Page 236 - Soil and water contamination, 2nd edition

Substance transport 223

e. Calculate the Peclet number for the situation in question d. What conclusion can you
draw about the contribution of dispersion to overall transport?
Molecular diffusion was not considered in this calculation.
f. Describe qualitatively the effect of molecular diffusion on the above calculations
(hint: see Table 11.1)

7. Estimate the mixing length in a river with the following characteristics: width = 4 m,
-1
water depth = 1 m, bed slope = 0.0001, and average flow velocity = 31 cm s .
8. Describe the principles of the method of characteristics to minimise numerical dispersion.

9. After a spill of 50 litres of a NAPL into a water-saturated soil with a porosity of 30
percent, the maximum relative saturation of the NAPL in the soil reaches 60 percent.
From Figure 11.14 determine:
a. The relative permeability of the soil with respect to water.
b. The relative permeability of the soil with respect to the NAPL.
The NAPL is slowly replaced by water (see the imbibition curve in Figure 11.14).
c. From Figure 11.14 determine the residual saturation of the NAPL.
d. Explain why the NAPL will not be completely replaced by water in the contaminated
soil and a NAPL residue will remain.
e. Calculate the volume of soil contaminated by the NAPL at residual saturation.

10. A landfill is situated just above the water table on a permeable, sandy subsoil. An aquitard
is present at about 40 m depth and dips to the west at 3 percent. Groundwater in the
-1
unconfined aquifer flows east at a velocity of 10 m y .
a. Sketch the chloride concentrations in the leachate in a west–east cross-section of the
unconfined aquifer 10 years after construction of the landfill.
-3
Leaking containers filled with tetrachloromethane (specific density = 1.59 g cm ) and
-3
toluene (specific density = 0.87 g cm ) are present in the landfill.
b. In the same figure, sketch the plumes of tetrachloromethane and toluene.
c. Based on your knowledge of organic pollutants (see Chapter 9) and the behaviour of
NAPLs in the subsurface, discuss the fate of the tetrachloromethane and toluene in
the long term.

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