Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 365

TABLE 10.12 (continued)
Comparison of CRF SOIL to CRF GW

Groundwater
Contaminant
Risk Factor a
(CRF GW )

Groundwater/
Soil Contaminant
Risk Factor Ratio
CRF GW /CRF SOIL

Soil Contaminant
Soil Risk Factor
(CRF SOIL )

Average Extent of
Contamination
from Source (m)

Contaminant
Type

Geological
Unit

SSC 0.0003 2,524 1.2 × 10 −7 25

Sand 0.0009 1,457 6.2 × 10 −7 25

Moraine 0.0008 1,548 5.2 × 10 −7 25

a Data from Kaufman, M.M. et al. Water, Air, and Soil Pollut ., 167, 365-386, 2005.
b Contaminant risk factors listed represent an average value of specific compounds listed in Tables 10.5 and
10.11.
c SC, sandy clay
d SSC, sandy and silty clay

contaminants are formed in the atmosphere through photochemical or chemical reactions
(USEPA 2008). Many of these contaminants travel between the air, soil, and surface water
and change states. For example, contaminants initially released into the soil or water may
volatilize and become airborne. Or in some cases, contaminants airborne in the vapor
phase may become deposited on the soil and sorb onto a soil particle only to become air-
borne again by wind action.
Given the composition and behavior of air pollutants, developing a model for evaluating
the contaminant risks to air must address contaminants in the vapor phase and contami-
nants attached to particulate matter. Once airborne, a combination of these seven factors
control whether there is an adverse effect from an air contaminant (McKone and Enoch
2002; USEPA 2008):

• Toxicity
• Mobility
• Persistence
• Volume released
• Time period of the release
• Distance to a specific receptor being evaluated
• Wind speed and direction

The last four factors in this list are environmental factors dependent upon site-specific
criteria, so they are not included in the development of the air contaminant risk factor
(CRF AIR ). These environmental factors are applicable when evaluating the impacts of an
actual release, as done with the definition of the nature and extent of a contaminant plume
within soil and groundwater.
Three familiar factors remain (toxicity, mobility, and persistence). In addition, there are
similarities between the physical and chemical attributes controlling the migration poten-
tial of a contaminant in air, groundwater, and soil including solubility, vapor pressure,
density, chemical stability, persistence, and adsorption potential (Kaufman et al. 2005;
USGS 2006; Rogers et al. 2007a). On the basis of these similarities, the environmental risk