2 *Gasoline contaminated groundwater has been transported under a residential dw

Question

2 *Gasoline contaminated groundwater has been transported under a residential dwelling from a nearby gasoline station. Two meters beneath the 100 m2 dirt floor of the residential basement, the concentration of hydrocarbon vapors in the soil air is 25ppm on a mass/mass basis. Assume the following: The basement is well ventilated so that the gasoline vapor concentration in the basement is much less than 25 ppm (i.e. close to 0). The molecular diffusion coefficient is 10-2 for gasoline vapor in the soil. Air density is 1.2 g/L Pressure = 1 atm Temperature = 20° C

a Estimate the flux density of gasoline vapor and the daily rate of vapor inflow transported into the basement by molecular diffusion.

b What would be the flux density of gasoline vapor through the basement floor if the basement was not well ventilated and the concentration was 25 ppm? (Show your work.)

Explanation / Answer

Concentration of hydrocarbon vapors in the soil air 25ppm = 25g per million grams of air

Concentration of vapor two meters beneath the 100 m2 dirt floor of the residential basement,

C ={(25g/106g)*1.2g/L}/1000L = 3*10-8 g/L

Upward concentration gradient of vapor, dC/dz

= (3*10-8 g/dm3)/20 dm = -1.5*10-9 g/dm4

Flux density, J

= -D(dC/dz)

= -(10-2 dm2/sec).( -1.5*10-9 g/dm4)

= 1.5*10-11 g/dm2.sec

The daily rate of vapor transport

= (1.5*10-11 g/dm2.sec) * (104 dm2).(24*60*60 sec)

= 0.0123 g/day

b. Flux density when the basement is not well ventilated, J

= -D(0/dz)....[when concentration of vapor is same below the basement and at the basement, i.e., 25 ppm)

= 0

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