McKay, Donald. "Front matter" Multimedia Environmental Models ...

8.5.3 Model
The Sediment model is available from the website in Windows- and DOS-based
versions. Input data are requested on the properties of the chemical, the dimensions
and properties of the media, and the prevailing concentrations. The Z and D values
are calculated, followed by fugacities and fluxes. It is also of interest to calculate
the overall steady-state mass balance, which is given by
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f W (D D + D T) = f S (D R + D T + D B + D S)
The steady-state water and sediment fugacities corresponding to the defined sediment
and water fugacities are deduced. Response times can be calculated for each medium
if the volumes are known.
It is noteworthy that, for a persistent, hydrophobic substance, it is likely that the
steady-state sediment fugacity will exceed that of the water. The principal loss
process of a persistent chemical from the sediment is likely to be D R, which must
be less than D D, because some sediment is buried, and the organic carbon content
of the resuspended material will be less than the deposited material because of
mineralization. As a result, a benthic organism that respires sediment pore water
may reach a higher fugacity and concentration than a corresponding organism in
the water column above. A compelling case can be made for monitoring benthic
organisms, because they are less mobile than fish and they are likely to build up
higher tissue concentrations of contaminants.
These sediment-water calculations can be invaluable for estimating the rate at
which “in-place” sediment concentrations, resulting from past discharges of persistent
substances, are falling. Often, the memory of past stupidities lingers longer in
sediments than in the water column.
8.6 QWASI MODEL OF CHEMICAL FATE IN A LAKE
8.6.1 Introduction
Having established air-water and sediment-water exchange models, it is relatively
straightforward to combine them in a lake model by adding reaction and advective
inflow and outflow terms. The result is the QWASI (Quantitative Water Air Sediment
Interaction) model of Mackay et al. (1983), which was applied to Lake Ontario
(Mackay, 1989). Other reports include an application to a variety of chemicals by
Mackay and Diamond (1989), to organochlorine chemicals produced by the pulp
and paper industry by Mackay and Southwood (1992), the use of spreadsheets to
aid fitting parameter values to the model (Southwood et al., 1989), to situations in
which surface microlayers are important (Southwood et al., 1999), and to metals by
Woodfine et al. (2000). Mackay et al. (1994) took the QWASI fugacity model and
replaced all fugacities by C/Z and all Z values by partition coefficients, then converted
all D values to rate constants. This “new” model, called the “rate constant”
model, gives identical results and is suitable for use by people who are too lazy to