Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 363

100,000

10,000

Hg
Chlordane
PCBs

1,000

Arsenic

100

DNAPL

PAHs

Lead

10

LNAPL
Chromium VI

1

Clay Silty clay Sand and

silty clay

Sand

FIGURE 10.6
CRF SOIL distribution by contaminants type.

• Composition
• Permeability
• pH
• Redox

These factors work to increase the retention capacity of soils composed of clay and decrease
the capacity in sandy soils. For example, clay soils generally have a higher contaminant
retention capacity and a much lower permeability than sandy soils. Thus, the CRF SOIL
reflects a decrease in risk as the mean grain size of soil increases.

10.5 Air Contaminant Risk Factor
Atmospheric contaminants vary dramatically and originate from anthropogenic and
natural sources. Anthropogenic sources are most prevalent in urban areas and include
stationary emitters such as manufacturing facilities, power generating plants, dry clean-
ers, and mobile emitters such as automobiles, trucks, buses and airplanes (USEPA 2008).
Natural sources include particles from volcanic eruptions, biological decay, forest fires,
and pollen. VOCs are also emitted by plants and trees. As an example of atmospheric
contaminant variety, consider the VOC isoprene emitted by some tree species (e.g., oaks)
(Sharkey et al. 2008). Isoprene is carcinogenic to humans if inhaled as a concentrated vapor
within a closed space (USEPA 2002b), but concentrations in the atmosphere do not reach
levels of concern.
Contaminants are released into the atmosphere as a gas, particulate matter, or as part of
a water droplet through sorption or solution (USEPA 2008). Primary contaminants include
heavy metals, volatile and semivolatile compounds and particulate matter. Secondary