Environmental Engineering Reference
In-Depth Information
• Hydrolysis
• Biodegradation
• Transformation
• Volatilization to the atmosphere
• Sorption of contaminants in sediment
• Bioaccumulation of contaminants by animal and plant life
• Dilution
Molecular diffusion and groundwater discharge and recharge are not shown in the figure.
Since most urban areas of the United States obtain their potable water from surface
sources, wastewater discharges are a concern. Treatment costs rise when the source of
supply is contaminated, and the risks of biological contamination also increase. Moreover,
as we noted in Chapters 2 and 3, a majority of urban areas are located along rivers and
streams and other surface water bodies such as the Great Lakes, and many of these water
bodies have already been degraded—some significantly (USGS 1995b).
8.3.4.1 Contaminant Persistence and Bioaccumulation
When certain contaminants are released to surface waters through overland flow, storm-
water runoff, or wastewater discharge, they may accumulate in sediments. Compounds
with a higher likelihood of accumulating have the following physical characteristics:
• Low solubility
• High molecular weight
• Low potential to degrade
• High sorption potential
Contaminants having these physical chemistry attributes typically do not sustain them-
selves in surface water unless the rate of flow is substantial. Even then, they may be carried
along the bottom of the stream or river until the carrying capacity of surface water is insuf-
ficient and the contaminants settle to the bottom. As shown in Figure 8.5, certain locations
in the stream bottom become a sink for these contaminants as they accumulate (USGS
1996a). If the source of contamination persists, greater amounts of the contaminant will be
deposited. The accumulation of contaminants in sediments increases the exposure risk to
aquatic and terrestrial plant and animal life. If any of the contaminants exhibit bioaccu-
mulation properties, contamination may proceed up the food chain from bottomdwelling
macroinvertebrates to small fish and eventually to larger fish, predatory birds, and other
organisms. Humans are situated at the top of food chain, and the potential risks to human
health must be considered when evaluating the fate and transport of contaminants in sur-
face water (USEPA 2009a, 2009b).
Contaminants considered to be bioaccumulative include (USEPA 2009b)
1. Mercury
2. Polychlorinated biphenyls (PCBs)
3. Chlordane
4. Dioxins
5. Dichlorodiphenyltrichloroethane (DDT)
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