-Environmental Engineering- Estimate the clean time in the presence of free prod
ID: 107865 • Letter: #
Question
-Environmental Engineering-
Estimate the clean time in the presence of free product after gasoline spill.
A gasoline spill occurred during filling of an UST (Underground Storage Tank) located at a gas station. The owner of the gas station had previously installed a soil venting well near the UST which can be used for this spill.
The following data was collected immediately after the spill:
Size of the contaminant plume (in vadose zone): 2000 ft3
Initial concentration of gasoline (fresh): 7500 mg/Kg
Desired final gasoline concentration: 150 mg/Kg
Assume: The gasoline will be weathered within 3 days after the gas extraction starts. Operating conditions for the soil venting well:
Pressure at the extraction well: 0.85 atm
Diameter of the soil venting well: 4 in
Permeability of the formation: 1 Darcy
Radius of influence of the venting well: 90 ft
Well screen length: 15 ft
Viscosity of air: 0.018 centipoise
Temperature: 20 C
Porosity: 0.35
Organic contaminant of soil: 0.01
Degree of water saturation: 0.30
Bulk density of soil: 1.8 g/cm3
Removal efficiency of the well: 0.15 (Rremoval = G Q where = 0.15)
Questions:
1. Estimate the extracted vapor flow rate of the soil venting well. Assume the pressure at the radius of influence is 1 atm. (Hint: a. First calculate the velocity of gas flow (uw) at the well. Refer to the example with the title: “Estimate the extracted vapor flow rate of a soil venting well” b. Estimate the flow rate at atmospheric pressure (Qatm).)
2. Estimate the time required until disappearance of the free product. Follow the procedure in the example titled “Estimate the clean time in the presence of free product phase”
3. Estimate the time required until the desired soil concentration is achieved (150 mg/Kg)
Explanation / Answer
1.The ectracted vapor flow is calculated using the velocity of gas flow and then the flow rate at atmospheric pressure is calculated.
The velocity of gas flow=(4*pressure at extraction well)/(pi*(diameter of soil venting well)2)
=(4*1)/(3.14*(0.332)2) (1 inch = 0.083ft,therefore 4 inch = 0.332ft)
=11.42
Flow rate at atmospheric pressure=(1/4)*(diameter)2*velocity*pi)
=(1/4)*0.332^2*11.42*3.14
=0.9952
2.From the above example of "estimate the clean time in the presence of free product phase",we find that within 3days the gasoline will be weathered.
The time for disapperence of free product=3 days approximately.
3.Time for desired soil concentration=(initial concentration/final concentration)*(1/3)
=(7500/150)*(1/3)
=16.66=17days
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