Page 280 - Fundamentals of Air Pollution 3E

IV. Modeling 237

to the atmosphere where surface water is contaminated by volatile sub-
stances. High temperatures, high surface area/volume ratios, high wind
conditions, and turbulent stream flow also enhance volatilization rates.
Contaminant transfer to bed sediments represents another significant
transfer mechanism, especially in cases where contaminants are in the
form of suspended solids or are dissolved hydrophobic substances that can
become adsorbed by organic matter in bed sediments. For the purposes of
this chapter, sediments and water are considered part of a single system
because of their complex interassociation. Surface water-bed sediment
transfer is reversible; bed sediments often act as temporary repositories
for contaminants and gradually rerelease contaminants to surface waters.
Sorbed or settled contaminants are frequently transported with bed
sediment migration or flow. Transfer of sorbed contaminants to bottom-
dwelling, edible biota represents a fate pathway potentially resulting in
human exposure. Where this transfer mechanism appears likely, the biotic
fate of contaminants should be assessed.

C. Soil and Ground Water Fate
The following numbered paragraphs refer to particular numbered boxes
in Fig. 16-5.

1. The fate of contaminants in the soil medium is assessed whenever
the contaminant release atmospheric, or fate screening, assessment results
show that significant contamination of soils is likely,
2. The most significant contaminant movement in soils is a function of
liquid movement. Dry, soluble contaminants dissolved in precipitation,
run-on, or human applied water will migrate through percolation into
the soil. Migration rates are a function of net water recharge rates and
contaminant solubility.
3. Important intermedia transfer mechanisms affecting soil contaminants
include volatilization or resuspension to the atmosphere and biouptake by
plants and soil organisms. These, in turn, introduce contaminants into the
food chain.

IV. MODELING

An extremely difficult task is the estimation of emissions from hazardous
waste sites. Frequently, both the amounts of materials existing within the
site and the compounds and mixtures that are represented are not known.
Even if both of these pieces of information are reasonably well known, the
conditions of the containers holding these chemicals are not initially known.