2.5 VOLATILIZATION
When hazardous materials are in accident, disposal, or storage areas such
as landfills, spill sites, contaminated soil, and ponds, the volatile constituents
may be emitted directly to the air in a gaseous form. The processes by
which this exchange of gaseous materials to the air occurs is called volatilization.
The volatilization pathway can dominate the potential human health impacts
of an environmental problem. A typical example of this kind of problem
is a landfill with volatile organic compounds that result in significant
exposures from inhaling these vapors. In addition, semivolatile emissions
of materials, such as PCBs, can be important sources of pollutant exposure
via the volatilization pathway (EPA 1988b).
MEPAS provides three methods to address volatilization.
The gaseous release rates may be
- input by the investigator
- computed by the model
- back-calculated from air concentration
monitoring data.
For
the input method, emission rates are obtained from sources such as flux
measurements at the contaminant source, or computation with an alternative
volatilization model. The computation method requires input of site-specific
information on the constituent and site characteristics. The back-calculation
method using monitoring data requires information on the size of the contaminated
area and ambient concentrations.
The mathematical formulations for estimating atmospheric volatilization
rates are described in the following sections. Gaseous emissions of constituents
from landfills, spills, and ponds are computed using physical characteristics
of the contaminant source and chemical properties of the contaminants.
A number of models have been proposed and summarized for computing volatilization
rates from soils and ponds with liquid mixtures of waste (see EPA
1980; Farino et al. 1983; Thibodeaux 1981; Shen 1981; Thibobeaux and Hwang
1982; Mackay and Leinoen 1975; Lyman et al. 1982; and Hwang and Falco 1986
as cited in EPA 1990). Different models have been developed to estimate
emission rates from different types of treatment and disposal activities.
The MEPAS volatilization mathematical formulations include models published
in the literature for estimating emission rates of volatile compounds from
soils, various treatment and disposal facilities, and spill sites. Knowledge
of the derivation of a model is important in selecting an appropriate model
to best represent the condition of the contaminants, the media from which
emissions occur, and their interactions at a specific site. Because of
many similarities among these models, it is important to understand the
assumptions made by the authors in deriving these models.
MEPAS includes mathematical formulations for computing volatilization rates
from several different types of contaminant sources based on the U.S. Environmental
Protection Agency's (EPA's) recommendations in the Superfund Exposure Assessment
Manual (EPA 1988b) and a guidance document (EPA 1990). The MEPAS volatilization
source models are for contamination in soil and liquids:
- contaminated soil gradient model (soil)
- land treatment facilities (soil)
- landfills without internal gas generation (soil)
- landfills with internal gas generation (soil)
- spill sites (pooled liquid)
- surface impoundments (liquid storage).
Note that the first
three models are all applicable to soil contamination. The formulations
given below should be used to help determine which source model applies
best to a particular soil contamination situation