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

leakage. It is often useful to assess the likely fate of the chemical, i.e., how fast the
rates of degradation, volatilization, and leaching in water are likely to be, and how
long it will take for the soil to “recover” to a specified or acceptable level of
contamination. Persistence is an important characteristic for pesticide selection.
Remedial measures such as excavation may be needed when recovery times are
unacceptably long.
Most modeling efforts in this context have been for agrochemical purposes, the
most comprehensive recent effort being described in a series of publications by Jury,
Spencer, and Farmer (1983, 1984a, 1984b, 1984c). Other notable models are
reviewed in these papers. The Soil model is essentially a very simplified version of
the Jury model (1983) and is a modification of a published herbicide fate model
(Mackay and Stiver, 1990). The reader is referred to the texts by Sposito (1989) and
Sawhney and Brown (1989) and the chapter by Green (1988) for fuller accounts of
chemical fate in soils. Cousins et al. (1999) have reviewed and modeled these
processes.
In the Soil model, only soil-to-air processes are treated; no air-to-soil transport
is considered. A second, more complex fugacity model SoilFug was developed by
DiGuardo et al. (1994a), which allows the user to calculate the fate of the pesticide
in a defined agricultural area over time with changing rainfall. The model gave
satisfactory predictions of pesticide runoff in agricultural regions in Italy and the
U.K. (Di Guardo et al., 1994a, 1994b). Both models are available from the website.
Only the Soil model is described here.
8.4.2 Process Description
In the Soil model, the soil matrix illustrated in Figure 8.2 is considered to consist
of four phases: air, water, organic matter, and mineral matter. The organic matter is
considered to be 56% organic carbon. The volume fractions of air and water are
defined, either by the user or by default values, as is the mass fraction organic carbon
(OC) content on soil basis. Assuming densities of 1.19, 1000, 1000, and 2500 kg/m 3
for air, water, organic matter, and mineral matter, respectively, enables the mass and
volume fractions of each phase, and the overall soil density, to be calculated.
The soil area and depth are specified, thus enabling the total volumes and mass
of soil and its component phases to be deduced. The amount of chemical present in
the soil is specified as a concentration or as an amount in units of kg/ha, which is
convenient for agrochemicals. The chemical is assumed to be homogenously distributed
throughout the entire soil volume.
The individual phase Z values are calculated, then the bulk Z value of the soil
Z TS is deduced. From the concentration, the fugacity is deduced, and the individual
phase quantities and concentration are calculated.
It is prudent to examine the fugacity to check that it is less than the vapor
pressure. If it exceeds the vapor pressure, phase separation of pure chemical will
occur; i.e., the capacity of all phases to “dissolve” chemical is exceeded. This can
occur in heavily contaminated soils that have been subject to spills, or when there
is heavy application of a pesticide. Essentially, the “solubility” of the chemical in
the soil is exceeded. This calculation of partitioning behavior provides an insight
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