The Canadian

Centre for Environmental Modelling and Chemistry

QWASI Model of Chemical Fate in Lakes


The "Quantitative Water Air Sediment Interaction" or "QWASI" model is written in BASIC and is designed to assist in understanding chemical fate in lakes.

It describes the steady state behaviour of an organic chemical in a lake subject to chemical inputs by direct discharge, inflow in rivers, and deposition from the atmosphere. Chemical is removed from the lake by evaporation, irreversible reaction in the water and sediment, outflow in the water, and sediment burial. The mass balance equations for the well-mixed water column and the well-mixed layer of surficial sediment also include sediment-water exchange by diffusion, deposition, and resuspension. The model, as supplied, treats PCBs in Lake Ontario.

The model is described in "Multimedia Environmental Models; The Fugacity Approach" by D. Mackay, Lewis Publishers, MI. 1991 on pages 203-210. Several other versions of the QWASI models have been published treating, for example, chemicals present in pulp mill effluents (Water Poll. Res. J. Canada 27: 509, 1992 by Southwood et al.). An unsteady state QWASI model applied to PCBs in Lake Ontario was published in Journal of Great Lakes Research in 1989 Vol. 15, p. 283.

Attempts have been made to apply the QWASI-type models to speciating chemicals:

  • Diamond et al. Chemosphere 25: 1907, 1992.
  • Mackay, D., Wania, F. and Schroeder, W.H., "Prospects for Modeling the Behavior and Fate of Mercury, Globally and in Aquatic System", in "Mercury as a Global Pollutant", Eds., Porcella, D.B., Huckabee, J.W. and Wheatley, B., Kluwer Academic Publishers, Water Air and Soil Pollution, Vol. 80, pp941-950, 1995.

    The required input data are:

  • chemical name
  • molecular mass
  • log Kow
  • data collection temperature
  • melting point
  • water solubility
  • vapour pressure
  • overall reaction rate half-lives in water and sediment
  • chemical concentration in air
  • chemical concentration in inflow water
  • direct discharges of chemical to the water
  • Model output includes:

  • partition coefficients
  • Z and D values
  • fugacities, concentrations and amounts in each compartment
  • System requirements to run the compiled program are minimal.

    Please read the QWASI 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 12, 2013.