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

applications. Citations are given to enable the reader to download these models from
the internet site of the Canadian Environmental Modelling Centre at Trent University,
namely http://www.trentu.ca/envmodel. Included are DOS and Windows Level I,
Level II, and Level III models, the EQC (EQuilibrium Criterion) model, the Generic
model, and ChemCAN, a Level III model that has data for regions of Canada but
can be (and has been) adapted to other regions.
The next group of models is used to explore how a chemical is migrating or
exchanging across the interface between two media, given the concentrations or
fugacity in both. No mass balance is necessarily sought—merely a knowledge of
how fast, and by what mechanism, the chemical is migrating. Compartment volumes
are not necessary, but they may be included for the purpose of calculating half-lives.
An example is air-water exchange in which both concentrations are defined and the
aim is to deduce in what direction and at what rate the chemical is moving. Often,
it is not clear if a substance in a lake is experiencing net input or output as a result
of exchange with the atmosphere. An important conclusion is that zero net flux does
not necessarily correspond to equilibrium or equifugacity. We refer to these as
intermedia exchange models.
The simplest mass balance model is a one-compartment “box” that receives
various defined inputs either as an emission term or as the product of a D value and
a fugacity from an adjoining compartment. The various D values for output or loss
processes are then calculated. The steady-state fugacity at which inputs and outputs
are equal is then deduced. An unsteady-state version of the model can also be devised.
Examples are a “box” of soil to which sludge or pesticide is applied, a one-compartment
fish with input of chemical from respired water and food, and a mass
balance for the water in a lake.
The complexity can be increased by adding more connected compartments. The
QWASI (Quantitative Water, Air, Sediment Interaction) model includes mass balances
in two compartments (water and sediment), the concentration in air being
defined. A river, harbour, or estuary can be treated as a series of connected Eulerian
QWASI boxes or using Lagrangian (follow a parcel of water as it flows) coordinates.
A sewage treatment plant (STP) model is described in which the compartments are
the three principal vessels in the activated sludge process. This illustrates that the
modeling concepts can also be applied to engineered systems. Indeed, such systems
are often easier to model, because they are well defined in terms of volumes, flows,
and other operating conditions such as temperature.
This multicompartment approach can be applied to chemical fate in organisms
ranging from plants to humans and whales. These are physiologically based pharmacokinetic
(PBPK) models.
Fairly complex models containing multiple compartments can be assembled,
an example being the POPCYCLING–BALTIC model of chemical fate in the
Baltic region. The ultimate model is one of chemical fate in the entire global
environment, GloboPOP. These models are available from a website at the University
of Toronto, to which a link is provided from the Trent University address
given earlier.
Where possible, references are given to published studies in which the models
have been applied. These reports give more detail than is possible here.
©2001 CRC Press LLC