Page 192 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 175

Solution:
(a) The flow rate has been determined in Example 5.10 to be 0.216
m /min, or 7.6 ft /min.
3
3
(b) The subsurface data are the same as those in Example 5.5, and
the extracted benzene vapor concentration has been determined
to be 47.5 mg/L, or 47.5 g/m .
3
(c) Assuming the overall efficiency factor is equal to 1, the removal
rate can be found from Equation (5.9) as:

R removal = [(η)(G)](Q)
= [(1.0)(47.5 g/m )](0.216 m /min)
3
3
= 10.26 g/min = 14,770 g/day = 32.5 lb/day

Discussion:
The estimated value of 32.5 lb/day is on the high side because the over-
all efficiency factor is assumed to be unity. In addition, the removal
rate would drop because the benzene concentration in the subsur-
face decreases as the venting project progresses.

5.2.6 Cleanup Time
Once the COC removal rate is determined, the cleanup time (T cleanup ) can be
estimated as:

T cleanup = M spill /R removal (5.10)
where M spill is the amount of spill to be removed. M spill can be found by using
Equation (5.11):

M spill = (X initial − X cleanup )(M ) = (X − X cleanup )[(V)(ρ )] (5.11)
s
t
s
where X initial is the average initial COC concentration in soil, X cleanup is the
soil cleanup level, M is the mass of the impacted soil, V is the volume of the
s
s
impacted soil, and ρ is the total bulk density of the soil. If the cleanup level
t
is very low compared to the initial COC concentration, it can be deleted from
Equation (5.11) as a factor of safety for design.
These two equations appear simple. However, estimation of the cleanup
time is complicated by the fact that the COC removal rate is changing. The
rate decreases as the amount of the COCs left in the soil decreases. One
approach is to divide the cleanup into several time intervals. The removal
rate for each interval is determined and used to estimate the cleanup time