Page 268 - Soil and water contamination, 2nd edition

Chemical transformation 255

e. In the above figure sketch the breakthrough curve of nitrate. Briefly explain the
processes responsible for the position and shape of the curve.

6. A road accident involving a tanker causes a spill of 10 000 l of benzene into the soil.
Quick cleanup recovers most of the substance, but some benzene reaches the shallow
water table. Some of this benzene dissolves in the groundwater. The log octanol–water
partition coefficient log K of benzene is 2.13. The aquifer material consists of sandy
ow
-3
sediment with a bulk density of 1.8 g cm and a porosity of 0.3. The weight fraction of
organic carbon in the aquifer sediment is 0.8 percent. The local horizontal groundwater
-1
flow velocity is 12 m y .
a. Estimate the maximum horizontal distance over which the benzene is transported in
10 years. Assume that the benzene transport is only affected by retardation. (Hint:
choose an appropriate relation between K and K from Table 13.1).
ow oc
b. Will the aquifer be contaminated over the entire aquifer depth? Motivate your answer.
c. Discuss other processes that affect the evolution of the benzene plume in the
subsurface environment.
7. Explain why a convex isotherm results in tailing of the breakthrough curve.

8. A canal discharges effluent from a wastewater treatment plant. At two locations in the
canal, water samples were collected and analysed for ammonium, nitrate, and phosphate.
The first sampling location was near the effluent outflow and the second sampling
location was 1 km further downstream. Flow velocity and water temperature were also
measured. The data are given in the table below.

Location NH 4 + NO 3 - PO 4 3- T U x
-1
-1
(mg l ) (mg l ) (mg l ) (°C) (m s )
-1
-1
Near effluent discharge << d.l. * 15.0 2.6 16.5 0.12
1 km downstream from effluent discharge << d.l. * 13.5 2.1 16.5 0.12
* d.l. = detection limit
a. Using the above data, calculate the denitrification rate constant at 20 °C.
-1
b. Calculate the distance needed to lower the nitrate concentration below 10 mg l ,
assuming that the other parameters remain constant.
By means of a field experiment the equilibrium phosphate concentration in the canal was
-1
determined to be 0.5 mg l .
c. Calculate the phosphate fixation rate constant.

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