Assignment – Benzene in soil
Vacuum ventilation is a method to strip substances with a suitable high henrys constant from the soil
water in the unsaturated zone. The process requires application of a vacuum to a soil environment by
applying suction to wells inserted in the unsaturated zone. Inlet of air to the soil can come from the soil
surface or inlet ventilation wells. The inlet well can be supplied with pressurized air forcing an air flow
into the soil. This is also referred to as air-sparging. Air sparging can also be applied below the ground
water table to strip substances from the groundwater. The soil must be sandy for this process to work
well, as the soil voids in silt and clay soils are normally too small and water-filled for the process to
work.
In a soil environment an oil contamination has led to a concentration of benzene in the soil water in the
unsaturated zone of 300 mg/L. The dimensionless Henrys constant for Benzene is 0.25. The
contaminated soil volume is estimated to be 10 m3. The porosity of the soil is 0.30 and the soil water
content is 150 l of water per m3 of soil.
A) If water and air is at equilibrium, calculate the maximum concentration of benzene in stripped
soil gas Cx,g.
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B) If the stripping is ideal (and not limited by diffusive Transport of benzene in the water phase),
estimate the quantity of air that must be flushed through the soil to achieve a benzene
concentration in the soil water of 10 mg/l. For this calculation we assume that the benzene is
not sorbed to the soil particles, but only found in the water and air. Furthermore, we assume that
the stripping is so slow that we constantly have equilibrium between soil water and gas. You
can make the calculation either as an integration of the stripping process, or numerically and
stepwise assuming first removal og 10 mg/l from the water and hence a water concentration of
290 mg/l – how much air is needed to remove the first 10 mg/l? Then continue with removal of
another 10 mg and so on. In the end sum up the calculated gas volumes for each step.
C) With a log(KOW) = 2.1 for benzene and a soil organic matter contend foc = 1 % calculate the Kd
using “Abduls formula” and estimate the concentration of benzene sorbed to the soil at
equilibrium before the stripping begins..
D) With a soil porosity of 0.3 and a water filled porosity of 0.15 and goiven a density of the soil
particles of 2.65 kg/l calculate the total quantity of benzene in the 10 m3 of soil if the
distribution of benzene is uniform and there is equilibrium between the gas, water and particle
phases (before the start of the stripping process).
E) A stripping process operated on/off for some time will typically show “rebound” as indicated in
the figure below. Can you give an explanation of the nature of the “rebound”?
(It may help you to know that the diffusion coefficient for benzene in air is 9.3 x 10-2 cm2/s and
for benzene in water (at 10 oC) is in the order of 0.66 x 10-5 cm2/s).
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