Page 277 - Fundamentals of Air Pollution 3E

III. Contaminant Fate Analysis 235

unsaturated soils. Contaminants settling to the surface through dry deposi-
tion may dissolve in or become suspended in surface waters or may be
leached into unsaturated soils and ground water by subsequent rainfall.
Dry deposition may also result in formation of a layer of relatively high
contamination at the soil surface. When such intermedia transfers are likely,
one should assess the fate of contaminants in the receiving media.

4. If areas identified as likely to receive significant atmospheric contami-
nant concentrations include areas supporting edible biota, the biouptake of
contaminants must be considered as a possible environmental fate pathway.
Direct biouptake from the atmosphere is a potential fate mechanism for
lipophilic contaminants. Biouptake from soil or water following transfer of
contaminants to these media must also be considered as part of the screen-
ing assessments of these media.

B. Surface Water Fate
The following numbered paragraphs refer to particular numbered boxes
in Fig. 16-4.

1. The aquatic fate of contaminants released from the CERCLA site as
well as those transferred to surface water from other media beyond site
boundaries must be considered.
2. Direction of contaminant movement is usually clear only for contami-
nants introduced into rivers and streams. Currents, thermal stratification
or eddies, tidal pumping, and flushing in impoundments and estuaries
render qualitative screening assessment of contaminant directional trans-
port highly conjectural for these types of water bodies. In most cases,
entire water bodies receiving contaminants must be considered potentially
significant human exposure points. More in-depth analyses or survey data
may subsequently identify contaminated and unaffected regions of these
water bodies.
3. Similarly, contaminant concentrations in rivers or streams can be
roughly assessed based on rate of contaminant introduction and dilution
volumes. Estuary or impoundment concentration regimes are highly depen-
dent on the transport mechanisms enumerated. Contaminants may be
localized and remain concentrated or may disperse rapidly and become
diluted to insignificant levels. The conservative approach is to conduct a
more in-depth assessment and use model results or survey data as a basis
for determining contaminant concentration levels.
4. Important intermedia transfer mechanisms that must be considered
where significant surface water contamination is expected include transfers
to ground water where hydrogeology of the area indicates significant sur-
face water-ground water exchange, transfers to biota where waters contam-
inated with lipophilic substances support edible biotic species, and transfer