access the 

data by marking columns and rows of data and copying it to the Windows clipboard so 

that it can be pasted into another application (i.e. spreadsheet).

Figure 4-21 Example of Tabular Data from Graph

The user has no ability to modify the data and have the results appear in the x/y plot.

Exporting Data

The user may export the data that is used to generate the active x/y plot to an external file 

to 1) comma delimited ASCII files and 2) Paradox database files.
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The comma delimited ASCII file type writes the data from the graph in column format,

each curve will consist of two columns (x & y values). The data is separated by commas 

and can be directly imported into most spreadsheet programs.

The Paradox file format creates a Paradox compatible database file of all the data in the 

x/y plot. This database table that is created can be read directly by Paradox or the

Deliberator program (a component of WASP). Each x/y pair from the graph is given it’s 

own record in the database. 

To initiate the export data option the user should press the export data icon on the x/y plot 

toolbar. This will bring up a file dialog box that allows the user to define the path and 

filename to save the exported data.

To select the file type in which to save the data the user should use the drop down list 

from the Save As type in the dialog box. Chose the appropriate type and give the file a 

name and press save.

4.4.11. Curve Calculations

A wide range of calculations can be performed on defined curves on the x/y plot. The 

curve calculation screen is entered by pressing the curve calculate button on the x/y plot 

toolbar. Curve calculations can only be performed on defined curves within the x/y plot 

window. The are several types of curve calculations that can be performed:

1) User Defined – functions that the user can derive to make calculation

2) Moving Average – user specifies the time interval for the moving average 

and generates a new curve depicting the results.

3) Frequency Distribution – the user specifies the number of concentration 

intervals to be used in the frequency distribution and calculation of the 

new curves.
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Figure 4-22 Example of Curve Calculation

The curve calculation screen has several dialog boxes that provide information to the user 

as well as allow the user to specify information and operations.

Curve Source

The curve source dialog allows the user to select which loaded data source will be used 

for defining a curve.

Calculation Type

The user has the option of selecting the type of calculation that should be performed. The 

calculation type is selected from a drop down picklist as illustrated in Figure 4-23. The 

user has the option of user defined, moving average or frequency distribution.
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Figure 4-23 Built in Curve Calculation Functions

Tolerance

The tolerance factor is used to sets the range of time that would consider point to be at the 

same time.

Number of Output Points

Defines the number of output points that will be produced for the curve calculations.

Calculation Names

The calculation name dialog box provides a means for the user to give a meaningful

name to the calculation. This is an important step, because once the calculation is made, 

to display the calculation in the x/y plot the user will need to add a curve to the current or 

new x/y plot window. To do this, the user would select the calculate type, from the curve 

parameter window. Once this is selected a picklist is provided of available calculations. 

This picklist contains the calculation name as defined by the user.
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Segment Time/Number

These dialog box sets the label displayed on the graph for the segment/time axis.

Data Name

This dialog box sets the label for the value in the x/y plot legend.

Value Expression

This dialog box is used to define the function that will be performed on the values.

Time Expression

This dialog box is used to define the function that will be performed on the time values.

The following tables provide information for the pre-defined variables and functions that

are available.

Variable Pre-Defined Variables

X Value of cell being calculated or X-value of curve point being calculated

Y Y-value of curve point being calculated

R Row number (1 - Number of rows) of cell being calculated

C Column number (1 - Number of columns) of cell being calculated

NUMPOINTS Returns the number of points in the reference curve.

I Current point number being calculated

REFCURVE Curve Selected in List box of Curve Calculator

PI PI (One half the circumference of a unit circle : 3.141592)

SECSPERDAY Number of seconds in a day (86400)

NUMCURVES Number of Curves on Graph

E Base of Natural Log

CELL(row, column) Value of cell located at (row, column)

ABS(x) Absolute value of the expression x

FLOOR(x) The largest integer not greater than the expression x

CEIL(x) The smallest integer not less than the expression x

POW(x, y) x raised to the y power

SQRT(x) The square root of the expression x

FT3M3(x) Conversion of the expression x from cubic feet to cubic meters

M3FT3(x) Conversion of the expression x from cubic meters to cubic feet

FTM(x) Conversion of the expression x from feet to meters 

MFT(x) Conversion of the expression x from meters to feet

FT2M2(x) Conversion of the expression x from square feet to square meters

M2FT2(x) Conversion of the expression x from square meters to square feet

MGDCFS(x) Conversion of the expression x from million-gals/day to cubic feet/sec

SIN(x) Sin of x (where x is in radians)

COS(x) Cos of x (where x is in radians)

LOG(x) Log of x
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LN(x) Natural Log of x

CFSGM3LBSDAY(x) Converts flow (cfs) * concentration (g/m3) to Lbs/day

CFSGM3KGDAY(x) Converts flow (cfs) * concentration (g/m3) to kg/day 

CMSGM3KGDAY(x) Converts flow (cms) * concentration (g/m3) to kg/day

MAX(c) Maximum Number in the Curve

MIN(c) Minimum Number in the Curve

STDDEV(c) Standard Deviation of Curve

AVG(c) Average Value for Curve

MEDIAN(c) Median Value for Curve

MEAN(c) Mean value for Curve

VARIANCE(c) Variance Value for Curve

MODE(c) Mode value for curve

X(c) X value of any curve at current point

Y(c) Y value of any curve at current point

NUMPOINTS(c) Number of Points in Curve

X(c, p) X value of any curve at any point 

Y(c, p) Y value of any curve at any point 

INTERPOLATE(c, x) Interpolates the value of curve c at domain value x 

DATE(m, d, y, h, m, s) Returns x value for a given date 

SUM(c) Sum of the values in the curve

Frequency/Running Average

These are built-in calculations. They work much the same as the user defined with the 

exception the user does not have to enter any functions, they are done automatically.

Once this dialog box has been completed and the user has pressed the Okay button, this 

partition calculation can now be plotted. To plot the partition calculation the user should 

press the Add Curve button. The curve parameter dialog box will appear. The user

should select the Calculate radio button. Once Calculate is selected the user should see 

the name of the calculation just performed appear in the file dialog box. The user can

then select the variable and location to plot. Once these are selected and the user presses 

"Okay", the x/y plot window will re-appear with the calculated data plotted as well.
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5. The Basic Water Quality Model 

WASP6 is a dynamic compartment model that can be used to analyze a variety of water 

quality problems in such diverse water bodies as ponds, streams, lakes, reservoirs, rivers, 

estuaries, and coastal waters. This section presents an overview of the basic water quality 

model. Subsequent chapters detail the transport and transformation processes in WASP6 

for various water quality constituents.

The equations solved by WASP6 are based on the key principle of the conservation of

mass. This principle requires that the mass of each water quality constituent being

investigated must be accounted for in one way or another. WASP6 traces each water

quality constituent from the point of spatial and temporal input to its final point of export, 

conserving mass in space and time. To perform these mass balance computations, the 

user must supply WASP6 with input data defining seven important characteristics:

• simulation and output control

• model segmentation

• advective and dispersive transport

• boundary concentrations

• point and diffuse source waste loads

• kinetic parameters, constants, and time functions

• initial concentrations

These input data, together with the general WASP6 mass balance equations and the

specific chemical kinetics equations, uniquely define a special set of water quality

equations. These are numerically integrated by WASP6 as the simulation proceeds in

time. At user-specified print intervals, WASP6 saves the values of all display variables

for subsequent retrieval by the post-processor program. These programs allow the user to 

interactively produce graphs and tables of variables of all display variables. 

5.1. General Mass Balance Equation

A mass balance equation for dissolved constituents in a body of water must account for 

all the material entering and leaving through direct and diffuse loading; advective and

dispersive transport; and physical, chemical, and biological transformation. Consider the

coordinate system shown in Equation 5-1, where the x- and y-coordinates are in the

horizontal plane, and the z-coordinate is in the vertical plane. 
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Figure 5-1 Coordinate System for Mass Balance Equation

The mass balance equation around an infinitesimally small fluid volume is: 

Equation 5-1 General Mass Balance Equation

 +S +S +S 

)

z

C

( E

z

)+

y

C

( E

y

)+

x

C

( E

x

 +

(U C)

z

(U C)-

y

(U C)-

x

= -

t

C

L B K

x y z

x y z

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

∂

where:

C = concentration of the water quality constituent, mg/L or g/m3

t = time, days

Ux,Uy,Uz = longitudinal, lateral, and vertical advective velocities, m/day

Ex,Ey,Ez = longitudinal, lateral, and vertical diffusion coefficients,

m

2

/day

SL = direct and diffuse loading rate, g/m3-day

SB = boundary loading rate (including upstream, downstream,

benthic, and atmospheric), g/m3-day

SK = total kinetic transformation rate; positive is source, negative is 
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sink, g/m3-day

By expanding the infinitesimally small control volumes into larger adjoining "segments,"

and by specifying proper transport, loading, and transformation parameters, WASP

implements a finite-difference form of Equation 5-1. For brevity and clarity, however,

the derivation of the finite-difference form of the mass balance equation will be for a

one-dimensional reach. Assuming vertical and lateral homogeneity, we can integrate

over y and z to obtain Equation 5-2

Equation 5-2 WASP Implementation of the Finite Difference Form of Mass Balance Equation

+ A( S +S )+ A S

x

C

-U AC +E A

x

(A C)=

t

x x  L B K











∂

∂

∂

∂

∂

∂

where:

A = cross-sectional area, m2

This equation represents the three major classes of water quality processes -- transport 

(term 1), loading (term 2), and transformation (term 3). The finite-difference form is

derived in Appendix E. The model network and the major processes are discussed in the 

following sections.

5.2. The Model Network

The model network is a set of expanded control volumes, or “segments,” that together

represents the physical configuration of the water body. As Figure 5-2 illustrates, the

network may subdivide the water body laterally and vertically as well as longitudinally. 

Benthic segments can be included along with water column segments. If the water

quality model is being linked to the hydrodynamic model, then water column segments 

must correspond to the hydrodynamic junctions. Concentrations of water quality

constituents are calculated within each segment. Transport rates of water quality

constituents are calculated across the interface of adjoining segments.
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Figure 5-2 Model Segmentation Schematic

Segments in WASP may be one of four types, as specified by the input variable ITYPE. 

A value of 1 indicates the epilimnion (surface water), 2 indicate hypolimnion layers

(subsurface), 3 indicate an upper benthic layer, and 4 indicate lower benthic layers. The 

segment type plays an important role in bed sedimentation and in certain transformation

processes. The user should be careful to align segments properly. The segment

immediately below each segment is specified by the input variable IBOTSG. This

alignment is important when light needs to be passed from one segment to the next in the 

water column, or when material is buried or eroded in the bed.

Segment volumes and the simulation time step are directly related. As one increase or

decreases, the other must do the same to insure stability and numerical accuracy. 

Segment size can vary dramatically, as illustrated in Figure 5-3. Characteristic sizes are 

dictated more by the spatial and temporal scale of the problem being analyzed than by the 

characteristics of the water body or the pollutant per se. For example, analyzing a

problem involving the upstream tidal migration of a pollutant into a water supply might

require a time step of minutes to an hour. By contrast, analyzing a problem involving the 

total residence time of that pollutant in the same water body could allow a time step of

hours to a day. The first network was used to study the general eutrophic status of Lake 

Ontario. The second network was used to investigate the lake-wide spatial and seasonal 

variations in eutrophication. The third network was used to predict changes in near-shore 

eutrophication of Rochester Embayment resulting from specific pollution control plans.
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Figure 5-3 Spatial Scales used in Lake Ontario Analysis

As part of the problem definition, the user must determine how much of the water quality

frequency distribution must be predicted. For example, a daily-average dissolved oxygen 

concentration of 5 mg/L would not sufficiently protect fish if fluctuations result in

concentrations less than 2 mg/L for 10% of the time. Predicting extreme concentration

values is generally more difficult than predicting average values. Figure 5-4 illustrates

typical frequency distributions predicted by three model time scales and a typical

distribution observed by rather thorough sampling, as they would be plotted on

probability paper. The straight lines imply normal distributions. Reducing the model

time step (and consequently segment size) allows better simulation of the frequency

distribution. This increase in predictive ability, however, also entails an increase in the

resolution of the input data.
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Figure 5-4 Frequency Distribution of Observed and Calculated Values of a Quality Variable

Once the nature of the problem has been determined, then the temporal variability of the 

water body and input loadings must be considered. Generally, the model time step must 

be somewhat less than the period of variation of the important driving variables. In some 

cases, this restriction can be relaxed by averaging the input over its period of variation. 

For example, phytoplankton growth is driven by sunlight, which varies diurnally. Most

eutrophication models, however, average the light input over a day, allowing time steps

on the order of a day.

Care must be taken so that important non-linear interactions do not get averaged out. 

When two or more important driving variables have a similar period of variation, then

averaging may not be possible. One example is the seasonal variability of light,

temperature, nutrient input, and transport in lakes subject to eutrophication. Another

example involves discontinuous batch discharges. Such an input into a large lake might

safely be averaged over a day or week, because large-scale transport variations are

relatively infrequent. The same batch input into a tidal estuary cannot safely be averaged, 

however, because of the semi-diurnal or diurnal tidal variations. A third example is

salinity intrusion in estuaries. Tidal variations in flow, volume, and dispersion can

interact so that accurate long-term predictions require explicit simulation at time steps on

the order of hours.

Once the temporal variability has been determined, then the spatial variability of the

water body must be considered. Generally, the important spatial characteristics must be 

homogeneous within a segment. In some cases, this restriction can be relaxed by

judicious averaging over width, depth, and/or length. For example, depth governs the

impact of reaeration and sediment oxygen demand in a column of water. Nevertheless, 

averaging the depth across a river would generally be acceptable in a conventional waste 
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load allocation, whereas averaging the depth across a lake would not generally be

acceptable. Other important spatial characteristics to consider (depending upon the

problem being analyzed) include temperature, light penetration, velocity, pH, benthic

characteristics or fluxes, and sediment concentrations.

The expected spatial variability of the water quality concentrations also affects the

segment sizes. The user must determine how much averaging of the concentration

gradients is acceptable. Because water quality conditions change rapidly near a loading

point and stabilize downstream, studying the effects on a beach a quarter-mile 

downstream of a discharge requires smaller segments than studying the effects on a beach 

several miles away. 

A final, general guideline may be helpful in obtaining accurate simulations: water

column volumes should be roughly the same. If flows vary significantly downstream,

then segment volumes should increase proportionately. The user should first choose the

proper segment volume and time step in the critical reaches of the water body (Vc, Ätc), 

and then scale upstream and downstream segments accordingly:

V =V Q i

/ Q c

i c

Of course, actual volumes specified must be adjusted to best represent the actual spatial

variability, as discussed above. This guideline will allow larger time steps and result in

greater numerical accuracy over the entire model network, as explained in the section on

"Simulation Parameters" in Chapter 2.

5.3. The Model Transport Scheme

Transport includes advection and dispersion of water quality constituents. Advection and 

dispersion in WASP are each divided into six distinct types, or "fields." The first

transport field involves advective flow and dispersive mixing in the water column. 

Advective flow carries water quality constituents "downstream" with the water and

accounts for instream dilution. Dispersion causes further mixing and dilution between

regions of high concentrations and regions of low concentrations. 

The second transport field specifies the movement of pore water in the sediment bed. 

Dissolved water quality constituents are carried through the bed by pore water flow and 

are exchanged between the bed and the water column by pore water diffusion. 

The third, fourth, and fifth transport fields specify the transport of particulate pollutants

by the settling, resuspension, and sedimentation of solids. Water quality constituents

sorbed onto solid particles are transported between the water column and the sediment

bed. The user can define the three solids fields as size fractions, such as sand, silt, and 

clay, or as inorganic, phytoplankton, and organic solids. 

The sixth transport field represents evaporation or precipitation from or to surface water

segments.
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Most transport data, such as flows or settling velocities, must be specified by the user in a 

WASP input dataset. For water column flow, however, the user may "link" WASP with a 

hydrodynamics model. If this option is specified, during the simulation WASP will read

the contents of a hydrodynamic file for unsteady flows, volumes, depths, and velocities.

5.4. Application of the Model

The first step in applying the model is analyzing the problem to be solved. What

questions are being asked? How can a simulation model be used to address these

questions? A water quality model can do three basic tasks-- describe present water

quality conditions, provide generic predictions, and provide site-specific predictions. The 

first, descriptive task is to extend in some way a limited site-specific database. Because 

monitoring is expensive, data seldom give the spatial and temporal resolution needed to

fully characterize a water body. A simulation model can be used to interpolate between

observed data, locating, for example, the dissolved oxygen sag point in a river or the

maximum salinity intrusion in an estuary. Of course such a model can be used to guide 

future monitoring efforts. Descriptive models also can be used to infer the important

processes controlling present water quality. This information can be used to guide not

only monitoring efforts, but also model development efforts.

Providing generic predictions is a second type of modeling task. Site-specific data may

not be needed if the goal is to predict the types of water bodies at risk from a new

chemical. A crude set of data may be adequate to screen a list of chemicals for potential 

risk to a particular water body. Generic predictions may sufficiently address the

management problem to be solved, or they may be a preliminary step in detailed

site-specific analyses.

Providing site-specific predictions is the most stringent modeling task. Calibration to a

good set of monitoring data is definitely needed to provide credible predictions. Because 

predictions often attempt to extrapolate beyond the present database, however, the model

also must have sufficient process integrity. Examples of this type of application include

waste load allocation to protect water quality standards and feasibility analysis for

remedial actions, such as tertiary treatment, phosphate bans, or agricultural

best-management practices.

Analysis of the problem should dictate the spatial and temporal scales for the modeling

analysis. Division of the water body into appropriately sized segments was discussed in

Section "Model Network." The user must try to extend the network upstream and

downstream beyond the influence of the waste loads being studied. If this is not possible, 

the user should extend the network far enough so that errors in specifying future

boundary concentrations do not propagate into the reaches being studied.

The user also should consider aligning the network so that sampling stations and points of 

interest (such as water withdrawals) fall near the center of a segment. Point source waste 

loads in streams and rivers with unidirectional flow should be located near the upper end 

of a segment. In estuaries and other water bodies with oscillating flow, waste loads are 

best centered within segments. If flows are to be input from DYNHYD, then a WASP 
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segment must coincide with each hydrodynamic junction. Benthic segments, which are

not present in the hydrodynamic network, may nevertheless be included in the WASP

network. WASP segment numbering does not have to be the same as DYNHYD junction 

numbering. Segments stacked vertically do not have to be numbered consecutively from

surface water segments down.

Once the network is set up, the model study will proceed through four general steps

involving, in some manner, hydrodynamics, mass transport, water quality

transformations, and environmental toxicology. The first step addresses the question of

where the water goes. This can be answered by a combination of gaging, special studies, 

and hydrodynamic modeling. Flow data can be interpolated or extrapolated using the

principle of continuity. Very simple flow routing models can be used; very complicated

multi-dimensional hydrodynamic models can also be used with proper averaging over

time and space. At present, the most compatible hydrodynamic model is DYNHYD.

The second step answers the question of where the material in the water is transported. 

This can be answered by a combination of tracer studies and model calibration. Dye and 

salinity are often used as tracers.

The third step answers the question of how the material in the water and sediment is

transformed and what its fate is. This is the main focus of many studies. Answers

depend on a combination of laboratory studies, field monitoring, parameter estimation,

calibration, and testing. The net result is sometimes called model validation or

verification, which are elusive concepts. The success of this step depends on the skill of

the user, who must combine specialized knowledge with common sense and skepticism

into a methodical process.

The final step answers the question of how this material is likely to affect anything of

interest, such as people, fish, or the ecological balance. Often, predicted concentrations 

are simply compared with water quality criteria adopted to protect the general aquatic

community. Care must be taken to insure that the temporal and spatial scales assumed in 

developing the criteria are compatible with those predicted by the model. Sometimes

principles of physical chemistry or pharmacokinetics are used to predict chemical body

burdens and resulting biological effects. The bioaccumulation model FGETS (Barber, et

al., 1991) and the WASTOX food chain model (Connolly and Thomann, 1985) are good 

examples of this.
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6. Chemical Tracer Transport

A chemical tracer is a nonreactive chemical that is passively transported throughout the

water body. Examples include salinity or chlorides. Special dyes are used as tracers,

although these often decay at a slow rate. Setting up and calibrating a tracer is the first 

step in simulating more complex water quality variables.

6.1. Overview of WASP6 Tracer Transport

A conservative tracer is generally simulated using the TOXI program. TOXI simulates

the transport and transformation of one to three chemicals and one to three types of solids 

classes (Table 6-1). To simulate a tracer, the user should bypass solids and simulate

chemical 1 with no decay. A tracer is affected by transport, boundary, and loading

processes only, as described below.

Table 6-1 State Variables in Organic Chemical Model (TOXI)

System Variable

1 Chemical 1

2 Solids 1

3 Solids 2

4 Solids 3

5 Chemical 2

6 Chemical 3

WASP6 uses a mass balance equation to calculate chemical mass and concentrations for 

every segment in a specialized network that may include surface water, underlying water, 

surface bed, and underlying bed. Simulated chemicals undergo several transport

processes 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. Dissolved chemicals migrate downward or upward through

percolation and pore water diffusion. 

The transport, boundary, and loading processes for tracer chemicals are described below. 

These same processes are also applied to the water quality variables described in

subsequent chapters.

6.2. Transport Processes

Advective water column flows directly control the transport of dissolved and particulate

pollutants in many water bodies. In addition, changes in velocity and depth resulting

from variable flows can affect such kinetic processes as reaeration, volatilization, and

photolysis. An important early step in any modeling study is to describe or simulate

water column advection properly. In WASP6, water column flow is input via transport

field one. Circulation patterns may be described (flow options 1 and 2) or simulated by a 

hydrodynamic model, such as DYNHYD. Flow options are specified in Section 3-13.
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For descriptive flows, WASP6 tracks each separate inflow specified by the user from its 

point of origin through the model network. For each inflow, the user must supply a

continuity or unit flow response function and a time function. The time function

describes the inflow as it varies in time. The continuity function describes the unit flow

response as it varies throughout the network. The actual flow between segments that

results from the inflow is the product of the time function and the continuity function. 

If several inflow functions are specified, then the total flow between segments is the sum

of the individual flow functions. Segment volumes are adjusted to maintain continuity. 

In this manner, the effect of several tributaries, density currents, and wind-induced gyres 

can be described. 

In flow (Figure 3-6) Net Flow Option, WASP6 sums all the flows at a segment interface 

to determine the direction of net flow, and then moves mass in the ONE direction. In

Gross Flow Option, WASP6 moves mass independently of net flow. For example, if

opposite flows are specified at an interface, WASP6 will move mass in BOTH

directions. This option allows the user to describe large dispersive circulation patterns.

6.2.1. Hydrodynamic Linkage

For unsteady flow in long networks, lag times may become significant, and

hydrodynamic simulations may be necessary to obtain sufficient accuracy. Realistic

simulations of unsteady transport can be accomplished by linking WASP6 to a

compatible hydrodynamic simulation. This linkage is accomplished through an external 

file chosen by the user at simulation time. The hydrodynamic file contains segment

volumes at the beginning of each time step, and average segment interfacial flows during

each time step. WASP6 uses the interfacial flows to calculate mass transport, and the 

volumes to calculate constituent concentrations. Segment depths and velocities may also

be contained in the hydrodynamic file for use in calculating reaeration and volatilization

rates.

The first step in the hydrodynamic linkage is to develop a hydrodynamic calculational

network that is compatible with the WASP6 network. The easiest linkage is with linknode hydrodynamic models that run on equivalent spatial networks. An example is given

in Figure 6-1. Note that each WASP6 segment corresponds exactly to a hydrodynamic 

volume element, or node. Each WASP6 segment interface corresponds exactly to a

hydrodynamic link, denoted in the figure with a connecting line.
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Figure 6-1 Link-Node Hydrodynamic Linkage with WASP

The hydrodynamic model calculates flow through the links and volume within the nodes. 

Within the hydrodynamic model, the user must specify the water quality time step, or the 

number of hydrodynamic time steps per water quality time step. The hydrodynamic

model must then write out node volumes at the beginning of each water quality time step, 

and average link flows during each water quality time step. The user in the

hydrodynamic model or in an external interface program must supply a network map

such as the one in Figure 6-1. This map is used to create a hydrodynamic file that

WASP6 can read and interpret. The hydrodynamic model DYNHYD5, supplied with

WASP6, contains subroutines to produce a proper WASP6 hydrodynamic file.

It is important to note that the hydrodynamic model has additional nodes outside of the

WASP6 network. These additional nodes correspond to WASP6 boundaries, denoted by 

nominal segment number "0." These extra hydrodynamic nodes are necessary because

while flows are calculated only within the hydrodynamic network, WASP6 requires

boundary flows from outside its network. 

Multidimensional hydrodynamic models can also be linked to WASP6. A compatible

two-dimensional network is illustrated in Figure 2.2. For the beginning of each water

quality time step, the volumes within a WASP6 segment must be summed and written to 

the hydrodynamic file. For the duration of each water quality time step, flows across the 

WASP6 segment boundaries must be averaged. All of the averaged flows across a

boundary must then be summed and written to the hydrodynamic file. Again, it is

important to note the presence of hydrodynamic elements outside the WASP6 network

generating boundary flows. The preprocessor will determine the boundary segments
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from reading the hydrodynamic linkage file. The user will then be able to enter

concentrations associated with each of these boundaries (Figure 3-15).

To implement the hydrodynamic linkage, the user must specify Hydrodynamic Linkage

and select a previously created hydrodynamic linkage file. Following the choice of a

proper file, the hydrodynamic file will reset the simulation time step. The time steps read 

in Figure 3-18 will be ignored but must still be entered, as this is where the user specifies 

the ending time for the simulation. Similarly, water column segment volumes will be

read from the hydrodynamic file. The user must nevertheless enter volumes for each

segment in the usual location. During the simulation, flows and volumes are read every

time step.

6.2.2. Hydraulic Geometry

A good description of segment geometry as a function of flow conditions can be

important in properly using WASP6 to simulate rivers. For flow option 3, velocity and 

depth are computed within the hydrodynamic model, and are read by WASP6. For flow 

options 1 and 2, a set of user-specified hydraulic discharge coefficients can be entered in 

Figure 3-9 that defines the relationship between velocity, depth, and stream flow in the

various segments. This method, described below, follows the implementation in

QUAL2E (Brown and Barnwell, 1987). In WASP6, these segment velocities and depths 

are only used for calculations of reaeration and volatilization rates; they are not used in

the transport scheme.

Discharge coefficients giving depth and velocity from stream flow are based on empirical

observations of the stream flow relationship with velocity and depth (Leopold and

Maddox, 1953). It is important to note that these coefficients are only important when

calculating reaeration or volatilization. The velocity calculations are not used in time of

travel, and will not affect the simulation of tracers. The equations relate velocity, channel 

width, and depth to stream flow through power functions:

Equation 6-1 Hydraulic Coefficients

V =a Q

b

Equation 6-2

D= c Q

b

Equation 6-3

B=e Q

f

where:

D is average depth, m
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B is average width, m

a, b, c, d, e, and f are empirical coefficients or exponents 

Given that area is a function of average width (B) and average depth (D),

Equation 6-4

A= D B

it is clear from continuity that:

Equation 6-5

Q=U A=U D B=(a Q ) (c Q ) (e Q )=(a c e) Q

b d f b+ d + f

• • • • • • •

and, therefore, the following relationships hold:

Equation 6-6

a c e=1

Equation 6-7

b+d + f =1

WASP6 only requires specification of the relationships for velocity, Equation 6-1, and

depth, Equation 6-2; the coefficients for Equation 6-3 are implicitly specified by

Equation 6-6 and Equation 6-7.

These options can be put into perspective by noting that, for a given specific channel

cross-section, the coefficients (a, c, e) and exponents (b, d, f) can be derived from

Manning’s equation. For example, if a channel of rectangular cross-section is assumed, 

then width (B) is not a function of stream flow (Q), the exponent (f) is zero (0.00) and the 

coefficient (e) is the width of the rectangular channel (B). By noting that hydraulic radius 

(R) is approximately equal to depth (D) for wide streams and that A = D B, the discharge 

coefficients for rectangular cross sections can be shown to be 0.4 for velocity and 0.6 for 

width.

Leopold et al. (1964) have noted that stream channels in humid regions tend towards a 

rectangular cross-section because cohesive soils promote steep side slopes whereas

noncohesive soils encourage shallow sloped, almost undefined banks.

Table 6-2 Comparison of Hydraulic Exponents

Channel Cross-Section

Exponent for 

(b) Velocity

Exponent

For (d)

Depth

Exponent for (f) 

Width

Rectangular 0.40 0.60 0.00
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Channel Cross-Section

Exponent for 

(b) Velocity

Exponent

For (d)

Depth

Exponent for (f) 

Width

Average of 158 U.S. Gaging Stations 0.43 0.45 0.12

Average of 10 Gaging Stations on Rhine 

River 0.43 0.41 0.13

Ephemeral Streams in Semiarid U.S. 0.34 0.36 0.29

Table 6-2 compares hydraulic exponents for a rectangular channel with data reported by

Leopold et al. (1964). Note that the average velocity exponent is relatively constant for 

all channel cross sections. The major variation occurs as a decrease in the depth exponent 

and concomitant increase in the width exponent as channel cross-sections change from

the steep side slopes characteristic of cohesive soils to the shallow slopes of arid regions 

with noncohesive soils.

For bodies of water such as ponds, lakes, and reservoirs, velocity and depth may not be a 

function of flow. For these cases, both the velocity and depth exponents (b and d) can be 

chosen to be zero (0.00). Because Q to the zero power is equal to one (1.0), the

coefficients a and c must be the velocity and depth, i.e.,

IF b = 0.0 THEN a = V, and

IF d = 0.0 THEN c = D. 

When the depth exponent is zero, WASP6 will adjust segment depths with segment

volumes assuming rectangular sides.

For site-specific river or stream simulations, hydraulic coefficients and exponents must

be estimated. Brown and Barnwell (1987) recommended estimating the exponents (b and 

d) and then calibrating the coefficients (a and c) to observed velocity and depth. The

exponents may be chosen based on observations of channel shape noted in a

reconnaissance survey. If cross sections are largely rectangular with vertical banks, the

first set of exponents shown should be useful. If channels have steep banks typical of

areas with cohesive soils, then the second set of exponents is appropriate. If the stream is 

in an arid region with typically noncohesive soils and shallow sloping banks, then the last 

set of exponents is recommended.

The key property of the channel that should be noted in a reconnaissance survey is the 

condition of the bank slopes or the extent to which width would increase with increasing

stream flow. Clearly the bank slopes and material in contact with the stream flow at the 

flow rate(s) of interest are the main characteristics to note in a reconnaissance. This gives 

general guidance but it should be noted that values are derived for bankful flows. Even

in streams with vertical banks, the low flows may be in contact with a sand bed having

shallow sloped, almost nonexistent banks more representative of ephemeral streams in

semi-arid areas.
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6.2.3. Pore Water Advection

Pore water flows into or out of the bed can significantly influence benthic pollutant

concentrations. Depending on the direction of these flows and the source of the

pollutants, pore water advection may be a source or sink of pollutants for the overlying

water column.

If benthic segments are included in the model network, the user may specify advective

transport of dissolved chemicals in the pore water. In WASP6, pore water flows are 

input via transport field two. Pore water advection transports water and dissolved

chemical; sediment and particulate chemical are not transported. The mass derivative of

chemical due to pore water flow from segment j to segment i is given by:

Equation 6-8

=Q f C / n

t

M

Dj jk jji

ik

∂

∂

where:

Mik = mass of chemical "k" in segment "i," g

Cjk = total concentration of chemical "k" in segment "j," mg/L 

(g/m3)

nj = porosity of segment j, Lw/L

fDj = dissolved fraction of chemical in segment "j"

Qji= pore water flow rate from j to i, m3/day

Dissolved fractions fD may be input by the user in Figure 3-11. In TOXI, these are

recomputed from sorption kinetics each time step.

WASP6 tracks each separate pore water inflow through the benthic network. For each

inflow (or outflow), the user must supply a continuity function and a time function. The 

actual flow through benthic segments that results from each inflow is a product of the

time function and the continuity function. If a flow originates in or empties into a

surface water segment, then a corresponding surface water flow function must be

described in flow field 1 that matches the pore water function.

6.2.4. Water Column Dispersion 

Dispersive water column exchanges significantly influence the transport of dissolved and

particulate pollutants in such water bodies as lakes, reservoirs, and estuaries. Even in

rivers, longitudinal dispersion can be the most important process diluting peak

concentrations that may result from unsteady loads or spills. Natural or artificial tracers

such as dye, salinity, or even heat are often used to calibrate dispersion coefficients for a 

model network.
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In WASP6, water column dispersion is input via transport field one in Figure 3-13. The 

user may define several groups of exchanges. For each group, the user must supply a

time function giving dispersion coefficient values (in m

2

/sec) as they vary in time. For

each exchange in the group, the user must supply an interfacial area, a characteristic

mixing length, and the adjoining segments between which the exchange takes place. The 

characteristic mixing length is typically the distance between the segment midpoints. 

The interfacial area is the area normal to the characteristic mixing length shared by the

exchanging segments (cross-sectional area for horizontal exchanges, or surface area for

vertical exchanges). The actual dispersive exchange between segments i and j at time t is 

given by:

Equation 6-9

( C - C )

L

E (t) A

=

t

M

jk ik

cij

ik ij • ij

∂

∂

where:

Mik = mass of chemical "k" in segment "i," g

Cik, Cjk= concentration of chemical "k" in segment "i" and "j," mg/L 

(g/m3)

Eij(t)= dispersion coefficient time function for exchange "ij", m2/day

Aij= interfacial area shared by segments "i" and "j," m2

Lcij= characteristic mixing length between segments "i" and "j," m

6.2.5. Pore Water Diffusion 

Diffusive pore water exchanges can significantly influence benthic pollutant 

concentrations, particularly for relatively soluble chemicals and water bodies with little

sediment loading. Depending on the dissolved concentration gradient, pore water

diffusion may be a source or sink of pollutants for the overlying water column.

If benthic segments are included in the model network, the user may specify diffusive

transport of dissolved chemicals in the pore water. In WASP6, pore water diffusion is

input via transport field two in Figure 3-13. The user may define several groups of

exchanges. 

For each exchange group, the user must supply a time function giving dispersion

coefficient values (in m

2

/sec) as they vary in time. For each exchange in the group, the 

user must supply an interfacial area, a characteristic mixing length, and the segments

between which exchange takes place. The characteristic mixing length is typically the

distance between two benthic segment midpoints (multiplied internally by the tortuosity,

which is roughly the inverse of porosity). For pore water exchange with a surface water 

segment, the characteristic mixing length is usually taken to be the depth of the surficial

benthic segment. The interfacial area is the surficial area of the benthic segment (which

is input by the user) multiplied internally by porosity. 
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There may be several surficial benthic segments underlying a water column segment,

representing discrete benthic deposits (or habitats). The concentration of chemical

diffusing is the dissolved fraction per unit pore water volume. The actual diffusive

exchange between benthic segments i and j at time t is given by:

Equation 6-10

















∂

∂

n

f C -

n

f C

L / n

E (t)A n =

t

M

i

Dik ik

j

Djk jk

cij ij

ik ij ij ij

where:

fDik,fDjk = dissolved fraction of chemical "k" in segments "i" and "j"

nij= average porosity at interface "ij", Lw/L

Eij(t)= diffusion coefficient time function for exchange "ij", m2/day

Aij= interfacial area shared by segments "i" and "j," m2

Lcij= characteristic mixing length between segments "i" and "j," m

6.2.6. Boundary Processes

A boundary segment is characterized by water exchanges from outside the network,

including tributary inflows, downstream outflows, and open water dispersive exchanges. 

WASP6 determines its boundary segments by examining the advective and dispersive 

segment pairs specified by the user. If an advective or dispersive segment pair includes 

segment number "0," the other segment number is a boundary segment. Thus, for

advective flows, the segment pair (0,1) denotes segment 1 as an upstream boundary 

segment; segment pair (5,0) denotes segment 5 as a downstream boundary segment. 

Boundary concentrations CBik (mg/L) must be specified for each simulated variable "k" at 

each boundary segment "i". These concentrations may vary in time. At upstream

boundary segments, WASP6 applies the following mass loading rates:

Equation 6-11

Vi S Bik=Q0i(t)•CBik

where:

SBik= boundary loading rate response of chemical "k" in segment 

"i," g/m3-day

Vi= volume of boundary segment "i," m3

Q0i(t)= upstream inflow into boundary segment "i," m3/day

At downstream boundary segments, WASP6 applies the following mass loading rates:
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Equation 6-12

V S = - Q 0(t) Cik Bik i

i •

where:

Qi0(t)= downstream outflow from boundary segment "i," m3/day

Cik= internal concentration of chemical "k" in segment "i," mg/L

Notice that the specified boundary concentration is not used to calculate the boundaryloading rate for the downstream boundary segment. If, however, the downstream outflow 

becomes negative, it becomes in reality an inflow. In this case, Equation 6-11 applies 

where Q0i = -Qi0.

At exchange boundary segments, WASP6 applies the following mass loading rates:

Equation 6-13

( C - C )

L

E (t) A

V S = Bk ik

ci0

i0 i0

i Bi

•

where terms are as defined above. When a boundary concentration exceeds the internal

concentration, mass is added to the boundary segment; when the boundary concentration

falls below the internal concentration, mass is lost from the boundary segment.

6.2.7. Loading Processes

WASP6 allows the user to specify loading rates for each variable. Two types of loadings 

are provided for -- point source loads and runoff loads. The user in the input dataset 

specifies the first set of loads. The second set of loads is read by WASP6 from a

nonpoint source-loading file created by an appropriate loading model. Both kinds of

loads, in kg/day, are added to the designated segments at the following rates:

Equation 6-14

V S =1000 L (t) i Lik • ik

where:

SLik= loading rate response of chemical "k" in segment "i," g/m3-

day

Lik(t)= loading rate of chemical "k" into segment "i," kg/day

Point source loads are input as a series of loading versus time values. During a

simulation, WASP6 interpolates between these points to provide time-variable loadings. 

The WASP6 calculational time step should be set by the user to a value that is divisible 

into the difference in time entries in the point source loading functions. If evenly

divisible time steps cannot be specified, the user should specify maximum time steps at
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least 5 times smaller than the point source time entries. If the user is specifying daily

load variations, for example, the maximum time step should be 0.2 days.

The user should understand that mass entered as loads is not directly accompanied with

inflow. No significant errors are introduced if the inflow associated with a loading is

small compared with the water body flow. If a loading is associated with significant

inflow, then the user should generally enter the associated flows separately under water

column advection, and treat the loading as a model boundary by specifying the boundary

concentration accompanying the inflow. If a large number of diffuse loads are being read 

in, the user can provide for the incremental flows using a flow continuity function that

increases downstream.

6.2.8. Nonpoint Source Linkage 

Realistic simulations of nonpoint source loadings can be accomplished by linking

WASP6 to a compatible surface runoff simulation. This linkage is accomplished through

a formatted external file chosen by the user at simulation time. The nonpoint sourceloading file contains information on which WASP6 systems and segments receive

nonpoint source loads, and a record of the nonzero loads by system, segment, and day.

If the user sets the nonpoint source loading flag by checking the dialog box in Figure 3-6, 

a menu of previously prepared nonpoint source files (*.NPS) is presented. Following the 

choice of a proper file, nonpoint source loads are read once a day throughout a simulation 

from a loading file generated by a previous loading model simulation. These loads are

treated as step functions that vary daily. When the user implements the nonpoint source 

loading option, model time steps should be divisible into 1 day. (Time steps do not have 

to be exactly divisible into a day; if time steps are small, any errors associated with

carrying the previous day's loading rate into a new day will be small.)

The external nonpoint source load file is a formatted ASCII file chosen by the user. This 

file contains information on which WASP systems and segments receive nonpoint source

loads, and a record of the nonzero loads by system, segment, and day.

Six records comprise the nonpoint source file. 

Nonpoint Source File Format Line 1

 NPSMOD = Name or description of nonpoint source model or method

of generation; this is echoed to the output file for the

record. (A15)

NUMSEG = Number of segments receiving nonpoint source loads. (I5)

INTOPT = Interpolation option; 1 = step function (only one in code

now). (I5)
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NUMSYS = Number of WASP systems receiving nonpoint source

loads. (I5)

Record 2--Loading Segments (I5)

LSEG(I) = segment number receiving loads. (I5)

Record 2 is repeated NUMSEG times.

Record 3--Loading Systems (20I5)

LSYS(I) = WASP system numbers receiving loads. (I5)

Record 4--System Descriptors (A15)

NAMESY(I) = Name or description of WASP systems receiving loads. 

(A15)

Record 4 is repeated NUMSYS times.

Records 5 and 6 are repeated as a unit for the number of days that nonzero loads occur:

Record 5--Loading Days (F10.0)

LDAY = Time in days for the following nonzero load. (F10.0)

Record 6--Nonpoint Source Loads (A15, 20F10.0)

NAMESY(I) = System name or description (not read in by WASP). (A15)

NPSWK(I,J) = Nonpoint source loads for WASP system "I" at all loading

segments "J", in the order presented in Record 2. Loads are 

in kg/day. (20F10.0)

Record 6 is repeated NUMSYS times.

Record 1 is input once. Record 2 is repeated NUMSEG times. Record 3 is then input 

once. Record 4 is repeated NUMSYS times. Records 5 and 6 are a set and are repeated 

(as a set) NUMSYS times. Within each set, Record 5 is entered once and Record 6 is 

repeated NUMSYS times.

6.2.9. Initial Conditions

Because WASP6 is a dynamic model, the user must specify initial conditions for each

variable in each segment. Initial conditions include the chemical concentrations at the

beginning of the simulation. The product of the initial concentrations and the initial

volumes give the initial constituent masses in each segment. For steady simulations,

where flows and loadings are held constant and the steady-state concentration response is 
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desired, the user may specify initial concentrations that are reasonably close to what the

final concentrations should be. For dynamic simulations where the transient

concentration response is desired, initial concentrations should reflect measured values at 

the beginning of the simulation.

In addition to chemical concentrations, the dissolved fractions must be specified for each

segment at the beginning of the simulation. For tracers, the dissolved fractions will

normally be set to 1.0. For tracers, as well as dissolved oxygen, eutrophication, and

sediment transport, the initial dissolved fractions remain constant throughout the

simulation. For organic chemical simulations, the dissolved fraction will be internally

calculated from partition coefficients and sediment concentrations.

The density of each constituent must be specified under initial conditions. For tracers,

this value should be set to 1.0. 

6.3. Model Implementation

To simulate a tracer with WASP6, use the preprocessor or text editor to create a TOXI 

input file. The preprocessor will create an input file with parameters in the proper fields. 

Using a text editor, the user must take care to enter parameters into the proper fields. The 

model input parameters are organized below as they are presented in the preprocessor. 

6.4. Model Input Parameters

This section summarizes the input parameters that must be specified in order to solve the 

WASP6 mass balance equation. Input parameters are prepared for WASP6 in four major 

sections of the preprocessor -- environment, transport, boundaries, and transformations.

6.4.1. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control

the simulation.

Simulation Type-- The user must specify which WASP6 model will be run with the

dataset. The present choices are "TOXI" or "EUTRO." (Figure 3-6)

Simulation Titles-- The user may specify a 2-line title for the simulation. This title may

include any descriptive information on the water body, time frame, pollutants, simulation

parameters, etc. The user may also specify the properly positioned names of the

simulation switches input in Record 4. This is for user convenience only. (Figure 3-6) 

Number of Segments-- The user must define the segments. The preprocessor

automatically counts the number of segments. (Figure 3-8)
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

6-14

Number of Systems-- The user must specify the number of model systems (state

variables) in the simulation. In the preprocessor, select "simulate" for Chemical 1, and

"bypass" for Chemicals 2 and 3 and Solids 1-3. (Figure 3-7)

Restart Option-- The user must specify the restart option, which controls the use of the 

simulation restart file. This restart file stores the final conditions from a simulation, and

can be used to input initial conditions in a sequential simulation. 1) neither read from nor 

write to the restart file; 2 write final simulation results to restart file; 3 = read initial

conditions from restart file created by earlier simulation, and write final simulation

results to new restart file. (Figure 3-6)

Mass Balance Analysis-- The user should specify the system number for which a global

mass balance analysis will be performed. A value of 0 will result in no mass balance

table being generated. (Figure 3-7) 

Negative Solution Option-- Normally, concentrations are not allowed to become

negative. If a predicted concentration at t + Ät is negative, WASP maintains its positive 

value by instead halving the concentration at time t. The negative solution option lets the 

user bypass this procedure, allowing negative concentrations. This may be desirable for

simulating dissolved oxygen deficit in the benthos, for example. Unchecked = prevents

negative concentrations; checked = allows negative concentrations. (Figure 3-6) 

Time Step Option-- The user must specify how time steps will be determined during the 

simulation. 1 user inputs time step history; 2 model calculates time step. (Figure 3-6) 

Advection Factor, dimensionless-- The advection factor õ can be specified to modify the 

finite difference approximation of ∂c/∂x used in the advection term by WASP. For õ = 0, 

the backward difference approximation is used. This is most stable, and is recommended 

for most applications. For õ = 0.5, the central difference approximation is used. This is 

unstable in WASP, and is not recommended.

A nonzero advection factor is helpful in situations where the network size and time step 

produce large numerical dispersion. A nonzero advection factor reduces the numerical

dispersion produced by a particular velocity, length, and time step combination. 

According to Bella and Grenney (1970):

Equation 6-15

[(1- 2 ) L -U t]

2

U

E num = n Δ

Table 6-3 Values of Numerical Dispersion (m2/sec)

U (m/sec)

 õ 0.1 0.2 0.4 0.6 0.8 1.0

Ät = 1000 sec

0.0 95 180 320 420 480 500

0.1 75 140 240 300 320 300
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

6-15

U (m/sec)

 õ 0.1 0.2 0.4 0.6 0.8 1.0

Ät = 1000 sec

0.0 95 180 320 420 480 500

0.2 55 100 160 180 160 100

0.3 35 60 80 60 0 --

0.4 15 20 0 -- -- --

Ät = 2000 sec

0.0 90 160 240 240 160 0

0.1 70 120 160 120 0 --

0.2 50 80 80 0 -- --

0.3 30 40 0 -- -- --

0.4 10 0 -- -- -- --

Ät = 4000 sec

0.0 80 120 80 -- -- --

0.1 60 80 0 -- -- --

0.2 40 40 -- -- -- --

0.3 20 0 -- -- -- --

0.4 0 -- -- -- -- --

Ät = 8000 sec

0.0 60 40 -- -- -- --

0.1 40 0 -- -- -- --

0.2 20 -- -- -- -- --

0.3 0 -- -- -- -- --

0.4 -- -- -- -- -- --

Note that a õ of 0 reduces this to Equation 6-20. Values of Enum for a length of 2000 

meters and various combinations of velocity and time step are provided in Table 6-3. For 

a particular velocity, say 0.4 m/sec, numerical increasing the time step can reduce

dispersion. For õ = 0, increasing the time step from 1000 to 4000 seconds decreases Enum

from 320 to 80 m2/sec. If the time step must be 1000 seconds, however, increasing õ can 

still reduce numerical dispersion. In this case, increasing õ from 0 to 0.4 decreases Enum

from 320 to 0 m2/sec. (Group A, Record 4, ADFC)

Initial Time, day, hour, minute-- the time at the beginning of the simulation must be

specified in order to synchronize all the time functions. The day, hour, and minute can be 

input. The beginning of the simulation is day 1. (Figure 3-6)

Final Time, days--The elapsed time at the end of the simulation must be specified. The 

end of the simulation occurs when the final time from the integration time step history is 

encountered. The final time is entered on the same record as the time step. (Figure 3-18)

Integration Time Step, days--A sequence of integration time steps (Ät) must be specified, 

along with the time interval over which they apply. If user specified time step option is 

used these time steps will be used during the simulation. If the WASP calculated time

step is used the model will calculate time steps internally; the time steps given here are

the maximum allowed (Figure 3-6 & Figure 3-18). 

Given specific network and transport parameters, time steps are constrained within a

specific range to maintain stability and minimize numerical dispersion, or solution

inaccuracies. To maintain stability at a segment, the advected, dispersed, and

transformed mass must be less than the resident mass:
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

6-16

Equation 6-16

( Σ Q C j+Σ R C j+Σ S K V j) Δt <V j C j

Solving for Ät and applying the criterion over the entire network with appropriate factors 

gives the maximum stable step size used by WASP6:

Equation 6-17





















Δ

∑ Q +∑ R +5∑( S V / C )

V

t =0.9

Kjk j j

k

ij

i

ij

i

j

max Min

For purely advective systems, Equation 17 sets the time step to 90% of the minimum

segment travel time. For purely dispersive systems, Equation 17 sets the time step to

90% of the minimum segment flushing time. For a linear reactive system with no

transport, Equation 6-17 sets the time step to 18% of the reaction time. Usually Ät is 

controlled by advective or dispersive flows.

Numerical dispersion is artificial mixing caused by the finite difference approximation

used for the derivatives. If the advection factor õ = 0, the backward difference

approximation of ∂c/∂x is used in the advection term, and

Equation 6-18

2

U L

E num =

where:

L = length of the segment, m

For the Euler scheme, the forward difference approximation of ∂c/∂t is used, and

Equation 6-19

2

U t

E =

2

num

Δ

The total numerical dispersion, then, is

Equation 6-20

(L -U t)

2

U

E num = Δ
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

6-17

Note that increasing the time step up to Äx/U (or V/Q) decreases numerical dispersion to 

0. The conditions for stability discussed above require a time step somewhat less than

V/Q for most segments. So to maintain stability and minimize numerical dispersion in a 

water body subject to unsteady flow, the sequence of time steps must be as large as

possible, but always less than Ätmax given in Equation 6-17. (Figure 3-18)

Print Intervals, days-- The user must specify the print intervals controlling the output

density in the print file transferred to the post-processor. The model will store all display 

variables in all segments after each print interval throughout the simulation. Different

print intervals can be specified for different phases in the simulation. At least two print 

intervals must be specified, one for time 0 and one for the final time. (Figure 3-19)

Segment Volumes, m3--Initial volumes for each segment must be specified. These can be 

calculated from navigation charts or from a series of transects measuring depth versus

width along the river. Sometimes, volumes can be estimated from the travel time of a

well-mixed cloud of dye through a reach. For simulations using hydrodynamic results,

volumes from the hydrodynamic summary file (#.HYD) are used and continuity is

maintained. (Figure 3-8)

6.4.2. Transport Parameters

This group of parameters defines the advective and dispersive transport of simulated 

model variables. Input parameters include advective flows, sediment transport velocities,

dispersion coefficients, cross-sectional areas, and characteristic lengths. Although the

nominal units expected by the model are SI, English or other units can be used along with 

proper specification of conversion factors.

Advective Flow, m

3

/sec--Steady or unsteady flows can be specified between adjoining

segments, as well as entering or leaving the network as inflow or outflow. The user must 

be careful to check for continuity errors, as the model does not require that flow

continuity be maintained. For example, the user may specify that more flow enter a

segment than leaves. For simulations using hydrodynamic results from the *.HYD file

flow continuity is automatically maintained. (Figure 3-14)

Dispersion Coefficients, m

2

/sec--Dispersive mixing coefficients can be specified between

adjoining segments, or across open water boundaries. These coefficients can model pore 

water diffusion in benthic segments, vertical diffusion in lakes, and lateral and

longitudinal dispersion in large water bodies. Values can range from 10-10 m2/sec for

molecular diffusion to 5x102 m2/sec for longitudinal mixing in some estuaries. Values

are entered as a time function series of dispersion and time, in days. (Figure 3-13)

Cross-Sectional Area, m

2

--Cross-sectional areas are specified for each dispersion

coefficient, reflecting the area through which mixing occurs. These can be surface areas 

for vertical exchange, such as in lakes or in the benthos. Areas are not modified during

the simulation to reflect flow changes. (Figure 3-13)
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6-18

Characteristic Mixing Length, m--Mixing lengths are specified for each dispersion 

coefficient, reflecting the characteristic length over which mixing occurs. These are

typically the lengths between the center points of adjoining segments. A single segment

may have three or more mixing lengths for segments adjoining longitudinally, laterally,

and vertically. For surficial benthic segments connecting water column segments, the

depth of the benthic layer is a more realistic mixing length than half the water depth.

(Figure 3-13)

6.4.3. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial

conditions. Boundary concentrations must be specified for any segment receiving flow

inputs, outputs, or exchanges. Initial conditions include not only initial concentrations,

but also the density and solids transport field for each solid, and the dissolved fraction in

each segment.

Boundary Concentrations, mg/L--Steady or time-variable concentrations must be

specified for each water quality constituent at each boundary. A boundary is a tributary 

inflow, a downstream outflow, or an open water end of the model network across which

dispersive mixing can occur. Advective and dispersive flows across boundaries are

specified by the transport parameters. Values are entered as a time function series of

concentrations and time, in days. (Figure 3-15)

Waste Loads, kg/day--Steady or time-variable loads may be specified for each water

quality constituent at several segments. These loads represent municipal and industrial

wastewater discharges, urban and agricultural runoff, precipitation, and atmospheric

deposition of pollutants. Values are entered as a time function series of loads and time, in 

days. (Figure 3-16)

Initial Concentrations, mg/L--Concentrations of each constituent in each segment must be 

specified for the time at which the simulation begins. For those water bodies with low

transport rates, the initial concentrations of conservative substances may persist for a long

period of time. Accurate simulation, then, would require accurate specification of initial

concentrations. If initial concentrations cannot be determined accurately, then longer

simulations should be run, and early predictions discounted. (Figure 3-10)

Dissolved Fractions--The initial fraction of chemical dissolved in the water portion of a

segment is input as a fraction of total chemical concentration. The dissolved fraction is

important in determining the amount of chemical transported by pore water flow and

dispersion, and by solids transport. Dissolved fractions may be computed from sorption

kinetics in the transformation subroutines. (Figure 3-11)

Solid Densities, g/cm3--The density of each type of solid is needed to compute the

porosity of bed segments. Porosity will be a function of sediment concentration and the 

density of each solid type. (Figure 3-7)
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

6-19

Maximum Concentrations, mg/L--Maximum concentrations must be specified for each

water quality constituent. The simulation is automatically aborted if a calculated

concentration falls outside these limits. This usually indicates computational instability,

and the time step must usually be reduced. (Figure 3-7)

6.4.4. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic

time functions for the water quality constituents being simulated. None are necessary for 

dissolved, conservative chemicals.

6.4.5. External Input Files

At the user's option, two external input files may be called upon and used by WASP6

during a simulation. These formatted files may be created by a simulation model, or by

output from a spreadsheet. As formatted ASCII files, they may be edited using standard 

text editors. Hydrodynamic files are denoted by *.HYD, where the user specifies a 1 to 8 

character name for *. Nonpoint source loading files are denoted by *.NPS. The contents 

and format for these files are specified on page 3-13.
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7-1

7. Sediment Transport

Sediment transport is potentially a very important process in aquatic systems. Excess sediment

can affect water quality directly. Water clarity and benthic habitats can be degraded. Sediment 

transport also influences chemical transport and fate. Many chemicals sorb strongly to sediment 

and thus undergo settling, scour, and sedimentation. Sorption also affects a chemical's transfer

and transformation rates. Volatilization and base-catalyzed hydrolysis, for example, are slowed

by sorption. Both sediment transport rates and concentrations must be estimated in most toxic

chemical studies.

In general, the stream transport capacity for suspended sediment is in excess of its actual load, 

and the problem is one of estimating sediment source loading--namely, watershed erosion. In

areas of backwater behind dams or in sluggish reaches, the stream transport capacity may drop 

enough to allow net deposition. Strongly sorbed pollutants may build up significantly. Because 

sediment transport can be complex, site-specific calibration of the settling, scour, and

sedimentation rates is usually necessary.

7.1. Overview of WASP Sediment Transport

Sediment size fractions, or solids types, are simulated using the TOXI program. Simulations

may incorporate total solids as a single variable, or, alternately, 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. 

WASP6 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

compartments as well as water column compartments. Bulk densities or benthic volumes are

adjusted throughout the simulation. 

The user can vary all solids transport rates in space and time. There are, however, no special

process descriptions for solids transport. Erosion rates, for example, are not programmed as a 

function of sediment shear strength and water column shear stress. Consequently, the TOXI

sediment model should be considered descriptive, and must be calibrated to site data.

7.1.1. Sediment Transport Processes

Water Column Transport
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Sediment and particulate chemicals in the water column may settle to lower water segments and 

deposit to surficial bed segments. Velocities and surface areas in transport fields 3, 4, and 5

describe settling, deposition, and scour rates in WASP6. Particulate transport velocities may

vary both in time and in space, and are multiplied by cross-sectional areas to obtain flow rates for 

solids and the particulate fractions of chemicals.

Settling velocities should be set within the range of Stoke's velocities corresponding to the

suspended particle size distribution:

Equation 7-1

( - ) d

18

8.64 g

V =

2

 s r p r w p

m

 where:

Vs= Stokes velocity for particle with diameter dp and density ñp, m/day

g= acceleration of gravity = 981 cm/sec2

ì= absolute viscosity of water = 0.01 poise (g/cm3-sec) at 20 °C

ñp= density of the solid, g/cm3

ñw= density of water, 1.0 g/cm3

dp= particle diameter, mm

Values of Vs for a range of particle sizes and densities are provided in Table 7-1.

Table 7-1 Stoke's Settling Velocities (in m/day) at 20°c

Particle

Diameter, mm

Particle Density, g/cm3

1.80 2.00 2.50 2.70

Fine Sand

 0.3 300.00 400.00 710.00 800.00

 0.05 94.00 120.00 180.00 200.00

Silt

 0.05 94.00 120.00 180.00 200.00

 0.02 15.00 19.00 28.00 32.00

 0.01 3.80 4.70 7.10 8.00

 0.005 0.94 1.20 1.80 2.00

 0.002 0.15 0.19 0.28 0.32

Clay

 0.002 0.15 0.19 0.28 0.32

 0.001 0.04 0.05 0.07 0.08

Benthic Exchange

The net scour and deposition velocities drive Benthic exchange of sediment and particulate

chemicals:
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

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Equation 7-2

W = A ( w S - w S ) Bs ij R i D j

where:

WBs = net sediment flux rate, g/day

S = sediment concentration, g/m3

wD = deposition velocity, m/day

wR = scour velocity, m/day

Aij = benthic surface area, m2

i = benthic segment

j = water segment

The deposition velocity can be calculated as the product of the Stokes settling velocity and the 

probability of deposition:

Equation 7-3

w D =V sa D

where:

áD = probability of deposition upon contact with the bed.

The probability of deposition depends upon the shear stress on the benthic surface and the

suspended sediment size and cohesiveness. Likewise, the scour velocity depends upon the shear 

stress, the bed sediment size and cohesiveness, and the state of consolidation of surficial benthic 

deposits. Figure 7-1 is offered as initial guidance in specifying initial deposition and scour

velocities. For example, coarse silt of 0.05 mm diameter may settle at 100 to 200 m/day, but 

should not deposit where mean stream velocity is above 0.5 cm/sec. Where mean velocity rises 

above 30 cm/sec, erosion is expected, and nonzero scour velocities should be specified. For fine 

silt of 0.005 mm diameter settling at 1 to 2 m/day, deposition is not expected, even under

quiescent conditions. Nonzero scour velocities should be specified where mean velocity is above 

2 m/sec. Site-specific calibration is necessary to refine the initial estimates.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

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Figure 7-1 Sediment Transport Regimes (Graf, 1971)

7.1.2. Sediment Loading

Sediment loading derives primarily from watershed erosion and bank erosion. These can be

measured or estimated by several techniques, and input into each segment as a point source load. 

For some problems, long-term average sediment loads can be calculated using the Universal Soil

Loss Equation (Wischmeier and Smith, 1978). A useful treatment of this process is given by

Mills et al. (1985). This technique works poorly for short term 

or inherently dynamic problems because much of the sediment loading occurs during a few

extreme storm or snow melt events. If available, suspended sediment data at local gaging

stations can be extrapolated to provide areawide-loading estimates. Alternatively, daily runoff

loads can be simulated with a watershed model and read in directly from an appropriately

formatted nonpoint source-loading file.
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7.1.3. The Sediment Bed

The bed sediment plays an important role in the transport and fate of water quality constituents. 

Sediment-sorbed pollutants may be buried in the bed by deposition and sedimentation, or they

may be released to the water column by scour. In WASP6, the movement of sediment in the bed 

is governed by one of two options. In the first option, bed segment volumes remain constant and 

sediment concentrations vary in response to deposition and scour. No compaction or erosion of

the segment volume is allowed to occur. In the second option, the bed segment volume is

compacted or eroded as sediment is deposited or scoured. Sediment concentration in the bed 

remains constant. In both options chemical may be transported through the bed by pore water 

flow and dispersion.

The Constant Bed Volume Option--The first bed option, referred to as the constant volume

option, allows the sediment concentration of the bed to change according to the net flux of

sediment. Bed segments are located in reference to the rising or falling bed surface. The rate at 

which the bed rises or falls is represented by a sedimentation velocity input in flow fields 3, 4, 

and 5 for each sediment size fraction. Sediment in the bed is added through deposition and lost 

through scour and sedimentation.

Assuming the depth of the bed remains constant and neglecting dispersive mixing, the mass

balance of sediment in a stationary upper bed is given by:

Equation 7-4

= w S - ( w + w ) S

t

S

d D j R s i

i

i

∂

∂

where:

ws = sedimentation velocity of the upper bed, m/day

Si = sediment concentration in the upper bed, g/m3

Sj = sediment concentration in the water, g/m3

di = depth of the upper bed, m

For a lower bed layer,

Equation 7-5

= w S - w S

t

S

d s i sk k

k

k

∂

∂

where:

Sk = sediment concentration in the lower bed, g/m3
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

7-6

wsk = sedimentation velocity of the lower bed, m/day

dk = depth of the lower bed, m

In most applications the sediment concentration of the bed will be nearly constant over time. In

this case the mass derivative ∂S/∂t will be zero. The resulting mass balance in the upper bed is:

Equation 7-6

w D S j=( wR+w s ) S i

In the lower bed,

Equation 7-7

ws Si= wsk Sk

It should be noted that under the constant volume option WASP6 does not require a balance of 

sediment fluxes into and out of a bed segment. The user should, therefore, take care that

deposition, scour, and sedimentation velocities reflect the intended mass flux of sediment in the

bed.

The constant volume option also has a provision for a movable upper bed layer. This layer is 

modeled by specifying a total advective flow rate (flow field one) between upper bed segments. 

Thus, when a flow rate Qij is specified from upper bed segment j to upper bed segment i, the 

sediment, pore water, and chemical in j are transported to i. To maintain a mass balance in

segment i, a similar flow rate should be specified out of i. This option allows for the lateral

transport of sediment across the upper bed, and can be used to represent bed load transport.

The Variable Bed Volume Option--The second bed volume option, referred to as the variable

bed volume option, allows bed volumes to change in response to deposition and scour. Two

types of bed layers are assumed: an upper uncompacted layer, and one or more lower compacted 

layers. When deposition exceeds scour, the upper layer increases in volume as the surface of the 

bed rises. After a period of time, the added volume of upper bed compresses and becomes part 

of the lower bed. When scour exceeds deposition, the volume of the upper layer decreases as the 

surface of the bed drops. When the upper layer erodes completely, the next layer of bed is

exposed to scour.

In locations where sediment deposition exceeds scour (Figure 7-2), bed compaction is triggered 

by a sedimentation time step. This sedimentation time step is input by the user and will

generally be much larger than the simulation time step. As sediment and sorbed chemical settle 

from the water column, the top bed segment increases in volume, sediment mass, and chemical 

mass. Sediment concentrations remain constant. The volume of the upper bed continues to

increase until the end of the sedimentation time step. At this time, the volume of the upper bed 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

7-7

that has been added by net deposition is compressed to the density of the lower bed. Since the 

porosity of the uncompressed bed is greater than the porosity of the compressed bed, pore water 

and dissolved chemical are squeezed into the water column.

Figure 7-2 WASP Sediment Burial

During compression, the lower bed segments rise to include the compressed portion of the upper 

bed. The volumes and sediment concentrations of these lower bed segments remain constant. A 

portion of the bottom bed segment is buried out of the network, however, as bed segments rise in 

response to sedimentation. Thus, chemical mass in the lower bed is added through compression

of the upper bed, and lost through sediment burial.

After compression, the top bed segment returns to its original predeposition volume. Sediment

and chemical concentrations in the upper bed are not changed by compaction. In the lower beds, 

segment volumes and sediment concentrations are unchanged. Chemical mass from the

compacted portion of the bed is added to the lower bed, and chemical mass in the bottom bed 

segment is buried out of the model network.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

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Over several sedimentation time steps, the density and volume of the upper bed segment remain 

constant, so that:

Equation 7-8

= A w S - A ( w + w ) S =0

t

V

S ij D j ij R s i

i

i

∂

∂

and

Equation 7-9

ws=( wD S j

- wR Si)/ Si

For a lower bed layer, volumes are held constant along with density. To maintain mass balance, 

the average sedimentation velocity is, effectively:

Equation 7-10

wsk = ws Si

/ Sk

For locations where sediment scour exceeds deposition, WASP responds as in Figure 7-2. As 

sediment and sorbed chemical erode from the bed, the top bed segment decreases in volume,

depth, chemical mass, and sediment mass. Its density remains constant. When the sediment

mass in the top bed layer equals zero, then segment renumbering is triggered. All the properties 

of the remaining bed segments, including chemical concentration, remain unaffected by

renumbering. The new top bed segment, for example, has the same depth, volume, and sediment 

and chemical concentration as the old second bed segment. A new bottom bed segment is

created with the same physical properties as the other bed segments. Its chemical concentration, 

however, is zero. Renumbering and creation of a new bottom segment completes the WASP6 

erosion cycle (or time step).

As a consequence of the way the variable bed volume option treats sedimentation, certain

constraints are imposed on the bed segment properties defined in the input data set. The density 

(or sediment concentration) of a top bed segment must be less than or equal to the density of the 

lower bed segments within a vertical stack. Since the compaction routine implicitly handles

sedimentation, no sedimentation velocities to lower beds may be specified in the sediment

transport fields. Finally, the user must simulate sediment as a state variable in order to use this 

option. Sediment is a state variable in the toxics program, but not the eutrophication program.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

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7.2. Model Implementation

To simulate sediment transport with WASP6, use the preprocessor or a text editor to create a 

TOXI input file. Simple datasets are provided for use as templates to edit and adapt. The model 

input dataset and the input parameters will be similar to those for the conservative tracer model

as described in Chapter 6. To those basic parameters, the user will add benthic segments and 

solids transport rates. During the simulation, solids variables will be transported both by the

water column advection and dispersion rates and by these solids transport rates.

In WASP6, solids transport rates in the water column and the bed are input via up to three solids 

transport fields. These fields describe the settling, deposition, scour, and sedimentation flows of

three kinds of solids. The transport of particulate chemicals or the particulate fraction of

simulated chemicals follows the solids flows. The user must specify the dissolved fraction (i.e.

0.0) and the solids transport field for each simulated solid under initial conditions. To simulate 

total solids, solids 1 must be used.

7.2.1. Model Input Parameters

This section summarizes the input parameters that must be specified in order to solve the

sediment balance equations in TOXI. Input parameters are prepared for WASP6 in four major

sections of the preprocessor -- environment, transport, boundaries, and transformation. Basic

model parameters are described in Chapter 6, and will not be repeated here. 

7.2.2. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control the

simulation.

Systems-- To simulate total solids only, select "simulate" for Solids 1 and "bypass" for the other 

five systems. To simulate two solids types, select "simulate" for both Solids 1 and Solids 2. To 

simulate three solids types, select "simulate" for all three. The chemical systems can be

simulated or bypassed. (Figure 3-7)

Bed Volume Option-- The user must determine whether bed volumes are to be held constant or 

allowed to vary. Volumes may be held constant by specifying 0, in which case sediment

concentrations and porosities in the bed segments will vary. Alternatively, sediment

concentrations and porosities may be held constant by specifying 1, in which case surficial bed 

segment volumes will vary. (Figure 3-6)

Bed Time Step-- While mass transport calculations are repeated every model time step, certain

benthic calculations are repeated only at this benthic time step, in days. If the constant bed

volume option is chosen, sediment concentrations are updated every model time step, but
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porosities are recalculated every benthic time step. If the variable bed volume is chosen, upper 

benthic segment volumes are updated every time step, with compaction occurring every benthic 

time step. (Figure 3-6)

Transport Parameters

Sediment Transport Velocities, m/sec-- Time variable settling, deposition, scour, and

sedimentation velocities can be specified for each type of solid. If the units’ conversion factor is 

set to 1.157e-5, then these velocities are input in units of m/day. These velocities are multiplied 

internally by cross-sectional areas and treated as flows that carry solids and sorbed chemical

between segments. Settling velocities are important components of suspended sediment

transport in the water column. Scour and deposition velocities determine the transfer of solids

and sorbed chemical between the water column and the sediment bed. Sedimentation velocities 

represent the rate at which the bed is rising in response to net deposition. (Figure 3-14)

Cross-Sectional Areas, m

2

-- The interfacial surface area must be specified for adjoining

segments where sediment transport occurs. These surface areas are multiplied internally by

sediment transport velocities to obtain sediment transport flows. ( Figure 3-14)

7.2.3. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial conditions. 

Boundary concentrations must be specified for any segment receiving flow inputs, outputs, or

exchanges. Initial conditions include not only initial concentrations, but also the density and

solids transport field for each solid, and the dissolved fraction in each segment.

Boundary Concentrations, mg/L-- At each segment boundary, time variable concentrations must

be specified for total solids, or for each solids type simulated. A boundary segment is

characterized by water exchanges from outside the network, including tributary inflows,

downstream outflows, and open water dispersive exchanges. (Figure 3-15)

Waste Loads, kg/day-- For each point source discharge, time variable sediment loads can be

specified for total solids, or for each solids type simulated. These loads can represent municipal

and industrial wastewater discharges, or urban and agricultural runoff. (Figure 3-16)

Solids Transport Field-- The transport field associated with total solids or each solids type must 

be specified under initial conditions.

Solid Density, g/cm3-- The average density of the total sediment, or the density of each solids

type must be specified. This information is used to compute the porosity of benthic segments. 

Porosity is a function of sediment concentration and the density of each solids type. (Figure 3-7)
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Initial Concentrations, mg/L-- Concentrations of total sediment or of each solids type in each

segment must be specified for the time at which the simulation begins. If the variable benthic

volume option is used, the benthic sediment concentrations specified here will remain constant

for the entire simulation. (Figure 3-10)

Dissolved Fraction-- The dissolved fraction of each solid in each segment should be set to 0. 

(Figure 3-11)

7.2.4. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. None are necessary for sediment

transport.
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8. Dissolved Oxygen

Dissolved oxygen (DO) is one of the most important variables in water quality analysis. Low

concentrations directly affect fish and alter a healthy ecological balance. Because DO is affected 

by many other water quality parameters, it is a sensitive indicator of the health of the aquatic

system.

DO have been modeled for over 70 years. The basic steady-state equations were developed and 

used by Streeter and Phelps (1925). Subsequent development and applications have added terms 

to their basic equation and provided for time-variable analysis. The equations implemented here 

are fairly standard. As explained below, the user may implement some or all of the processes 

that are described with terms in these equations.

8.1. Overview of WASP6 Dissolved Oxygen

Dissolved oxygen and associated variables are simulated using the EUTRO program. Several

physical-chemical processes can affect the transport and interaction among the nutrients,

phytoplankton, carbonaceous material, and dissolved oxygen in the aquatic environment. Figure 

8-1 presents the principal kinetic interactions for the nutrient cycles and dissolved oxygen.
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Figure 8-1 EUTRO State Variable Interactions

EUTRO can be operated 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 and documented at the end of this section: (1)

Streeter-Phelps, (2) modified Streeter-Phelps, (3) full linear DO balance, and (4) nonlinear DO

balance. The actual simulation of phytoplankton is described in Chapter 5.
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8.2. Dissolved Oxygen Processes

Five EUTRO state variables can participate directly in the DO balance: phytoplankton carbon,

ammonia, nitrate, carbonaceous biochemical oxygen demand, and dissolved oxygen. The

reduction of dissolved oxygen is a consequence of the aerobic respiratory processes in the water 

column and the anaerobic processes in the underlying sediments. Because both these sets of

processes can contribute significantly, it is necessary to formulate their kinetics explicitly. 

Figure 8-2 Oxygen balance equations

The methodology for the analysis of dissolved oxygen dynamics in natural waters, particularly in

streams, rivers, and estuaries is reasonably well-developed (O'Connor and Thomann, 1972). The 

major and minor processes incorporated into EUTRO are discussed below. The reader should 

refer to the kinetic equations summarized in Figure 8-2, and the reaction parameters and

coefficients in Table 8-1. 
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Table 8-1 CBOD and DO Reaction Terms

Value from Potomac 

Estuary Model

Description Notation Units

Oxygen to carbon ratio aOC 32/12 mg O2/mg C

Phytoplankton nitrogen-carbon ratio aNC 0.25 mg N/mg C

Deoxygenation rate @ 20°C, Temp. coeff. kd Èd 0.21-0.16

 1.047

day-1 -

Half saturation constant for oxygen

limitation

KBOD 0.5 mg O2/L

Nitrification rate @ 20°C, Temp. coeff. k12 È12 0.09-0.13

 1.08

day-1 -

Half saturation constant for oxygen

limitation

KNIT 0.5 mg N/L

Denitrification rate @ 20°C, Temp. coeff. k2D È2D -

 1.08

day-1 -

Half saturation constant for oxygen

limitation

KNO3 0.1 mg N/L

Phytoplankton growth rate GP1 0.1-0.5 day-1

Phytoplankton resp-iration rate, 20°C, 

Temperature coeff.

k1R 

È1R

 0.125

 1.045

day-1 -

Sediment Oxygen Demand, Temp. coeff. SOD Ès 0.2-4.0

 1.08

g/m2-day

-

Reaeration rate @ 20°C, Temp. coeff. k2 Èa 1.028 day-1

-

DO saturation Cs Eq. 4.8 mg O2/L

Fraction dissolved CBOD fD5 0.5 none

Organic matter settling velocity vs3 - m/day

8.2.1. Reaeration

Oxygen deficient, i.e., below saturation, waters are replenished via atmospheric reaeration. The 

reaeration rate coefficient is a function of the average water velocity, depth, wind, and

temperature. In EUTRO, the user may specify a single reaeration rate constant, spatiallyvariable reaeration rate constants, or allow the model to calculate variable reaeration rates based 

upon flow or wind. Calculated reaeration will follow either the flow-induced rate or the windinduced rate, whichever is larger.

EUTRO calculates flow-induced reaeration based on the Covar method (Covar, 1976). This

method calculates reaeration as a function of velocity and depth by one of three formulas --

Owens, Churchill, or O'Connor- Dobbins, respectively:

Equation 8-2

k (20 C)= 5.349 v D

-1.85

j

0.67

qj ° j
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8-5

Equation 8-3

k (20 C)= 5.049 v D

-1.67

j

0.97

qj ° j

or

k (20 C)= 3.93 v D

-1.50

j

0.50

qj ° j

where:

kqj = flow-induced reaeration rate coefficient at 20°C, day-1

vj = average water velocity in segment j, m/sec

Dj = average segment depth, m

The Owens formula is automatically selected for segments with depth less than 2 feet. For

segments deeper than 2 feet, the O'Connor-Dobbins or Churchill formula is selected based on a 

consideration of depth and velocity. Deeper, slowly moving rivers require O'Connor-Dobbins; 

moderately shallow, faster moving streams require Churchill. Segment temperatures are used to

adjust the flow-induced kqj(20 °C) by the standard formula:

Equation 8-4

° Θ

T-20

qj qj C) a k (T)=k (20

where:

T = water temperature, °C

kqj(T) = reaeration rate coefficient at ambient segment temperature, day-1

Èa = temperature coefficient, unitless

Wind-induced reaeration is determined by O'Connor (1983). This method calculates reaeration

as a function of wind speed, air and water temperature, and depth using one of three formulas:

Equation 8-5

C (100 W) D

100 D

86400

k = d

1/3

W

a

1/2

W

OW

2/3

j

wj •

Γ































 k

r

r

n
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8-6

Equation 8-6

[ ]

C 

z

D TERM2=

C 

D TERM1=

where

(TERM1 100W ) +(TERM2 100W )

100 D

86400

k =

d

W W

a a

0

OW

1/2

d

u

1/3

W

a

1/2

W

OW

2/3

-1 -1

-1

j

wj

















Γ

































•

r n

r n

k

k

r

r

n

or

Equation 8-7

C 100W

z

D

100 D

86400

k = d

W W

a a

e

OW

1/2

j

wj 















r n

r n

k

where:

kwj= wind-induced reaeration rate coefficient, day-1

W= time-varying wind speed at 10 cm above surface, m/sec

Ta= air temperature, °C

ña= density of air, a function of Ta, g/cm3

ñw= density of water, 1.0 g/cm3

va= viscosity of air, a function of Ta, cm2/s

vW= viscosity of water, a function of T, cm2/s

DOW= diffusivity of oxygen in water, a function of T, cm2/s

ê= von Karman's coefficient, 0.4

vt= transitional shear velocity, set to 9, 10, and 10 for small, medium, and 

large scales, cm/s

vc= critical shear velocity, set to 22, 11, and 11 for small, medium, and 

large scales, cm/s

ze= equivalent roughness, set to 0.25, 0.35, and 0.35 for small, medium, 

and large scales, cm

z0= effective roughness, a function of ze, Ã, Cd, vt, va, and W, cm

ë= inverse of Reynold's number, set to 10, 3, and 3 for small, medium, 
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and large scales

Ã= nondimensional coefficient, set to 10, 6.5, and 5 for small, medium, 

and large scales

Ãu= nondimensional coefficient, a function of Ã, vc, Cd, and W

Cd= drag coefficient, a function of ze, Ã, va, ê, vt, and W

Equation 8-5 is used for wind speeds of up to 6 m/sec, where interfacial conditions are smooth

and viscous forces dominate momentum transfer. Equation 8-7 is used for wind speed over 20 

m/sec, where interfacial conditions are rough and momentum transfer is dominated by turbulent

eddies. Equation 8-6 is used for wind speeds between 6 and 20 m/sec, and represents a transition 

zone in which the diffusional sublayer decays and the roughness height increases.

The user is referred to O'Connor (1983) for details on the calculation of air density, air and water 

viscosity, the drag coefficient, the effective roughness, and Ãu. Small scale represents laboratory 

conditions. Large scale represents open ocean conditions. Medium scale represents most lakes 

and reservoirs.

Dissolved oxygen saturation, Cs, is determined as a function of temperature, in degrees K, and 

salinity S, in mg/L (APHA, 1985):

Equation 8-8

 - 0.5535 S (0.031929 - 19.428 T +3867.3T )

 +( 1.2438 10 )T -( 8.6219 10 )T

C = -139.34+( 1.5757 10 )T - ( 6.6423 10 )T 

-2

K

-1

K

-4

K

-3 11

K

10

-2

K

-1 7

K

5

s

• •

ln • •

8.2.2. Carbonaceous Oxidation

The long history of applications has focused primarily on the use of BOD as the measure of the 

quantity of oxygen demanding material and its rate of oxidation as the controlling kinetic

reaction. This has proven to be appropriate for waters receiving a heterogeneous combination of

organic wastes of municipal and industrial origin since an aggregate measure of their potential 

effect is a great simplification that reduces a complex problem to one of tractable dimensions.
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Figure 8-3 BOD Sources in the Aquatic Environment

The oxidation of carbonaceous material is the classical BOD reaction. Internally the model uses 

ultimate carbonaceous biochemical oxygen demand CBOD as the indicator of equivalent oxygen

demand for the carbonaceous material. A principal source of CBOD, other than man-made 

sources and natural runoff, is detrital phytoplankton carbon, produced as a result of algal death. 

The primary loss mechanism associated with CBOD is oxidation:

Equation 8-9

C x H y O z →CO 2+ H 2 O

The kinetic expression for carbonaceous oxidation in EUTRO contains three terms -- a first order 

rate constant, a temperature correction term, and a low DO correction term. The first two terms 

are standard. The third term represents the decline of the aerobic oxidation rate as DO levels 

approach 0. The user may specify the half-saturation constant KBOD, which represents the DO 

level at which the oxidation rate is reduced by half. The default value is zero, which allows this 

reaction to proceed fully even under anaerobic conditions.

Direct comparisons between observed BOD5 data and model output cannot be made using the 

internal CBOD computed by EUTRO, since field measurements may be tainted by algal
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respiration and the decay of algal carbon. Therefore a correction must be made to the internally

computed model CBOD so that a valid comparison to the field measurement may be made. This 

results in a new variable, known as the bottle BOD5, which is computed via equation 4.10.

Equation 8-10

+a C (1- e ) 

C (1- e )

14

64 Bottle BOD =C (1 - e )+

-5 k

oc 4

5 k-

1

5 k-

5 5

1R

dbot nbot

where:

C5 = the internally computed CBOD, mg/L

C1 = the internally computed NH3, mg/L

C4 = the phytoplankton biomass in carbon units, mg/L

aoc = the oxygen to carbon ratio, 32/12 mg O2/mg C

kdbot = the laboratory "bottle" deoxygenation rate constant, day

-1

knbot = the laboratory "bottle" nitrification rate constant, day-1

k1R = the algal respiration rate constant at 20°C, day-1

Equation 8-11 can provide a low estimate of the observed bottle BOD because it does not include 

a correction for the decay of detrital algal carbon, which in turn depends upon the number of

non-viable phytoplankton. Please note that laboratory "bottle" CBOD and nitrification rates are

used here, as specified by the user. The default laboratory rate constant for nitrification is 0, 

reflecting the use of a nitrifying inhibitor.

8.2.3. Nitrification

Additional significant losses of oxygen can occur as a result of nitrification: 

Equation 8-11

NH + 2O NO + H 0 +H

+

2 3 2

+

3 →

Thus for every mg of ammonia nitrogen oxidized, 2 (32/14) mg of oxygen are consumed.

The kinetic expression for nitrification in EUTRO contains three terms -- a first order rate

constant, a temperature correction term, and a low DO correction term. The first two terms are 

standard. The third term represents the decline of the nitrification rate as DO levels approach 0. 

The user may specify the half-saturation constant KNIT, which represents the DO level at which

the nitrification rate is reduced by half. The default value is zero, which allows this reaction to 

proceed fully even under anaerobic conditions.
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8.2.4. Denitrification

Under low DO conditions, the denitrification reaction provides a sink for CBOD:

Equation 8-12

5 CH 0 +5 H O +4 NO +4 H 5 CO 2+2 N 2+12 H 2O

+

2 2 3 →

Thus for each mg of nitrate nitrogen reduced, 5/4 (12/14) mg of carbon are consumed, which

reduces CBOD by 5/4 (12/14) (32/12) mg. Denitrification is not a significant loss in the water 

column, but can be important when simulating anaerobic benthic conditions.

The kinetic expression for denitrification in EUTRO contains three terms -- a first order rate

constant (with appropriate stoichiometric ratios), a temperature correction term, and a DO

correction term. The first two terms are standard. The third term represents the decline of the 

denitrification rate as DO levels rise above 0. The user may specify the half-saturation constant 

KNO3, which represents the DO level at which the denitrification rate is reduced by half. The 

default value is zero, which prevents this reaction at all DO levels.

8.2.5. Settling

Under quiescent flow conditions, the particulate fraction of CBOD can settle downward through

the water column and deposit on the bottom. In water bodies, this can reduce carbonaceous 

deoxygenation in the water column significantly. The deposition of CBOD and phytoplankton,

however, can fuel sediment oxygen demand in the benthic sediment. Under high flow

conditions, particulate CBOD from the bed can be resuspended.

The kinetic expression for settling in EUTRO is driven by the user-specified particulate settling

velocity vs3 and the CBOD particulate fraction (1 - fD5), where fD5 is the dissolved fraction. 

Settling velocities that vary with time and segment can be input as part of the advective transport 

field. Resuspension can also be input using a separate velocity time function. Segment-variable 

dissolved fractions are input with initial conditions.

8.2.6. Phytoplankton Growth

A byproduct of photosynthetic carbon fixation is the production of dissolved oxygen. The rate of 

oxygen production (and nutrient uptake) is proportional to the growth rate of the phytoplankton

since its stoichiometry is fixed. Thus, for each mg of phytoplankton carbon produced by growth, 

32/12 mg of O2 are produced. An additional source of oxygen from phytoplankton growth

occurs when the available ammonia nutrient source is exhausted and the phytoplankton begins to 

utilize the available nitrate. For nitrate uptake the initial step is a reduction to ammonia that

produces oxygen:
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Equation 8-13

2 NO 3 → 2 NH 3+30 2

Thus, for each mg of phytoplankton carbon produced by growth using nitrate, aNC mg of

phytoplankton nitrogen are reduced, and (48/14) aNC mg of O2 are produced. 

Phytoplankton Respiration

Oxygen is diminished in the water column as a result of phytoplankton respiration, which is

basically the reverse process of photosynthesis:

Equation 8-14

C 4+O 2 →CO 2

where C4 is phytoplankton carbon, in mg/L. Thus for every mg of phytoplankton carbon

consumed by respiration, 32/12 mg of oxygen are also consumed.

8.2.7. Phytoplankton Death

The death of phytoplankton provides organic carbon, which can be oxidized. The kinetic

expression in EUTRO recycles phytoplankton carbon to CBOD using a first order death rate and 

the stoichiometric oxygen to carbon ratio 32/12.

8.2.8. Sediment Oxygen Demand

The decomposition of organic material in benthic sediment can have profound effects on the

concentrations of oxygen in the overlying waters. The decomposition of organic material results 

in the exertion of an oxygen demand at the sediment-water interface. As a result, the areal fluxes 

from the sediment can be substantial oxygen sinks to the overlying water column. 

EUTRO provides two options for oxygen fluxes: descriptive input and predictive calculations. 

The first option is used for networks composed of water column segments only. The kinetic

equation is given in Figure 4.2. Observed sediment oxygen demand fluxes must be specified for 

water segments in contact with the benthic layer. Seasonal changes in water temperature can

affect SOD through the temperature coefficient.

The calculational framework incorporated for benthic-water column exchange draws principally

from a study of Lake Erie, which incorporated sediment-water column interactions, performed

by Di Toro and Connolly (1980). For a single benthic layer with thickness, Dj, the CBOD and 

DO mass balance equations are summarized in Figure 4.3. The equivalent SOD generated for 
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the overlying water column segment is also given. Subscripts "j" and "i" refer to a benthic

segment and the overlying water column segment, respectively.

WASP6 allows a more detailed parameterization of settling into the benthos that includes not

only a downward settling velocity but an upward resuspension velocity as well. In this context, 

then, the net particulate flux to the sediment is due to the difference between the downward

settling flux and the upward resuspension flux.

One of the first decisions to be made regarding the benthic layer is to determine its depth. Two 

factors influence this decision. The first is to adequately reflect the thickness of the active layer, 

the depth to which the sediment is influenced by exchange with the overlying water column. 

Secondly one wishes the model to reflect a reasonable time history or "memory" in the sediment 

layer. Too thin a layer and the benthos will "remember" or be influenced by deposition of

material that would have occurred only within the last year or two of the period being analyzed; 

too thick a layer and the model will "average" too long a history, not reflecting substantial

reductions resulting from reduced discharges from sewage treatment plants. The choice of

sediment thickness is further complicated by spatially variable sedimentation rates. The benthic

layer depths, together with the assigned sedimentation velocities, provide for a multi-year 

detention time or "memory', providing a reasonable approximation of the active layer in light of

the observed pore water gradients.

The decomposition reactions that drive the component mass balance equations are the anaerobic 

decomposition of the phytoplankton carbon, and the anaerobic breakdown of the benthic organic 

carbon. Both reactions are sinks of oxygen and rapidly drive its concentration negative,

indicating that the sediment is reduced rather than oxidized. The negative concentrations

computed can be considered the oxygen equivalents of the reduced end products produced by the 

chains of redox reactions occurring in the sediment (Figure 8-4).
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Figure 8-4 Benthic layer oxygen balance equations

Because the calculated concentration of oxygen is positive in the overlying water, it is assumed

that the reduced carbon species (negative oxygen equivalents) that are transported across the

benthic water interface combine with the available oxygen and are oxidized to CO2 and H2O 

with a consequent reduction of oxygen in the overlying water column.

Table 8-2 summarize the benthic CBOD and DO reactions and parameters. Illustrative

parameter values from an early Potomac Estuary modeling study are provided.

Table 8-2 Benthic Layer CBOD and DO Reaction Terms

Description Notation Value Units

Organic carbon (as CBOD) decomposition rate kDS 0.0004 day-1

Temperature coefficient ÈDS 1.08 none

Denitrification rate k2D day-1

Temperature coefficient È2D none

Phytoplankton decomposition rate kPzD day-1
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Description Notation Value Units

Temperature coefficient ÈPzD none

Diffusive exchange coefficient EDIF 2.0 x 10-4 m2/day

Benthic layer depth Dj 0.2-0.7 m

Benthic layer j

Water column i

8.3. Model Implementation

To simulate dissolved oxygen with WASP6, use the preprocessor to create a EUTRO input

dataset. For the portions of the dataset describing environment, transport, and boundaries,

EUTRO model input will be similar to that for the conservative tracer model as described in

Chapter 2. To those basic parameters, the user will add combinations of transformation

parameters and perhaps solids transport rates.

EUTRO kinetics can be implemented using some or all of the processes and kinetic terms

described above to analyze dissolved oxygen problems. For convenience, four levels of

complexity are identified here: (1) Streeter-Phelps, (2) modified Streeter- Phelps, (3) full linear 

DO balance, and (4) nonlinear DO balance. Please note that the discrete levels of simulation

identified here are among a continuum of levels that the user could implement. 

The four-implementation levels are described briefly below, along with the input parameters

required to solve the DO balance equations in EUTRO. Input parameters are prepared for

WASP6 in four major sections of the preprocessor -- environment, transport, boundaries, and

transformation. Basic model parameters are described in Chapter 2, and will not be repeated

here. Six of the eight EUTRO state variables that can participate in DO balance simulations,

with abbreviations used in this text, are listed in Table 8-3. 

Table 8-3 Summary of EUTRO Variables used in DO Balance

 Variable Notation Concentration Units

1. Ammonia Nitrogen NH3 C1 mg N/L

2. Nitrate Nitrogen NO3 C2 mg N/L

4. Phytoplankton Carbon PHYT C4 mg C/L

5. Carbonaceous BOD CBOD C5 mg O2/L

6. Dissolved Oxygen DO C6 mg O2/L

6. Organic Nitrogen ON C7 mg N/L

8.3.1. Streeter-Phelps

The simplest dissolved oxygen balance solves the Streeter-Phelps BOD-DO equations in a

slightly modified form. 
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8-15

Equation 8-15

(1- f )C

D

v

S = - k C -

D5 5

 s3

5

T-20

k5 d Θd

Equation 8-16

D

SOD S =+ k ( C - C ) - k C -

T

5

T-20

 s 6 d d

T -20

k6 2 Θ2 Θ

where Ski is the source/sink term for variable "i" in a segment, in mg/L-day. Kinetic rate

constants and coefficients are as defined in Table 4.1, except that C5 is interpreted as total (not 

just carbonaceous) biochemical oxygen demand, BOD. These equations are usually applied in

well-defined low flow design conditions.

8.3.2. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control the

simulation.

Systems-- Select "simulate" for CBOD and DO and "bypass" for the other six systems. For this 

implementation, the CBOD system is used to represent total ultimate BOD. Figure 3-7

Segments-- Water column segments should be defined in the standard fashion. If BOD settling

is to be simulated, the user should add a single benthic segment underlying all water column

segments. This benthic segment will merely act as a convenient sink for settling BOD. Model

calculations within this benthic segment should be ignored. Figure 3-8

8.3.3. Transport Parameters

This group of parameters defines the advective and dispersive transport of simulated model

variables.

Number of Flow Fields-- To simulate settling, the user should select solids 1 flow under

advection. The user should also select water column flow. Figure 3-14

Particulate Transport, m3/sec-- Time variable settling and resuspension rates for particulate BOD

can be input using the Solids 1 continuity array BQ and the time function QT. For each solids 

flow field, cross-sectional exchange areas (m2) for adjacent segment pairs are input using the

spatially variable BQ. Time-variable settling velocities can be specified as a series of velocities, 
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8-16

in m/sec, versus time. If the units conversion factor is set to 1.157e-5, then these velocities are 

input in units of m/day. These velocities are multiplied internally by cross-sectional areas and

treated as flows that carry particulate organic matter out of the water column. Figure 3-14

8.3.4. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial conditions. 

Boundary concentrations must be specified for any segment receiving flow inputs, outputs, or

exchanges. Initial conditions include not only initial concentrations, but also the density and

solids transport field for each solid, and the dissolved fraction in each segment.

Boundary Concentrations, mg/L-- At each segment boundary, time variable concentrations must

be specified for BOD and DO. A boundary segment is characterized by water exchanges from

outside the network, including tributary inflows, downstream outflows, and open water

dispersive exchanges. Figure 3-15

Waste Loads, kg/day-- For each point source discharge, time variable BOD and DO loads can be 

specified. These loads can represent municipal and industrial wastewater discharges, or urban

and agricultural runoff. (Figure 3-16)

Solids Transport Field-- The transport field associated with particulate BOD settling must be

specified under initial conditions. Field 3 is recommended. (Figure 3-14)

Solid Density, g/cm3-- A value of 0 can be entered for the nominal density of BOD and DO. 

This information is not used in EUTRO. (Figure 3-10)

Initial Concentrations, mg/L-- Concentrations of BOD and DO in each segment must be

specified for the time at which the simulation begins. Concentrations of zero for nonsimulated

variables -- NH3, NO3, PO4, PHYT, ON, and OP -- will be entered by the preprocessor. ((Figure 

3-10)

Dissolved Fraction-- The dissolved fraction of BOD and DO in each segment must be specified. 

Values for DO should be 1.0. Only the particulate fraction of BOD will be subject to settling. 

(Figure 3-11Figure 3-11)

8.3.5. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. Parameter values are entered for

each segment. Specified values for constants apply over the entire network for the whole 

simulation. Kinetic time functions are composed of a series of values versus time, in days. 
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Water Temperature, °C-- Segment variable water temperatures can be specified using the

parameter TMPSG (parameter TMPFN and time functions TEMP(1-4) should be omitted). 

Temperatures will remain constant in time. 

Sediment Oxygen Demand, g/m2-day-- Segment variable sediment oxygen demand fluxes can be 

specified using the parameter SOD1D. Values should be entered for water column segments that 

are in contact with the bottom of the water body. 

BOD Deoxygenation Rate, day-1-- The BOD deoxygenation rate constant and temperature

coefficient can be specified using constants KDC and KDT, respectively.

Reaeration Rate, day-1-- There are three options for specifying reaeration rate constants in

EUTRO. In the first option, a single reaeration rate constant can be specified using constant K2. 

An internal temperature coefficient of 1.028 is used with this option.

If K2 is not entered (or is set to 0), the second option is attempted by EUTRO. In this option, 

variable reaeration rate constants can be input using parameter REARSG and time function

REAR. The product of spatially variable REARSG and time-variable REAR gives the segment

and time specific reaeration rate constants used by EUTRO. These reaeration values are not

modified by a temperature function.

The third option is invoked if neither K2 nor REARSG is entered. In this option, reaeration rates 

will be calculated from water velocity, depth, wind velocity, and water and air temperature. The 

actual reaeration rate used by EUTRO will be either the flow or wind-induced value whichever is 

largest.

For rivers, segment water velocities and depths are calculated as a function of flow using the

hydraulic coefficients entered under the topic "environment". For lakes and estuaries, ambient

velocities in m/sec can be input using parameter VELFN and time functions VEL(1-4). The

parameter VELFN indicates which velocity function will be used by the model for each segment. 

Values of 1.0, 2.0, 3.0, or 4.0 will call time functions VELN(1), VELN(2), VELN(3), and

VELN(4), respectively. Water velocities should then be entered via these time functions as a

series of velocity versus time values. 

For open bodies of water, wind-driven reaeration can be significant. The user should input

ambient wind speed, in m/sec, and air temperature, in °C, using time functions WIND and

AIRTMP. The default values for wind speed and air temperature are 0.6 m/sec and 15 C. The 

scale of the water body should be input using constant WTYPE. Values of 1.0, 2.0, and 3.0 

indicate laboratory scale, lake and reservoir scale, and open ocean scale, respectively. The

default value is 2.

For estuaries, where salinity affects DO saturation significantly, salinity values in g/L can be

input using parameter SAL and time function SALFN. The product of spatially-variable SAL
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8-18

and time-variable SALFN gives the segment and time specific salinity values used by EUTRO.

Average segment salinity values can be input to SAL, while relative variations in time, if

significant, can be input to SALFN. 

For northern climates, where ice cover can affect reaeration during winter months, the user may

input the fraction of water surface available for reaeration using time function XICECVR. A

value of 1.0 indicates that the entire surface area is available for reaeration. The time variable

value of XICECVR will be multiplied by the reaeration rate constants for options 1 and 3. For

option 2, it is assumed that ice cover is built into the time function REAR.

WTYPE and K2 are identified in EUTRO as constants.. VELFN, SAL, and REARSG are

identified in EUTRO as parameters. WIND, VELN(1-4), SALFN, AIRTMP, XICECVR, and 

REAR are identified in EUTRO as time functions. 

8.4. Modified Streeter-Phelps

The modified Streeter-Phelps equations divide biochemical oxygen demand into carbonaceous

and nitrogenous fractions, and allow time-variable temperatures to be specified. This allows for 

more realistic calibration to observed data. Waste load allocations, however, are usually

projected for design low-flow conditions (Figure 8-5).
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Figure 8-5 Modified Streeter Phelps

Equation 8-17

(1- f )C

D

v

S = - k C -

D5 5

 s3

5

T-20

k5 d Θ d

Equation 8-18

(1- f )C

D

v

S =- k C -

D1 1

 s3

1

T -20

k1 n Θn
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8-20

Equation 8-19

Θ Θ

Θ Θ

T-20

1 s

T -20

n n

5

T-20

 s 6 d d

T -20

k6 2 2

D

SOD

k C -

14

64

-

S =+ k ( C - C )- k C

where Ski is the source/sink term for variable "i" in a segment, in mg/L-day. Kinetic rate

constants and coefficients are as defined in Table 4.1, except for the following:

C1= nitrogenous biochemical oxygen demand (NBOD), as expressed by 

TKN, mg/L (use System 1)

kn= nitrogenous deoxygenation rate constant, day-1

Èn= temperature coefficient

fD1= NBOD dissolved fraction

To implement these equations in EUTRO, System 1 (nominally NH3) must be interpreted as

nitrogenous BOD rather than ammonia. Here, NBOD is expressed by total Kjeldahl nitrogen

(TKN). If directly measured NBOD data are available, 4.57 before use in this model should

divide values. Likewise, System 1 model predictions should be multiplied by 4.57 before

comparison with NBOD data.

8.4.1. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control the

simulation.

Systems-- Select "simulate" for NH3, CBOD and DO and "bypass" for the other five systems. 

For this implementation, the NH3 system is used to represent nitrogenous BOD, as expressed by 

TKN. 

Segments-- Water column segments should be defined in the standard fashion. If CBOD or

NBOD settling is to be simulated, the user should add a single benthic segment underlying all

water column segments. This benthic segment will merely act as a convenient sink for settling

BOD. Model calculations within this benthic segment should be ignored. 

8.4.2. Transport Parameters

This group of parameters define the advective and dispersive transport of model variables.

Number of Flow Fields-- To simulate settling, the user should select solids 1 flow under

advection. The user should also select water column flow. 
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Particulate Transport, m

3

/sec-- Time variable settling and resuspension rates for particulate

CBOD and NBOD can be input using the Solids 1 continuity array BQ and the time function QT. 

For each solids flow field, cross-sectional exchange areas (m2) for adjacent segment pairs are

input using the spatially-variable BQ. Time-variable settling velocities can be specified as a

series of velocities, in m/sec, versus time. If the units conversion factor is set to 1.157e-5, then 

these velocities are input in units of m/day. These velocities are multiplied internally by crosssectional areas and treated as flows that carry particulate organic matter out of the water column. 

8.4.3. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial conditions. 

Boundary concentrations must be specified for any segment receiving flow inputs, outputs, or

exchanges. Initial conditions include not only initial concentrations, but also the density and

solids transport field for each solid, and the dissolved fraction in each segment.

Boundary Concentrations, mg/L-- At each segment boundary, time variable concentrations must 

be specified for CBOD, NBOD, and DO. The NH3 system is used to represent NBOD, which is 

expressed as TKN. A boundary segment is characterized by water exchanges from outside the 

network, including tributary inflows, downstream outflows, and open water dispersive

exchanges. 

Waste Loads, kg/day-- For each point source discharge, time variable CBOD, NBOD, and DO 

loads can be specified. These loads can represent municipal and industrial wastewater

discharges, or urban and agricultural runoff. The NH3 system is used to represent NBOD, which 

is expressed as TKN. 

Solids Transport Field-- The transport field associated with particulate CBOD and NBOD

settling must be specified under initial conditions. Field 3 is recommended for both.

Solid Density, g/cm3-- A value of 0 can be entered for the nominal density of CBOD, NBOD, 

and DO. This information is not used in EUTRO. 

Initial Concentrations, mg/L-- Concentrations of CBOD, NBOD, and DO in each segment must 

be specified for the time at which the simulation begins. The NH3 system is used to represent 

NBOD, which is expressed as TKN. Concentrations of zero for non-simulated variables -- NO3, 

PO4, PHYT, ON, and OP -- will be entered by the preprocessor.

Dissolved Fraction-- The dissolved fraction of CBOD, NBOD, and DO in each segment must be 

specified. Values for DO should be 1.0. Only the particulate fraction of CBOD and NBOD will 

be subject to settling. 
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8-22

8.4.4. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. Parameter values are entered for

each segment. Specified values for constants apply over the entire network for the whole

simulation. Kinetic time functions are composed of a series of values versus time, in days.

Water Temperature, °C-- Time and segment variable water temperatures can be specified using

the parameters TMPSG and TMPFN, and the time functions TEMP(1-4). If temperatures are to 

remain constant in time, then the user should enter segment temperatures using the parameter

TMPSG. TMPFN and TEMP(1-4) should be omitted. 

If the user wants to enter time-variable temperatures, then values for the parameter TMPSG

should be set to 1.0. The parameter TMPFN indicates which temperature function will be used 

by the model for each segment. Values of 1.0, 2.0, 3.0, or 4.0 will call time functions TEMP(1), 

TEMP(2), TEMP(3), and TEMP(4), respectively. Water temperatures should then be entered via 

these time functions as a series of temperature versus time values. The product of TMPSG and 

the selected TEMP function will give the segment and time specific water temperatures used by

EUTRO.

TMPSG and TMPFN are identified in EUTRO as parameters 3 and 4, respectively. TEMP(1-4) 

are identified in EUTRO as time functions 1-4.

Sediment Oxygen Demand, g/m2-day-- Segment variable sediment oxygen demand fluxes and

temperature coefficients can be specified using the parameters SOD1D and SODTA,

respectively. Values should be entered for water column segments that are in contact with the 

bottom of the water body. If temperatures remain constant in time, then SODTA can be omitted.

CBOD Deoxygenation Rate, day-1-- The CBOD deoxygenation rate constant and temperature 

coefficient can be specified using constants KDC and KDT, respectively.

 NBOD Deoxygenation Rate, day-1-- The NBOD deoxygenation rate constant and temperature

coefficient can be specified using constants K12C and K12T, respectively.

Reaeration Rate, day-1-- There are three basic options for specifying reaeration -- a single rate 

constant, segment and time variable rate constants, and flow and wind calculated rate constants. 

These options are described in the Streeter-Phelps section.

8.5. Full Linear DO Balance

The full DO balance equations divide the NBOD process into mineralization and nitrification,

and add the effects of photosynthesis and respiration from given phytoplankton levels (Figure 

8-6):
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Figure 8-6 Full Linear DO

Equation 8-20

(1- f )C

D

v

s = - k C -

D7 7

 s3

7

T-20

k7 71 Θ71

Equation 8-21

S =+ k C - k C1

T-20

7 12 12

T -20

k1 71 Θ71 Θ

Equation 8-22

S =+k C 1

T -20

k2 12 Θ12

Equation 8-23

(1- f )C

D

v

S = - k C -

D5 5

 s3

5

T-20

k5 d Θ d
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8-24

Equation 8-24

C 

12

32

+( k - k )

D

SOD

-

k C

14

64

S =+ k ( C - C ) - k C -

4

T - 20

1R 1R

T - 20

1c 1c

T - 20

s

1

T-20

5 12 12

T -20

 s 6 d d

T -20

k6 2 2

Θ Θ Θ

Θ Θ Θ

where Ski is the source/sink term for variable "i" in a segment, in mg/L-day. Kinetic rate

constants and coefficients are as defined in Table 4.1. In addition, the following are used:

k71= organic nitrogen mineralization rate constant, day-1

È71= temperature coefficient

k1C= average phytoplankton growth rate constant, day-1 (user must input 

light and nutrient limited value)

È1C= temperature coefficient

fD7= organic nitrogen dissolved fraction

Constant phytoplankton concentrations to be used in the DO balance are input under initial

conditions as μg/L chlorophyll a. If the carbon to chlorophyll ratio is not input, then a default 

value of 30 is used. The particulate fractions of CBOD and ON are associated with transport 

field 3, organic matter settling. 

8.5.1. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control the

simulation.

Systems-- Select "simulate" for NH3, NO3, CBOD, DO, and ON. Select "constant" for PHYT, 

and "bypass" for PO4 and OP. 

Segments-- Water column segments should be defined in the standard fashion. If CBOD or ON 

settling is to be simulated, the user should add a single benthic segment underlying all water

column segments. This benthic segment will merely act as a convenient sink for settling organic 

matter. Model calculations within this benthic segment should be ignored. 

8.5.2. Transport Parameters

This group of parameters defines the advective and dispersive transport of model variables.

Number of Flow Fields-- To simulate settling, the user should select solids 1 flow under

advection. The user should also select water column flow. 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

8-25

Particulate Transport, m

3

/sec-- Time variable settling and resuspension rates for particulate

CBOD and ON can be input using the Solids 1 continuity array BQ and the time function QT. 

For each solids flow field, cross-sectional exchange areas (m2) for adjacent segment pairs are

input using the spatially-variable BQ. Time-variable settling velocities can be specified as a

series of velocities, in m/sec, versus time. If the units conversion factor is set to 1.157e-5, then 

these velocities are input in units of m/day. These velocities are multiplied internally by crosssectional areas and treated as flows that carry particulate organic matter out of the water column. 

8.5.3. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial conditions. 

Boundary concentrations must be specified for any segment receiving flow inputs, outputs, or

exchanges. Initial conditions include not only initial concentrations, but also the density and

solids transport field for each solid, and the dissolved fraction in each segment.

Boundary Concentrations, mg/L-- At each segment boundary, time variable concentrations must

be specified for NH3, NO3, ON, CBOD, and DO. A boundary segment is characterized by

water exchanges from outside the network, including tributary inflows, downstream outflows,

and open water dispersive exchanges. 

Waste Loads, kg/day-- For each point source discharge, time variable NH3, NO3, ON, CBOD, 

and DO loads can be specified. These loads can represent municipal and industrial wastewater 

discharges, or urban and agricultural runoff.

Solids Transport Field-- The transport field associated with particulate CBOD and ON settling

must be specified under initial conditions. Field 3 is recommended for both. 

Solid Density, g/cm3-- A value of 0 can be entered for the nominal density of NH3, NO3, ON, 

CBOD, and DO. This information is not used in EUTRO. 

Initial Concentrations, mg/L-- Concentrations of NH3, NO3, ON, CBOD, and DO in each

segment must be specified for the time at which the simulation begins. Average concentrations

of PHYT, expressed as μg/L chlorophyll a, must be specified as well. These are converted to 

mg/L phytoplankton carbon in EUTRO using a default carbon to chlorophyll ration of 30. 

Phytoplankton concentrations will remain constant throughout the simulation and affect DO

through photosynthesis and respiration. Concentrations of zero for non-simulated variables --

PO4 and OP -- will be entered by the preprocessor.

Dissolved Fraction-- The dissolved fraction of NH3, NO3, ON, CBOD, and DO in each segment 

must be specified. Values for DO should be 1.0. Only the particulate fraction of CBOD and ON 

will be subject to settling. 
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8.5.4. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. Parameter values are entered for 

each segment. Specified values for constants apply over the entire network for the whole

simulation. Kinetic time functions are composed of a series of values versus time, in days.

Water Temperature, °C-- Time and segment variable water temperatures can be specified using

the parameters TMPSG and TMPFN, and the time functions TEMP(1-4), as described in the 

modified Streeter-Phelps section. 

Sediment Oxygen Demand, g/m2-day-- Segment variable sediment oxygen demand fluxes and

temperature coefficients can be specified using the parameters SOD1D and SODTA,

respectively. Values should be entered for water column segments that are in contact with the

bottom of the water body. 

Nitrogen Mineralization Rate, day-1-- The mineralization rate constant and temperature 

coefficient for dissolved organic nitrogen can be specified using constants K71C and K71T,

respectively. 

Nitrification Rate, day-1-- The nitrification rate constant and temperature coefficient for dissolved 

ammonia nitrogen can be specified using constants K12C and K12T, respectively. 

CBOD Deoxygenation Rate, day-1-- The CBOD deoxygenation rate constant and temperature

coefficient can be specified using constants KDC and KDT, respectively. 

Reaeration Rate, day-1-- There are three basic options for specifying reaeration -- a single rate 

constant, segment and time variable rate constants, and flow and wind calculated rate constants. 

These options are described in the Streeter-Phelps section.

Photosynthesis Rate, day-1-- The average phytoplankton growth rate constant and temperature

coefficient can be input using constants K1C and K1T, respectively. For DO balance

simulations where phytoplankton dynamics are bypassed, the growth rate constant must reflect

average light and nutrient limitations in the water body. 

Respiration Rate, day-1-- The average phytoplankton respiration rate constant and temperature

coefficient can be input-using constants K1RC and K1RT, respectively. 

8.6. Nonlinear DO Balance

The nonlinear DO balance equations add feedback from DO concentrations to terms in the linear 

DO balance equations presented above. This feedback can become important in inhibiting
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nitrification and carbonaceous oxidation and in promoting denitrification where low DO

concentrations occur.

For this level of analysis, the linear DO balance equations presented above are supplemented

with nonlinear terms for carbonaceous oxidation, nitrification, and denitrification. These terms

are presented in Table 8-1. The environment, transport, and boundary parameters required to 

implement the nonlinear DO balance are the same as those in the linear DO balance presented

above. The user should supplement the transformation parameters presented above with the

following.

Nitrification Rate, day-1-- The nitrification rate constant and temperature coefficient for dissolved 

ammonia nitrogen can be specified using constants K12C and K12T, respectively. The halfsaturation constant for oxygen limitation of nitrification can be specified using constant KNIT. 

The default value for KNIT is 0.0, indicating no oxygen limitation. 

Denitrification Rate, day-1-- The denitrification rate constant and temperature coefficient for

dissolved nitrate nitrogen can be specified using constants K20C and K20T, respectively. The 

half-saturation constant for oxygen limitation of denitrification can be specified using constant

KNO3. The default value for KNO3 is 0.0, indicating no denitrification at oxygen

concentrations above 0.0. 

CBOD Deoxygenation Rate, day-1-- The CBOD deoxygenation rate constant and temperature

coefficient can be specified using constants KDC and KDT, respectively. The half-saturation 

constant for oxygen limitation of carbonaceous deoxygenation can be specified using constant

KBOD. The default value for KBOD is 0.0, indicating no oxygen limitation. 
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9-1

9. Eutrophication

Nutrient enrichment and eutrophication are continuing concerns in many water bodies. High

concentrations of nitrogen and phosphorus can lead to periodic phytoplankton blooms and an

alteration of the natural trophic balance. Dissolved oxygen levels can fluctuate widely, and low

DO concentrations in bottom waters can result.

Eutrophication has been modeled for approximately 30 years. The equations implemented here

were derived from the Potomac Eutrophication Model, PEM (Thomann and Fitzpatrick, 1982),

and are fairly standard. Sections of this text are modified from the PEM documentation report.

9.1. Overview of WASP6 Eutrophication

The nutrient enrichment, eutrophication, and DO depletion processes are simulated using the

EUTRO program. Several physical-chemical processes can affect the transport and interaction

among the nutrients, phytoplankton, carbonaceous material, and dissolved oxygen in the aquatic 

environment. Figure 9-1 presents the principal kinetic interactions for the nutrient cycles and

dissolved oxygen.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-2

Figure 9-1 EUTRO State Variables

EUTRO can be operated by the user at various levels of complexity to simulate some or all of

these variables and interactions. Four levels for simulating the DO balance were described in

Chapter 8. Three levels of complexity for simulating eutrophication are identified and

documented at the end of this section: (1) simple eutrophication kinetics, (2) intermediate

eutrophication kinetics, and (3) intermediate eutrophication kinetics with benthos. The user

should become familiar with the full capabilities of EUTRO even if simpler simulations are

planned.

EUTRO simulates the transport and transformation reactions of up to eight state variables,

illustrated in Figure 9-1. They can be considered as four interacting systems: phytoplankton

kinetics, the phosphorus cycle, the nitrogen cycle, and the dissolved oxygen balance. The

general WASP6 mass balance equation is solved for each state variable. To this general

equation, the EUTRO subroutines add specific transformation processes to customize the general

mass balance for the eight state variables in the water column and benthos. Following a short

summary of the material cycles, the rest of this section covers the specific details for the several

transformation sources and sinks.
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9.1.1. Phosphorus Cycle

Dissolved or available inorganic phosphorus (DIP) interacts with particulate inorganic

phosphorus via a sorption-desorption mechanism. DIP is taken up by phytoplankton for growth, 

and is incorporated into phytoplankton biomass. Phosphorus is returned from the phytoplankton

biomass pool to dissolved and particulate organic phosphorus and to dissolved inorganic

phosphorus through endogenous respiration and nonpredatory mortality. Organic phosphorus is

converted to dissolved inorganic phosphorus at a temperature-dependent mineralization rate.

9.1.2. Nitrogen Cycle

The kinetics of the nitrogen species are fundamentally the same as the phosphorus system. 

Ammonia and nitrate are taken up by phytoplankton for growth, and incorporated into

phytoplankton biomass. The rate at which each is taken up is a function of its concentration

relative to the total inorganic nitrogen (ammonia plus nitrate) available. Nitrogen is returned

from the phytoplankton biomass pool to dissolved and particulate organic nitrogen and to

ammonia through endogenous respiration and nonpredatory mortality. Organic nitrogen is

converted to ammonia at a temperature dependent mineralization rate, and ammonia is then

converted to nitrate at a temperature- and oxygen-dependent nitrification rate. Nitrate may be 

converted to nitrogen gas in the absence of oxygen at a temperature- and oxygen-dependent 

denitrification rate.

9.1.3. Dissolved Oxygen

Dissolved oxygen is coupled to the other state variables. The sources of oxygen considered are 

reaeration and evolution by phytoplankton during growth. The sinks of oxygen are algal

respiration, oxidation of detrital carbon and carbonaceous material from waste effluents and

nonpoint discharges, and nitrification. These processes are discussed in Chapter 8.

9.1.4. Phytoplankton Kinetics

Phytoplankton kinetics assumes a central role in eutrophication, affecting all other systems. An

overview of this system is given in Figure 5.2.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-4

Figure 9-2 Phytoplankton kinetics

It is convenient to express the reaction term of phytoplankton, Sk4j, as a difference between the 

growth rate of phytoplankton and their death and settling rates in the volume Vj. That is:

Equation 9-1

S k4j=( G p1j - D p1j- k s4j ) P j

where:

Sk4j = reaction term, mg carbon/L-day

Pj = phytoplankton population, mg carbon/L

Gp1j = growth rate constant, day-1

Dp1j = death plus respiration rate constant, day-1
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-5

ks4j = settling rate constant, day-1

j = segment number, unitless

The subscript 1 identifies the quantities as referring to phytoplankton type 1, (only one type is

considered in this particular model); the subscript j refers to the volume element being 

considered. The balance between the magnitude of the growth rate and death rate (together with 

the transport, settling, and mixing) determines the rate at which phytoplankton mass is created in

the volume element Vj. In subsequent text and in figures, subscripts i and j will be omitted

unless needed for clarity.

9.1.5. Phytoplankton Growth

The growth rate of a population of phytoplankton in a natural environment is a complicated

function of the species of phytoplankton present and their differing reactions to solar radiation, 

temperature, and the balance between nutrient availability and phytoplankton requirements. The

available information is not sufficiently detailed to specify the growth kinetics for individual

algal species in a natural environment. Rather than considering the problem of different species 

and their associated environmental and nutrient requirements, this model characterizes the

population as a whole by the total biomass of the phytoplankton present.

A simple measure of total biomass that is characteristic of all phytoplankton, chlorophyll a, is

used as the aggregated variable. The principal advantages are that the measurement is direct; it 

integrates cell types and ages, and it accounts for cell viability. The principal disadvantage is

that it is a community measurement with no differentiation of functional groups (e.g., diatoms,

blue-greens); also, it is not necessarily a good measurement of standing crop in dry weight or

carbon units because the chlorophyll-to-dry-weight and carbon ratios are variable and non-active 

chlorophyll (phaeopigments) must be measured to determine viable chlorophyll concentrations.

As can be seen from the above discussion, no simple aggregate measurement is entirely

satisfactory. From a practical point of view, the availability of extensive chlorophyll data

essentially dictates its use as the aggregate measure of the phytoplankton population or biomass

for calibration and verification purposes. For internal computational purposes, however,

EUTRO uses phytoplankton carbon as a measure of algal biomass. Using either a fixed or

variable carbon to chlorophyll mechanism (discussed subsequently), phytoplankton chlorophyll a

may be computed and used as the calibration and verification variable to be compared against

observed chlorophyll a field data.

With a choice of biomass units established, a growth rate that expresses the rate of production of 

biomass as a function of the important environmental variables (temperature, light, and nutrients)

may be developed. The specific growth rate, GPlj, in segment j is related to k1c, the maximum 

20°C growth rate at optimum light and nutrients, via the following equation.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-6

Equation 9-2

G P1j= k 1c X RTj X RIj X RNj

where:

XRTj= the temperature adjustment factor, dimensionless

XRIj= the light limitation factor as a function of I, f, D, and Ke, 

dimensionless:

XRNj= the nutrient limitation factor as a function of dissolved inorganic 

phosphorus and nitrogen (DIP and DIN), dimensionless:

T= ambient water temperature, °C

I= incident solar radiation, ly/day

f= fraction day that is daylight, unitless

D= depth of the water column or model segment, m

K= total light extinction coefficient, m-1

DIP= dissolved inorganic phosphorus (orthophosphate) available for

growth, mg/L

DIN= dissolved inorganic nitrogen (ammonia plus nitrate) available for 

growth, mg/L

An initial estimate of k1c can be made based upon previous studies of phytoplankton dynamics 

and upon reported literature values (such as Bowie et al., 1985) and subsequently refined during 

the calibration and verification process. This maximum growth rate constant is adjusted

throughout the simulation for ambient temperature, light, and nutrient conditions.

Temperature -- Water temperature has a direct effect on the phytoplankton growth rate. The

selected maximum growth rate is temperature-corrected using temporally- and spatially-variable 

water column temperatures as reported in field studies. The temperature correction factor is

computed using:

Equation 9-3

Θ

T -20 X RTj = 1c

where:

È1c = temperature coefficient, unitless

Light -- In the natural environment, the light intensity to which the phytoplankton are exposed is 

not uniformly at the optimum value. At the surface and near-surface of the air-water interface, 

photoinhibition can occur at high light intensities, whereas at depths below the euphotic zone

light is not available for photosynthesis due to natural and algal-related turbidity. 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-7

Modeling frameworks developed by Di Toro et al. (1971), and by Smith (1980), extending upon 

a light curve analysis formulated by Steele (1962), account for both the effects of supersaturating

light intensities and light attenuation through the water column. The instantaneous 

depth-averaged growth rate reduction developed by Di Toro is presented in Equation 9-4 and is 

obtained by integrating the specific growth rate over depth:

Equation 9-4

)]

I

I

( - K D) - ( -

I

I

f [ -

K D

e

X =

 s

a

e

 s

a

e

RI exp exp exp













where:

Ia= the average incident light intensity during daylight hours just below 

the surface, assumed to average 0.9 I/f, ly/day

Is= the saturating light intensity of phytoplankton, ly/day

Ke= the light extinction coefficient, computed from the sum of the

non-algal light attenuation, Ke

'

, and the phytoplankton self-shading 

attenuation, Keshd (as calculated by Equation 5.5), m-1

Equation 9-5

K =0.0088 P +0.054 P

0.67

eshd Chl Chl

PChl= phytoplankton chlorophyll concentration, μg/L

Typical clear sky values of surface light intensity for different latitudes and months are provided

in Table 9-1.

Table 9-1Example Solar Radiation

Time

of Day Season

Annual

Mean

Latitude Spring Summer Fall Winter

30°N Mean1 680 750 530 440 600

Mid-Day2 2100 2200 1700 1400 1900

40°N Mean 650 740 440 320 540

Mid-Day 1900 2100 1400 1000 1600

50°N Mean 590 710 330 190 460

Mid-Day 1700 1900 1000 650 1300
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-8

1

calculated seasonal means under a clear sky, representing upper limits for solar radiant energy at 

sea level. Reference: Weast and Astle (1980).

2Mid-day flux extended over a 24-hour period, assuming an atmospheric turbidity of 0,

precipitable water content of 2 cm, and atmospheric ozone content of .34 cm NTP. Reference:

Robinson (1966).

Equation 9-4 is quite similar in form to that developed by Smith, which is also available as an

option in this model:

Equation 9-6

)]

I

I

(- K D) - (-

I

I

[ -

K D

e

X (t)=

 s

o

e

 s

o

e

RI exp exp exp













where:

Equation 9-7

 =0, t= f - 1

 , t=0 - f

f

t

f

I

2

I o

(t)= 



























p p

SIN

and

Equation 9-8

K f

k X e

I =

c u

1c RT c

 s

Φ

Θ

max

where:

Io= the time variable incident light intensity just below the surface, 

assumed to follow a half sin function over daylight hours, ly/day

Ömax= the quantum yield, mg carbon fixed per mole of light quanta

absorbed

Kc= the extinction coefficient per unit of chlorophyll, m2/mg chlorophyll 

a

Ke= the light extinction coefficient, computed from the sum of the

non-algal light attenuation, Ke

'

, and the phytoplankton self-shading 

attenuation, Keshd (as calculated by Equation 5.9), m-1
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-9

Equation 9-9

K eshd= Kc PChl

fu= units conversion factor (0.083, assuming 43% incident light is visible 

and 1 mole photons is equivalent to 52,000 cal), mole photons/m2-ly

Èc= the ratio of carbon to chlorophyll in the phytoplankton, (mg

carbon/mg chlorophyll a)

e= the base of natural logarithms (2.71828), unitless

Equations 9.6 - 9.9 give a light limitation coefficient that varies over the day with incident light. 

This term is numerically integrated over the day within the computer program to obtain daily

average light limitation:

Equation 9-10

X = X RI (t)dt

1

o

RI ∫

The term Is, the temperature-dependent light saturation parameter is an unknown in the Di Toro 

light formulation, and must be determined via the calibration-verification process. In the Smith

formulation, this term is calculated from parameters that are reasonably well documented in the

literature. As Smith (1980) points out, since the early experiments of Warburg and Negelein

(1923), maximum photosynthetic quantum yield (Ömax) has been measured for a wide range of

conditions (reviewed by Kok, 1960), and a nearly temperature-independent value of 0.08 to 0.1 

mole O2 per mole of photons absorbed is now widely accepted for photosynthesizing plants in

general in the laboratory. Bannister (1974a) gives good arguments for adopting 0.06 mole

carbon (0.07 mole O2) per mole of photons as the maximum yield for plankton in nature. 

Reported values for Kc generally fall in the range 0.01 to 0.02 m

2mg-1

, and 0.016 m2mg-1 has 

been suggested as the approximate average (Bannister, 1974b). 

A second feature incorporated in the modeling framework derived from Smith's work is the

calculation of a variable carbon to chlorophyll ratio based on the assumption that adaptive

changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient

conditions of light and temperature. Smith found that phytoplankton adjusts chlorophyll

composition so that Is roughly equals 30% of the average available light. The expression used to 

calculate the carbon to chlorophyll ratio is presented in Equation 9-11 :

Equation 9-11













•

•

Φ

Θ •

K D

1- e

I

k X e

K f

=0.3

e

-K D

a

1c RT

c u

c

e

max
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-10

where the latter term is the average daily solar radiation within a segment during daylight hours, 

in ly/day. Note that substituting Equation 5.11 into 5.8 gives an Is equal to 30% of the average 

available light.

A review of reported carbon/chlorophyll ratios in nature (Eppley and Sloane, 1966) suggests that 

physiological factors (in part the energy cost of synthesizing chlorophyll as compared with other

cellular compounds) come into play to prevent Èc from going much below 20, even in very low 

light. This lower limit of 20 has been included when determining a value for Èc. Previously

reported values of Èc from algal composition studies conducted by EPA Region III's Central

Regional Laboratory (CRL) are compared in Table 5.2 to calculated values of using Equation

5.11. There is general agreement between the measured and calculated values. Unfortunately, 

no winter algae composition studies were available for comparison purposes.

Nutrients -- The effects of various nutrient concentrations on the growth of phytoplankton have

been investigated and the results are quite complex. As a first approximation to the effect of

nutrient concentration on the growth rate, it is assumed that the phytoplankton population in

question follows Monod growth kinetics with respect to the important nutrients. That is, at an 

adequate level of substrate concentration, the growth rate proceeds at the saturated rate for the 

ambient temperature and light conditions present. At low substrate concentration, however, the

growth rate becomes linearly proportional to substrate concentration. Thus, for a nutrient with

concentration Nj in the j

th segment, the factor by which the saturated growth rate is reduced is: 

Nj/(Km + Nj). The constant, Km (called the Michaelis or half-saturation constant) is the nutrient 

concentration at which the growth rate is half the saturated growth rate. Because there are two 

nutrients, nitrogen and phosphorus, considered in this framework, the Michaelis-Menten 

expression is evaluated for the dissolved inorganic forms of both nutrients and the minimum

value is chosen to reduce the saturated growth rate, as given by Equation 9-12.

Carbon/Chlorophyll a

μg C/μg Chlorophyll a

Sampling Period Observed

Mean

Observed

Range

Predicted

Range

July 20-Oct. 6,19701 45 25-68 24-28

August 1-29, 19772 28 12-37 23-26

Sept. 7-28, 19782 21 15-27 26-30

Sept. 7-28, 19783 26-30

1. Elemental analysis of blue-green algae 

2. Laboratory elemental analysis of overall phytoplankton population 

3. Estimates of cell composition based upon field data 

Equation 9-12

















K + DIP

DIP

 ,

K +DIN

DIN

X =

mN mP

RN Min
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-11

At the user's discretion, the multiplicative formulation for nutrient limitation may be selected. 

This formulation multiplies the two terms in 5.12. It is not generally recommended.

Figure 9-3 presents plots of G(N) versus DIN and DIP with KmN = 25 μg-N/L and KmP = 1 

μg-P/L, respectively. The upper plot shows the standard Michaelis-Menten response curve to 

various concentrations of the inorganic nutrients. As can be seen, no significant reduction in

growth rate is achieved until DIN is less than 200 μg/L (0.2 mg/l) or until DIP is less than 8 μg/L 

(0.008 mg/l).

The lower plot on Figure 9-3 uses an expanded nutrient scale and shows the Michaelis-Menten 

formulation in a slightly different format. Here the impact of the function may be evaluated

quite readily. For example, a particular reach of the water body may have concentrations of DIN 

equal to 100 μg/L. This corresponds to a 20% reduction in the growth rate (XRN = 0.8). In order 

for phosphorus to become the limiting nutrient in the same reach, dissolved inorganic

phosphorus must reach a level of 4 μg/L or less. It should also be noted that if upstream nitrogen 

controls were instituted such that DIN was reduced to 60 μg/L for that same reach, then a further 

reduction in DIP to 2.5 μg/L would be required to keep phosphorus as the limiting nutrient. In

other words, as water column concentrations of DIP begin to approach growth limiting levels

due to continued reduction in point source phosphorus effluents, any nitrogen control strategies

that might be instituted would require additional levels of phosphorus removal in order to keep

phosphorus as the limiting nutrient.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-12

Figure 9-3 Effects of nutrient limitation on growth rate

9.1.6. Phytoplankton Death 

Numerous mechanisms have been proposed that contribute to the biomass reduction rate of

phytoplankton: endogenous respiration, grazing by herbivorous zooplankton, and parasitization. 

The first two mechanisms have been included in previous models for phytoplankton dynamics,

and they have been shown to be of general importance.

The endogenous respiration rate of phytoplankton is the rate at which the phytoplankton oxidize 

their organic carbon to carbon dioxide per unit weight of phytoplankton organic carbon. 

Respiration is the reverse of the photosynthesis process and, as such, contributes to the reduction 

in the biomass of the phytoplankton population. If the respiration rate of the phytoplankton as a 

whole is greater than the growth rate, there is a net loss of phytoplankton carbon or biomass. 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-13

The endogenous respiration rate is temperature dependent (Riley et at., 1949) and is determined 

via Equation 5.13:

Equation 9-13

° Θ

( T -20)

1R 1R C) 1R k (T)= k (20

where:

k1R(20°C)= the endogenous respiration rate at 20°C, day-1

k1R(T)= the temperature corrected rate, day-1

È1R= temperature coefficient, dimensionless

Reported values of endogenous respiration at 20° vary from 0.02 day-1 to 0.60 day-1, with most 

values falling between 0.05 day-1 and 0.20 day-1 (Bowie et al., 1985). Di Toro and Matystik 

(1980) report a value of 1.045 for È1R. The total biomass reduction rate for the phytoplankton in 

the jth segment is expressed via Equation 5.14: 

Equation 9-14

D =k (T)+k +k Z(t) 1j 1R 1D 1G

where: 

D1j= biomass reduction rate, day-1

k1D= death rate, representing the effect of parasitization, i.e., the infection 

of algal cells by other microorganisms, and toxic materials, such as 

chlorine residual, day-1

k1G= grazing rate on phytoplankton per unit zooplankton population,

L/mgC-day

Z(t)= herbivorous zooplankton population grazing on phytoplankton,

mgC/L

Note that the zooplankton population dynamics are described by the user, not simulated. If

population fluctuations are important in controlling phytoplankton levels in a particular body of

water, the user may want to simulate zooplankton and their grazing. On the other hand, many

studies need only a constant first order-grazing rate constant, where grazing rates are assumed

proportional to phytoplankton levels. In that case, k1G can be set to the first order constant with 

Z(t) omitted (default value = 1). Reported grazing rates vary from 0.1 to 1.5 L/mgC-day (Bowie 

et al., 1985).
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9-14

9.1.7. Phytoplankton Settling

The settling of phytoplankton is an important contribution to the overall mortality of the

phytoplankton population, particularly in lakes and coastal oceanic waters. Published values of

the settling velocity of phytoplankton, mostly under quiescent laboratory conditions, range from

0.07-18 m/day. In some instances, however, the settling velocity is zero or negative. Actual

settling in natural waters is a complex phenomenon, affected by vertical turbulence, density

gradients, and the physiological state of the different species of phytoplankton. Although the

effective settling rate of phytoplankton is greatly reduced in a relatively shallow, well-mixed 

river or estuary due to vertical turbulence, it still can contribute to the overall mortality of the

algal population. In addition, the settling phytoplankton can be a significant source of nutrients

to the sediments and can play an important role in the sediment oxygen demand. In EUTRO,

phytoplankton are equated to solid type 2. Time and segment-variable phytoplankton settling

velocities can be input by the user, then, using transport field 4, so that:

Equation 9-15

D

v

k =

j

 s4ij

 s4j

where:

ks4j= the effective phytoplankton settling or loss rate, day-1

vs4ij= the net settling velocity of phytoplankton from segment j to segment 

i, m/day

Dj= depth of segment j, equal to volume/surface area, m

9.1.8. Summary

This completes the specification of the growth and death rates of the phytoplankton population in

terms of the physical variables: light, temperature, and the nutrient concentrations present. 

Table 9-2 summarizes the variables and parameters in the net growth equations. With these

variables known as a function of time, it is possible to calculate the phytoplankton chlorophyll

throughout the year.

Table 9-2 Phytoplankton net growth terms

 Exogenous Variables

Description Notation Values Units

Extinction Coefficient Ke 0.1-5 m-1

Segment Depth D 0.1-30 m

Water Temperature T 0-35 oC

Fraction of day that is daylight f 0.3-0.7 -

Average Daily Surface Solar Radiation Ia 200-750 langleys/day
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-15

 Exogenous Variables

Description Notation Values Units

Zooplankton Population Z 0 mgC/L

Rate Constants

Description Notation Values Units

Maximum Growth Rate k1c 2.0 day-1

Temperature Coefficient È1c 1.068 none

Maximum Photosynthetic Quantum Yield Ömax 720.0 mg C/mole photon

Phytoplankton Self-Light Attenuation Kc 0.017 m2/mg Chl a

Carbon-Chlorophyll Ratio Èc 20-50 -

Saturating Light Intensity Is 200-500 langleys/day

Half-Saturation Constant for Nitrogen KmN 25.0 μg N/L

Half-Saturation Constant for Phosphorus KmP 1.0 μg P/L

Endogenous Respiration k1R 0.125 day-1

Temperature Coefficient È1R 1.045 none

Settling Velocity vs4 0.1 m/day

Death Rate k1D 0.02 day-1

Grazing Rate k1G 0 L/mgC-day

The nutrients are not known a priori, however, because they depend upon the phytoplankton

population that develops. These systems are interdependent and cannot be analyzed separately. 

It is necessary to formulate a mass balance for the nutrients as well as the phytoplankton in order 

to calculate the chlorophyll that would develop for a given set of environmental conditions.

9.1.9. Stoichiometry and Uptake Kinetics

A principal component in the mass-balance equations written for the nutrient systems included in

the eutrophication framework is the nutrient uptake kinetics associated with phytoplankton

growth. To specify the nutrient uptake kinetics associated with this growth, however, it is

necessary to specify the population stoichiometry in units of nutrient uptake/mass of population

synthesized. For carbon as the unit of population biomass, the relevant ratios are the mass of

nitrogen and phosphorus per unit mass of carbon. A selection of these ratios presented by Di

Toro et al. (1971) indicates that their variability is quite large. The use of constant ratios in the 

analysis, then, is questionable.

Upon further investigation, however, it is clear that the reason these ratios vary is the varying

cellular content of nutrients, which is, in turn, a function of the external nutrient concentrations

and the past history of the phytoplankton population. Large ratios of carbon to nitrogen or

phosphorus correspond to that nutrient limiting growth; small ratios reflect excess nutrients. 

Thus, the choice of the relevant ratios can be made with the specific situation in mind.

The operational consequence of this choice is that the population stoichiometry under

non-limiting conditions may be underestimated, but under limiting conditions should be

estimated correctly. Hence the trade off is a probable lack of realism during a portion of the year 

versus a correct estimate of phytoplankton biomass during periods of possible nutrient

limitations. Because this is usually the critical period and because most questions to be answered 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-16

are usually sensitive to maximum summer populations, this choice is a practical expedient. A

comparison of carbon-to-nitrogen and carbon-to-phosphorus ratios measured in the Potomac

Estuary is provided in Table 9-3.

Table 9-3 Carbon to nitrogen, carbon to phosphorus ratios

 Phosphorus/Carbon mg 

P/mg C

 Nitrogen/Carbon

 mg N/mg C

Sampling Period Observed

 Mean

 Observed

 Range

Observed

 Mean

Observed

 Range

July 20-Oct. 6, 19701 0.023 0.010-0.046 0.26 0.10-0.48

August 1-29, 19772 0.024 0.012-0.028 0.24 0.15-0.36

Sept. 7-28, 19782 0.030 0.017-0.047 0.26 0.18-0.35

Sept. 7-28, 19782 0.031 0.26

Model 0.025 0.25

1. Elemental analysis of blue-green algae

2. Laboratory elemental analysis of overall phytoplankton population

3. Estimates of cell composition based upon field data

Once the stoichiometric ratios have been determined, the mass balance equations may be written 

for the nutrients in much the same way as is done for the phytoplankton biomass. The primary

interaction between the nutrient systems and the phytoplankton system is the reduction or sink of

nutrients associated with phytoplankton growth. A secondary interaction occurs wherein the

phytoplankton system acts as a source of nutrients due to release of stored cellular nitrogen and 

phosphorus during algal respiration and death.

9.2. The Phosphorus Cycle

Three phosphorus variables are modeled: phytoplankton phosphorus, organic phosphorus, and

inorganic (orthophosphate) phosphorus. Organic phosphorus is divided into particulate and

dissolved concentrations by spatially variable dissolved fractions. Inorganic phosphorus also is

divided into particulate and dissolved concentrations by spatially variable dissolved fractions,

reflecting sorption. The phosphorus equations are summarized in Figure 9-4.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-17

Figure 9-4 Phosphorus cycle equations

Table 9-4 Phosphorus reaction terms

Description Notation Value Units

Phytoplankton biomass as carbon Pc - mg C/L

Specific phytoplankton growth rate Gp1j (eq 5.2) day-1

Phytoplankton loss rate Dp1j (eq 5.14) day-1

Phosphorus to carbon ratio aPC 0.025 mg P/mg C

Dissolved organic phosphorus mineralization at 20°C k83 0.22 day-1

Temperature coefficient È83 1.08 none

Half saturation constant for phytoplankton limitation of 

phosphorus recycle

KmPc 1.0 mg C/L

Fraction of dead and respired phytoplankton recycled to 

the organic phosphorus pool

fop 0.5 none

 . . . recycled to the

phosphate phosphorus pool

(1-fop) 0.5 none

Fraction dissolved inorganic phosphorus in the water

column

fD3 0.85,

0.70

none

Fraction dissolved organic phosphorus fD8 - none

Organic matter settling velocity vs3 - m/day

Inorganic sediment settling velocity vs5 - m/day
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-18

9.2.1. Phytoplankton Growth

As phytoplankton grow, dissolved inorganic phosphorus is taken up, stored and incorporated into 

biomass. For every mg of phytoplankton carbon produced, aPC mg of inorganic phosphorus is 

taken up.

9.2.2. Phytoplankton Death

As phytoplankton respire and die, biomass is recycled to nonliving organic and inorganic matter. 

For every mg of phytoplankton carbon consumed or lost, aPC mg of phosphorus is released. A 

fraction fop is organic, while (1 - fop) is in the inorganic form and readily available for uptake by 

other viable algal cells. In work on the Great Lakes, fop was assigned at 50% (Di Toro and 

Matystik, 1980).

9.2.3. Mineralization

Nonliving organic phosphorus must undergo mineralization or bacterial decomposition into

inorganic phosphorus before utilization by phytoplankton. In their work on Lake Huron and

Saginaw Bay, Di Toro and Matystik (1980) proposed a nutrient recycle formulation that was a 

function of the localized phytoplankton population. This proposal was based on both an analysis 

of available field data and the work of others (Hendry, 1977; Lowe, 1976; Henrici, 1938; Menon, 

1972; and Rao, 1976) that indicated bacterial biomass increased as phytoplankton biomass

increased. EUTRO uses a saturating recycle mechanism, a compromise between conventional 

first-order kinetics and a second order recycle mechanism wherein the recycle rate is directly

proportional to the phytoplankton biomass present, as had been indicated in pure culture,

bacteria-seeded, laboratory studies (Jewell and McCarty, 1971). 

Saturating recycle permits second order dependency at low phytoplankton concentrations, when

Pc << KmPc, where KmPc is the half-saturation constant for recycle, and permits first order recycle 

when the phytoplankton greatly exceed the half-saturation constant. Basically, this mechanism

slows the recycle rate if the phytoplankton population is small, but does not permit the rate to

increase continuously as phytoplankton increase. The assumption is that at higher population

levels, recycle kinetics proceed at the maximum first order rate. The default value for KmPc is 0, 

which causes mineralization to proceed at its first order rate at all phytoplankton levels.

9.2.4. Sorption

There is an adsorption-desorption interaction between dissolved inorganic phosphorus and

suspended particulate matter in the water column. The subsequent settling of the suspended

solids together with the sorbed inorganic phosphorus can act as a significant loss mechanism in

the water column and is a source of phosphorus to the sediment. Because the rates of reaction 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-19

for adsorption-desorption are in the order of minutes versus reaction rates in the order of days for 

the biological kinetics, an equilibrium assumption can be made. This equilibrium reaction

implies that the dissolved and particulate phosphorus phases "instantaneously" react to any

discharge sources of phosphorus or runoff or shoreline erosion of solids so as to redistribute the 

phosphorus to its "equilibrium" dissolved and solids phase concentrations.

Consider CDIP to be the concentration of dissolved inorganic phosphorus in the water column. It 

interacts with the particulate concentration, CPIP. The interaction may be an

adsorption-desorption process with the solids or an assimilation-depuration process with the

phytoplankton. If the total suspended solids are considered, the particulate concentration can be 

defined as:

Equation 9-16

C PIP=C

,

PIP M

where:

CPIP

' = concentration of phosphorus sorbed to solids, mg P/kg M

M = concentration of solids, kg/L

The total inorganic phosphorus is then the sum of dissolved inorganic and the particulate

inorganic phosphorus

Equation 9-17

C 3= C DIP+C PIP

The underlying assumption that is made, as mentioned previously, is "instantaneous equilibrium"

between the adsorption-desorption processes. The equilibrium between the dissolved inorganic

phosphorus in the water column and the mass concentration of inorganic phosphorus of the

solids is usually expressed in terms of a partition coefficient:

Equation 9-18

C

C

K =

DIP

,

PIP

PIP

where:

KPIP = partition coefficient for particulate phosphorus, (mg P/kg M) 

per (mg P/L), or (L/kg M)

Substituting Equation 9-18 into Equation 8-16 gives:
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-20

Equation 9-19

C PIP= K PIP M C DIP

Equation 5.19 is the linear portion of the Langmuir isotherm. Although not always

representative of actual conditions, it is a reasonable approximation when the sorbed phosphorus 

concentration is much less than the ultimate adsorbing capacity of the solids. Combining

Equations 5.17 and 5.19, the total concentration may be expressed as

Equation 9-20

C 3= C DIP+ K PIP M C DIP

The dissolved and particulate fractions may be expressed, respectively, as

Equation 9-21

1+ K M

1

=

C

C

f =

3 PIP

DIP

D3

Equation 9-22

1+ K M

K M

=

C

C

f =

PIP

PIP

3

PIP

p3

A wide range of partition coefficients is found in the literature. Thomann and Fitzpatrick (1982) 

report values between 1,000 and 16,000. Using a range in partition coefficients from 1,000 -

16,000 and a range of inorganic solids of from 10 to 30 mg/L in the water column leads to a 

range in the fraction particulate inorganic phosphorus of from 0.01 to 0.33. In EUTRO, the

dissolved and particulate phosphorus phases are assigned as spatially-variable, time-constant 

fractions of the total inorganic phosphorus:

Equation 9-23

C DIP,i= f

D3iC3i

Equation 9-24

CPIP,i=(1- f

D3i)C 3i

 where:
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-21

C3i= the total inorganic phosphorus in segment i, mg/L

fD3i= the fraction of the total inorganic phosphorus assigned to the

dissolved phase in segment i

CDIP,i= the equilibrium dissolved inorganic phosphorus in segment i,

available for algal uptake, mg/L

CPIP,i= the equilibrium sorbed inorganic phosphorus in segment i, which 

may then settle to the sediment layer from the water column, mg/L.

9.2.5. Settling

Particulate organic and inorganic phosphorus settle according to user-specified velocities and 

particulate fractions. Particulate organic phosphorus is equated to solid type 1, which represents 

organic matter. Time and segment-variable organic matter settling velocities, vs3, can be input

by the user using transport field 3. Segment-variable organic phosphorus dissolved fractions,

fD8j, are input with initial conditions.

Particulate inorganic phosphorus is equated to solid type 3, which represents inorganic sediment. 

Time and segment variable inorganic phosphorus settling velocities, vs5, can be input by the user 

using transport field 5. Segment-variable inorganic phosphorus dissolved fractions, fD3, are input 

with initial conditions. 

9.3. The Nitrogen Cycle

Four nitrogen variables are modeled: phytoplankton nitrogen, organic nitrogen, ammonia, and

nitrate. A summary is illustrated in Figure 9-5. Table 9-5 summarizes the terms used in the 

nitrogen system kinetics.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

9-22

Figure 9-5 Nitrogen cylce equations

Table 9-5 Nitrogen reaction terms

 Value from

Potomac

Estuary Model

Description Notation Units

Nitrogen to carbon ratio aNC 0.25 mg N/gm C

Organic nitrogen mineralization rate 

 @ 20°C

k71 0.075 day-1

Temperature coefficient È71 1.08 -

Nitrification rate k12 0.09-0.13 day-1

Temperature coefficient È12 1.08 -

Half saturation constant for oxygen KNIT 2.0 mg 02/L
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limitation of nitrification

Denitrification rate at

20°C

k2D 0.09 day-1

Temperature coefficient È2D 1.045 -

Michaelis constant for denitrification KNO3 0.1 mg O2/L

Fraction of dead and respired phytoplankton 

recycled ...

to the organic nitrogen pool fON 0.5 -

to the ammonia nitrogen pool (1-fON) 0.5 -

Preference for ammonia uptake term PNH3 eq. 5.30 -

Fraction dissolved organic nitrogen fD7 1.0 -

Organic matter settling velocity vs3 - m/day

9.3.1. Phytoplankton Growth

As phytoplankton grow, dissolved inorganic nitrogen is taken up and incorporated into biomass. 

For every mg of phytoplankton carbon produced, aNC mg of inorganic nitrogen is taken up. Both 

ammonia and nitrate are available for uptake and use in cell growth by phytoplankton; however, 

for physiological reasons, the preferred form is ammonia nitrogen. The ammonia preference

term PNH3 is given in Figure 9-6. 
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Figure 9-6 Ammonia preference

The behavior of this equation, for a Michaelis value, KmN, of 25 μg N/L, is shown in Figure 5.6. 

The behavior of this equation is most sensitive at low values of ammonia or nitrate. For a given

concentration of ammonia, as the available nitrate increases above approximately the Michaelis

limitation, the preference for ammonia reaches an asymptote. Also as the concentration of

available ammonia increases, the plateau levels off at values closer to unity, i.e., total preference 

for ammonia.

9.3.2. Phytoplankton Death

As phytoplankton respire and die, living organic material is recycled to nonliving organic and

inorganic matter. For every mg of phytoplankton carbon consumed or lost, aNC mg of nitrogen is 

released. During phytoplankton respiration and death, a fraction of the cellular nitrogen fon is 

organic, while (1 - fon) is in the inorganic form of ammonia nitrogen. The fraction recycled to 

the inorganic pool for Great Lakes models has been assigned at 50% (Di Toro and Matystik,

1980).
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9.3.3. Mineralization

Nonliving organic nitrogen must undergo mineralization or bacterial decomposition into

ammonia nitrogen before utilization by phytoplankton. In EUTRO, the first order, temperaturecorrected rate constant is modified by a saturated recycle term, as explained in the phosphorus

mineralization section. This mechanism slows the mineralization rate if the phytoplankton 

population is small, but does not permit the rate to increase continuously as phytoplankton

increase. The default value for the half-saturation constant KmPc is 0, which causes

mineralization to proceed at its first order rate at all phytoplankton levels.

9.3.4. Settling 

Particulate organic nitrogen settles according to user-specified velocities and particulate

fractions. Particulate organic nitrogen is equated to solid type 1, which represents organic

matter. Time and segment-variable organic matter settling velocities, vs3, can be input by the

user using transport field 3. Segment-variable organic nitrogen dissolved fractions, fD7, are input 

with initial conditions.

9.3.5. Nitrification

Ammonia nitrogen, in the presence of nitrifying bacteria and oxygen, is converted to nitrate

nitrogen (nitrification). The process of nitrification in natural waters is carried out by aerobic

autotrophs; Nitrosomonas and Nitrobacter predominate in fresh waters. It is a two-step process 

with Nitrosomonas bacteria responsible for the conversion of ammonia to nitrite and Nitrobacter 

responsible for the conversion of nitrite to nitrate. 

Essential to this reaction process are aerobic conditions. Also this process appears to be affected 

by high or low values of pH that inhibit Nitrosomonas growth, particularly for pH below 7 and 

greater than 9. As with phytoplankton, the nitrifying bacterial populations are sensitive to flow. 

During periods of high flow or storm runoff, upstream bacteria may be advected downstream,

with some lag time after a flow transient before they can build up to significant levels again.

The process of nitrification in natural waters, then, is complex, depending on dissolved oxygen,

pH, and flow conditions, which in turn leads to spatially and temporally varying rates of

nitrification. To properly account for this complex phenomenon in the modeling framework

would be difficult and would require a database that is usually unavailable. 

The kinetic expression for nitrification in EUTRO contains three terms -- a first order rate

constant, a temperature correction term, and a low DO correction term. The first two terms are 

standard. The third term represents the decline of the nitrification rate as DO levels approach 0. 

The user may specify the half-saturation constant KNIT, which represents the DO level at which
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the nitrification rate is reduced by half. The default value is zero, which allows this reaction to 

proceed fully even under anaerobic conditions.

9.3.6. Denitrification

Denitrification refers to the reduction of NO3 (or NO2) to N2 and other gaseous products such as 

N2O and NO. This process is carried out by a large number of heterotrophic, facultative

anaerobes. Under normal aerobic conditions found in the water column, these organisms use

oxygen to oxidize organic material. Under the anaerobic conditions found in the sediment bed or 

during extremely low oxygen conditions in the water column, however, these organisms are able 

to use NO3 as the electron acceptor.

The process of denitrification is included in the modeling framework simply as a sink of nitrate. 

The kinetic expression for denitrification in EUTRO contains three terms -- a first order rate

constant, a temperature correction term, and a DO correction term. The first two terms are

standard. The third term represents the decline of the denitrification rate as DO levels rise above 

0. The user may specify the half-saturation constant KNO3, which represents the DO level at

which the denitrification rate is reduced by half. The default value is zero, which prevents this 

reaction at all DO levels. Denitrification is assumed to always occur in the sediment layer where 

anaerobic conditions always exist. 

9.4. The Dissolved Oxygen Balance

Five state variables participate in the DO balance: phytoplankton carbon, ammonia, nitrate,

carbonaceous biochemical oxygen demand, and dissolved oxygen. A summary is illustrated in

Figure 9-1. The reduction of dissolved oxygen is a consequence of the aerobic respiratory

processes in the water column and the anaerobic processes in the underlying sediments. Both

these processes contribute significantly and, therefore, it is necessary to formulate their kinetics

explicitly. 

The dissolved oxygen processes in EUTRO are discussed in Chapter 8. The CBOD and DO 

reaction terms are summarized in Table 8-1.

9.4.1. Benthic - Water Column Interactions

The decomposition of organic material in benthic sediment can have profound effects on the

concentrations of oxygen and nutrients in the overlying waters. The decomposition of organic

material releases nutrients to the sediment interstitial waters and also results in the exertion of an

oxygen demand at the sediment-water interface. As a result, the areal fluxes from the sediment 

can be substantial nutrient sources or oxygen sinks to the overlying water column. Additionally, 

the occurrence of anoxia, due in part to the sediment oxygen demand, may dramatically increase 
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certain nutrient fluxes through a set of complex redox reactions that change the state and

concentrations of various nutrients and metals thereby releasing bound nutrients. The relative 

importance of the sediment oxygen demand and nutrient fluxes vis-a-vis future nutrient control

strategies requires the incorporation of a dynamic sediment layer and its associated interactions

with the overlying water column in a framework that is consistent with that discussed in the

previous sections.

EUTRO provides two options for nutrient and oxygen fluxes: descriptive input and predictive

calculations (Figure 9-7). The first option is used for networks composed of water column 

segments only. Spatially variable observed fluxes must be specified for ammonia, phosphate,

and sediment oxygen demand. Time functions may be specified for ammonia and phosphate,

reflecting seasonal changes. Seasonal changes in water temperature can affect SOD through its 

temperature coefficient.

Figure 9-7 Sediment - Water Exchange
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9.4.2. Benthic Simulation

The calculational framework incorporated for benthic-water column exchange draws principally

from a study of Lake Erie, which incorporated sediment-water column interactions, performed

by Di Toro and Connolly (1980). For a surficial benthic layer with thickness Dj, the nitrogen and 

phosphorus mass balance equations are summarized in Figure 9-8 and Table 9-6. The benthic 

CBOD and DO equations were summarized in Figure 8-4 and Table 8-2 in the previous chapter. 

Figure 9-8 Benthic nutrient equations

Table 9-6 Benthic nutrient reaction coefficients

 Value from Potomac 

Estuary Study

Description Notation Units
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 Value from Potomac 

Estuary Study

Anaerobic algal decomposition rate kPZD 0.02 day-1

Temperature coefficient ÈPZD 1.08 none

Organic nitrogen decomposition rate kOND 0.0004 day-1

Temperature coefficient ÈOND 1.08 none

Organic phosphorus decomposition rate kOPD 0.0004 day-1

Temperature coefficient ÈOPD 1.08 none

Fraction inorganic phosphorus dissolved in 

benthic layer

fD3j 0.045-0.001 none

Diffusive exchange coefficient EDIF 2-2.5 x 10-4 m2/day

Benthic layer depth Dj 0.1-0.3 m

Benthic layer j

Water column i

WASP6 allows a more detailed parameterization of settling into the benthos that includes not

only a downward settling velocity but an upward resuspension velocity as well. In this context, 

then, the net particulate flux to the sediment is due to the difference between the downward

settling flux and the upward resuspension flux.

Benthic Depth -- One of the first decisions to be made regarding the benthic layer is to determine 

its depth. Two factors influence this decision. The first is to adequately reflect the thickness of

the active layer, the depth to which the sediment is influenced by exchange with the overlying

water column. Secondly one wishes the model to reflect a reasonable time history or "memory"

in the sediment layer. Too thin a layer and the benthos will "remember" or be influenced by

deposition of material that would have occurred only within the last year or two of the period

being analyzed; too thick a layer and the model will "average" too long a history, not reflecting, 

as in the case of phosphorus, substantial reductions in sedimentary phosphorus resulting from

reduced phosphorus discharges from sewage treatment plants. The choice of sediment thickness 

is further complicated by spatially variable sedimentation rates. The benthic layer depths,

together with the assigned sedimentation velocities, provide for a multi-year detention time or

"memory", providing a reasonable approximation of the active layer in light of the observed pore 

water gradients.

Benthic Nitrogen -- The next consideration is the application of these mass balance equations to 

the nitrogen species in a reducing sediment (Berner, 1974). Particulate organic nitrogen is
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hydrolyzed to ammonia by bacterial action within the benthos. In addition to the ammonia

produced by the hydrolysis of particulate organic nitrogen in the benthos, ammonia is generated

by the anaerobic decomposition of algae. In a study of this reaction, Foree and McCarty (1970) 

showed that the anaerobic rate of decay of algae is substantial (0.007-0.022 day-1). However, the 

end product initially is not exclusively ammonia. Rather, a fraction of the algal nitrogen

becomes particulate organic nitrogen, which must undergo hydrolysis before becoming

ammonia.

Ammonia produced by the hydrolysis of non-algal organic nitrogen and the decomposition of

detrital algal nitrogen may then be exchanged with the overlying water column via diffusion. No 

nitrification occurs in the sediment due to the anaerobic conditions present in the sediment. 

Denitrification, the conversion of nitrate to nitrogen gas, may occur, however. Nitrate is present 

in the benthos due to diffusive exchange with the overlying water column.

The analysis of the benthic nitrogen concentrations and the resulting flux of ammonia is

relatively straightforward because of the simplicity of the kinetics: hydrolysis and anaerobic

algal decay produce a stable end product, ammonia, which does not undergo further reactions in

the anaerobic sediment. The equations resulting from the above framework are presented in

Figure 9-8, and the coefficients are summarized in Table 6-1.

Benthic Phosphorus -- A complete analysis of the phosphorus fluxes from sediments would 

require a rather complex and elaborate computation of solute-precipitate chemistry and its

interaction with the mass transport of the dissolved species. The reasons for this are twofold: 

first, it is well known (Nriagu, 1972) that for phosphorus the formation of precipitates affects the 

interstitial water concentrations, thereby affecting the interstitial water transport of the various

phosphorus forms or species; second, the dissolved concentrations are affected by the redox

reactions, which in turn, affect the phosphorus fluxes that occur during aerobic and anaerobic

conditions. (Phosphorus fluxes are enhanced under anaerobic conditions.)

A computation of solute-precipitate chemistry was judged to be outside the scope of this model. 

Instead, a simplified approach was taken, which to a large degree relies on empiricism. 

Anaerobic decomposition of detrital algal phosphorus is assumed to occur using the same rate

expressions and rate constants as those for detrital algal nitrogen, yielding both organic and

inorganic phosphorus. Anaerobic decomposition of organic phosphorus then proceeds. A

spatially-variable fraction of the end product, dissolved inorganic phosphorus, remains in the

interstitial water and is not involved in the formation of precipitates and is not sorbed onto the 

benthic solids. This spatial variation reflects the ionic chemical makeup of the benthos in

various regions of the water body.

Using observed total and interstitial dissolved inorganic phosphorus values, the fraction

dissolved inorganic phosphorus can be assigned to each segment, with the particulate and

dissolved inorganic phosphorus computed for each time step in a manner similar to the overlying

water column inorganic phosphorus (Equation 9-26through Equation 9-30). Exchange of the
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dissolved phosphorus forms with the overlying water column is also similar to that of ammonia, 

nitrate, and dissolved oxygen. Mass flux equations are presented in Figure 9-8. The effects of 

anoxia upon sediment phosphorus flux were not included in the modeling framework. The

approach used to generate sediment phosphorus flux, although not entirely satisfactory, is at least 

consistent with the framework within which the fluxes of other materials are being generated.

Benthic Carbon -- The reactions that convert algal and refractory carbon to their end products are 

complex. The initial step in which the algal and refractory carbon are converted to reactive

intermediates appears to be similar to the refractory organic and algal nitrogen degradation, and

in the subsequent calculations, the rates for carbon and nitrogen decomposition are assumed to

be equal. The reactive intermediates, however, participate in further reactions: for example,

volatile acids react to become methane, and the mechanisms that control these reactions are

somewhat uncertain. In addition, few measurements of these intermediate species are available

and a calculation that incorporates their concentrations explicitly would of necessity be

speculative. Thus, one uses a simplified, yet realistic, formulation of these reactions.

The method proposed by Di Toro and Connolly (1980), and highlighted here, is based upon

separating the initial reactions that convert sedimentary organic material into reactive

intermediates and the remaining redox reactions that occur. Then using a transformation variable 

and an orthogonality relationship, Di Toro and Connolly derive mass balance equations that are

independent of the details of the redox equations. Rather they are only functions of the

component concentration, and it suffices to compute only the component concentrations, which

can be treated in exactly the same way as any other variable in the mass transport calculation.

The convenient choices of components for the calculation are those that parallel the aqueous

variables -- carbonaceous BOD and dissolved oxygen. Restricting the calculation to these

components, however, eliminates the possibility of explicitly including the effects of other

reduced species such as iron, manganese, and sulfide, which play a role in overall redox

reactions and may be involved in the generation of sediment oxygen demand. This

simplification appears reasonable in light of the preliminary nature of the benthic calculation.

The decomposition reactions that drive the component mass balance equations are the anaerobic 

decomposition of the algal carbon, and the anaerobic breakdown of the benthic organic carbon. 

Both reactions are sinks of the oxygen and rapidly drive its concentration negative, indicating

that the sediment is reduced rather than oxidized. The negative concentrations computed can be 

considered the oxygen equivalents of the reduced end products produced by the chains of redox 

reactions occurring in the sediment.

Because the calculated concentration of oxygen is positive in the overlying water, it is assumed

that the reduced carbon species (negative oxygen equivalents) that are transported across the

benthic water interface combine with the available oxygen and are oxidized to CO2 and H2O 

with a consequent reduction of oxygen in the overlying water column. The sediment mass
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balance equations for carbonaceous BOD and DO, together with the equation for sediment

oxygen demand, are presented in Figure 4.3 and Table 4.2.

9.5. Model Implementation

To simulate eutrophication with WASP6, use the preprocessor to create a EUTRO input dataset. 

For the portions of the dataset describing environment, transport, and boundaries, EUTRO model

input will be similar to that for the conservative tracer model as described in Chapter 6. To those 

basic parameters, the user will add combinations of transformation parameters and perhaps solids 

transport rates.

EUTRO kinetics can be implemented using some or all of the processes and kinetic terms

described above to analyze eutrophication problems. For convenience, three levels of

complexity are identified here: (1) simple eutrophication kinetics, (2) intermediate

eutrophication kinetics, and (3) intermediate eutrophication kinetics with benthos. Please note

that the discrete levels of simulation identified here are among a continuum of levels that the user 

could implement. 

The three-implementation levels are described briefly below, along with the input parameters

required to solve the eutrophication equations in EUTRO. Input parameters are prepared for

WASP6 in four major sections of the preprocessor -- environment, transport, boundaries, and

transformation. Basic model parameters are described in Chapter 5, and will not be repeated 

here. The eight state variables, with abbreviations used in this text, are listed in Table 9-7. 

Table 9-7 Summary of EUTRO variables

 Variable Notation Concentration Units 

1. Ammonia Nitrogen NH3 C1 mg N/L

2. Nitrate Nitrogen NO3 C2 mg N/L

3. Inorganic Phosphorus PO4 C3 mg P/L

4. Phytoplankton Carbon PHYT C4 mg C/L

5. Carbonaceous BOD CBOD C5 mg O2/L

6. Dissolved Oxygen DO C6 mg O2/L

7. Organic Nitrogen ON C7 mg N/L

8. Organic Phosphorus OP C8 mg P/L
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9.6. Simple Eutrophication Kinetics

Simple eutrophication kinetics simulates the growth and death of phytoplankton interacting with

one of the nutrient cycles. Growth can be limited by the availability of inorganic nitrogen or

inorganic phosphorus, and light. Equations include phytoplankton kinetics:

Equation 9-25

C

D

v

S = G - D - 4

 s4

k4 P1 P1 











and either the phosphorus cycle:

Equation 9-26

(1 - f )C

D

v

S =+ D a C - k C -

D8 8

 s3

8

T -20

k8 P1 PC 4 83 Θ 83

Equation 9-27

(1- f )C

D

v

S =+k C - G a C -

D3 3

 s5

8 P1 PC 4

T -20

k3 83 Θ83

or the nitrogen cycle:

Equation 9-28

(1 - f )C

D

v

S =+ D a C - k C -

D7 7

 s3

7

T -20

k7 P1 NC 4 71Θ71

Equation 9-29

S =+k C - G a P C - k C1

T -20

7 P1 PC NH 4 12 12

T -20

k1 71Θ71 3 Θ

Equation 9-30

S =+k C1

- G P1 aNC (1 - P NH )C 4

T-20

k2 12 Θ12 3
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where Ski is the source/sink term for variable "i" in a segment, in mg/L-day. Kinetic rate

constants and coefficients are as defined in Table 9-5, Table 9-4 and Table 9-3.

Phytoplankton plus either three nitrogen variables or two phosphorus variables are used in

simple eutrophication simulations. While phytoplankton is simulated internally as mg/L carbon,

initial concentrations and boundary concentrations are input by the user as μg/L chlorophyll a. 

EUTRO converts these input concentrations to internal concentrations using a user-specified 

carbon to chlorophyll ratio. If the carbon to chlorophyll ratio is not input, then a default value of 

30 is used. Internal concentrations of phytoplankton nitrogen and phytoplankton phosphorus are 

calculated from user-specified nitrogen to carbon and phosphorus to carbon ratios. If these ratios 

are not input, then default values of 0.25 and 0.025 are used.

Simple eutrophication kinetics assumes that death returns phytoplankton nitrogen and

phosphorus entirely to the organic nitrogen and organic phosphorus pools. Mineralization is a

simple first order function that is unaffected by phytoplankton levels, and nitrification is a simple 

first order function unaffected by dissolved oxygen. Denitrification is not simulated.

Light limitation is described by the Di Toro formulation, Equation 9-4, and the user must

calibrate the saturating light intensity Is.

The particulate fractions of ON and OP are associated with transport field 3, organic matter

settling. Particulate PHYT is associated with transport field 4. The particulate fraction of PO4 is 

associated with transport field 5, inorganic settling.

9.6.1. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control the

simulation.

Systems-- Select "simulate" for PHYT and either ON, NH3, and NO3, or OP and PO4. Select 

"constant" for the nonsimulated nutrients and "bypass" for CBOD and DO. During calibration,

the user may select "constant" or "bypass" for any selected variables. 

Segments-- Water column segments should be defined in the standard fashion. If settling is to be 

simulated (i.e., for ON, OP, PHYT, or PO4), the user should add a single benthic segment

underlying all water column segments. This benthic segment will merely act as a convenient

sink for settling organic matter. Model calculations within this benthic segment should be

ignored. 

9.6.2. Transport Parameters

This group of parameters defines the advective and dispersive transport of model variables.
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Number of Flow Fields-- To simulate settling of ON and OP, the user should select solids 1 flow 

under advection. To simulate settling of PHYT, the user should select solids 2 flow. To

simulate PO4 settling, the user should select solids 3 flow. The user should also select water 

column flow. 

Particulate Transport, m

3

/sec-- Time variable settling and resuspension rates for solids 1, solids

2, and solids 3 can be input using the continuity array BQ and the time function QT. For each

solids flow field, cross-sectional exchange areas (m2) for adjacent segment pairs are input using

the spatially-variable BQ. Time-variable settling velocities can be specified as a series of

velocities, in m/sec, versus time. If the units conversion factor is set to 1.157e-5, then these 

velocities are input in units of m/day. These velocities are multiplied internally by crosssectional areas and treated as flows that carry particulate organic matter out of the water column. 

9.6.3. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial conditions. 

Boundary concentrations must be specified for any segment receiving flow inputs, outputs, or

exchanges. Initial conditions include not only initial concentrations, but also the density and

solids transport field for each solid, and the dissolved fraction in each segment.

Boundary Concentrations, mg/L-- At each segment boundary, time variable concentrations must

be specified for PHYT, expressed as μg/L chlorophyll a. Time variable concentrations must also 

be specified for either ON, NH3, and NO3, or OP and PO4. A boundary segment is

characterized by water exchanges from outside the network, including tributary inflows,

downstream outflows, and open water dispersive exchanges. 

Waste Loads, kg/day-- For each point source discharge, time variable PHYT, ON, NH3, NO3, 

OP, and PO4 loads can be specified. These loads can represent municipal and industrial

wastewater discharges, or urban and agricultural runoff. If any phytoplankton loads are

specified, they should be in units of kg carbon/day. 

Solids Transport Field-- The transport fields associated with particulate settling must be specified 

under initial conditions. Solids 1 (Field 3) is recommended for ON and OP. Solids 2 (Field 4) is 

recommended for PHYT. Solids 3 (Field 5) is recommended for PO4. 

Solid Density, g/cm3-- A value of 0 can be entered for the nominal density of PHYT, ON, NH3, 

NO3, OP, and PO4. This information is not used in EUTRO. 

Initial Concentrations, mg/L-- Concentrations of PHYT, expressed as μg/L chlorophyll a and

either ON, NH3, and NO3, or OP and PO4 in each segment must be specified for the time at 

which the simulation begins. For the nonsimulated nutrients held constant, average

concentrations must be specified. These nutrient concentrations will remain constant throughout

the simulation and can affect PHYT through growth rate limitation (although nonsimulated
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nutrients should be in excess and therefore not affect growth). Concentrations of zero for

bypassed variables -- CBOD and DO -- will be entered by the preprocessor.

Dissolved Fraction-- The dissolved fraction of PHYT, ON, NH3, NO3, OP, and PO4 in each

segment must be specified. The dissolved fraction of PHYT should be set to 0. Only the

particulate fractions of the nutrients will be subject to settling. 

9.6.4. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. Parameter values are entered for

each segment. Specified values for constants apply over the entire network for the whole

simulation. Kinetic time functions are composed of a series of values versus time, in days.

Water Temperature, C-- Time and segment variable water temperatures can be specified using

the parameters TMPSG and TMPFN, and the time functions TEMP(1-4). If temperatures are to 

remain constant in time, then the user should enter segment temperatures using the parameter

TMPSG. TMPFN and TEMP(1-4) should be omitted. 

If the user wants to enter time-variable temperatures, then values for the parameter TMPSG

should be set to 1.0. The parameter TMPFN indicates which temperature function will be used 

by the model for each segment. Values of 1.0, 2.0, 3.0, or 4.0 will call time functions TEMP(1), 

TEMP(2), TEMP(3), and TEMP(4), respectively. Water temperatures should then be entered via 

these time functions as a series of temperature versus time values. The product of TMPSG and 

the selected TEMP function will give the segment and time specific water temperatures used by

EUTRO.

TMPSG and TMPFN are identified in EUTRO as parameters 3 and 4, respectively. TEMP(1-4) 

are identified in EUTRO as time functions 1-4. 

Solar Radiation, langleys/day-- Time-variable solar radiation at the water surface can be

described using time functions ITOT and FDAY. Seasonally-varying values of solar radiation at 

the surface can be entered using ITOT with a series of radiation versus time values. FDAY gives 

the seasonally-varying fraction of day that is daylight, entered as a series of fraction versus time 

values. Internally, EUTRO uses the quotient ITOT/FDAY for the average radiation intensity

during daylight hours. 

Light Extinction, m

-1

-- Time and segment variable light extinction coefficients can be specified

using the parameters KESG and KEFN, and the time functions KE(1-5). If extinction 

coefficients are to remain constant in time, then the user should enter segment coefficients using

the parameter KESG. KEFN and KE(1-4) should be omitted. 
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If the user wants to enter time-variable extinction coefficients, then values for the parameter 

KESG should be set to 1.0. The parameter KEFN indicates which light extinction function will

be used by the model for each segment. Values of 1.0, 2.0, 3.0, 4.0, or 5.0 will call time

functions KE(1), KE(2), KE(3), KE(4), and KE(5), respectively. Light extinction coefficients

should then be entered via these time functions as a series of coefficient versus time values. The 

product of KESG and the selected KE function will give the segment and time specific light

extinction coefficients used by EUTRO.

KESG and KEFN are identified in EUTRO as parameters 5 and 6, respectively. KE(1-4) are 

identified in EUTRO as time functions 8-12. 

Growth Rate, day-1-- The maximum phytoplankton growth rate constant and temperature

coefficient can be input using constants K1C and K1T, respectively. 

Carbon to Chlorophyll Ratio, mg C/mg Chl-- The average carbon to chlorophyll weight ratio in

phytoplankton can be specified using constant CCHL. A default value of 30 is provided for in

EUTRO. 

Light Limitation-- Available light is specified using time functions describing seasonal light at

the water surface and segment- and time-variable light extinction coefficients. These are

described above.

The Di Toro light limitation option can be specified using a value of 1.0 for LGHTS. The

saturating light intensity can then be specified using constant IS1. Default values for LGHTS

and IS1 are 1 and 300, respectively. 

Respiration Rate, day-1-- The average phytoplankton respiration rate constant and temperature

coefficient can be input-using constants K1RC and K1RT, respectively. 

Death Rate, day-1-- The non-predatory phytoplankton death rate constant can be input using

constant K1D. No temperature dependence is assumed. 

Phosphorus to Carbon Ratio, mg P/mg C-- The average phosphorus to carbon weight ratio in

phytoplankton can be specified using constant PCRB. The EUTRO default value for PCRB is

0.025. 

Phosphorus Mineralization Rate, day-1-- The mineralization rate constant and temperature

coefficient for dissolved organic phosphorus can be specified using constants K83C and K83T,

respectively. 

Phosphorus Half-Saturation Constant, mg P/L-- The phosphorus half-saturation constant for

phytoplankton growth can be specified using constant KMPG1. When inorganic phosphorus

concentrations are at this level, the phytoplankton growth rate is reduced by half. 
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Nitrogen to Carbon Ratio, mg N/mg C-- The average nitrogen to carbon weight ratio in

phytoplankton can be specified using constant NCRB. The EUTRO default value for NCRB is 

0.25.

Nitrogen Mineralization Rate, day-1-- The mineralization rate constant and temperature

coefficient for dissolved organic nitrogen can be specified using constants K71C and K71T,

respectively. 

Nitrification Rate, day-1-- The nitrification rate constant and temperature coefficient for dissolved 

ammonia nitrogen can be specified using constants K12C and K12T, respectively. 

Nitrogen Half-Saturation Constant, mg N/L-- The nitrogen half-saturation constant for

phytoplankton growth can be specified using constant KMNG1. When inorganic nitrogen

concentrations are at this level, the phytoplankton growth rate is reduced by half. This parameter 

also affects ammonia preference PNH3 as outlined in Figures 5.5 and 5.6. When KMNG1 = 0, 

PNH3 = 1.0. When KMNG1 becomes very large, PNH3 approaches a value of C1/(C1 + C2). 

9.7. Intermediate Eutrophication Kinetics

Intermediate eutrophication kinetics simulates the growth and death of phytoplankton interacting

with the nitrogen and phosphorus cycles and the dissolved oxygen balance. Growth can be

limited by the availability of inorganic nitrogen, inorganic phosphorus, and light.

Intermediate eutrophication kinetics adds CBOD and DO equations as well as certain nonlinear

terms and functions to the simple eutrophication kinetics described above. The oxygen balance

equations and kinetic parameters are summarized in Figure 8-2 and Table 8-1. The phosphorus 

cycle equations and kinetic parameters are summarized in Figure 9-4 and Table 9-4. The

nitrogen cycle equations and parameters are summarized in Figure 9-5 and Table 9-5. 

Phytoplankton equations are presented throughout Section 9.1.4, with parameters summarized in

Table 9-2. 

Either the Di Toro or the Smith formulation can describe light limitation. The Smith formulation

implements equations Equation 9-5 through Equation 9-7. These equations predict the carbon to 

chlorophyll ratio based on the availability of light, and then predict the saturating light intensity

based on the carbon to chlorophyll ratio. 

Other terms included in the intermediate kinetics equations are the phytoplankton effect on

mineralization of organic phosphorus and nitrogen, the dissolved oxygen limitation on

nitrification, the denitrification reaction, and zooplankton grazing. The nonlinear DO balance

equations can become important in inhibiting nitrification and carbonaceous oxidation and in

promoting denitrification where low DO concentrations occur.
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All eight state variables are simulated in intermediate eutrophication simulations. During

calibration of the model to observed data, however, the user may want to bypass certain variables 

or hold them constant. Nutrients can be held at observed concentrations, for instance, while

phytoplankton growth and death rates are calibrated.

9.7.1. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control the

simulation.

Systems-- Select "simulate" for all variables. During calibration, the user may select "constant"

or "bypass" for any selected variables. 

Segments-- Water column segments should be defined in the standard fashion. If settling is to be 

simulated (i.e., for ON, OP, PHYT, PO4, or CBOD), the user should add a single benthic

segment underlying all water column segments. This benthic segment will merely act as a

convenient sink for settling organic matter. Model calculations within this benthic segment

should be ignored.

9.7.2. Transport Parameters

This group of parameters defines the advective and dispersive transport of model variables.

Number of Flow Fields-- To simulate settling of ON, OP, and CBOD, the user should select

solids 1 flow under advection. To simulate settling of PHYT, the user should select solids 2 

flow. To simulate PO4 settling, the user should select solids 3 flow. The user should also select 

water column flow. 

Particulate Transport, m

3

/sec-- Time variable settling and resuspension velocities can be

specified for particulate ON, OP, CBOD, PHYT, and PO4, as described in the simple

eutrophication section above.

9.7.3. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial conditions. 

Boundary concentrations must be specified for any segment receiving flow inputs, outputs, or

exchanges. Initial conditions include not only initial concentrations, but also the density and

solids transport field for each solid, and the dissolved fraction in each segment.

Boundary Concentrations, mg/L-- At each segment boundary, time variable concentrations must

be specified for PHYT, expressed as μg/L chlorophyll a. Time variable concentrations must also 

be specified for either ON, NH3, NO3, OP, PO4, CBOD, and DO. A boundary segment is 
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characterized by water exchanges from outside the network, including tributary inflows,

downstream outflows, and open water dispersive exchanges. 

Waste Loads, kg/day-- For each point source discharge, time variable PHYT, ON, NH3, NO3, 

OP, PO4, CBOD, and DO loads can be specified. These loads can represent municipal and 

industrial wastewater discharges, or urban and agricultural runoff. If any phytoplankton loads

are specified, they should be in units of kg carbon/day. 

Solids Transport Field-- The transport fields associated with particulate settling must be specified 

under initial conditions. Solids 1 (Field 3) is recommended for ON, OP, and CBOD. Solids 2 

(Field 4) is recommended for PHYT. Solids 3 (Field 5) is recommended for PO4. 

Solid Density, g/cm3-- A value of 0 can be entered for the nominal density of PHYT, ON, NH3, 

NO3, OP, PO4, CBOD, and DO. This information is not used in EUTRO. 

Initial Concentrations, mg/L-- Concentrations of all variables in each segment must be specified

for the time at which the simulation begins. Concentrations of PHYT are expressed as μg/L

chlorophyll a.

Dissolved Fraction-- The dissolved fraction of each variable in each segment must be specified. 

The dissolved fraction of PHYT should be set to 0, and the dissolved fraction of DO should be 

set to 1. Only the particulate fractions of CBOD and the nutrients will be subject to settling. 

9.7.4. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. Parameter values are entered for

each segment. Specified values for constants apply over the entire network for the whole

simulation. Kinetic time functions are composed of a series of values versus time, in days.

Water Temperature, C-- Time and segment variable water temperatures can be specified using

the parameters TMPSG and TMPFN, and the time functions TEMP(1-4), as described in the 

simple eutrophication section above.

Solar Radiation, langleys/day-- Time-variable solar radiation at the water surface can be

described using time functions ITOT and FDAY, as described in the simple eutrophication

section above. 

Light Extinction, m

-1

-- Time and segment variable light extinction coefficients can be specified

using the parameters KESG and KEFN, and the time functions KE(1-5), as described in the

simple eutrophication section above.
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Growth Rate, day-1-- The maximum phytoplankton growth rate constant and temperature

coefficient can be input using constants K1C and K1T, respectively. 

Carbon to Chlorophyll Ratio, mg C/mg Chl-- The average carbon to chlorophyll weight ratio in

phytoplankton can be specified using constant CCHL. A default value of 30 is provided for in

EUTRO. If the Smith light limitation option is chosen, then CCHL will be variable, recalculated 

daily throughout the simulation. 

Light Limitation-- Available light is specified using time functions describing seasonal light at

the water surface and segment- and time-variable light extinction coefficients. These are

described above.

The Di Toro light limitation option can be specified using a value of 1.0 for LGHTS. The

saturating light intensity, in langleys/day can then be specified using constant IS1. Default

values for LGHTS and IS1 are 1 and 300, respectively. 

The Smith light limitation option can be specified using a value of 2.0 for LGHTS. Two other 

parameters must then be specified. The maximum quantum yield constant, in mg C/mole

photons, can be specified using constant PHIMX. The chlorophyll extinction coefficient, in (mg

chl a/m3)

-1m-1

, can be specified using constant XKC. Default values for PHIMX and XKC are 

720 and 0.017, respectively.

Nitrogen Half-Saturation Constant, mg N/L-- The nitrogen half-saturation constant for

phytoplankton growth can be specified using constant KMNG1. When inorganic nitrogen

concentrations are at this level, half reduces the phytoplankton growth rate. This parameter also 

affects ammonia preference PNH3 as outlined in Figures 5.5 and 5.6. When KMNG1 = 0, PNH3 = 

1.0. When KMNG1 becomes very large, PNH3 approaches a value of C1/(C1 + C2). 

Phosphorus Half-Saturation Constant, mg P/L-- The phosphorus half-saturation constant for

phytoplankton growth can be specified using constant KMPG1. When inorganic phosphorus 

concentrations are at this level, half reduces the phytoplankton growth rate. 

Nutrient Limitation Option-- The nutrient limitation formulation can be specified using constant

NUTLIM. A value of 0 selects the minimum formulation, which is recommended. A value of

1.0 selects the multiplicative formulation. The default value is 0. 

Respiration Rate, day-1-- The average phytoplankton endogenous respiration rate constant and

temperature coefficient can be input-using constants K1RC and K1RT, respectively. 

Death Rate, day-1-- The non-predatory phytoplankton death rate constant can be input using

constant K1D. No temperature dependance is assumed. 

Grazing Rate, day-1-- Zooplankton grazing can be specified using parameter ZOOSG, time

function ZOO, and constant K1G. Time- and segment-variable herbivorous zooplankton
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populations are described as the product of the time variable population ZOO, in mg zooplankton 

C/L, and segment specific ratios ZOOSG. The grazing rate per unit zooplankton population, in 

L/mg zooplankton C-day, can be input using constant K1G. The resulting grazing rate constant 

for phytoplankton is the product of the variable zooplankton population and the unit grazing rate. 

(Note that ZOO can also be expressed as cells/L if K1G is expressed as L/cell-day). 

Phosphorus to Carbon Ratio, mg P/mg C-- The average phosphorus to carbon weight ratio in

phytoplankton can be specified using constant PCRB. The EUTRO default value for PCRB is

0.025. 

Phytoplankton Phosphorus Recycle-- The fraction of dead and respired phytoplankton

phosphorus that is recycled to the organic phosphorus pool can be specified using constant FOP. 

The default value is 1. The fraction of phytoplankton phosphorus recycled directly to inorganic 

phosphorus is 1 - FOP. 

Phosphorus Mineralization Rate, day-1-- The mineralization rate constant and temperature

coefficient for dissolved organic phosphorus can be specified using constants K83C and K83T,

respectively. Phytoplankton effects on mineralization can be described using constant KMPHY,

the half-saturation constant for mineralization dependence on phytoplankton, in mg C/L. This

causes mineralization rates to increase as phytoplankton levels increase. If KMPHY is zero, 

there is no phytoplankton effect on mineralization. If KMPHY is large, then large concentrations 

of phytoplankton are needed to drive mineralization, and thus relatively low phytoplankton

levels can lead to low mineralization rates. 

Benthic Phosphorus Flux, mg/m2-day-- The segment- and time-variable benthic phosphorus flux

can be specified using parameter FPO4 and time function TFPO4. The product of the spatiallyvariable FPO4 and time-variable TFPO4 gives the segment and time specific benthic flux for 

PO4 used by EUTRO. Flux versus time values can be entered using TFPO4, while unitless

segment ratios can be entered using FPO4. Values should be entered for water column segments 

that are in contact with the bottom of the water body. 

Nitrogen to Carbon Ratio, mg N/mg C-- The average nitrogen to carbon weight ratio in

phytoplankton can be specified using constant NCRB. The EUTRO default value for NCRB is 

0.25. 

Phytoplankton Nitrogen Recycle-- The fraction of dead and respired phytoplankton nitrogen that 

is recycled to the organic nitrogen pool can be specified using constant FON. The default value 

is 1. The fraction of phytoplankton nitrogen recycled directly to ammonia is 1 - FON. 

Nitrogen Mineralization Rate, day-1-- The mineralization rate constant and temperature

coefficient for dissolved organic nitrogen can be specified using constants K71C and K71T,

respectively. Phytoplankton effects on mineralization can be described using constant KMPHY, 
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the half-saturation constant for mineralization dependence on phytoplankton, as explained above

in the phosphorus mineralization section. 

Nitrification Rate, day-1-- The nitrification rate constant and temperature coefficient for dissolved 

ammonia nitrogen can be specified using constants K12C and K12T, respectively. The halfsaturation constant for oxygen limitation of nitrification can be specified using constant KNIT. 

The default value for KNIT is 0.0, indicating no oxygen limitation. 

Denitrification Rate, day-1-- The denitrification rate constant and temperature coefficient for

dissolved nitrate nitrogen can be specified using constants K20C and K20T, respectively. The

half-saturation constant for oxygen limitation of denitrification can be specified using constant

KNO3. The default value for KNO3 is 0.0, indicating no denitrification at oxygen

concentrations above 0.0. 

Benthic Nitrogen Flux, mg/m2-day-- The segment- and time-variable benthic nitrogen flux can

be specified using parameter FNH4 and time function TFNH4. The product of the spatiallyvariable FNH4 and time-variable TFNH4 gives the segment and time specific benthic flux for

NH3 used by EUTRO. Flux versus time values can be entered using TFNH4, while unitless

segment ratios can be entered using FNH4. Values should be entered for water column segments 

that are in contact with the bottom of the water body. 

Sediment Oxygen Demand, g/m2-day-- Segment variable sediment oxygen demand fluxes and

temperature coefficients can be specified using the parameters SOD1D and SODTA,

respectively. Values should be entered for water column segments that are in contact with the

bottom of the water body. 

Reaeration Rate, day-1-- There are three basic options for specifying reaeration -- a single rate 

constant, segment and time variable rate constants, and flow and wind calculated rate constants. 

These options are described in Section 4.2, under the Streeter-Phelps transformation parameters.

CBOD Deoxygenation Rate, day-1-- The CBOD deoxygenation rate constant and temperature

coefficient can be specified using constants KDC and KDT, respectively. The half-saturation 

constant for oxygen limitation of carbonaceous deoxygenation can be specified using constant

KBOD. The default value for KBOD is 0.0, indicating no oxygen limitation. 

9.8. Intermediate Eutrophication Kinetics with Benthos

Simulating benthic interactions requires the addition of benthic segments to the model network. 

All state variables are simulated in the benthic segments. Dissolved fractions of NH3, NO3,

PO4, CBOD, DO, ON, and OP may exchange with the water column by diffusion. Particulate 

fractions of PHYT, PO4, CBOD, ON, and OP may deposit to or be scoured from the benthic 

segments. Benthic layer decomposition rates for OP, ON, PHYT, and CBOD must be specified. 
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The equations used are those presented in Figure 8-4 and Figure 9-8. Rate parameters are

summarized in Table 8-2 and Table 9-6.

Many of the environment, transport, boundary, and transformation parameters required to

implement this option are the same as those in the intermediate eutrophication option presented

above. The benthic nitrogen and phosphorus flux functions should be omitted, and the following 

should be modified or added.

Segments-- Water column segments should be defined in the standard fashion. In addition, the 

user should add a benthic segment underlying each water column segment (or stack of water

column segments). These benthic segments will receive settling organic and inorganic matter

from the water column above, and can return material to the water column via resuspension or by 

pore water diffusion. 

Phytoplankton Decomposition, day-1-- The user may specify the rate constant and temperature

coefficient for phytoplankton decomposition in benthic segments using constants KPZDC and

KPZDT. 

Carbonaceous BOD Decomposition, day-1-- The user may specify the rate constant and

temperature coefficient for CBOD decomposition in benthic segments using constants KDSC

and KDST. 

Organic Nitrogen Decomposition, day-1-- The user may specify the rate constant and temperature 

coefficient for organic nitrogen decomposition in benthic segments using constants KONDC and

KONDT. 

Organic Phosphorus Decomposition, day-1-- The user may specify the rate constant and

temperature coefficient for organic phosphorus decomposition in benthic segments using

constants KOPDC and KOPDT. 
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10. Simple Toxicants

Some organic and inorganic chemicals can cause toxicity to aquatic organisms, or bioconcentrate 

through the food chain. Humans may be affected by ingesting contaminated water or fish. 

Criteria for protecting human health and indigenous aquatic communities have been promulgated

for specific chemicals and for general toxicity. 

The simulation of toxicants has become common only in the past decade. Near-field mixing

zone models simulate the dilution and dispersal of waste plumes, along with associated toxicants. 

Far-field models, such as WASP6, simulate the transport and ultimate fate of chemicals

throughout a water body. At a minimum, these models simulate the water column and a bed

layer, and include both chemical degradation and sorption to solids. The simpler models use

first-order decay constants and equilibrium partition coefficients. More complex models may

employ second-order decay mechanisms and either nonlinear sorption isotherms or first-order 

sorption and desorption rate constants.

Several physical-chemical processes can affect the transport and fate of toxic chemicals in the

aquatic environment. Some chemicals undergo a complex set of reactions, while others behave

in a more simplified manner. WASP6 allows the simulation of a variety of processes that may

affect toxic chemicals. 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.

Although the potential amount and variety of data used by WASP6 is large, data requirements 

for any particular simulation can be quite small. For example, it is possible to simulate a

chemical using no reactions, or using only sorption and one or two transformation reactions that 

significantly affect a particular chemical. Indeed, for empirical studies, all chemical constants,

time functions, and environmental parameters can be ignored and a simple user-specified 

transformation rate constant used. Thus, WASP6 can be used as a first-order water pollutant 

model to conduct simulations of dye tracers, salinity intrusion, or coliform die-off.

Table 10-1 Overview of Simple WASP6 Toxicants

SYSTEM VARIABLE

1 CHEMICAL 1

2 SOLIDS 1

3 SOLIDS 2

4 SOLIDS 3

5 CHEMICAL 2

6 CHEMICAL 3
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Simple toxicants and associated solids are simulated using the TOXI program. TOXI simulates

the transport and transformation of one to three chemicals and one to three types of particulate 

material (solids classes, Table 10-1). The three chemicals may be independent or they may be 

linked with reaction yields, such as a parent compound-daughter product sequence. The

simulation of solids is described in Chapter 3. The simulation of simple toxicants is described 

below. The simulation of more complex organic chemicals is described in Chapter 11.

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 simplified transformation processes are described by first-order rate

equations.

WASP6 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 as detailed in Chapter 

7.

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 TOXI. 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 can

affect key environmental characteristics, such as pH or bacterial populations, thus altering

transformation rates.

Table 10-2 Concentration related symbols used in mathematical equations

Symbol Definition Units

Cij Concentration of total chemical i in segment j. mgc/L

Cwij Concentration of dissolved chemical i in segment j. mgc/L

C'wij Concentration of dissolved chemical i in water in segment j. C'wij = Cwij/nj mgc/Lw

Csij Concentration of sorbed chemical i on sediment type "s" in segment j. mgc/L

C'

sij Concentration of sorbed chemical i on sediment type "s" in segment j. mgc/kgs
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Symbol Definition Units

C'

sij = Csij/Msij

msj Concentration of sediment type "s" in segment j. mgs/L

Msj Concentration of sediment type "s" in segment j. Mj = mj ⋅ 10-6 kgs/L

M'

sj Concentration of sediment type "s" in water in segment j. kgs/Lw

nj Porosity or volume water per volume segment j. Lw/L

Kpsij Partition coefficient of chemical i on sediment type "s" in segment j. Lw/kgs

fDij Fraction of chemical i in segment j in dissolved phase -

fsij Fraction of chemical i in segment j in solid phase "s" -

In TOXI, it is convenient to define concentration related symbols as in Table 10-2. Please note 

that in the general development of the equations below, subscripts "i" and "j" are sometimes

omitted for convenience.

10.1. Simple Transformation Kinetics

TOXI allows the user to specify simple first-order reaction rates for the transformation reactions 

of each of the chemicals simulated. First order rates may be applied to the total chemical and 

varied by segment. Alternatively, constant first order rates may be specified for particular

processes, including biodegradation, hydrolysis, photolysis, volatilization, and oxidation. These

constant rates may be used exclusively or in combination with model computed rates as

described in Chapter 11. For example, the user may specify a first-order rate for biodegradation 

and have TOXI compute a loss rate for volatilization. 

10.1.1. Option 1: Total Lumped First Order Decay

The simplest rate expression allowed by TOXI is lumped, first-order decay. This option allows 

the user to specify spatially-variable first order decay rate constants (day-1) for each of the

chemicals simulated. Because these are lumped decay reactions, chemical transformations to

daughter products are not simulated.

Equation 10-1

| = K C

t

C

reaction ij ij

ij

∂

∂

where:

Kij = lumped first order decay constants (day-1) for chemical i in 

segment j. 

The lumped decay rate constant is a model parameter that may be varied between model

segments. If a lumped decay rate constant is specified, the chemical will react at that rate

regardless of other model input. 
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10.1.2. Option 2: Individual First Order Transformation

This option allows the user to input a global first-order reaction rate constant separately for each 

of the following processes: volatilization, water column biodegradation, benthic biodegradation,

alkaline hydrolysis, neutral hydrolysis, acid hydrolysis, oxidation, photolysis, and an extra

reaction. The total reaction is then based on the sum of each of the individual reactions as given 

by

Equation 10-2

| = K C

t

C

ki ij

N

k=1

reaction

ij

∂ ∑

∂

where:

Kki = first order transformation constants for reaction k of

chemical i, day-1

The user may input half-lives rather than first-order decay rate constants. If half-lives are

provided for the transformation reactions, they will be converted internally to first order rate

constants and used as above:

Equation 10-3

K ki=0.693 / T Hki

where:

THki = half-life of reaction k for chemical i, days. 

10.2. Equilibrium Sorption

Sorption is the bonding of dissolved chemicals onto solid phases, such as benthic and suspended 

sediment, biological material, and sometime dissolved or colloidal organic material. Sorption

can be important in controlling both the environmental fate and the toxicity of chemicals. 

Sorption may cause the chemical to accumulate in bed sediment or bioconcentrate in fish. 

Sorption may retard such reactions as volatilization and base hydrolysis, or enhance other

reactions including photolysis and acid-catalyzed hydrolysis.

Sorption reactions are usually fast relative to other environmental processes, and equilibrium

may be assumed. For environmentally relevant concentrations (less than 10-5 M or one-half 

water solubility), equilibrium sorption is linear with dissolved chemical concentration

(Karickhoff, 1984) or:
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Equation 10-4

Cs′= K ps •Cw′

At equilibrium, then, the distribution among the phases is controlled by the partition coefficients

Kps. As developed in Chapter 7, the total mass of chemical in each phase is controlled by Kps

and the amount of solid phase present (ignoring here any DOC phase), so that

Equation 10-5

n+ K M

n

f =

ps s

s

D ∑ •

and

Equation 10-6

n+ K M

K M f =

ps s

s

ps s

 s

•

•

∑

These fractions are determined in time and space throughout a simulation from the partition 

coefficients, internally calculated porosities, and simulated sediment concentrations. Given the

total concentration and the phase fractions of chemical i in segment j, the dissolved and sorbed 

concentrations are uniquely determined:

Equation 10-7

C =C f wij ij Dij •

Equation 10-8

C =C f sij ij sij •

In addition to the assumption of instantaneous equilibrium, implicit in the use of these equations

is the assumption of reversibility. Laboratory data for very hydrophobic chemicals suggest,

however, that a hysteresis exists, with desorption being a much slower process than adsorption. 

Karickhoff suggests that this effect may be the result of intraparticle kinetics in which the

chemical is slowly incorporated into components of the sorbant. This phenomenon is not well

understood and no quantitative modeling framework is available to characterize it.
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Values for the partition coefficients can be obtained from laboratory experiments or field data. 

TOXI allows the input of either a single constant partition coefficient, or a set of spatiallyvariable partition coefficients. These options are described under "Model Implementation"

below. The calculation of partition coefficients for organic chemicals is described in Chapter 7.

10.3. Transformations and Daughter Products

The three chemicals that may be simulated by TOXI may be independent, or they may be linked 

with reaction yields, such as a parent compound-daughter product sequence. Linked

transformations may be implemented by simulating two or three chemicals and by specifying

appropriate yield coefficients for each process:

Equation 10-9

S = K kc C cY kc1 , c= 2,3

c k

kc1 ∑ ∑

Equation 10-10

S = K kc C c Y kc2 , c=1,3

c k

kc2 ∑∑

Equation 10-11

S = K kc C c Y kc3 , c=1,2

c k

kc3 ∑∑

where:

Skci= production of chemical "i" from chemical "c" undergoing reaction 

"k," mgi/L-day

Kkc = effective rate coefficient for chemical "c," reaction "k," day-1

Ykci = yield coefficients for production of chemical "i" from

chemical "c" undergoing reaction "k," mgi/mgc

8 illustrates some of the linked reactions that can be simulated by specifying appropriate yield

coefficients.

Potential reaction products in WASP
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10.4. Model Implementation

To simulate simple toxicants with WASP6, use the preprocessor to create a TOXI input file. The 

model input dataset and the input parameters will be similar to those for the conservative tracer 

model as described in Chapter 2. To those basic parameters, the user will add benthic segments; 

solids transport rates, and transformation parameters. During the simulation, solids and toxicants 

will be transported both by the water column advection and dispersion rates and by these solids 

transport rates.

In WASP6, solids transport rates in the water column and the bed are input via up to three solids 

transport fields, as described in Chapter 3. The transport of the particulate fraction of toxicants 

follows the solids flows. The user must specify the dissolved fraction (i.e. 0.0) and the solids

transport field for each simulated solid under initial conditions. To simulate total solids, solids 1 

must be used.

10.4.1. Model Input Parameters

This section summarizes the input parameters that must be specified in order to solve the simple 

toxicant equations in TOXI. The user is referred to Chapter 3 for a summary of input parameters 

for the sediment balance equations. Input parameters are prepared for WASP6 in four major

sections of the preprocessor -- environment, transport, boundaries, and transformation. Basic

model parameters are described in Chapter 2, and will not be repeated here. 

10.4.2. Environment Parameters

These parameters define the basic model identity, including the segmentation, and control the

simulation.

Systems-- To simulate a toxicant, select "simulate" for chemical 1 and "bypass" for chemical 2

and chemical 3. To simulate total solids along with the toxicant, select "simulate" for solids 1

and "bypass" for solids 2 and solids 3. To simulate two or more toxicants or solids, select

"simulate" for the appropriate variable. 

Bed Volume Option-- The user must determine whether bed volumes are to be held constant or 

allowed to vary. Volumes may be held constant by specifying 0, in which case sediment

concentrations and porosities in the bed segments will vary. Alternatively, sediment

concentrations and porosities may be held constant by specifying 1, in which case surficial bed 

segment volumes will vary. 

Bed Time Step-- While mass transport calculations are repeated every model time step, certain

benthic calculations are repeated only at this benthic time step, in days. If the constant bed 

volume option is chosen, sediment concentrations are updated every model time step, but
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10-8

porosities are recalculated every benthic time step. If the variable bed volume is chosen, upper 

benthic segment volumes are updated every time step, with compaction occurring every benthic 

time step. 

10.4.3. Transport Parameters

Number of Flow Fields-- Under advection, the user has a choice of up to six flow fields. To 

simulate surface water toxicant and solids transport, select water column flow. When simulating

total solids, the user should also select solids 1 flow. To simulate three sediment types, the user 

should select solids 1 flow, solids 2 flow, and solids 3 flow. 

Water Column Flows, m3/sec-- Time variable water column flows can be specified, as detailed in 

Chapter 6.2. 

Sediment Transport Velocities, m/sec-- Time variable settling, deposition, scour, and

sedimentation velocities can be specified for each type of solid. If the units conversion factor is 

set to 1.157e-5, then these velocities are input in units of m/day. These velocities are multiplied 

internally by cross-sectional areas and treated as flows that carry solids and sorbed chemical

between segments. Settling velocities are important components of suspended sediment

transport in the water column. Scour and deposition velocities determine the transfer of solids

and sorbed chemical between the water column and the sediment bed. Sedimentation velocities 

represent the rate at which the bed is rising in response to net deposition. 

Cross-Sectional Areas, m

2

-- The interfacial surface area must be specified for adjoining

segments where sediment transport occurs. These surface areas are multiplied internally by

sediment transport velocities to obtain sediment transport flows.

Number of Exchange Fields-- Under dispersion, the user has a choice of up to two exchange

fields. To simulate surface water toxicant and solids dispersion, select water column dispersion. 

To simulate exchange of dissolved toxicants with the bed, the user should also select pore water 

diffusion. 

Water Column Dispersion, m

3

/sec-- Time variable water column dispersion can be specified, as 

detailed in Chapter 2. 

Pore Water Diffusion Coefficients, m

2

/sec-- Time variable pore water diffusion coefficients can 

be specified for dissolved toxicant exchange within the bed or between the bed and the water

column. If the units’ conversion factor is set to 1.157e-5, then these coefficients are input in units 

of m

2

/day. Diffusion coefficients are multiplied internally by cross-sectional areas divided by

characteristic mixing lengths, and are treated as flows that carry dissolved toxicants between

benthic segments and the water column. 
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Cross-Sectional Areas, m

2

-- The interfacial surface area must be specified for adjoining 

segments where pore water diffusion occurs. These surface areas are multiplied internally by

diffusion coefficients and divided by characteristic mixing lengths to obtain pore water exchange

flows. 

Characteristic Mixing Lengths, m-- The characteristic mixing length must be specified for

adjoining segments where pore water diffusion occurs. The value for a mixing length is typically

equal to the average depth of the pore water segments involved in the exchange. These mixing

lengths are divided into the product of the diffusion coefficients and cross-sectional areas to

obtain pore water exchange flows. 

10.4.4. Boundary Parameters

This group of parameters includes boundary concentrations, waste loads, and initial conditions. 

Boundary concentrations must be specified for any segment receiving flow inputs, outputs, or

exchanges. Initial conditions include not only initial concentrations, but also the density and

solids transport field for each solid, and the dissolved fraction in each segment.

Boundary Concentrations, mg/L-- At each segment boundary, time variable concentrations must

be specified for each toxicant and for each solids type simulated. A boundary segment is

characterized by water exchanges from outside the network, including tributary inflows, 

downstream outflows, and open water dispersive exchanges. 

Waste Loads, kg/day-- For each point source discharge, time variable toxicant and solids loads 

can be specified. These loads can represent municipal and industrial wastewater discharges, or

urban and agricultural runoff. 

Solids Transport Field-- The transport field associated with total solids or each solids type must 

be specified under initial conditions. 

Solid Density, g/cm3-- The average density of the total sediment, or the density of each solids 

type must be specified. This information is used to compute the porosity of benthic segments. 

Porosity is a function of sediment concentration and the density of each solids type.

Initial Concentrations, mg/L-- Concentrations of toxicant and each solids type in each segment

must be specified for the time at which the simulation begins. If the variable benthic volume

option is used, the benthic sediment concentrations specified here will remain constant for the

entire simulation. 

Dissolved Fraction-- The dissolved fraction of each solid in each segment should be set to 0. 

The dissolved fraction of toxicant will be controlled by the partition coefficient and solids

concentrations. 
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10.4.5. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. None are necessary for sediment

transport.

First-Order Degradation-- There are two options to input first-order toxicant degradation:

Option 1: Total Lumped First Order Decay

The use of the simple lumped first-order decay rate requires the user to input a decay rate

constant for the chemical for each model segment. If a simple lumped first order rate is specified 

for a particular chemical, the chemical will decay at that rate regardless of other input. For

example, if both a lumped decay rate and either a simple first order or second order

transformation rate are specified, the simple first or second order rates will only be used if the

lumped rate is zero. 

Option 2: Individual First Order Transformation

Constant C1 C2 C3 REACTION

KV, day-1 140 740 1340 Volatilization

THV, day 145? 745? 1345?

KBW, day-1 141 741 1341 Water Column

 Biodegradation

THBW, day 143 743 1343

KBS, day-1 142 742 1342 Benthic

 Biodegradation

THBS, day 144 744 1344

KHOH, day-1 181 781 1381 Alkaline 

 Hydrolysis

THHOH, day 252 852 1452

KHN, day-1 182 782 1382 Neutral

 Hydrolysis

THHN, day 253 853 1453

KHH, day-1 183 783 1383 Acid 

 Hydrolysis

THHH, day 254 854 1454

KO, day-1 256 856 1456 Oxidation

THO, day 257 857 1457

KF, day-1 287 887 1482 Photolysis

THF, day 289 889 1489

KE, day-1 571 1171 1771 Extra Reaction

THE, day 572 1172 1772

The use of the simple first-order transformation rate requires the user to input a global rate

constant (day-1) or half-life (day) for each particular processes simulated. If a simple first-order 
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transformation rate is specified, it will take priority over other input for that particular processes. 

For example, if both a first order and a second order transformation rate constant is specified, the 

second order rate will only be used if the first-order rate constant is zero. First-order 

transformation rate constant numbers are given in Table 6.3. 

Partition Coefficients-- TOXI allows the input of either a single constant partition coefficient, or

a set of spatially-variable partition coefficients:

Option 1: Constant Partition Coefficient. 

 C1 C2 C3

Solids 1 111 711 1311

Solids 2 116 716 1316

Solids 3 121 721 1321

This option allows the user to directly input constant partition coefficients that apply over the

entire model network. These partition coefficients are input using the set of constants PIXC, in 

units of Lw/kgs (not in log units). If only one chemical and one solids type is being simulated, 

then the partition coefficient can be input by specifying a value for Constant 111 -- PIXC(1,1). 

All other partitioning information should be omitted (i.e. - LKOW, LKOC, and FOC).

If three chemicals are being simulated, the user may specify values for their partition coefficients 

to solids 1 using three separate PIXC values -- Constants 111, 711, and 1311, respectively.

If multiple solids types are being simulated, then separate partition coefficients may be input for 

each of the three solids types. The constant partition coefficients for chemical 1 to solids type 2 

and 3 can be input by specifying appropriate PIXC values for Constants 116 and 121,

respectively.

Constant numbers for partitioning of chemical i to solid j are summarized in Table 6.4.

Option 2: Spatially-Variable Partition Coefficients. 

This option allows the user to directly input spatially-variable partition coefficients for chemical

1. These partition coefficients are input using the parameter FOC, in units of Lw/kgs (not in log 

units). If only one chemical and one solids type is being simulated, then the partition coefficients 

can be input by specifying segment-variable values for Parameter 7 -- FOC(ISEG,1). Constant 

101, LKOC, should be given a small nonzero value, such as 1.0e-20.

If multiple solids types are being simulated, then separate sets of partition coefficients may be

input for each of the three solids types. The constant partition coefficients for chemical 1 to

solids type 2 and 3 can be input by specifying segment-variable values for FOC(ISEG,2) and 

FOC(ISEG,3) -- Parameters 8 and 9, respectively.

FROM to C1 to C2 to C3 REACTION
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C1 176 177 Water Column

Biodegradation

 YBWci

C2 776 777

C3 1376 1377

C1 178 179 Benthic

Biodegradation

 YBSci

C2 778 779

C3 1378 1379

C1 246 247 Alkaline 

 Hydrolysis

 YHOHci

C2 846 847

C3 1446 1447

C1 248 249 Neutral

 Hydrolysis

 YHNci

C2 848 849

C3 1448 1449

C1 250 251 Acid 

 Hydrolysis

 YHHci

C2 850 851

C3 1450 1451

C1 281 282 Oxidation

 YOXci

C2 881 882

C3 1481 1482

C1 566 567 Photolysis

 YFci

C2 1166 1167

C3 1766 1767

C1 596 597 Extra 

 Reaction

 YEci

C2 1196 1197

C3 1796 1797

Reaction Yields-- The input yield constants that may be specified are YHOHci, YHNci, YHHci, 

YBWci, YBSci, YFci, YOXci, and YEci where c is the chemical reactant (1, 2, or 3) and i is the 

chemical product (1, 2, or 3) in units of mgc/mgi. Yield coefficients may be provided for all

possible combinations of chemicals and for the reactions, as listed in Table 6.5.
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11. Organic Chemicals

In modern technological societies, synthetic organic chemicals have been manufactured, used,

and disposed of in large quantities. The large number and variety of organic compounds include 

such major classes as pesticides, polychlorinated biphenyls, halogenated aliphatic hydrocarbons,

halogenated ethers, monocyclic aromatics, phthalate esters, polycyclic aromatic hydrocarbons,

and nitrosamines. Organic chemicals can enter the aquatic environment by various pathways,

including point source waste discharges and nonpoint source runoff. Some of these organic

chemicals can cause toxicity to aquatic organisms, or bioconcentrate through the food chain. 

Humans may be affected by ingesting contaminated water or fish. Criteria for protecting human

health and indiginous aquatic communities have been promulgated for some 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. WASP6 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.

WASP6 allows the simulation of a variety of processes that may affect toxic chemicals. 

However, WASP6 makes relatively few assumptions concerning the particular processes

affecting the transport, transformations, and kinetic 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.

Although the potential amount and variety of data used by WASP6 is large, data requirements

for any particular simulation can be quite small. Most often, organic chemical simulations use

only sorption and one or two transformation processes that significantly affect a particular

chemical. What is gained by the second-order process functions and resulting input data burden 

is the ability to extrapolate more confidently to future conditions. The user must determine the

optimum amount of empirical calibration and process specification for each application.

Overview of WASP6 Organic Chemicals

SYSTEM VARIABLE

1 CHEMICAL 1

2 SOLIDS 1

3 SOLIDS 2

4 SOLIDS 3

5 CHEMICAL 2

6 CHEMICAL 3
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Organic chemicals and associated solids are simulated using the TOXI program. TOXI

simulates the transport and transformation of one to three chemicals and one to three types of

particulate material (solids classes, Table 7.1). The three chemicals may be independent or they

may be linked with reaction yields, such as a parent compound-daughter product sequence. The 

simulation of solids is described in Chapter 3. The simulation of organic chemicals is described 

below. Organic chemical process routines are closely derived from the Exposure Analysis

Modeling System EXAMS (Burns, et al., 1982; Burns and Cline, 1985).

Each organic chemical may exist as a neutral compound and up to four ionic species. The

neutral and ionic species can exist in five phases: dissolved, sorbed to dissolved organic carbon 

(DOC), and sorbed to each of the up to three types of solids (8). 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

(WASP system) representing total concentration is required for each chemical. The model, then, 

is composed of up to six systems -- three chemicals and three solids -- for which the general

WASP6 mass balance equation is solved.

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 TOXI through the use of model parameters 

and time functions. They are specified to the model (described) rather than simulated. They

may be varied over space (e.g. between model segments) and/or over time. Examples of the 

concentrations or properties that are described to the model are provided in Table 7.2.

Parameter or Time

Function Units

Time

Variable

Affected Kinetic

Processes

Water Temperature oC Y All

Dissolved Organic

Carbon

mg/L N Sorption, Photolysis

Fraction Organic

Carbon

none N Sorption

pH - Y Hydrolysis

Oxidant Concentration moles/L N Oxidation

Bacterial Concentration variable Y Biodegradation

Extra Property variable N Extra 2nd Order Reaction

Wind Velocity m/sec Y Volatilization
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Air Temperature oC Y Volatilization

Chlorophyll a

Concentration

mg/L Y Photolysis

Normalized Light

Intensity

None Y Photolysis (Option 2

Only)

11.1. TOXI Reactions and Transformations

In an aquatic environment, an organic chemical may be transferred between phases and may be 

degraded by any of a number of chemical and biological processes. Ionization may speciate the 

chemical into multiple forms. Transfer processes defined in the model include sorption and

volatilization. Defined transformation processes include biodegradation, hydrolysis, photolysis,

and chemical oxidation. Sorption and ionization are treated as equilibrium reactions. All other

processes are described by rate equations. Rate equations may be quantified by first-order 

constants or by second-order chemical specific constants and environment-specific parameters

that may vary in space and time.

WASP6 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, settles to and erodes from benthic segments, and moves

between benthic segments through net sedimentation, erosion, or bed load. Chapter 3 details the 

TOXI sediment transport processes.

In a simulation, the chemical can undergo several physical or chemical transformations. It is

convenient to group these into fast and slow reactions. Fast reactions have characteristic reaction 

times that are much faster than, or on the same order as the model time step, and are handled 

with the assumption of local equilibrium. Slow reactions have characteristic reaction times much

longer than the model time step. These are handled with the assumption of local first order

kinetics using a lumped rate constant specified by the user, or calculated internally, based on

summation of several process rates, some of which are second-order. Thus, the effective first 

order decay rate can vary with time, and space, and is recalculated as often as necessary

throughout a simulation. 

The chemical is advected and dispersed among water segments, and exchanged with surficial

benthic segments by dispersive mixing. Sorbed chemical settles through water column segments 

and deposits to or erodes from surficial benthic segments. Within the bed, dissolved chemical

migrates downward or upward through percolation and pore water diffusion. Sorbed chemical
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migrates downward or upward through net sedimentation or erosion. Both rate constants and

equilibrium coefficients must be estimated in most toxic chemical studies. Although these can

be calculated internally from chemical properties and local environmental characteristics,

site-specific calibration or testing is desirable.

Some limitations should be kept in mind when applying TOXI. 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 can

affect key environmental characteristics, such as pH or bacterial populations, thus altering

transformation rates. TOXI does not include such feedback phenomena.

11.2. Model Implementation

To simulate organic chemicals with WASP6, use the preprocessor or text editor to create a TOXI 

input file. The model input dataset and the input parameters will be similar to those for the

conservative tracer model as described in Chapter 2. To those basic parameters, the user will add 

benthic segments, solids transport rates, and transformation parameters. During the simulation,

solids and organic chemicals will be transported both by the water column advection and

dispersion rates and by these solids transport rates.

In WASP6, solids transport rates in the water column and the bed are input via up to three solids 

transport fields, as described in Chapter 3. The transport of the particulate fraction of organic 

chemicals follows the solids flows. The user must specify the dissolved fraction (i.e. 0.0) and the 

solids transport field for each simulated solid under initial conditions. To simulate total solids,

solids 1 must be used.

11.2.1. Model Input Parameters

Input parameters are prepared for WASP6 in four major sections of the preprocessor --

environment, transport, boundaries, and transformation. The organic chemical input parameters

comprising the first three sections are identical to those in the simple toxicant model. The user is 

referred to Section 6.2 for a summary of these input parameters. This section, and the rest of this 

chapter, describes the organic chemical reaction parameters.

11.2.2. Transformation Parameters

This group of parameters includes spatially variable parameters, constants, and kinetic time

functions for the water quality constituents being simulated. The organic chemical reactions and

model input parameters are described in individual sections below. Because water temperature 

can affect every chemical reaction, it is described here.
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Water Temperature, °C-- Water temperature can vary in space and time, affecting the rates of all 

chemical reactions. Time and segment variable water temperatures can be specified using the

parameters TEMP and TMPFN, and the time functions TEMPN(1-4). If temperatures are to 

remain constant in time, then the user should enter segment temperatures using the parameter

TEMP. TMPFN and TEMPN(1-4) should be omitted. 

If the user wants to enter time-variable temperatures, then values for the parameter TEMP should 

be set to 1.0. The parameter TMPFN indicates which temperature function will be used by the 

model for each segment. Values of 1.0, 2.0, 3.0, or 4.0 will call time functions TEMPN(1), 

TEMPN(2), TEMPN(3), and TEMPN(4), respectively. Water temperatures should then be

entered via these time functions as a series of temperature versus time values. The product of

TEMP and the selected TEMPN function will give the segment and time specific water

temperatures used by TOXI.

TEMP and TMPFN are identified in TOXI as parameters 3 and 2, respectively. TEMPN(1-4) 

are identified in TOXI as time functions 1-4. (Group G, Record 4, PARAM(I,3), PARAM(I,2); 

Group I, Record 2, VALT(1-4,K))

Notation

In TOXI, it is convenient to define concentration related symbols as in Table 7.3. Please note 

that in the general development of the equations in the sections below, subscripts "i" and "j" are 

sometimes omitted for convenience.

Symbol Definition Units

Cij Concentration of total chemical i in segment j. mgc/L

Cwij Concentration of dissolved chemical i in segment j. mgc/L

C'wij Concentration of dissolved chemical i in water in segment j; Cwij/nj mgc/Lw

Csij Concentration of sorbed chemical i on sediment type "s" in segment j. mgc/L

C'

sij Concentration of sorbed chemical i on sediment type "s" in segment j; 

Csij/Msij

mgc/kgs

CBj Concentration of DOC-sorbed chemical i in segment j. mgc/L

C'Bj Concentration of DOC-sorbed chemical i in segment j; CBj/Bj mgc/kgB

msj Concentration of sediment type "s" in segment j. mgs/L

Msj Concentration of sediment type "s" in segment j; mj ⋅ 10-6 kgs/L

M'

sj Concentration of sediment type "s" in water in segment j; Msj/n kgs/Lw

Bj Concentration of DOC in segment j. kgB/L

B'

j Concentration of DOC in water in segment j; Bj/n kgB/Lw

nj Porosity or volume water per volume segment j. Lw/L

Kpsij Partition coefficient of chemical i on sediment type "s" in segment j. Lw/kgs

KpBij Partition coefficient of chemical i on DOC in segment j. Lw/kgB

fDij Fraction of chemical i in segment j in dissolved phase -

fBij Fraction of chemical i in segment j in DOC-sorbed phase -

fsij Fraction of chemical i in segment j in solid phase "s" -
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11-6

11.3. Ionization

Ionization is the dissociation of a chemical into multiple charged species. In an aquatic

environment some chemicals may occur only in their neutral form while others may react with

water molecules to form positively (cationic) or negatively (anionic) charged ions. These

reactions are rapid and are generally assumed to be at (local) equilibrium. At equilibrium, the

distribution of chemicals between the neutral and the ionized species is controlled by the pH and 

temperature of the water and the ionization constants.

Ionization can be important because of the different toxicological and chemical properties of the

neutral and ionized species. For example, in some cases only the neutral form of the chemical

may react or be transported through biotic membranes resulting in toxicity. As a result, it is

often necessary to compute the distribution of chemicals among ionic forms as well as to allow

them to react or transform at different rates. For example, in TOXI different sorption and

reaction constants (e.g. for hydrolysis, biodegradation, photolysis, etc.) may be specified for each

ionic form of the chemical. 

11.3.1. Overview of TOXI Ionization Reactions

In TOXI, each of the three possible chemicals being simulated may occur in up to five forms,

including 1) the neutral molecule, 2) singly charged cations, 3) doubly charged cations, 4) singly

charged anions, and 5) doubly charged anions. Each of the neutral or ionic species may also

occur in the dissolved phase or sorbed to dissolved organic carbon (DOC) or the three solids 

types. A total of 25 forms of each chemical may occur. Each chemical form may have different 

reactivities as reflected by different degradation or transformation rates. TOXI makes no direct 

assumptions as to the formation of the ionic species or their reactivity. The user controls the

formation by specification of model input. 

A chemical being modeled by TOXI is presumed to exist as neutral molecules that may, or may

not, react with water molecules to form singly and, possibly, doubly charged cations and anions. 

To illustrate, an organic acid (A--) may react with water as described by:

Equation 11-1

AH + H O AH +OH + -

2 2 _ 3

Equation 11-2

AH + H O AH +OH ++ -

2 4

+

3 _
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11-7

Equation 11-3

AH +H 0 AH + H O

+

3

-

2 2 _

Equation 11-4

AH +H O A +H O

+

3

--

2

- _

so that the chemical may exist in from one to a maximum of five species simultaneously (A--, 

AH-, AH2, AH3

+

, AH4

++). The law of mass action can be used to describe local chemical

equilibrium for each of these reactions:

Equation 11-5

[ AH ]

[ AH ][OH ]

K =

2

+ -

3

b 1

Equation 11-6

[ AH ]

[ AH ][OH ]

K = +

3

++ -

4

b 2

Equation 11-7

[ AH ]

[ AH ][ H ]

K =

2

- +

a 1

Equation 11-8

[ AH ]

[ A ][ H ]

K = -

-- +

a 2

where K is the equilibrium constant for the formation of the acid (Kai), or anionic species, or the 

base (Kbi) or cationic species. 

The total concentration of the particular chemical is the sum of the concentration of each of these 

forms, as given by
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11-8

Equation 11-9

C= AH + AH + AH + AH + A

++ - --

4

+

2 3

which may be combined with the law of mass action to form

Equation 11-10













[ H ]

K K +

[ H ]

K +

[OH ]

K K +

[OH ]

K C= AH + 2a1 a2

+

a1

- 2

b1 b2

-

b1

2

By definition, [H+] = 10-pH and [OH-] = 1014-pH; the bracketed term in equation 7.10, denoted D, 

can be written

Equation 11-11













(10 )

K 1 K 2

+

10

K 1

+

(10 )

K 1K 2

+

10

K 1

D= - pH 2

a a

- pH

a

pH - 14 2

b b

pH -14

b

Equations 7.10 and 7.11 may be combined with equations 7.5 - 7.8 and solved for the fraction of 

the total chemical f

k

 occurring in each of the chemical species k, given the total chemical

concentration, the pH, and the equilibrium constants:

Equation 11-12

D

1

f =

0

Equation 11-13

D

K 1 / 10 f =

pH - 14

+ b

Equation 11-14

D

K 1K 2/(10 )

f =

pH - 14 2

++ b b
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11-9

Equation 11-15

D

K 1/ 10 f =

-pH

- a

Equation 11-16

D

K 1 K 2/(10 )

f =

-pH 2

-- a a

The rates of chemical reactions may also vary with temperature so that the equilibrium constants 

are a function of temperature. The functional dependence of these constants on temperature may

be described by the Van't Hoff equation: 

Equation 11-17

RT

E =

d T

d K

2

K

ai

K

i

ln

or in its integrated form:













• 











•

T T

T -T

2.303 R

E = - pK +

T T

T -T

2.303 R

E

K (T )= K (T )+

k Ri

ai K Ri

ai

K Ri

ai K Ri

i K i Ri log log

where 

Ki = equilibrium constant 

Ai = frequency factor

Eai = standard enthalpy change for reaction, kcal/mole 

R = the universal gas constant, kcal/mole °K 

TK = water temperature, °K.

TRi = reference temperature at which input ionization reaction

constant was observed, °K.

Common

Range

 S.I.

 Units

Description Notation

Negative log of hydrogen ion activity [H+] pH 5-9 -

Negative log of ionization constants for acid pKai - -

Negative log of ionization constants for base pKbi - -

Enthalpy change for ionization reactions Eai 4-8 kcal/mole

Water temperature T 4-30 °C

Reference temperature TRi 20-25 °C
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11.4. Implementation

The data required for the implementation of ionization in TOXI are summarized in Table 7.4. 

They include first identifying whether or not a particular ionic specie is to be included in the

simulation and then, if a particular specie is selected, the information necessary to compute its

formation. For example, to compute particular ionic specie, it is necessary to input the pK

(negative log) of the equilibrium constant for the formation of the acid and/or base, and the

activation energy used in the Van't Hoff Equation to adjust the equilibrium constant with

temperature. If the activation energy is not input, then no temperature correction will occur. If 

no data are input for ionization, none will occur and the reactions and transformations will be

applied to the total or dissolved form of the chemical, as appropriate.

In addition to the constants for the formation of the ionic species, the pH and temperature (if the 

rate is to be temperature corrected) are required. The pH and temperature are model parameters, 

which are specified for each model segment. They may be constant or time variable. 

If ionization is specified in input, separate transformation and reaction rates may be specified for 

each ionic specie. For example, where necessary, different sorption, biodegradation, hydrolysis,

oxidation, and photolysis constants may be specified for each ionic specie, providing

considerable flexibility in the model application.

IONIC SPECIE

"i"

C1 C2 C3 INPUT 

VARIABLE

+ 85 685 1285 Species Flag, 

SFLGi

 

++ 86 686 1286

- 87 687 1287

-- 88 688 1288

+ 91 691 1291 Negative Log

 of Ionization

 Constant,

 PKAi 

++ 92 692 1292

- 93 693 1293

-- 94 694 1294

+ 95 695 1295 Ionization

 Reaction

 Enthalpy,

 EPKAi,

 kcal/mole

++ 96 696 1296

- 97 697 1297

-- 98 698 1298

All 99 699 1299 Reference Temp TREFI, 

°C
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-11

The transformation input parameters for ionization are summarized below. Constant numbers

are given in Table 7.5.

Ionization Switches-- The user may choose to simulate ionic species by specifying values of 1.0 

for constant SFLG.

Ionization Constants-- For each ionic specie being simulated, the user should provide a value for 

the negative log of the frequency factor in the Van't Hoff equation using constant PKA. If the

activation energy is 0, then this is equivalent to the pKa or pKb.

Reaction Enthalpy, kcal/mole-- To simulate temperature dependence for ionization, the user can

specify the standard enthalpy change of the dissociation reaction using constant EPKA. Higher

reaction enthalpies cause more temperature dependence.

pH-- The user may specify segment and time variable bacterial concentrations using parameter

11, PH, and time functions 10 and 11, PHNW and PHNS. If pH is to remain constant in time, 

the user should enter segment mean values using parameter PH. PHNW and PHNS should be 

omitted.

The user may enter time-variable water column and benthic pH values via time functions PHNW 

and PHNS, respectively, as a series of concentration versus time values. Parameter PH will then 

represent the ratio of each segment pH to the time function values. The product of PH and the 

PHNW or PHNS function gives the segment and time specific pH values used by TOXI. (Group 

G, Record 4, PARAM(I,11); Group I, Record 2, VALT(10,K), VALT(11,K))

11.5. Equilibrium Sorption

Sorption is the bonding of dissolved chemicals onto solid phases, such as benthic and suspended 

sediment, biological material, and sometime dissolved or colloidal organic material. Sorption

can be important in controlling both the environmental fate and the toxicity of chemicals. 

Sorption may cause the chemical to accumulate in bed sediment or bioconcentrate in fish. 

Sorption may retard such reactions as volatilization and base hydrolysis, or enhance other

reactions including photolysis and acid-catalyzed hydrolysis.

Sorption reactions are usually fast relative to other environmental processes, and equilibrium

may be assumed. For environmentally relevant concentrations (less than 10-5 M or one-half 

water solubility), equilibrium sorption is linear with dissolved chemical concentration 

(Karickhoff, 1984) or:

Equation 11-18

C = K C

,

ps w

,

s •
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-12

At equilibrium, then, the distribution among the phases is controlled by the partition coefficients

Kps. The total mass of chemical in each phase is controlled by Kps and the amount of solid phase 

present (including any DOC phase).

In addition to the assumption of instantaneous equilibrium, implicit in the use of equation 7.19 is 

the assumption of reversibility. Laboratory data for very hydrophobic chemicals suggest,

however, that a hysteresis exists, with desorption being a much slower process than adsorption. 

Karickhoff suggests that this effect may be the result of intraparticle kinetics in which the

chemical is slowly incorporated into components of the sorbant. This phenomenon is not well

understood and no quantitative modeling framework is available to characterize it.

11.5.1. Overview of TOXI Sorption Reactions

Dissolved chemical in water column and benthic segments interacts with sediment particles and 

dissolved organic carbon to form five phases-- dissolved, DOC-sorbed, and sediment-sorbed 

(three sediment types "s"). The reactions can be written with respect to unit volume of water:

Equation 11-19

M +C C s /n ,

 w

,

 s ↔

Equation 11-20

B +C C B

/n ,

 w

, ↔

where n is the porosity (volume of water divided by total volume), 

The forward reaction is sorption and the backward reaction is desorption. These reactions are 

usually fast in comparison with the model time step, and can be considered in local equilibrium. 

The phase concentrations Cw, Cs, and CB are governed by the equilibrium partition coefficients 

Kps0 and KpB (L/kg):

Equation 11-21

C

C =

M C

C /n

K = ,

 w

,

 s

,

 w

,

 s

 s

ps0

Equation 11-22

C

C =

B C

C /n

K = ,

 w

,

B

,

 w

,

B

pB
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-13

These equations give the linear form of the Freundlich isotherm, applicable when sorption sites

on sediment and DOC are plentiful:

Equation 11-23

C = K C

,

ps w

,

 s

Equation 11-24

C = K C

,

pB w

,

B

The total chemical concentration is the sum of the five phase concentrations

Equation 11-25

C=C n+ C M +C B

,

 s B

,

 s

s

,

 w ∑

Substituting in equations 7.24 and 7.25, factoring, and rearranging terms gives the dissolved

fraction fD:

Equation 11-26

n+K B+ K M

n

=

C

C n

f =

ps s

s

pB

,

 w

D

• ∑ •

Similarly, the sediment-sorbed and DOC-sorbed fractions are

Equation 11-27

n+ K B + K M

K M =

C

C M f =

ps s

s

pB

 s ps s

,

 s

 s

• •

•

∑

Equation 11-28

n+ K B + K M

K B

=

C

C B

f =

ps s

s

pB

pB

,

B

B

• •

•

∑
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-14

These fractions are determined in time and space throughout a simulation from the partition

coefficients, internally calculated porosities, simulated sediment concentrations, and specified

DOC concentrations. Given the total concentration and the five phase fractions, the dissolved, 

sorbed, and biosorbed concentrations are uniquely determined:

Equation 11-29

C =C f w D

•

Equation 11-30

C =C f

 s s •

Equation 11-31

C =C f B B

•

These five concentrations have units of mg/L, and can be expressed as concentrations within

each phase:

Equation 11-32

C =C w /n ,

 w

Equation 11-33

C =C s / M s

,

 s

Equation 11-34

C =C B

/B ,

B

These concentrations have units of mg/Lw, mg/kgs, and mg/kgB, respectively.

In some cases, such as near discharges, the user may have to alter input partition coefficients to 

describe the effect of incomplete sorption. As guidance, Karickhoff and Morris (1985) found

that typical sorption reaction times are related to the partition coefficient:

Equation 11-35

k =0.03 k ps

-1

d
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-15

where kd

-1

 is the desorption rate constant, hr-1.

Thus, compounds with high, medium, and low Kow's of 105, 103, and 10 sorbing onto 2% organic 

sediment should have reaction times of a day, a half hour, and seconds. Given that time to

equilibrium is roughly three times the reaction time, the three compounds should reach

equilibrium within 3 days, 1 hour, and 30 minutes.

11.5.2. Computation of Partition Coefficients

Values for the partition coefficients can be obtained from laboratory experiments. For organic

chemicals, lab studies have shown that the partition coefficient is related to the hydrophobicity of

the chemical and the organic matter content of the sediment. TOXI provides several optional

methods for the description or computation of the partition coefficients. These options are

identified by the data input, as described below. 

11.5.3. Option 1. Measured Partition Coefficients. 

This option allows the user to directly input a partition coefficient. Separate partition

coefficients may be input for each of the three solids types. The partition coefficient is input in

units of Lw/kgs (not in log units). 

11.5.4. Option 2. Input of Organic Carbon Partition Coefficient. 

Normalization of the partition coefficient by the organic-carbon content of the sediment has been

shown to yield a coefficient, KOC (the organic carbon partition coefficient), that is relatively

independent of other sediment characteristics or geographic origin. Many organic pollutants of

current interest are non-polar, hydrophobic compounds whose partition coefficients correlate

quite well with the organic fraction of the sediment. Rao and Davidson (1980) and Karickhoff et 

al. (1979) have developed empirical expressions relating equilibrium coefficients to laboratory

measurements leading to fairly reliable means of estimating appropriate values. The correlations

used in TOXI are

Equation 11-36

K ps0= f

ocs K oc

Equation 11-37

K pB=1.0 K oc

where:
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-16

Koc = organic carbon partition coefficient, Lw/kgoc

focs = organic carbon fraction of sediment

1.0 = organic carbon fraction of DOC

Option 3. Computation of the Organic Carbon Partition Coefficient. 

Correlation of Koc with the water solubility of the chemical or the octonal/water partition

coefficient of the chemical has yielded successful predictive tools for incorporating the

hydrophobicity of the chemical in an estimate of its partitioning. If no log Koc values are

available, one is generated internally using the following correlation with the octanol-water 

partition coefficient Kow (Lw/Loct):

Equation 11-38

log K oc= a o+a1log K ow

where ao and a1 are typically considered to be log 0.6 and 1.0, respectively. Once the value of 

Koc is determined, the computation of the partition coefficient proceeds as in Option 2.

11.5.5. Option 4. Computation of Solids Dependant Partitioning. 

The value of the partition coefficient is dependent on numerous factors in addition to the fraction

organic carbon of the sorbing particles. Of these, perhaps the most potentially significant and the 

most controversial is the effect of particle concentration, which was first presented by O'Connor 

and Connolly (1980). Based on empirical evidence, O'Connor and Connolly concluded that the 

partition coefficient was inversely related to the solids concentration. Much research has been

conducted to prove or disprove this finding. At present, the issue remains contentious. A

particle interaction model has been proposed (Di Toro, 1985) which describes the effects of

particle concentration. This model was shown to be in conformity with observations for a large 

set of adsorption-desorption data. At present, this should be considered an empirical

relationship. The equation defining partition coefficient is:

Equation 11-39

 s ps0 n x

ps0

ps 1+M K /

K

K =

where:

Kps0 = limiting partition coefficient with no particle interaction (focs

Koc for neutral organic chemicals)

Ms = solids concentration, kg/L

õx = ratio of adsorption to particle-induced desorption rate
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-17

Di Toro found that õx was of order 1 over a broad range of chemical and solids types. This 

formulation has been included in TOXI. If õx is specified to be 1.0, then TOXI will predict a 

maximum particulate fraction in the water column of 0.5 for all hydrophobic chemicals (KpsOMs

> 10). 

Implementation

 Common 

 Range

 S.I.

 Units

Description Notation

Suspended sediment concentration ms 10-100 mg/L

Benthic sediment concentration MB 0.5-2 kg/L

Dissolved organic carbon DOC, B 0-10 mg/L

Partition coefficient, 

phase i

 Kpi 10-1-105 L/kg

Lumped metal distribution coefficient KD 100-105 L/kg

Octanol-water partition coefficient Kow 100-106 -

Organic carbon fraction, phase i foci 0.005-0.5 -

Particle interaction parameter õx 1-1012 -

 VARIABLE C1 C2 C3 DEFINITION

 LKOW 84 684 1284 Log10 octanol-water partition coefficient

 LKOC 101 701 1301 Log10 organic carbon partition coefficient

 A0 102 702 1302 Intercept in the Kow - Koc correlation

 A1 103 703 1303 Slope in the Kow - Koc correlation

 NUX1 106 706 1306 Solids dependent partitioning parameter

 PIXC1,1 111 711 1311 Solids independent (limiting) partition

coefficient to solids 1

 PIXC2,1 116 716 1316 Solids independent (limiting) partition

coefficient to solids 2

 PIXC3,1 121 721 1321 Solids independent (limiting) partition

coefficient to solids 3

TOXI data specifications for sorption are summarized in Table 7.6. For each chemical modeled, 

up to 20 partition coefficients are defined representing the five species of chemical (neutral plus

four ionic) and the four sorbants (DOC and three types of solids). Normally, only a subset of 

these would be used, as defined by those species and solids being modeled. Sorption of the

neutral chemical to DOC and the solids is defined by the foc of the sorbant (assumed to be 1 for 

DOC), the octanol-water partition coefficient of the chemical (Kow), the user-defined relationship 

between Kow and Koc, and the particle interaction parameter õx values for each species. The input 

ionic species partition coefficients are used as the limiting partition coefficients in equation 7.40. 

Constant numbers for the different coefficient options are given in Table 7.7.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-18

11.5.6. Option 1: Measured Partition Coefficients. 

For each chemical simulated, separate partition coefficients may be entered for sorption of the

neutral molecule and up to 4 ionic species onto each of the three possible solids types and DOC. 

The partition coefficient is input in units of Lw/kgs (not in log units). If a partition coefficient is 

specified it will be used regardless. The user is referred to Chapter 6 for details on directly 

specifying partition coefficients.

Solids Partition Coefficient, L/kg-- The user may directly specify partition coefficients to solids

using constant PIXC. Constant numbers for sorption of the neutral molecule are given in Table 

7.7. 

DOC Partition Coefficient-- The user may specify partition coefficients for sorption of ionic

species to DOC using constant PIDOC. 

11.5.7. Option 2: Input of Organic Carbon Partition Coefficient. 

Under this option the user inputs the log (base 10) of the organic carbon partition coefficient 

(Koc). In addition, the user should also input the fraction organic carbon for each of the solids

types simulated. The fraction organic carbon for dissolved organic carbon is assumed to be 1.0. 

The fraction organic carbon and dissolved organic carbon concentration are model parameters,

which may be specified for each model segment. If a value for the partition coefficient (Kp, 

Option 1) is input, then Koc will not be used.

Organic Carbon Partition Coefficient, L/kg-- The user may specify the log10 of the organic

carbon partition coefficient using constant LKOC. Constant numbers are given in Table 7.7.

Fraction Organic Carbon-- The user should specify the segment variable fraction organic carbon

for each solids type simulated using parameters FOC(I,1), FOC(I,2), and FOC(I,3). Parameter 

numbers for solids 1, 2, and 3 are 7, 8, and 9, respectively.

Dissolved Organic Carbon, mg/L-- The user may specify segment variable dissolved organic

carbon concentrations using parameter 6, DOC.

11.5.8. Option 3: Computation of the Organic Carbon Partition Coefficient. 

Under this option, the user allows the model to compute the Koc from a specified octanol water 

partition coefficient (Kow). The model then computes the Koc using equation 7.39. This option

will not be used if values for the log (Koc) are input. 

Octanol-Water Partition Coefficient, Lw/Lo-- The user may specify the log10 of the octanol-water 

partition coefficient using constant LKOC. Constant numbers are given in Table 7.7.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-19

Correlation Coefficients-- The user should specify correlation coefficients relating Kow with Koc

using constants A0 and A1. A0 and A1 are the intercept and the slope in the correlation

described by equation 7.39. Default values are log 0.6 and 1.0, respectively. If these constants 

are not entered, then, the correlation becomes Koc = 0.6 Kow. Constant numbers are given in

Table 7.7.

Fraction Organic Carbon-- The user should specify the segment variable fraction organic carbon

for each solids type simulated using parameters FOC(I,1), FOC(I,2), and FOC(I,3). Parameter 

numbers for solids 1, 2, and 3 are 7, 8, and 9, respectively.

Dissolved Organic Carbon, mg/L-- The user may specify segment variable dissolved organic

carbon concentrations using parameter 6, DOC.

11.5.9. Option 4: Solids Dependant Partitioning.

The user may include the effect of solids concentration on adsorption by using a value of õx of 

order 1 (see Di Toro, 1985 for more detail). If the user does not provide an input value for õx, 

the default value will eliminate any solids effect on the partition coefficient. Since collision

induced desorption is only expected to occur in the water column, solids dependant partitioning

is only computed for water column segments (where porosity is greater that 0.99). In addition to 

the partical interaction parameter, the user must provide for a partition coefficient following

option 1, 2, or 3 described above.

Particle Interaction Parameter-- The user may implement solids dependent partitioning by

specifying an appropriate value for constant NUX. A value of order 1 will cause the input

partition coefficient to decrease with increasing suspended solids, following equation 7.40. 

Larger values of NUX will reduce the solids effect on partitioning. The default value of 1012

effectively eliminates this behavior. Constant numbers for the solids effect on the neutral

molecule are given in Table 7.7. 

11.6. Volatilization

Volatilization is the movement of chemical across the air-water interface as the dissolved neutral

concentration attempts to equilibrate with the gas phase concentration. Equilibrium occurs when

the partial pressure exerted by the chemical in solution equals the partial pressure of the chemical

in the overlying atmosphere. The rate of exchange is proportional to the gradient between the

dissolved concentration and the concentration in the overlying atmosphere and the conductivity

across the interface of the two fluids. The conductivity is influenced by both chemical properties 

(molecular weight, Henry's Law constant) and environmental conditions at the air-water interface 

(turbulence-controlled by wind speed, current velocity, and water depth). 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-20

11.6.1. Overview of TOXI Volatilization

The dissolved concentration attempts to equilibrate with the gas phase concentration, as

illustrated in 8 and given by

Equation 11-40

























∂

∂

RT

H

C f C -

D

K

| = 

t

C

K

a

d

v

volat

where 

Kv = the transfer rate, m/day 

D = segment depth, m 

fd = fraction of the total chemical that is dissolved, 

Ca = atmospheric concentration, ug/L

R = universal gas constant, 8.206x10-5 atm-m

3

/mole oK 

TK = water temperature, oK

H = Henry's law coefficient for the air-water partitioning of the 

chemical, atm-m

3

/mole. 

Equilibrium occurs when the dissolved concentration equals the partial pressure divided by

Henry's Law Constant.

In TOXI, the dissolved concentration of a chemical in a surface water column segment can

volatilize at a rate determined by the two-layer resistance model (Whitman, 1923). The

two-resistance method assumes that two "stagnant films" are bounded on either side by well

mixed compartments. Concentration differences serve as the driving force for the water layer

diffusion. Pressure differences drive the diffusion for the air layer. From mass balance

considerations, it is obvious that the same mass must pass through both films, thus the two

resistances combine in series, so that the conductivity is the reciprocal of the total resistance:

Equation 11-41





























R T

H

K =( R +R ) = K + K

K

G

-1

-1

L

-1

L G

-1

v

where 

RL = liquid phase resistance, day/m 

KL = liquid phase transfer coefficient, m/day 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-21

RG = gas phase resistance, day/m 

KG = gas phase transfer coefficient, m/day.

There is actually yet another resistance involved, the transport resistance between the two 

interfaces, but it is assumed to be negligible. This may not be true in two cases: very turbulent 

conditions and in the presence of surface-active contaminants. Although this two-resistance 

method, the Whitman model, is rather simplified in its assumption of uniform layers, it has been

shown to be as accurate as more complex models.

The value of Kv, the conductivity, depends on the intensity of turbulence in a water body and in 

the overlying atmosphere. Mackay and Leinonen (1975) have discussed conditions under which 

the value of Kv is primarily determined by the intensity of turbulence in the water. As the

Henry's Law coefficient increases, the conductivity tends to be increasingly influenced by the

intensity of turbulence in water. As the Henry's Law coefficient decreases, the value of the

conductivity tends to be increasingly influenced by the intensity of atmospheric turbulence.

Because Henry's Law coefficient generally increases with increasing vapor pressure of a

compound and generally decreases with increasing solubility of a compound, highly volatile low

solubility compounds are most likely to exhibit mass transfer limitations in water and relatively

nonvolatile high solubility compounds are more likely to exhibit mass transfer limitations in the 

air. Volatilization is usually of relatively less magnitude in lakes and reservoirs than in rivers

and streams.

In cases where it is likely that the volatilization rate is regulated by turbulence level in the water 

phase, estimates of volatilization can be obtained from results of laboratory experiments. As

discussed by Mill et al. (1982), small flasks containing a solution of a pesticide dissolved in

water that have been stripped of oxygen can be shaken for specified periods of time. The amount 

of pollutant lost and oxygen gained through volatilization can be measured and the ratio of

conductivities (KVOG) for pollutants and oxygen can be calculated. As shown by Tsivoglou

and Wallace (1972), this ratio should be constant irrespective of the turbulence in a water body. 

Thus, if the reaeration coefficient for a receiving water body is known or can be estimated and 

the ratio of the conductivity for the pollutant to reaeration coefficient has been measured, the

pollutant conductivity can be estimated.

The input computed volatilization rate constant is for a temperature of 20°C. It is adjusted for 

segment temperature using the equation:

Equation 11-42

Θ

T -20 K v,T = K 20

where 

Èv = temperature correction factor
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-22

T = water temperature, °C.

Directly input volatilization rates are not temperature adjusted.

Computation of the Transfer Rates

There have been a variety of methods proposed to compute the liquid (KL) and gas phase (KG) 

transfer coefficients, several of which are included in TOXI. The particular method to be

employed is identified by the model through the user's selection of one of six volatilization

options, each of which is briefly described below. 

11.6.2. Volatilization Option 1.

This option allows the use of measured volatilization rates. The rates (Kv, m/day) are input as a 

parameter (which may be varied by segments) and may be time variable.

11.6.3. Volatilization Option 2.

This option allows the user to input an oxygen reaeration constant that is then adjusted to

represent the liquid film transfer constant for the particular chemical. The adjustment is made in

one of two ways. First, the user may input a measured ratio of oxygen to chemical exchange so 

that the rate (KL) is computed from

Equation 11-43

K L= K a • K vo

where 

Ka = reaeration velocity, m/day

Kvo = ratio of volatilization rate to reaeration rate. 

If Kvo is not provided, TOXI will compute the ratio based on the molecular weights of O2 and 

that of the chemical as shown below

Equation 11-44

K L= K a 32/ M w

where Mw = molecular weight of the chemical, g/mole. 

Under this option, the gas transfer rate (KG) is calculated using O'Conner's method (see Option 

4).
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-23

11.6.4. Volatilization Option 3.

If this option is specified, the liquid film transfer coefficient will be computed as in Option 2. 

However, the gas film transfer coefficient will be computed using Mackay's method (see Option

5).

11.6.5. Volatilization Option 4.

The liquid and gas film transfer coefficients computed under this option vary with the type of

waterbody. The type of waterbody is specified as one of the volatilization constants and can

either be a flowing stream, river or estuary or a stagnant pond or lake. The primary difference is 

that in a flowing waterbody the turbulence is primarily a function of the stream velocity, while

for stagnant waterbodies wind shear may dominate. The formulations used to compute the

transfer coefficients vary with the waterbody type as shown below.

a) Flowing Stream, River or Estuary: For a flowing system (type 0) the transfer coefficients are 

controlled by flow induced turbulence. For flowing systems, the liquid film transfer coefficient

(KL) is computed using the Covar method (Covar, 1976) in which the equation used varies with

the velocity and depth of the segment. First the transfer coefficient for dissolved oxygen is

computed using the formulations provided below and then KL calculated from equation 7.44 or 

7.45. 

For segments with depths less than 0.61 m the Owens formula is used to calculate the oxygen

reaeration rate:

Equation 11-45

D

u

K =5.349 0.85

0.67

a

where 

u = velocity of the water, m/s

D = segment depth, m

For segments with a velocity less than 0.518 m/s or a depth (m) greater than 13.584 u2.9135, the 

O'Connor-Dobbins formula is used:

Equation 11-46

8.64 10

D

D u

K =

 w 4

0.5

L  •










DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-24

where Dw is the diffusivity of the chemical in water (m2/s), computed from

Equation 11-47

M

22 10

D = 2/3

w

-9

w

•

In all other cases, the Churchill formula is used to calculate reaeration rate:

Equation 11-48

D

u

K =5.049 0.673

0.969

a

The gas transfer coefficient (KG) is assumed constant at 100 m/day for flowing systems.

b) Stagnant Lake or Pond: For a stagnant system (type 1) the transfer coefficients are controlled 

by wind induced turbulence. For stagnant systems, the liquid film transfer coefficient (KL) is

computed using the O'Connor equations:

Equation 11-49

K = u S cw

-0.67

2

0.33

 w

a

0.5

*

L

l

k

r

r

















Equation 11-50

K =u Sc a

-0.67

2

0.33

*

G

l

k

where u* is the shear velocity (m/s) computed from

Equation 11-51

u =C W 10

0.5

d

*

where 

Cd = drag coefficient (0.0011)

W10 = wind velocity 10 m above water surface, m/sec

ña = density of air, internally calculated from air temperature,
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-25

kg/m3

ñw = density of water, internally calculated from water temperature, 

kg/m3

ê = von Karmen's constant (0.74)

ë2 = dimensionless viscous sublayer thickness (4) 

Sca and Scw are air and water Schmidt Numbers, computed from

Equation 11-52

D

S =

a a

a

ca

r

m

Equation 11-53

D

S =

 w w

 w

cw

r

m

where 

Da = diffusivity of chemical in air, m2/sec

Dw = diffusivity of chemical in water, m2/sec

μa = viscosity of air, internally calculated from air temperature, 

kg/m-sec 

μw = viscosity of water, internally calculated from water

temperature, kg/m-sec 

The diffusivity of the chemical in water is computed using Equation 7.48 while the diffusivity of

the chemical in air (Da, m2/sec) is computed from

Equation 11-54

M

1.9 10

D = 2/3

w

-4

a

•

Thus KG is proportional to wind and inversely proportional to molecular weight to the 4/9 power. 

11.6.6. Volatilization Option 5.

As with Option 4, the liquid and gas film transfer coefficients computed under this option vary

with the type of waterbody. The type of waterbody is specified to the water as one of the

volatilization constants and can either be a flowing stream, river or estuary or a stagnant pond or 

lake. The primary difference is that in a flowing waterbody the turbulence is primarily a
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-26

function of the stream velocity, while for stagnant waterbodies wind shear may dominate. The 

formulations used to compute the transfer coefficients vary with the waterbody type as shown

below.

a. Flowing Stream, River or Estuary. The liquid and gas film transfer coefficients for flowing

waterbodies are computed identically to those described under Option 4.

b) Stagnant Pond or Lake. Under this option, the liquid and gas film transfer coefficients are

computed using formulations described by Mackay and Yeun (1983). The Mackay equations

are:

Equation 11-55

K =10 +0.00341u Sc u >.3 m /s *

 w

-6 * -0.5

L

Equation 11-56

K =10 +0.0144 u Sc u <.3 m /s *

 w

-6 * -0.5

L

2.2

Equation 11-57

K =10 +0.0462 u Sc a

-3 * -0.67

G

Implementation

Description Notation Range Units

Measured or calibrated conductance Kv 0.6-25 m/day

Henry's Law Constant H 10-7-10-1 atm-m3/mole

Concentration of chemical in atmosphere Ca 0-1000 μg/L

Molecular weight Mw 10-103 g/mole

Reaeration coefficient (conductance of

oxygen)

Ka 0.6-25 m/day

Experimentally measured ratio of

volatilization to reaeration

kvo 0-1

Current velocity ux 0-2 m/sec

Water depth D 0.1-10 m

Water temperature T 4-30 °C

Wind speed 10 m above surface W10 0-20 m/sec

Although there are many calculations involved in determining volatilization, most are performed

internally using a small set of data. TOXI volatilization data specifications are summarized in

Table 7.8. Not all of the constants are required. If Henry's Law constant is unknown, it will be 

calculated internally from vapor pressure and solubility (provided in input). If Kvo is not

measured, it will be calculated internally from molecular weight and specified or computed

liquid film transfer coefficients. Volatilization is only allowed for surficial water column
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11-27

segments as identified by the segment type specified in input. The segment types are: 1) Surface 

water segments (Type 1), 2) Subsurface water segments (Type 2), Surficial sediment segments

(Type 3), and 4) subsurface sediment segments (Type 4). 

Transformation input parameters that must be specified by the user are given below for each

volatilization option. Constant numbers are listed in Table 7.9. Three constants should be input 

for all volatilization options: the volatilization option number, Henry's Law Constant, and the

atmospheric chemical concentration. Segment depths (from Data Group C) must be specified.

 VARIABLE C1 C2 C3 DEFINITION

 WTYPE 2 2 2 Water body type (0 = flowing; 1 = quiescent)

 AIRTMP 5 5 5 Multiplier for air temperature time function

 ATMOS 8 608 1208 Atmospheric concentration of chemical, ug/L

 MOLWT 81 681 1281 Molecular weight of chemical

 SOLG 82 682 1282 Solubility of chemical in water, mg/L

 VAPRG 83 683 1283 Vapor pressure of chemical, torr

 XV 136 736 1336 Volatilization option:

0 = none; 1 = measured; 2 = measured

reaeration + O'Connor; 3 = measured

reaeration + MacKay; 4 = calculated by

O'Connor; 5 = calculated by MacKay

 HENRY 137 737 1337 Henry's Law constant, atm-m3/mole

 KLT 138 738 1338 Volatilization temperature correction factor

 KVOG 139 739 1339 Measured ratio of volatilization to reaeration 

rate

Volatilization Option-- The user should chose the volatilization option using constant XV. 

Specifying a value of 0 will prevent volatilization from occurring. Values of 1 - 5 will invoke 

volatilization options 1 - 5 as outlined in the text above: (1) volatilization rates are input directly; 

(2) volatilization is computed from input reaeration rate constants and O'Connor's equation for

gas transfer; (3) volatilization is computed from input reaeration rate constants and MacKay's

equation for gas transfer; (4) in flowing systems, volatilization is computed using reaeration rates 

calculated from Covar's method and a gas transfer rate of 100 m/day; in quiescent systems,

volatilization is computed from O'Connor's equations for liquid and gas transfer; (5) in flowing

systems, volatilization is computed using reaeration rates calculated from Covar's method and a

gas transfer rate of 100 m/day; in quiescent systems, volatilization is computed from MacKay's

equations for liquid and gas transfer.
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11-28

Henry's Law Constant, atm-m

3

/mole-- The user should specify Henry's Law constant for airwater partitioning of the chemical using constant HENRY. 

Atmospheric Concentration, ug/L-- The user should specify the mean atmospheric concentration

of chemical using constant ATMOS. If this concentration is 0, then volatilization will always

cause a loss of chemical from the water body.

11.6.7. Volatilization Option 1

In this option, variable volatilization rate constants can be input directly.

Volatilization Rates, m/day-- When XV is set to 1, the user may then input segment and time 

variable volatilization rates using parameter 5, REAR, and time function 12, REARN. The

product of spatially-variable REAR and time-variable REARN gives the segment and time

specific volatilization rate constants used by TOXI. These volatilization values are not modified 

by a temperature function.

11.6.8. Volatilization Option 2

In this option, volatilization rates are calculated from user-input reaeration rate constants and

O'Connor's method for gas transfer. Input data required for option 2 are listed below. For

flowing systems, wind speed and air temperature are not used and may be omitted.

Water Body Type-- The user should specify the water body type using constant WTYPE. A

value of 0 indicates a flowing water body, such as a stream, river, or estuary. A value of 1 

indicates a quiescent water body, such as a pond, reservoir, or lake.

Reaeration Rates, m/day-- When XV is set to 2, the user may then input segment and time

variable reaeration rates using parameter 5, REAR, and time function 12, REARN. The product 

of spatially-variable REAR and time-variable REARN gives the segment and time specific

reaeration rate constants used by TOXI. These reaeration values are not modified by a

temperature function.

Ratio of Volatilization to Reaeration-- The user may specify an experimentally-measured ratio of

volatilization to reaeration using constant KVOG. If this constant is not given, the ratio will be 

calculated from molecular weight.

Molecular Weight g/mole-- The user may specify the molecular weight using constant MOLWT. 

This constant is used to calculate the ratio of volatilization to reaeration if an experimentallymeasured value is not provided. It is also used in the calculation of diffusivities.

Wind Speed, m/sec-- The user may specify the segment and time variable wind speed using

parameter 4, WVEL, and time function 9, WINDN. The product of spatially-variable WVEL
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11-29

and time-variable WINDN gives the segment and time specific reaeration rate constants used by 

TOXI. Wind speed should be measured at 10 m height above the water surface.

Air Temperature, °C-- The user may specify time-variable air temperature using constant

AIRTMP and time function 13, AIRTMPN. The ambient air temperature is calculated as the

product of AIRTMP and AIRTMPN. For a constant air temperature, AIRTMPN can be omitted. 

For variable air temperatures, the user should set AIRTMP to 1.0 and input a series of air

temperature versus time values via AIRTMPN.

11.6.9. Volatilization Option 3

In this option, volatilization rates are calculated from user-input reaeration rate constants and

MacKay's method for gas transfer. Input data required for the same as for option 2, listed above. 

For flowing systems, wind speed and air temperature are not used and may be omitted.

11.6.10. Volatilization Option 4

In this option, volatilization rates in flowing systems are calculated using reaeration rates

calculated from Covar's method and a gas transfer rate of 100 m/day. In quiescent systems,

volatilization is computed from O'Connor's equations for liquid and gas transfer. Input data

required for option 4 are listed below. For flowing systems, wind speed and air temperature are 

not used and may be omitted. For quiescent systems, water velocity may be omitted.

Water Velocity, m/sec-- Variable current velocities are calculated from flow using hydraulic

geometry coefficients as described in Chapter 2. For most situations, no further input is required 

from the user. If an estuary is being simulated under tidal-average conditions, however, the net 

flows do not provide realistic ambient water velocities for use in volatilization calculations. In

this case, the user should enter time and segment variable water velocities using parameter 1,

VELFN and time functions 5-8, VELN(1-4).

 The parameter VELFN indicates which velocity function will be used by the model for each

segment. Values of 1.0, 2.0, 3.0, or 4.0 will call time functions VELN(1), VELN(2), VELN(3), 

and VELN(4), respectively. Water velocities should then be entered via these time functions as a 

series of velocity versus time values. 

Water Body Type-- see Option 2 above.

Ratio of Volatilization to Reaeration-- see Option 2 above.

Molecular Weight g/mole-- see Option 2 above.

Wind Speed, m/sec-- see Option 2 above.
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11-30

Air Temperature, C-- see Option 2 above.

11.6.11. Volatilization Option 5

In this option, volatilization rates in flowing systems are calculated using reaeration rates

calculated from Covar's method and a gas transfer rate of 100 m/day. In quiescent systems,

volatilization is computed from MacKay's equations for liquid and gas transfer. Input data

required for option 5 are the same as for option 4 above. For flowing systems, wind speed and 

air temperature are not used and may be omitted. For quiescent systems, water velocity may be 

omitted.

11.7. Hydrolysis

Hydrolysis, or reaction of the chemical with water, is known to be a major pathway for

degradation of many toxic organics. Hydrolysis is a reaction in which cleavage of a molecular

bond of the chemical and formation of a new bond with either the hydrogen or the hydroxyl

component of a water molecule occurs. Hydrolytic reactions are usually catalyzed by acid

and/or base and the overriding factor affecting hydrolysis rates at a given temperature is

generally hydrogen or hydroxide ion concentration (Wolfe, 1980). An example reaction is

shown in 8. The reaction can be catalyzed by hydrogen ions or proceed by consuming hydroxide 

ions. 8 illustrates the effects of base hydrolysis on carbaryl, neutral hydrolysis on

chloromethane, and acid and base hydrolysis on 2,4-D.

11.7.1. Overview of TOXI Hydrolysis Reactions

Hydrolysis may be simulated by TOXI using simple decay. Alternatively, hydrolysis can be

simulated using rates that are first order for the neutral chemical and second order for its ionic

forms. The second order rates are pH and temperature dependant. 

11.7.2. Option 1. First Order Hydrolysis.

Under this option, the user inputs a first order rate constant for either neutral, alkaline, or acid 

hydrolysis. The first order rate term constant is then applied to the total chemical concentration

(see Section 6.3). 

11.7.3. Option 2. Second Order Hydrolysis.

Under this option, hydrolysis by specific-acid-catalyzed, neutral, or base pathways is considered 

for the various species and phases of each chemical. The reactions are first order for the neutral 

chemical and second order for the acidic or basic forms of the chemical:
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-31

Equation 11-58

K = k f nij ij

i j

HN ∑∑

Equation 11-59

K = k [ H ] f

ij

+

aij

i j

HH ∑ ∑

Equation 11-60

K = k [OH ] f

ij

-

bij

i j

HOH ∑∑

where:

KHN = net neutral hydrolysis rate constant, day-1

KHH = net acid catalyzed hydrolysis rate constant, day-1

KHOH = net base catalyzed hydrolysis rate constant, day-1

kaij,kbij = specific acid catalyzed and base rate constants for ionic specie 

i in phase j, respectively, molar-1 day-1

knij = neutral rate constant for ionic specie i in phase j, day-1

fij = fraction of chemical as ionic specie i in phase j

The rates are also affected by temperature. TOXI adjusts the rates using the temperature-based 

Arrhenius function

Equation 11-61

k(T k)= k (T R)exp [1000 E aH (T K - T R) /( RT k T R)]

where:

TK = water temperature, °K

TR = reference temperature for which reaction rate is reported, °K

EaH = Arrhenius activation energy for hydrolysis reaction, kcal/mole 

°K

R = 1.99 cal/mole °K

1000 = cal/kcal

Implementation
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-32

Description Notation Range Units

Negative log of hydrogen ion activity [H+] pH 5-9 -

Acid hydrolysis rate constant for specie i,

phase j

kHAij 0-107

Neutral hydrolysis rate constant for specie i, 

phase j

kHNij 0-102 day-1

Base hydrolysis rate constant for specie i,

phase j

kHBij 0-107

Water temperature T 4-30 °C

Activation energy for hydrolysis reaction for 

specie i

EaHi 15-25

TOXI hydrolysis data specifications are summarized in Table 7.10. In addition, the simple first 

order rates may be specified as described under Option 1 and the section on simple TOXI

reactions. If no hydrolysis data are input, then the effect of hydrolysis will not be included in

simulations.

Option 1.

Under this option, the user inputs one or more of the following: an acid, neutral, and base

hydrolysis rate constant. 

First-Order Hydrolysis Rate Constants, day-1-- The user may input overall base, neutral, and acid 

hydrolysis rate constants using constants 181, 182, and 183 for chemical 1, constants 781, 782, 

and 783 for chemical 2, and constants 1381, 1382, and 1383 for chemical 3. The rates are first 

order, and are applied to the total chemical. If any one of these first order rates are specified in 

input they will be used regardless of whether other hydrolysis constants are specified.

Option 2. 

PHASE "i" or

REACTION "k"

C1 C2 C3 INPUT

 VARIABLE

all 184 784 1384 Reference

 Temperature

 TREFH, °C

aqueous 186 786 1386 Alkaline 

 Hydrolysis

 KH2O1,i,1

 M-1day-1

DOC 191 791 1391

sediment 196 796 1396

aqueous 201 801 1401 Neutral

 Hydrolysis

 KH2O2,i,1

 day-1

DOC 206 806 1406

sediment 211 811 1411

aqueous 216 816 1416 Acid 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-33

 Hydrolysis

 KH2O3,i,1

 M-1day-1

DOC 221 821 1421

sediment 226 826 1426

Alkaline 231 831 1431 Activation Energy Eak, 

kcal/mole °K

Neutral 236 836 1436

Acid 241 841 1441

Under this option, the reaction coefficients can be specified as constants. If the chemical

simulated does not ionize (as controlled by input of the ionization constants), then acid, base and 

neutral hydrolysis constants may be input for the dissolved, DOC sorbed and sediment sorbed

phases of the chemical, as summarized in Table 7.11. If ionization of the chemical is allowed, 

then constants may be input for the dissolved, DOC sorbed and sediment sorbed phases of each 

ionic specie simulated. In addition, the pH must be supplied in order to compute acid and base 

hydrolysis. The pH is input as a parameter, which must be specified for each model segment and 

may be constant or time variable. Separate pH time functions may be specified for surface water 

and benthic segments.

If the user wants TOXI to adjust the rates based on temperature, then non-zero activation

energies should be specified which would invoke the temperature-based Arrhenius function. 

Activation energies may be specified for each ionic specie and each hydrolysis reaction (acid,

neutral, base) simulated. If no activation energies are given, then rates constants will not be

adjusted to ambient water temperatures. 

Base Hydrolysis Rate Constants, M-1day-1-- The user may specify second order base hydrolysis 

rate constants for each phase (dissolved, DOC-sorbed, and sediment-sorbed) and each ionic

specie using constant KH20. Constant numbers for the neutral molecule are summarized in

Table 7.11. KH201,1,1 refers to the dissolved neutral chemical; KH201,2,1 refers to the DOCsorbed neutral chemical; KH201,3,1 refers to the sediment-sorbed neutral chemical. 

Neutral Hydrolysis Rate Constants, day-1-- The user may specify first order neutral hydrolysis

rate constants for each phase (dissolved, DOC-sorbed, and sediment-sorbed) and each ionic

specie using constant KH20. Constant numbers for the neutral molecule are summarized in

Table 7.11. KH202,1,1 refers to the dissolved neutral chemical; KH202,2,1 refers to the DOCsorbed neutral chemical; KH202,3,1 refers to the sediment-sorbed neutral chemical. 

Acid-Catalyzed Hydrolysis Rate Constants, M-1day-1-- The user may specify second order acidcatalyzed hydrolysis rate constants for each phase (dissolved, DOC-sorbed, and sedimentsorbed) and each ionic specie using constant KH20. Constant numbers for the neutral molecule 

are summarized in Table 7.11. KH203,1,1 refers to the dissolved neutral chemical; KH203,2,1

refers to the DOC-sorbed neutral chemical; KH203,3,1 refers to the sediment-sorbed neutral

chemical. 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-34

Arrhenius Activation Energy, kcal/mole-°K-- The user may specify activation energies for each

chemical using constant EHOH. Constant numbers are summarized in Table 7.11. If EHOH is 

omitted or set to 0, hydrolysis rates will not be affected by temperature.

Reference Temperature,°C-- The user may specify the reference temperature at which hydrolysis 

rates were measured using constant TREFH. Constant numbers are summarized in Table 7.11. 

If a reference temperature is not supplied, then a default of 20 °C is assumed.

pH-- The user may specify time and segment variable pH values using parameter 11, PH, and 

time functions 10 and 11, PHNW and PHNS. The pH in a water segment will be the product of 

PH and PHNW; the pH in a benthic segment will be the product of PH and PHNS. For constant 

pH, the user should enter values via parameter PH. Time functions should be omitted. For time 

variable pH, the user should enter a series of pH versus time values via PHNW and PHNS. The 

parameter PH values will then represent the ratio of pH in each segment to the time function.

11.8. Photolysis

Photodegradation (photolysis) is the transformation or degradation of a compound that results

directly from the adsorption of light energy. An example of several photochemical pathways is

given in 8. It is a function of the quantity and wavelength distribution of incident light, the light 

adsorption characteristics of the compound, and the efficiency at which absorbed light produces

a chemical reaction. Photolysis is classified into two types that are defined by the mechanism of 

energy absorption. Direct photolysis is the result of direct absorption of photons by the toxic

chemical molecule. Indirect or sensitized photolysis is the result of energy transfer to the toxic

chemical from some other molecule that has absorbed the radiation.

11.8.1. Overview of TOXI Photolysis Reactions

Photolysis is the transformation of a chemical due to absorption of light energy. The first order 

rate coefficient for photolysis can be calculated from the absorption rate and the quantum yield

for each ionic specie and phase:

Equation 11-62

K = k f ai ij ij

i j

pG ∑ ∑ f

where:

KpG = first order photolysis rate coefficient at ambient light

intensity, day-1

kai = specific sunlight absorption rate for specie i, E/mole-day or 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-35

(E/L)/(mole/L)/day

Öij = reaction quantum yield for specie i in phase j, mole/E

fij = fraction of chemical as specie i in phase j

The user may specify that the model calculate the first order photolysis rate constant or the user 

may provide a near water surface rate (for presumed cloudless conditions). If the user- supplied 

rate constant is representative of conditions at a location other than the water body being

modeled, the model corrects the rate for the difference in latitude between the two and any

difference in cloud cover. The options for computing the losses due to photolysis are briefly

described below.

11.8.2. Photolysis Option 1.

Under this option, the photolysis rate is calculated from molar absorptivities, calculated light

intensity, and quantum yield of the chemical. To calculate the rate constant, TOXI divides the 

wavelength spectrum between 280 and 800 nm into 46 intervals. For each interval the user must 

specify a molar absorptivity. The light intensity at each of the 46 wavelengths is internally

calculated from the location of the water body (i.e., latitude), the time of year, and the

atmospheric conditions (air mass type, relative humidity, atmospheric turbidity and ozone

content, cloudiness). The location and time of year are used to define the light intensity at the 

outer edge of the atmosphere. The atmospheric conditions are used to define the light decay

through the atmosphere. The light intensities and the molar absorptivities are used with a user

defined optical path (d) to calculate the specific sunlight absorption rate. The first order rate

constant is then calculated using equation 7.63. This calculation was taken directly from

EXAMS II (Burns and Cline, 1985) and is based on formulations published by Green, Cross and 

Smith (1980).

The specific sunlight absorption rate is the integral or summation over all bandwidths of the

average light multiplied by the molar absorptivity and the optical path:

Equation 11-63

k = I d (2303)(86400) / (6.022 x 10 )

23

Gk ki

k

ai ∑ e

where:

IGk = average light intensity of wavelength k, photons/cm2-sec

åki = decadic molar absorptivity of wavelength k by specie 

i, L/mole-cm-ln 10

d = ratio of the optical path to the vertical path, cm/cm

2303 = (cm3/L) (ln 10/ln e)

86400 = sec/day

6.022 x 1023 = Avagadro's number, photons/E
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-36

Light extinction is calculated with the integrated Beer-Lambert formulation for each wavelength

k:

Equation 11-64

d K D

1- (- d K D)

=

I

I

e

e

ok

Gk exp

where:

Iok = light intensity of wavelength k just below water surface, 

photons/cm2-sec

Ke = spatially variable light extinction coefficient, m-1

D = depth of water segment, m

Specific Light Extinction Coefficients

Number Wavelength

Pure Water

 l/m

Chlorophyll

 L/gm-m

 DOC

L/mg-m

Solids

L/mg-m

1 280.0 0.288 145 7.90 0.34

2 282.5 0.268 138 7.65 0.34

3 285.0 0.249 132 7.41 0.34

4 287.5 0.231 126 7.11 0.34

5 290.0 0.215 120 6.95 0.34

6 292.5 0.194 115 6.73 0.34

7 295.0 0.174 109 6.52 0.34

8 297.5 0.157 106 6.30 0.34

9 300.0 0.141 101 6.12 0.34

10 302.5 0.133 95 5.94 0.34

11 305.0 0.126 90 5.76 0.34

12 307.5 0.119 85 5.57 0.34

13 310.0 0.105 80 5.39 0.34

14 312.5 0.0994 78 5.22 0.34

15 315.0 0.0952 75 5.06 0.34

16 317.5 0.0903 72 4.90 0.34

17 320.0 0.0844 70 4.74 0.34

18 323.1 0.0793 68 4.56 0.34

19 330.0 0.0678 64 4.17 0.34

20 340.0 0.0561 59 3.64 0.34

21 350.0 0.0463 55 3.15 0.34

22 360.0 0.0379 55 2.74 0.34

23 370.0 0.0300 51 2.34 0.34

Specific Light Extinction Coefficients

Number Wavelength

Pure Water

 l/m

Chlorophyll

 L/gm-m

 DOC

L/mg-m

Solids

L/mg-m

24 380.0 0.0220 46 2.00 0.34

25 390.0 0.0191 42 1.64 0.34

26 400.0 0.0171 41 1.39 0.34

27 410.0 0.0162 39 1.19 0.34

28 420.0 0.0153 38 1.02 0.34

29 430.0 0.0144 35 0.870 0.34
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-37

Specific Light Extinction Coefficients

Number Wavelength

Pure Water

 l/m

Chlorophyll

 L/gm-m

 DOC

L/mg-m

Solids

L/mg-m

1 280.0 0.288 145 7.90 0.34

30 440.0 0.0145 32 0.753 0.34

31 450.0 0.0145 31 0.654 0.34

32 460.0 0.0156 28 0.573 0.34

33 470.0 0.0156 26 0.504 0.34

34 480.0 0.0176 24 0.444 0.34

35 490.0 0.0196 22 0.396 0.34

36 503.75 0.0295 19 0.357 0.34

37 525.0 0.0492 14 0.282 0.34

38 550.0 0.0638 10 0.228 0.34

39 575.0 0.0940 8 0.188 0.34

40 600.0 0.244 6 0.158 0.34

41 625.0 0.314 5 0.0 0.34

42 650.0 0.349 8 0.0 0.34

43 675.0 0.440 13 0.0 0.34

44 706.25 0.768 3 0.0 0.34

45 750.0 2.47 2 0.0 0.34

46 800.0 2.07 0 0.0 0.34

Iok is calculated for each wavelength based upon the time of year, latitude, ground elevation,

cloud cover, air mass type, relative humidity, atmospheric turbidity, and ozone content. The

atmospheric characteristics can vary monthly, or be specified as an annual average. The value of 

d, the ratio of the optical path to the vertical depth is difficult to compute, but a probable best 

value is 1.19 (Hutchinson, 1967). However, in the presence of a large concentration of scattering 

particles, it may approach 2.0. In order to ensure that an improper value is not loaded and used 

in computations, the input value is checked and set to 1.19 if the input is invalid. 

The photolysis rate constants for each water column segment are determined from the calculated 

near-surface rate constant and the rate of light decay in the water column (Ke). The value of Ke

is calculated for each wavelength based on a formulation taken from EXAMS II:

Equation 11-65

K e= K ew+h1CHL+h2 DOC +h3 m

where

Kew = pure water extinction coefficient, 1/m

CHL = phytoplankton chlorophyll concentration, mg/L

DOC = dissolved organic carbon concentration, mg/L

m = solids concentration, mg/L

ç1,ç2,ç3 = specific extinction coefficients, L/mg-m
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-38

Values of Kew, ç1, ç2, ç3 for each of the 46 wavelengths are supplied in the program as data 

statements in subroutine BEER and are shown in Tables 7.12 and 7.13. Segment average

photolysis rate constants are computed for each wavelength and then summed to yield an overall 

rate. 

11.8.3. Photolysis Option 2.

Under this option, a reference surface sunlight absorption rate kaRi (E/mole-day) is input by the 

user for each specie simulated. As with EXAMSII, the input rate is then adjusted as shown

below 

Equation 11-66

k = k I

( I G / I o)( 1- 0.056 C ) X L

,

aRi o

i j

ai ∑∑

where 

Io

, = user specified normalized light intensity time function, which 

is the ratio of ambient light intensity to the reference light intensity.

C = cloud cover (in tenths, 0-10)

XL = latitude correction factor, calculated by:

Equation 11-67













19169.65+87054.63 (0.039 L )

19169.65+87054.63 (0.039 L)

X =

Rf

L

cos

cos

where

L = latitude of the waterbody

LRf = reference latitude at which the surface photolysis rate was 

measured. 

The average light intensity attenuation, IG/Io, is computed as above from the Beer-Lambert 

formulation (equation 7.65). Therefore, the light intensity has a value for each model segment

ranging from zero to one.

The extinction coefficient may be directly specified as a model parameter, which may be varied 

by model segment. If the extinction coefficient is not specified, it is determined from a userspecified wavelength of maximum light absorption for the particular chemical species (neutral,

anionic or cationic) using equation 7.66 and the values listed in Tables 7.12 and 7.13. If the

wavelength of maximum absorption is outside of the relevant spectral range (280-825 nm) then

TOXI assumes a wavelength of 300 nm. 
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

11-39

After adjusting the reference sunlight absorption rate to ambient conditions, the first order

photolysis rate is computed from these and reaction quantum yields following equation 7.63. 

Photolysis option 2 is often implemented using reference first order photolysis rate constants

rather than reference sunlight absorption rates. If reference first order rate constants are input for 

kaRi, then equation 7.67 calculates kai as first order rate constants (day-1) adjusted to ambient light 

conditions. The overall first order photolysis rate constant is then calculated following equation

7.63 where quantum yields are set to 1.0.

Description Notation Range Units

Observed rate constant for a chemical at reference

light intensity IR

KpR 0 - 10 day-1

Observed sunlight absorption rate for a chemical at 

reference light intensity IR

kaR ? E/mole-day

Reference light intensity causing photolysis rate KpR or 

absorption rate kaR

IR 10-7-2x10-6 E/cm2-sec

Ratio of surface light intensity to reference light

intensity (Io/IR)

Io

, 0 - 10 -

Light extinction coefficient in water column Ke 0.1 - 5 m-1

Chlorophyll a concentration CHL 10-3 - 10-1 mg/L

Dissolved organic carbon DOC 0 - 10 mg/L

Depth of water column segment D 0.1 - 10 m

Reaction quantum yield fraction for specie i in phase j Öij 0 - 0.5 moles/E

Molar absorptivity by wavelength k by specie i åki 0 - ? L/mole-cm-ln 10

Waterbody elevation ELEVG 0 - 5000 m

Waterbody latitude L 0 - 90 degrees

Reference latitude LRf 0 - 90 degrees

Cloud cover, fraction of sky CG 0 - 10 tenths

Air type (rural, urban, maritime, or tropospheric) AIRTYG 1 - 4 -

Relative humidity RHUMG 0 - 100 percent

Atmospheric turbidity, in equivalent aerosol layer

thickness

ATURBG 0 - ? km

Ozone content OZONEG 0 - ? cm NTP

Implementation 

The TOXI photolysis data specifications are summarized in Table 7.14. In addition, an overall

first-order rate constant may be supplied by the user for each chemical, as presented in Chapter 

6. If the overall first order rate constant is specified, it will be used regardless of other input 

specifications. For the photolysis computations described in this chapter, input requirements are

described below.

11.8.4. Photolysis Option 1

In option 1, TOXI computes the sunlight absorption and the surface photolytic decay rate.

Photolysis Option-- The user should select the photolysis option using constant XPHOTO: 0 = 

no photolysis; 1 = photolysis rates will be computed from molar absorptivity; 2 = photolysis
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rates will be extrapolated from measured surface rates. Use constant numbers 286, 886, and 

1486 for chemicals 1, 2, and 3, respectively.

Molar Absorptivity, L/mole-cm-ln10-- The user may specify molar absorptivity values for each

ionic specie over 46 wavelengths using constant ABS. The wavelengths by number are listed in

Tables 7.12 and 7.13. Absorptivity values for each ionic specie apply across all phases (aqueous, 

DOC-sorbed, sediment-sorbed). Constant numbers for the neutral ionic specie are summarized

in Table 7.15. 

Quantum Yield, moles/einstein-- The user may specify reaction quantum yield values for each

phase (dissolved, DOC-sorbed, sediment-sorbed) and each ionic specie using constant

QUANTG. Constant numbers for the neutral molecule are summarized in Table 7.15. 

QUANTG11 refers to the dissolved neutral chemical; QUANTG21 refers to the DOC-sorbed 

neutral chemical; QUANTG31 refers to the sediment-sorbed neutral chemical.

Julian Date-- The user should specify the Julian date for the beginning of the simulation using

constant 1 - TO.

Elevation, m-- The user should specify the average ground 

VARIABLE C1 C2 C3

ABS1,L 301-346 901 - 946 1501-1546

QUANTG11 551 1151 1751

QUANTG21 556 1156 1756

QUANTG31 561 1161 1761

elevation using constant 3 - ELEVG.

Latitude, degrees and tenths-- The user should specify the latitude of the waterbody using

constant 4 - LATG.

Light Option-- Using constant 6 - XLITE, the user has a choice of options controlling how 

TOXI computes and uses light intensity: 0 = do not compute light; 1 = compute annual average 

light intensity; 2 = compute average light intensity for the month indicated by TO; 3 = compute 

monthly light intensity as a step function.

Optical Path-- The user may specify the ratio of the optical path to the vertical depth using

constant 7 - DFACG. A default value of 1.17 is assumed.
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Cloud Cover, tenths-- The user should specify the mean monthly or annual average cloud cover 

using constant CLOUDG. Monthly values can be entered using constant numbers 11-22; the 

annual average can be entered using number 23.

Air Type-- The user should specify the mean air mass type using constant AIRTYG. Values of

1, 2, 3, or 4 will select rural, urban, maritime, or tropospheric, respectively. Monthly values can

be entered using constant numbers 24-35; the annual average can be entered using number 36.

Relative Humidity, percent-- The user should specify the mean monthly daylight relative

humidity using constant RHUMG. Monthly values can be entered using constant numbers 37-

48; the annual average can be entered using number 49.

Atmospheric Turbidity, km-- The user should specify the mean atmospheric turbidity (in

equivalent aerosol layer thickness, km) using constant ATURBG. Monthly values can be

entered using constant numbers 50-61; the annual average can be entered using number 62.

Ozone Content, cm NTP-- The user should specify the mean ozone content (cm NTP) using

constant OZONEG. Monthly values can be entered using constant numbers 63-74; the annual

average can be entered using number 75.

Dissolved Organic Carbon, mg/L-- The user may specify segment variable dissolved organic

carbon concentrations using parameter 6 - DOC. (Group G, Record 4, PARAM(I,6))

Chlorophyll a, mg/L-- Time and segment variable phytoplankton chlorophyll a concentrations

can be specified using parameter 10, CHPHL and time function 14, CHLN. If chlorophyll

concentrations are to remain constant in time, the user should enter segment mean concentrations 

using parameter CHPHL. CHLN should be omitted.

The user may enter time-variable chlorophyll a concentrations via time function CHLN as a

series of concentration versus time values. Parameter CHPHL will then represent the ratio of

each segment concentration to the time function values. The product of CHPHL and the CHLN 

function gives the segment and time specific chlorophyll concentrations used by TOXI. (Group

G, Record 4, PARAM(I,10); Group I, Record 2, VALT(14,K)

11.8.5. Photolysis Option 2

In option 2, TOXI extrapolates either observed sunlight absorption rates or photolytic decay rates 

under "reference" conditions to ambient conditions. Required input data are described below.

Photolysis Option-- The user should select the photolysis option using constant XPHOTO: 0 = 

no photolysis; 1 = photolysis rates will be computed from molar absorptivity; 2 = photolysis

rates will be extrapolated from measured surface rates. Use constant numbers 286, 886, and

1486 for chemicals 1, 2, and 3, respectively.
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Measured Photolysis Rate, day-1-- The user may specify the measured photolysis rate constant 

under reference conditions using constant KDPG. Values for the neutral molecule of chemicals

1, 2, and 3 can be entered using constants 291, 891, and 1491, respectively. Separate values can 

be entered for each ionic specie. If a reference first order rate constant is input, the quantum

yield should be set to 1.0.

Measured Sunlight Absorption Rate, einstein/mole-day-- The user may specify measured

sunlight absorption rates under reference conditions using constant KDPG. Values for the

neutral molecule of chemicals 1, 2, and 3 can be entered using constants 291, 891, and 1491, 

respectively. Separate values can be entered for each ionic specie. If a reference sunlight

absorption rate is input, the corresponding quantum yield must be specified.

Quantum Yield, moles/einstein-- The user may specify reaction quantum yield values for each

phase (dissolved, DOC-sorbed, sediment-sorbed) and each ionic specie using constant

QUANTG. Constant numbers for the neutral molecule are summarized in Table 7.15. 

QUANTG11 refers to the dissolved neutral chemical; QUANTG21 refers to the DOC-sorbed 

neutral chemical; QUANTG31 refers to the sediment-sorbed neutral chemical. Separate values

can be entered for each ionic specie.

Reference Latitude, degree and tenths-- The user may specify the latitude at which the reference 

surface water photolytic rates were measured using constant RFLATG. Values for chemicals 1, 

2, and 3 can be entered using constant numbers 288, 888, and 1488, respectively. 

Maximum Absorption Wavelength, nm-- The user should specify the wavelength of maximum

absorption using constant LAMAXG. Values for the neutral specie of chemicals 1, 2, and 3 can 

be entered using constants 296, 896, and 1496, respectively. Separate values can be entered for 

each ionic specie.

Latitude, degrees and tenths-- The user should specify the latitude of the waterbody using

constant 4 - LATG.

Cloud Cover, tenths-- The user should specify the mean monthly or annual average cloud cover 

using constant CLOUDG. Monthly values can be entered using constant numbers 11-22; the 

annual average can be entered using number 23.

Light Intensity-- The user can specify time-variable normalized light intensity (dimensionless)

using time function 15, PHTON. This function is used to adjust the measured rate constant

under controlled reference light intensity to a predicted rate constant under ambient light

intensity. The default value for this function is 1.0.

Light Extinction Coefficient, m

-1

-- The user can specify segment light extinction coefficients for

the photochemically active light using parameter 12, XKE2. When this number is zero, the

extinction coefficients are calculated from solids, DOC and chlorophyll a concentrations for the 

wavelength of maximum absorption. DOC and chlorophyll a are specified as model parameters 
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which may vary between segments and over time. Their input is describe in the Photolysis

Option 1 section above. Light is set to zero under ice cover, which is assumed when water

temperatures reach 0 °C.

11.9. Oxidation

Chemical oxidation of organic toxicants in aquatic systems can be a consequence of interactions 

between free radicals and the pollutants. Free radicals can be formed as a result of

photochemical reactions. Free radicals that have received some attention in the literature include 

alkylperoxy radicals, RO2.; OH radicals; and singlet oxygen.

11.9.1. Overview of TOXI Oxidation Reactions

In TOXI, oxidation is modeled as a general second-order process for the various species and 

phases of each chemical:

Equation 11-68

K = [ RO ] k f oij ij

i j

o 2 ∑∑

where:

Ko = net oxidation rate constant, day-1

[RO2] = molar concentration of oxidant, moles/L

koij = second order oxidation rate constant for chemical as specie i 

in phase j, L/mole-day

The reaction coefficients may be specified as constants, with activation energy constants left as

0. If the user wants TOXI to determine rates based on the temperature based Arrhenius function, 

then non-zero activation energies specified, as constants will invoke the following calculation for 

each rate constant k:

Equation 11-69

k(T K )= k(T R)exp [1000 E ao (T K -T R)/( RT K T R)]

where:

Eao = Arrhenius activation energy for oxidation reaction, kcal/mole-°K

Activation energies may be specified for each ionic specie simulated. If no activation energies

are given, then rate constants will not be adjusted to ambient water temperatures.
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Because of the large number of alkylperoxy radicals that potentially exist in the environment, it

would be impossible to obtain estimates of kox for each species. Mill et al. (1982) propose

estimation of a rate coefficient using t-butyl hydroperoxide as a model oxidizing agent. They

argue that other alkylperoxides exhibit similar reactivities to within an order of magnitude. The 

second-order rate coefficients are input to TOXI as constants.

In addition to estimating a rate coefficient, an estimate of free radical concentrations must be

made to completely define the expression for free radical oxidation. Mill et al. (1982) report 

RO2 concentrations on the order of 10-9 M and OH concentrations on the order of 10-17 M for a 

limited number of water bodies. Zepp and Cline (1977) report an average value on the order of 

10-12 M for singlet oxygen in water bodies sampled. The source of free radicals in natural waters 

is photolysis of naturally occurring organic molecules. If a water body is turbid or very deep,

free radicals are likely to be generated only near the air-water interface, and consequently,

chemical oxidation will be relatively less important. In such cases, the concentrations cited

above are appropriate in only the near-surface zones of water bodies. The molar oxidant

concentrations are input to TOXI using parameter OXRADG (ISEG). 

Implementation

Description Notation Range Units

Oxidation rate constant for specie i, phase j koij L/mole-day

Activation energy for oxidation of specie i Eaoi 15-25 kcal/mole °K

Water temperature T 4-30 °C

Concentration of oxidants [RO2] 10-17-10-8 moles/L

TOXI oxidation data specifications are summarized in Table 7.16. The water temperature and

concentration of oxidants are input parameters, which may be specified for each model segment. 

The temperature may be time variable as well (input as a time series). If an activation energy is 

not supplied, no temperature corrections will be performed. Input data are described below.

VARIABLE C1 C2 C3

TREFO 258 858 1458

KOX2011 261 861 1461

KOX2021 266 866 1466

KOX2031 271 871 1471

 EOX1 276 876 1476

Oxidation Rate, L/mole-day-- The user may specify second order oxidation rate constants for

each phase (dissolved, DOC-sorbed, and sediment-sorbed) and each ionic specie using constant 

KOX20. Constant numbers for the neutral molecule are summarized in Table 7.17. KOX2011

refers to the dissolved neutral chemical; KOX2021 refers to the DOC-sorbed neutral chemical; 

KOX2031 refers to the sediment-sorbed neutral chemical. 

Activation Energy, kcal/mole-°K-- The user may specify activation energies for each chemical

using constant EOX. Constant numbers are summarized in Table 7.17. If EOX is omitted or set 

to 0, oxidation rates will not be affected by temperature.
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Reference Temperature,°C-- The user may specify the reference temperature at which oxidation

rates were measured using constant TREFO. Constant numbers are summarized in Table 7.17. 

If a reference temperature is not supplied, then a default of 20 °C is assumed.

Oxidant Concentration, mole/L-- The user should specify segment variable oxidant

concentrations using parameter 13, OXRAD. (Group G, Record 4, PARAM(I,13)

11.10. Biodegradation

Bacterial degradation, sometimes referred to as microbial transformation, biodegradation or

biolysis, is the breakdown of a compound by the enzyme systems in bacteria. Examples are

given in 8. Although these transformations can detoxify and mineralize toxins and defuse

potential toxins, they can also activate potential toxins.

Biodegradation encompasses the broad and complex processes of enzymatic attack by organisms 

on organic chemicals. Bacteria, and to a lesser extent fungi, are the mediators of biological

degradation in surface water systems. Dehalogenation, dealkylation, hydrolysis, oxidation, 

reduction, ring cleavage, and condensation reactions are all known to occur either metabolically

or via organisms that are not capable of utilizing the chemical as a substrate for growth. 

Two general types of biodegradation are recognized--growth metabolism and cometabolism. 

Growth metabolism occurs when the organic compound serves as a food source for the bacteria. 

Adaptation times from 2 to 20 days were suggested in Mills et al., 1985. Adaptation may not be 

required for some chemicals or in chronically exposed environments. Adaptation times may be 

lengthy in environments with a low initial density of degraders (Mills et al., 1985). For cases

where biodegradation is limited by the degrader population size, adaptation is faster for high

initial microbial populations and slower for low initial populations. Following adaptation,

biodegradation proceeds at fast first-order rates. Cometabolism occurs when the organic

compound is not a food source for the bacteria. Adaptation is seldom necessary, and the

transformation rates are slow compared with growth metabolism.

The growth kinetics of the bacterial population degrading a toxic chemical are not well

understood. The presence of competing substrates and of other bacteria, the toxicity of the

chemical to the degrading bacteria, and the possibilities of adaptation to the chemical or cometabolism make quantification of changes in the population difficult. As a result, toxic

chemical models assume a constant biological activity rather than modeling the bacteria directly. 

Often, measured first order biodegradation rate constants from other aquatic systems are used

directly.
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11.10.1. Overview of TOXI Biodegradation Reactions

In TOXI, first order biodegradation rate constants or half lives for the water column and the

benthos may be specified. If these rate constants have been measured under similar conditions, 

this first order approach is likely to be as accurate as more complicated approaches. If first order 

rates are unavailable, or if they must be extrapolated to different bacterial conditions, then the

second-order approach may be used. It is assumed that bacterial populations are unaffected by 

the presence of the compound at low concentrations. Second-order kinetics for dissolved, DOCsorbed, and sediment-sorbed chemical are considered:

Equation 11-70

K = P (t) k f j =1,2 Bij ij

i j

Bw bac ∑∑

Equation 11-71

K = P (t) k f j = 3 Bij ij

i j

Bs bac ∑∑

where:

KBw = net biodegradation rate constant in water, day-1

KBs = net biodegradation rate constant on sediment, day-1

kBij = second order biodegradation rate constant for specie 

i, phase j, ml/cell-day

Pbac(t) = active bacterial population density in segment, cell/ml

fij = fraction of chemical as specie i in phase j

In TOXI, the biodegradation rate may be adjusted by temperature as shown below

Equation 11-72

k (T)=k Q

(T-20)/10

Bij Bij Tij

where:

QTij = "Q-10" temperature correction factor for biodegradation of 

specie i, phase j

T = ambient temperature in segment, °C

The temperature correction factors represent the increase in the biodegradation rate constants

resulting from a 10°C temperature increase. Values in the range of 1.5 to 2 are common.
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Environmental factors other than temperature and population size can limit bacterial rates. 

Potential reduction factors must be considered externally by the user. Nutrient limitation can be 

important in oligotrophic environments.

Low concentrations of dissolved oxygen can also cause reductions in biodegradation rates and

this effect is not simulated in TOXI. Below DO concentrations of about 1 mg/L, the rates start to 

decrease. When anoxic conditions prevail, most organic substances are biodegraded more

slowly. Because biodegradation reactions are generally more difficult to predict than physical

and chemical reactions, site-specific calibration becomes more important. 

Biodegradation can be implemented using segment variable first order rate constants rather than 

bacterial populations. If first order rate constants are input for Pbac, then second order rate

constants kBij should be set to 1.0 in equations 7.71 and 7.72.

Implementation

TOXI biodegradation data specifications are summarized in Table 7.18. The second order rate 

constants for water and for bed segments can be specified as constants. Temperature correction

factors can be left at 0. If the user wants TOXI to correct the rate constants for ambient segment 

temperatures, then nonzero temperature correction factors should specified as constants. User

input for implementing biodegradation is given below.

Description Notation

 Common

 Range Units

Observed first order degradation rate in water column KBw 0-0.5 day-1

Observed first order degradation rate in benthos KBs 0-0.5 day-1

Bacterial activity or concentration of bacterial agent Pbac 102-107 cells/mL

Observed second-order rate coefficients for specie i kBij 0-10-6 mL/cell- day

Biodegradation temperature coefficients for specie i, phase j QTij 1.5-2.5 -

Water temperature T 4-30 °C

VARIABLE C1 C2 C3

KBW 141 741 1341

KBS 142 742 1342

KBIO2O11 146 746 1346

KBIO2021 151 751 1351

KBIO2031 156 756 1356

Q10DIS1 161 761 1361

Q10DOC1 166 766 1366
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Q10PAR1 171 771 1371

First Order Rates, day-1-- The user may specify first order biodegradation rate constants for water 

column and benthic segments using constants KBW and KBS. If nonzero values are specified 

for these constants, they will be used directly, bypassing second order calculations. Constant

numbers are given in Table 7.19.

Second Order Rate Coefficients, mL/cell-day-- The user may specify second order

biodegradation rate constants for each phase (dissolved, DOC-sorbed, and sediment-sorbed) and 

each ionic specie using constant KBIO20. Constant numbers for the neutral molecule are

summarized in Table 7.19. KBIO2011 refers to the dissolved neutral chemical; KBIO2021 refers 

to the DOC-sorbed neutral chemical; KBIO2031 refers to the sediment-sorbed neutral chemical. 

Temperature Coefficients-- The user may specify temperature correction factors for the

dissolved, DOC-sorbed, and sediment-sorbed phase of each chemical using constants Q10DIS, 

Q102DOC, and Q10PAR, respectively. These constants represent the multiplication factor for

biodegradation rates corresponding to a 10 °C temperature increase. Constant numbers are

summarized in Table 7.19. If Q10 values are omitted or set to 0, biodegradation rates will not be 

affected by temperature.

Bacterial Population Levels, cell/mL-- The user may specify segment and time variable bacterial

concentrations using parameter 14, BAC, and time funcions 16 and 17, BACNW and BACNS. 

Typical population size ranges are given in Table 7.19. 

If bacterial concentrations are to remain constant in time, the user should enter segment mean

concentrations using parameter BAC. BACNW and BACNS should be omitted.

Water Body Type

Bacterial Numbers

(cells/ml) Ref.

Oligotrophic Lake 50 - 300 a

Mesotrophic Lake 450 - 1,400 a

Eurtophic Lake 2000 - 12,000 a

Eutrophic Reservoir 1000 - 58,000 a

Dystrophic Lake 400 - 2,300 a

Lake Surficial Sediments 8x109 - 5x1010

 cells/100 g dry wt

a

40 Surface Waters 500 - 1x106 b

Stream Sediments 107 - 108

 cells/100 g

c

Rur River (winter) 3x104 d

References:

aWetzel (1975). Enumeration techniques unclear
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b

Paris et al (1981). Bacterial enumeration using plate counts.

cHerbes & Schwall (1978). Bacterial enumeration using plate counts.

dLarson et al. (1981). Bacterial enumeration using plate counts.

The user may enter time-variable water column and benthic bacterial concentrations via time

functions BACNW and BACNS, respectively, as a series of concentration versus time values. 

Parameter BAC will then represent the ratio of each segment concentration to the time function

values. The product of BAC and the BACNW or BACNS function gives the segment and time 

specific bacterial concentrations used by TOXI. (Group G, Record 4, PARAM(I,14); Group I, 

Record 2, VALT(16,K), VALT(17,K))

11.11. Extra Reaction

An extra second-order reaction is included in TOXI. The second order reaction allows the user 

to simulate the effect of processes not considered by TOXI. The reaction depends upon a rate 

constant and a environmental parameter which may be taken to represent, for example, some

reducing or oxidizing agent. The rate of reaction may also vary with temperature. 

11.11.1. Overview of TOXI Extra Reaction

TOXI allows the user to specify an additional second order reaction for the various species and 

phases of each chemical:

Equation 11-73

K = [E] k f eij ij

i j

E ∑∑

where:

KE = net extra reaction rate constant, day-1

[E] = intensity of environmental property driving this reaction

keij = second order rate constant for chemical as specie in phase j, 

in [E]-1 day-1

fij = fraction of chemical as specie i in phase j

The reaction coefficients may be specified as constants, with activation energy constants left as

0. If the user wants TOXI to determine rates based on the temperature based Arrhenius function, 

then non-zero activation energies specified, as constants will invoke the following calculation for 

each rate constant k:
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Equation 11-74

k e

(T K )=k e(T R)exp [1000 E ae (T K -T R)/( RT K T R)]

where:

Eae = Arrhenius activation energy for extra reaction, kcal/mole-°K

Activation energies may be specified for each ionic specie simulated. If no activation energies 

are given, then rate constants will not be adjusted to ambient water temperatures.

An example of a kinetic process that may be modeled as this extra reaction is reduction. If

reduction is modeled, [E] may be interpreted as the concentration of environmental reducing

agents RH2, so that

Equation 11-75

C+ RH 2 → P

and

[E] = Concentration of RH2, moles/L

ke = second order rate constant, L/mole-day

P = reduced product

The identity of the reducing agent and the second order rate constant must be identified and

quantified by laboratory kinetics studies. If both the environmental oxidizing and reducing

agents are in excess, then two chemicals may be simulated as a redox pair:

Equation 11-76

C 1+ RO 2 _ C 2+ RH 2

where:

C1 = reduced chemical

C2 = oxidized chemical

RO2 = oxidizing agent

RH2 = reducing agent

Laboratory kinetics studies can control the concentrations of RO2 and RH2 to determine rate

constants for both oxidation and reduction. These may be specified as constants kox and kE. 

Yield coefficients Y012 and YE21 must also be specified as constants. The spatially variable

concentrations [RO2] and [RH2] must be specified as parameters.

Implementation
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The input data requirements for the second order reactions include the second order reaction rate 

constants that may be specified for each specie and sorbed form (dissolved, DOC sorbed and

sorbed to particulate). If the rates are to be temperature corrected, then the user may supply the 

reference temperature at which the extra reaction rates were measured and the activation energy

for the reaction. The rates will then be adjusted using a temperature-based Arrhenius function. 

If activation energy is not supplied, no temperature corrections will be performed. The "extra"

property of the aquatic environment that affects the extra reaction is specified to the model as a 

parameter which may vary between segments. The units of the "extra" property must be

consistent with those used for the second-order rate constant. The product of the extra property 

and second-order rate constant must have units of day-1. The temperature may be time variable 

as well (input as a time series). Input data are described below.

VARIABLE C1 C2 C3

TREFE 573 1173 1773

KE2011 576 1176 1776

KE2021 581 1181 1781

KE2031 586 1186 1786

 EEX1 591 1191 1791

Extra Reaction Rate, L/mole-day-- The user may specify second order extra rate constants for 

each phase (dissolved, DOC-sorbed, and sediment-sorbed) and each ionic specie using constant 

KE20. Constant numbers for the neutral molecule are summarized in Table 7.21. KE2011 refers 

to the dissolved neutral chemical; KE2021 refers to the DOC-sorbed neutral chemical; KE2031

refers to the sediment-sorbed neutral chemical. 

Activation Energy, kcal/mole-°K-- The user may specify activation energies for each chemical

using constant EEX. Constant numbers are summarized in Table 7.21. If EEX is omitted or set 

to 0, oxidation rates will not be affected by temperature.

Reference Temperature,°C-- The user may specify the reference temperature at which oxidation

rates were measured using constant TREFE. Constant numbers are summarized in Table 7.21. 

If a reference temperature is not supplied, then a default of 20 °C is assumed.

Extra Environmental Concentration, mole/L-- The user should specify segment variable extra

environmental concentrations using parameter 15, EXENV. (Group G, Record 4, PARAM(I,15)
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12. REFERENCES

Alexander, M. 1980. Biodegradation of Toxic Chemicals in Water and Soil. In:

Dynamics, Exposure, and Hazard Assessment of Toxic Chemicals, R. Haque,

editor. Ann Arbor Science, Ann Arbor, MI.

Ambrose, R.B. 1987. Modeling Volatile Organics in the Delaware Estuary. American

Society of Civil Engineers. Journal of Environmental Engineering, V. 113, No. 4, 

pp 703-721.

Ambrose, R.B. et al. 1988. WASP4, A Hydrodynamic and Water Quality Model--Model 

Theory, User's Manual, and Programmer's Guide. U.S. Environmental Protection

Agency, Athens, GA. EPA/600/3-87-039.

APHA (American Public Health Association). 1985. Standard Methods for the

Examination of Water and Wastewater, 15th Edition. APHA. Washington, DC.

Bannister, T.T. 1974a. Production Equations in Terms of Chlorophyll Concentration,

Quantum Yield, and Upper Limit to Production. Limnol. Oceanogr. 19:1-12.

Bannister, T.T. 1974b. A General Theory of Steady State Phytoplankton Growth in a

Nutrient Saturated Mixed Layer. Limnol. Oceanogr. 19:13-30.

Barber, M.C., L.A. Suarez, and R.R. Lassiter. 1991. Modelling Bioaccumulation of

Organic Pollutants in Fish with an Application to PCBs in Lake Ontario

Salmonids. Canadian J. Fisheries and Aquatic Sciences. Vol. 48, pp. 318-337.

Bella, D.A. and W.J. Grenney. 1970. Finite-Difference Convection Errors. Journal of

the Sanitary Engineering Division, ASCE, Vol. 96, No. SA6, pp. 1361-1375.

Berner, R.A. 1974. Kinetic Models for the Early Digenesis of Nitrogen Sulfur,

Phosphorus, and Silicon in Anoxic Marine Sediments. In: The Sea, Vol. 5, ed. 

E.D. Goldberg. J. Wiley and Sons. New York.

Bowie, G.L., W.B. Mills, D.B. Porcella, C.L. Campbell, J.R. Pagenkopf, G.L. Rupp,

K.M. Johnson, P.W.H. Chan, S.A. Gherini and C.E. Chamberlin. 1985. Rates,

Constants, and Kinetics Formulations in Surface Water Quality Modeling. 

Second Edition. U.S. Environmental Protection Agency Athens, GA. 

EPA-600/3-85-040.

Brown, L.C. and T.O. Barnwell. 1987. The Enhanced Stream Water Quality Models

QUAL2E and QUAL2E-UNCAS: Documentation and User Manual. U.S.

Environmental Protection Agency, Athens, GA. EPA/600/3-87-007.

Burns, L.A., D.M. Cline, and R.R. Lassiter. 1982. Exposure Analysis Modeling System

(EXAMS): User Manual and System Documentation, U.S. Environmental

Protection Agency, Athens, GA. EPA-600/3-82-023.
DRAFT: Water Quality Analysis Simulation Program (WASP) Version 6.0

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