Page 356 - Materials Chemistry, Second Edition

L1644_C08.fm Page 320 Tuesday, October 21, 2003 3:03 PM

observed that air dispersion in the south was influenced by topography, more con-
cretely by the elevation present 1 km away in the south. On the other hand, it can
be observed that the wind blowing from the southeast influenced the air dispersion
to the northwest direction.
In order to assess the air concentrations of 1,1,1-trichloroethane, medium con-
centrations for the entire study area were estimated. The medium air concentration
–2
3
of the pollutant was 9.69 × 10 ng/m ; it was estimated with the annual averages
in all 2584 Cartesian receptors representing the study area.
8.2.4.5 Exposure Calculation Model

For the evaluation of the exposure of the population living in the area, a multiple
pathway exposure, transport and transformation model (CalTOX) was used. In
Chapter 4 this model is compared with other multimedia model EUSES. This model
includes modules for the distribution of substances in the environment, exposure
scenario models for humans and the environment, human risk estimation and efforts
to quantify and reduce uncertainty in multimedia. It has been designed to assist in
assessing human exposure, define soil clean-up goals at (uncontrolled) hazardous
waste sites and improve the quality of risk assessment information, especially as
required for regulatory decisions. The models and data sets have been compiled as
Microsoft Excel 4.0 (or higher) spreadsheets. They are available together with
information and documentation via the Internet through various websites.
Each pathway can be included or excluded separately in the calculations, depend-
ing on the scope of the study. The exposure model defines air, drinking water, food,
and soil as the four main sources for human exposure to a substance via different
pathways such as inhalation, ingestion, or dermal contact.
Figure 8.7 shows the eight compartments implemented in CalTOX: air, surface
water, ground water, sediment, surface soil, root-zone soil, vadose-zone soil and plants.
Usually a chemical equilibrium between the phases is assumed. Unidirectional trans-
port is assumed only from soil to water and from upper to lower soil zones, mainly
because of the much higher diffusion speed in these directions compared to the other
direction. The equations used in CalTOX to estimate exposure and risk are taken from
the U.S. Environmental Protection Agency Risk Assessment Guidance for Superfund
(U.S. EPA, 1989) and from the California Department of Toxic Substances Control
(DTSC, 1992a,1992b). They are based on conservation of mass.
The release of a substance can be continuous (to air, water and surface soil) or
a batch process with an initial concentration within deeper soil zones. The exposure
model of CalTOX calculates daily human doses for various pathways (e.g., inhalation
of air, ingestion of soil, milk, meat, etc.) based on daily intake rates and predicted
concentrations in the respective exposure medium. Finally, risk values based on the
doses are calculated.

8.2.4.6 Spatial Scale, Time Scale and Assessable Substances

There is only one spatial scale because CalTOX is intended to be used site specif-
ically (i.e., to assess a specific existing site rather than a big area such as an entire

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