The Canadian

Centre for Environmental Modelling and Chemistry

Level III Model

Version 2.70 - March 2002

New in version 2.70

A Level III simulation describes a situation which is one step more complex and realistic than the Level II model. Like the Level II model, chemical is continuously discharged at a constant rate and achieves a steady state condition in which input and output rates are equal. The loss processes are degrading reactions and advection. Unlike the Level II model, equilibrium between media is not assumed and, in general, each medium is at a different fugacity. A mass balance applies not only to the system as a whole, but to each compartment. Rates of intermedia transport are calculated using D values which contain information on mass transfer coefficients, areas, deposition and resuspension rates, diffusion rates, and soil runoff rates. It is now essential to define inputs to each medium separately, whereas in Level II only the total input rate was requested.

Mass balances are calculated for the four bulk media of air (gas + aerosol), water (solution + suspended sediment + biota), soil, (solids + air + water), and sediment (solids + pore water). Equilibrium exists within, but not between media. For example, sediment solids and pore water are at equilbrium, but sediment is not necessarily at equilibrium with the overlying water.

Physical-chemical properties are used to quantify a chemical's behaviour in an evaluative environment. Three types of chemicals are treated in this model: chemicals that partition into all media (Type 1), involatile chemicals (Type 2), and chemicals with zero, or near-zero, solubility (Type 3). The model can not treat ionizing or speciating substances. The Level III model assumes a simple, evaluative environment with user-defined volumes and densities for the following homogeneous environmental media (or compartments): air, water, soil, sediment, suspended sediment, fish and aerosols.

This model gives a more realistic description of a chemical's fate including the important degradation and advection losses and the intermedia transport processes. The distribution of the chemical between media depends on how the chemical enters the system, e.g. to air, to water, or to both. This mode of entry also affects persistence or residence time.

Three persistences are calculated, an overall value, TO, and individual persistences attributable to reaction only, TR, and advection only, TA. Note that 1/TO equals the sum of 1/TR and 1/TA.

The rates of intermedia transport are controlled by a series of 12 transport velocities. Reaction half-lives are requested for all 7 media. The advective residence time selected for air also applies to aerosols and the residence time for water applies to suspended sediment and fish. The advective residence time of aerosols, suspended sediment and fish cannot be specified independently of the air and water residence times.

Features of the Level III Program:
Provides a database of chemicals and chemical properties.
Permits temporary additions/changes of chemicals and their properties to a simulation.
Permits permanent additions, changes and deletions of chemicals and their properties to the database.
Provides a database of environments and environmental properties.
Permits temporary additions/changes of environments and their properties to a simulation.
Permits permanent additions, changes and deletions of environments and their properties to the database.
Provides context-sensitive Help.
Displays and prints the Level III model calculations, as performed by the program.
Allows the printing of simulation tables, the summary diagram, and a small selection of charts.
Allows the program results to be saved as a comma separated value file, readable by spreadsheet programs such as Excel.

This program was based on the following publication:

Mackay, D.2001. "Multimedia Environmental Models: The Fugacity Approach - Second Edition", Lewis Publishers, Boca Raton, pp.1-261.

The required input data are:
Chemical Properties:

  • chemical name
  • molecular mass
  • data temperature
  • reaction half-life estimates for

  • - air
    - water
    - soil
    - sediment
    - aerosols
    - suspended sediment
    - aquatic biota
  • Type 1 chemicals

  • - water solubility
    - vapour pressure
    - log Kow
    - melting point
  • Type 2 and 3 chemicals

  • - partition coefficients

    Environmental Properties:

  • areas and depths for all bulk media
  • volume fractions for all subcompartments
  • densities for all subcompartments
  • organic carbon content (soil, sediment & suspended sediment only)
  • fish lipid content (Type I chemicals only)
  • advective flow residence times for air (including aerosols), and water (including suspended sediment and aquatic biota)
  • advective flow residence time for sediment burial
  • transport velocities
  • - air side air-water mass transfer coefficient - water side air-water mass transfer coefficient - rain rate - aerosol deposition velocity (wet and dry combined) - soil air phase diffusion mass transfer coefficient - soil water phase diffusion mass transfer coefficient - soil air boundary layer mass transfer coefficient - sediment-water mass transfer coefficient - sediment deposition velocity - sediment resuspension velocity - soil water runoff rate - soil solids runoff rate


  • chemical input rates for each bulk medium or compartment
  • inflow concentrations in air and water
  • Model Output:

  • partition coefficients (Type 1)
  • Z values
  • fugacity of each medium
  • intermedia transport rates and D values
  • reaction and advection D values and loss rates
  • residence times or persistences (overall, reaction, and advection)
  • concentrations and amounts for each medium
  • a summary diagram
  • charts of key results
  • This program is only available in compiled form. A "readme.txt" file with more detailed technical information is included in the zipped file.

    Minimum sytem requirements:
    Pentium-75MHz with 8 Mb of RAM running Windows 95. On some systems it may be necessary to adjust your screen resolution.

    For non-Windows users the BASIC, evaluative, Level I, II and III fugacity models are available.

    Please read the LEVEL III SOFTWARE LICENSE before downloading the software. Use of the software constitutes your agreement to abide by the terms and conditions set out in the license agreement.


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    Last updated June 10, 2013.