Page 39 - Materials Chemistry, Second Edition

22 Practical Design Calculations for Groundwater and Soil Remediation

Discussion:
1. The total bulk density of soil was assumed to be 112 lb/ft , which
3
is equal to 1.79 g/cm (= 112/62.4)
3
2. The density of TPH was assumed to be 50 lb/ft , which is equal
3
to 0.80 g/cm (= 50/62.4)
3
2.2.3 Amount of Impacted Soil in the Vadose Zone
Chemicals that leaked from USTs might move beyond the tank pit. If subsur-
face contamination is suspected, soil borings are drilled to assess the extent
of contamination in the vadose zone. Soil-boring samples are then taken at a
fixed interval, e.g., every 5 or 10 ft, and analyzed for soil properties. Selected
samples are submitted to certified laboratories and analyzed for COCs. From
these data, a fence diagram is often developed to delineate the extent of the
COC plume.
When selecting remedial alternatives, an engineer needs to know the ver-
tical and areal extents of the plume, types of subsurface soil, types of COCs,
mass and/or volume of the impacted soil, and mass of COCs in different
phases. If the location of the plume is shallow (not deep from the ground
surface level [gsl]) and the amount of the impacted soil is not extensive, exca-
vation coupled with on-site aboveground treatment or off-site treatment/
disposal may be a viable option. On the other hand, in situ remediation alter-
natives, such as soil venting, would be more favorable if the volume of the
impacted soil is large and deep. Therefore, a good estimate of the amount of
impacted soil left in the vadose zone is important for remediation consider-
ations. This section describes the methodology for such calculations.
As mentioned, a fence diagram is often drawn to illustrate the vertical and
areal extents of the plume. Based on the information from the diagram, the
following procedure can be used to estimate the amount of the impacted soil
in the vadose zone:

Step 1: Determine the area of the plume at each sampling depth, A . i
Step 2: Determine the thickness interval for each area calculated above, h . i
Step 3: Determine the volume of the impacted soil, V , using the follow-
s
ing formula:

s V = ∑ A i i h (2.9)
i
Step 4: Determine the mass of the impacted soil, M , by multiplying V
s
s
with the total bulk density of soil, ρ , as:
t
M =ρ ×V (2.10)
s t s