Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 348

Values for the organic carbon partition coefficient can be obtained from numerous
sources including USEPA (1996a), Wiedemeier (1999), USEPA (2002a), Suthersan and
Payne (2005). To obtain the best representation, values for the fraction of organic carbon
should be collected in the field. If field collection is not possible, standard values and
ranges can be obtained from USEPA (1996a), Wiedemeier (1999); USEPA (2002a); Suthersan
and Payne (2005).
The retardation factor represents the ratio between the rate of groundwater movement
and the rate of contaminant movement. When the retardation value equals 1, the rate of
groundwater movement equals the rate of contaminant movement and no retardation is
expected. A retardation value > 1 indicates groundwater movement is greater than contam-
inant movement, so increasing values indicate greater contaminant retardation (USEPA
1989, 2009b).
Persistence values are obtained from the literature and expressed as first-order decay
rates in years (Howard et al. 1991; USEPA 1996a,b, 2000). The first-order decay rates
selected for each compound represent the most conservative values of the spectrum of
data available.
Finally, the CRF GW is calculated in Equation 10.5 by multiplying the inverse of the chemi-
cal compound's toxicity (T), by the inverse of its mobility (M) and its persistence (P).

1 1

CRF

GW =

× × ( ) P (10.5)

( T M ) ( )

where
CRF GW is the contaminant risk ractor for groundwater
T is the toxicity
M is the mobility
P is the persistence

The inverse of the toxicity value must be used because the integer values assigned for
toxicity decrease with increasing carcinogenicity (USEPA 2009a). The inverse of the mobil-
ity values must also be used because the calculated values of retardation decrease with
increasing mobility.
Here is an example calculation of the CRF GW for a chemical XYZ in a geological unit
composed of sand:

1 1

CRF for chemical XYZ = × × ( ) P

GW

( T M ) ( )

Step 1 : Obtain toxicity value:
The toxicity of XYZ chemical was obtained from the literature and has a value of 0.04.
Step 2 : Determine the mobility value:
We need the Henry's law constant and the retardation factor. Let us calculate the retar-
dation factor first. We start with the distribution coefficient (Kd) using Equation 10.4:

Kd = ( Foc Koc )( )

Foc was obtained through analysis of several soil samples in the study area and was found
to be 0.0003 kg/kg. Koc for chemical XYZ was obtained from the literature and has a value
of 58.9 L/kg (USEPA 2002a). We can now calculate the distribution coefficient