Page 27 - Materials Chemistry, Second Edition

10 Practical Design Calculations for Groundwater and Soil Remediation

(as adsorbed) as well as in the soil moisture (as dissolved). Both the adsorbed
and dissolved COCs in this soil sample would then be extracted and quanti-
fied as a whole. The COC concentration in soil (X) would be reported in the
unit of “mass of COC/mass of soil,” such as milligrams per kilogram. The
mass of a COC in soil can be obtained by multiplying its concentration in soil
with the mass of soil (M ):
s
Mass of COC in soil = (mass of soil) × (COC concentration in soil) = (M )(X)
s
(2.6)
The mass of soil can be estimated as the multiplication product of the
volume and bulk density of the soil. Bulk density is the mass of a material
divided by the total volume it occupies. For civil engineering practices, the
reported values of bulk density are often on a dry-soil basis, i.e., “dry” bulk
density (= mass of dry soil ÷ volume as a whole). However, the COC con-
centration in soil is usually based on the mass of wet soil (soil + moisture).
Consequently, the bulk density used to calculate the soil mass for subse-
quent estimation of COC mass should be the “wet” bulk density (= mass of
dry soil plus moisture ÷ volume as a whole). The “wet” bulk density is also
referred to as “total” bulk density. In this book, ρ is the symbol for total bulk
t
density and ρ is the symbol for (dry) bulk density. The mass of COC in soil
b
can also be found as:
Mass of COC in soil = [(soil volume)(total bulk density)]
× (COC concentration in soil)
= [(V)(ρ )](X) = (M )(X) (2.7)
s
t
s
COC concentration in air (G) is often expressed in vol/vol (such as ppmV)
or in mass/vol (such as mg/m ). In calculation of mass, we need to convert
3
the concentration into the mass/vol basis using Equation (2.1) or (2.2). Mass
of the COC in air can then be obtained by multiplying the mass concentra-
tion with the volume of the air (V ):
a
Mass of COC in air = (air volume) × (COC concentration in mass/volume)
= (V )(G) (2.8)
a

Example 2.1: Mass and Concentration Relationship
Which of the following media contains the largest amount of xylenes
[C H (CH ) ]?
6
3 2
4
(a) 1 million gallons of water containing 10 ppm of xylene
(b) 100 cubic yards of soil (total bulk density = 1.8 g/cm ) having 10 ppm
3
of xylenes