Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 368

Step 3 : Determine the mobility and persistence values for the particulate phase using
Equation 10.9:

1

Mobility in theparticulatephase M ( ) = × Koc

particulate

SpG

SpG for Chemical XYZ = 0.88
Koc for Chemical XYZ = 1.92
Therefore

1
0 88

Mobility in theparticulatephase M particulate ( ) = × 1 92 . = 2 18 .

.

Persistence in the particulate phase (P gas ) = literature value of 1.0 years

M ( . 2 18 ) × P ( . ) 0 2 = 2 18 . × 1 0 . = 2 18 .

particulate particulate

Step 4 : Determine the CRF air using Equation 10.10:

1

CRF for chemical XYZ = × [( M × P ) + ( M × P

iculate )] AIR gas gas particulate part

( T )

1
0 04

CRF for chemical XYZ = × [( . 0 228 4 × ) + ( . 2 18 × 1 0 . )]

AIR

( . )

1
0 04

CRF for chemical XYZ = × [( . 0 91 ) + ( . 2 18 )]

AIR

( . )

CRF for chemical XYZ = 77 25 .

AIR

This sample calculation for CRF AIR demonstrates how the risk posed by the chemical in
the particulate phase is much greater than the risk posed by the chemical in the gas phase
(2.18 vs. 0.912). This occurs in most cases because the contaminant is much more persistent
as a particulate than in the gas phase, and as a result, there is an increased probability of
exposure with a particulate form of a chemical. This trend is common with most contami-
nants, especially organics.
Table 10.13 displays the CRF AIR values for LNAPLs, DNAPLs, PAHs, selected metals,
PCBs, and the pesticide chlordane.
As shown in column 2, lead had the lowest contaminant risk factor for air compared to
any of the contaminants evaluated. The reason is not because lead is less toxic, but because
lead is not volatile and has the second highest specific gravity of any compound evaluated.
The specific gravity of lead is 11.34; mercury is the only compound evaluated with a higher
specific gravity (13.6). Lead is routinely released into the environment from air emission
sources (USEPA 2008), but most airborne lead is rapidly deposited on the ground surface.
The particulate to gas ratios shown in column 5 are significant because the contaminants
shown here are commonly found in U.S. urban areas, where particulate matter tends to
concentrate. With the exception of benzene and mercury, the risk of exposure to contami-
nants in the atmosphere is highest from those contaminants sorbed to particulate matter.