Determining the partitioning of contaminants between soil and water at a site is an important part of any assessment.  The following calculator uses the EPA soil/water partition equation for organic contaminants (Soil Screening Guidance Technical Support Document, EPA, 1996). This equation is used to relate the contaminant concentrations in soil moisture to the total concentration detected in a soil sample.

Where:           C_t = soil concentration (mg/kg)

                     C_w = aqueous concentration (mg/L)

                     k_oc = soil organic carbon/water partition coeffiecient (L/kg)

                     f_oc = fraction organic carbon (kg/kg)

                     P_w = water-filled soil porosity

                     P_a = air filled soil porosity

                     P_b = dry soil bulk density (kg/L)

                     H_d = dimensionless Henry’s Law

 

C_w: 

k_oc:   

f_oc: 

P_w:   

P_a: 

P_b:   

H_d: 

 

C_t:   

 

This equation can be used for several regulatory purposes, including RBCA in several states.  The newly released RBCA Tool Kit for Chemical Releases v2.0 makes calculations and report writing for RBCA regulations significantly easier.

Example

There is a known Benzene source, with a concentration of 6.04 x 10^-2 mg/L in a sand aquifer (properties of sand: foc=0.06, Pw=0.079, Pa=0.321, Pb=1.5 kg/L).  The koc and Hd of Benzene are found to be 6.17 x 10¹ L/kg and 2.28 x 10^-1 respectively.  What is the expected concentration of Benzene in the sand?

The solution is 2.85 x 10^-2 mg/kg.

 

 

References

 

EPA, 1996. Soil Screening Guidance Technical Support Document.