Page 81 - Materials Chemistry, Second Edition

64 Practical Design Calculations for Groundwater and Soil Remediation

(c) The total mass of the COC in soil can be found using Equation
(2.7) as

(V)(ρ )](X) = (M )(X)
t
s
s
= [(1 m )(1,800 kg/m )](1.83 mg/kg)
3
3
= 3,284 mg (for benzene)
= [(1 m )(1,800 kg/m )](268 mg/kg)
3
3
= 482,000 mg (for pyrene)
Discussion:
1. This example illustrates the differences between X and S. For
benzene, the values of X and S are relatively close. For pyrene,
the ratio of the X and S values is essentially the ratio of the dry
bulk density and total bulk density, mainly because the majority
of the pyrene compounds are adsorbed on the surface of the soil
grains.
2. Neglecting the mass in the void has an insignificant impact on
the estimated values of X.
3. The calculated values of total mass in soil are essentially the
same as those in Example 2.37.

Example 2.39: Relationship between Soil Vapor Concentration
and Soil Sample Concentration
The vapor concentrations of benzene and pyrene in the void space of the
vadose zone underneath a landfill are 100 ppmV and 10 ppbV, respectively,
from a soil gas survey. The total porosity of the vadose zone is 40%, and 30%
of the void is occupied by water. The (dry) bulk density of the soil is 1.6 g/
cm and the total bulk density is 1.8 g/cm . The values of the dimensionless
3
3
Henry’s constant for benzene and pyrene are 0.22 and 0.0002, respectively.
Values of K for benzene and pyrene are 1.28 and 717, respectively.
p
Soil samples were taken from the location where the soil gas probe was
located and then analyzed in a laboratory for the COC concentrations in soil.
Estimate the COC concentrations in soil.

Solution:
Basis: 1 L of soil

(a) Let us work on benzene first. We have to convert the vapor con-
centration in ppmV into mg/L first. From Example 2.37, G = 0.324
mg/L for benzene.