Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 347

Contaminants released into the environment only pose a risk to humans if there is a
completed exposure pathway. In general terms, the CRF GW is expressed in Equation 10.1
(Kaufman et al. 2005; Rogers et al. 2007a).

CRF

GW =

toxicity mobility persistence × × (10.1)

Toxicity values are obtained from the USEPA Integrated Risk Information System (IRIS)
(2009a). This database is updated weekly, and often more frequently. The toxicity values
selected should be the most conservative for each exposure pathway—ingestion, dermal
adsorption, and inhalation. Using the most conservative value is appropriate since expo-
sure to contaminated groundwater can occur in each of these pathways. For instance, der-
mal adsorption can occur during washing, ingestion can occur through drinking, and
inhalation can occur during showering.
Mobility is derived from Henry's law constant and the retardation factor shown in
Equation 10.2 (Kaufman et al. 2005; Rogers et al. 2007a).

M H R = ( )( )

(10.2)

where
M is the mobility
H is the Henry's law constant
R is the retardation factor

Henry's law constant (H) (atm/mol/m 3 ) is a measure of the tendency for substances to
volatilize, and is very useful in assessing the mobility of specific contaminants because
solubility affects the volatilization of contaminants into the atmosphere (Sander 1999). It
is related to vapor pressure (VP) (atm.), molecular weight (MW) (g/mol), and solubility in
water (W s ) (g/L).
Henry's law constants can be obtained from several sources including USEPA (1996a);
Sander (1999); Wiedemeier (1999); Suthersan and Payne (2005); Payne et al. (2008).
The retardation factor is represented by Equation 10.3.

( ρ b Kd )( )

R = + 1 (10.3)

η

where
R is the retardation factor
ρb is the bulk density of aquifer matrix (g/cm 3 )
Kd is the distribution coefficient (mL/g)
η is the effective porosity (calculated as a percent value)

The distribution coefficient is calculated using Equation 10.4.

Kd = ( Foc Koc )( ) (10.4)

where
Kd is the distribution coefficient
Foc is the organic carbon partition coefficient (kg/kg)
Koc is the fraction of total organic carbon in soil (L/kg)