Page 191 - Materials Chemistry, Second Edition

174 Practical Design Calculations for Groundwater and Soil Remediation

Discussion:
1. The extracted vapor flow rate in this example is relatively small,
at 7.6 ft /min. However, the calculated removal rates, 920 lb/day
3
for the fresh gasoline and 155 lb/day for the weathered gasoline,
are extraordinarily high. If the removal rate can be sustained
at this level, the site would be cleaned up in a matter of days.
Unfortunately, this is not the case. It normally takes months, if
not longer, for a typical soil-venting project to reach completion.
The overall efficiency factor was set as unity, which is extraordi-
narily high.
2. Since gasoline is a mixture of compounds, the removal rate
will drop, as the more volatile ones have left the formation (as
indicated by the five times lower removal rate of the weathered
gasoline). However, the value of 155 lb/day corresponding to the
weathered gasoline is still on the high side because the limita-
tions of mass transfer were not included in this calculation. The
removal rate should drop further after the free-product phase
disappears.

Example 5.15: Estimate the COC Removal Rate (in the
Absence of the Free-Product Phase)

A subsurface is impacted by benzene. The average benzene concentration of
the soil samples, taken from the impacted zone, was 500 mg/kg. A soil-vent-
ing well (4-in. diameter) was installed at a site. The pressure in the extraction
well is 0.9 atm, and the radius of influence of this soil-venting well has been
determined to be 50 ft.
Estimate the benzene removal rate at the beginning of the project using the
following information:

• Permeability of the formation = 1 darcy
• Well screen length = 20 ft
• Viscosity of air = 0.018 centipoise
• Temperature of the formation = 20°C
• Porosity = 0.35
• Organic content = 0.03
• Water saturation = 45%
• Dry bulk density of soil = 1.6 g/cm 3
• Total bulk density of soil = 1.8 g/cm 3