Environmental Engineering Reference
In-Depth Information
and Mexico, but it was banned in 1982 due to its toxicity to animals and all usage
stopped by 1986. A total of about 490 kt (kt = 10 6 kg) was used in the United
states from 1947 to 1986 (Li, 2001) and about 10-100 kt were used in Mexico
during the same time. No toxaphene was used in Canada.
Despite the fact that toxaphene has not been used in North America for nearly 3
decades, significant concentrations are found in the atmosphere, soil, lakes, fish,
and animals at locations far from the regions where it was used. Although virtually
all toxaphene use was in the southern US and Mexico, it is found in the upper
Great Lakes in such high concentrations that restrictions have been placed on the
consumption of fish caught there. Thus it is clear that this pesticide continues to be
emitted from soils where it was used, transported over long distances and depo-
sited at locations many hundreds or even thousands of km from the location of its
original application.
The slow emission and gradual dispersion over great distances is a general
characteristic of a wide range of semi-volatile organic materials (SVOMs) that
have been dispersed into the soil and water. In order to quantify this behaviour and
thereby assess the dangers to human populations posed by such materials, we have
developed a modelling system consisting of the US EPA's MM5/SMOKE/CMAQ
regional chemical transport model (CTM), which is coupled in-line with an air-
soil exchange Pesticide Emission Model (PEM). The resulting system is capable
of simulating the transport of heat, moisture and SVOM in the soil; SVOM emis-
sion from the soil to the atmosphere; transport and chemical reaction of the
SVOM in the atmosphere and its deposition to terrestrial and water surfaces. In
this report, we describe its application to the pesticide toxaphene, but with suitable
modification of certain physical parameters, the model is also capable of simu-
lating the behaviour of a wide range of other SVOMs.
2. Modelling Details and Simulations
The SMOKE/CMAQ regional CTM includes all relevant atmospheric species
such as ozone, OH radicals and aerosols. Thus it accounts for the chemical
removal of the SVOM by reaction with OH as well as its physical partitioning
between the gas phase and three aerosol size modes. The PEM soil model uses 17
soil texture classes having different hydraulic properties and tillage characteristics,
all of which affect the rate of volatilization to the atmosphere. PEM is driven by
the same meteorology as the CTM, so it is capable of simulating the diurnal and
seasonal emission variations realistically. The combined system has high vertical
resolution: 22 sigma layers in the atmosphere up to a pressure level of 100 mb and
49 layers in the soil down to a depth of 1 m.
The 1999 US and 1995 Canadian criteria emission inventories were used directly
in the SMOKE emissions processor. The pesticide emissions calculated by PEM
were also introduced into SMOKE through an interface developed for this purpose
and combined with the other atmospheric species.
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