Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 370

This is dramatically demonstrated by PAHs and PCBs—compounds having a mean par-
ticulate to gas ratio of 1000-1. Also displaying impressively large particulate to gas ratios
are chlordane (500:1) and DNAPL compounds (100:1). If an equal weighting were used
between the gas phase and particulates, the differential would be much greater. This is
because the values of persistence in the gas phase used to calculate the CRF AIR values are
in days and the values of persistence in the particulate phase are in years.
The LNAPL VOCs provide a good example of how the presence of particulate matter can
transform a short-lived group of chemicals into a significant atmospheric risk. This group
of compounds includes benzene, toluene, ethyl benzene, and xylenes and is commonly
referred to as BTEX. They are common components of gasoline and highly volatile, toxic,
and flammable, but degrade rapidly in the gaseous phase due to photolysis. Their persis-
tence increases significantly when sorbed onto a particle, and this is reflected by their mod-
erate values for CRF AIR in column 2. Without this sorption, these values would be very low.

10.6 Discussion and Implications
Table 10.14 compares the contaminant risk factors for groundwater, soil, and air. DNAPLs
and chromium VI have the highest potential to contaminate groundwater, mercury,
PCBs, and chlordane have the highest potential to contaminate and remain in soil or sedi-
ment, and several compounds including LNAPLs and DNAPLs, PCBs, PAHs, chlordane,
arsenic, chromium VI, and mercury have the potential to contaminate the air, especially
when attached to particulate matter.
An examination of the DNAPL compounds listed in Tables 10.3, 10.9, and 10.13, reveals
that the compound vinyl chloride exhibits the highest groundwater, soil, and air contami-
nant risk factors compared to the other DNAPL compounds evaluated. This dubious dis-
tinction occurs because vinyl chloride has the following physical/chemical attributes: (1) it
is the most toxic DNAPL compound evaluated (USEPA 2009a) and is also the most mobile,
(2) it is the most soluble DNAPL compound in water compared to the other DNAPL
compounds evaluated, and (3) it has the highest vapor pressure so it evaporates readily
(ATSDR 2006b).
Chromium VI is also a compound of special note. It has the highest groundwater and
air risk factors because of its high toxicity and relatively high solubility in water. Because

TABLE 10.14
Contaminant Risk Factors for Groundwater, Soil, and Air for Each Contaminant Group

Contaminant Type Groundwater CRF (CRF GW ) Soil CRF (CRF SOIL ) Air CRF (CRF AIR )

DNAPL 978.00 1,508.00 67.52

LNAPL 0.47 10.16 20.23

PAH 0.001 142.00 124.82

Total PCBs 0.0004 5,264.00 37.00

Chlordane 0.0001 7,501.00 173.00

Arsenic 27.24 3,723.00 44.00

Chromium VI 1,926.00 5.17 210.00

Lead 0.55 52.44 3.45

Mercury 2.56 22,013.00 166.00