Page 349 - Soil and water contamination, 2nd edition

336 Soil and Water Contamination

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which were probably derived from the oxidation of sediment organic matter. NH
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concentrations in the groundwater also decreased throughout the plume transect after cessation
of wastewater emissions, from peak concentrations ranging from 100 to >400 μM in the anoxic
zone in 1996 to less than 90 μM at the distal end of the plume transect in 2004 (Repert et al.,
2006). It is likely that after the source has been fully depleted and aerobic conditions have
taken over in the aquifer, the aquifer will have permanently changed features. For example, Fe
and Mn oxyhydroxides may have been depleted in the central part of the plume and enriched
further downgradient due to the migration of reduced Fe and Mn (Christensen et al., 2000).

17.10 FURTHER READING ON CONTAMINANTS IN GROUNDWATER
• Appelo, C.A.J. and Postma, D., 2005, Geochemistry, Groundwater and Pollution, 2nd
edition, (Leiden NL: Balkema/Taylor and Francis).
• Ginn, J.S. and Russel Boulding, J., 2003, Practical Handbook of Soil, Vadose Zone, and
Ground-Water Contamination: Assessment, Prevention, and Remediation, 2nd edition,
(Boca Raton FL: Lewis/CRC).
• Mayer, A.S. and Hassanizadeh S.M. (Eds.), 2005, Soil and Groundwater Contamination:
Nonaqueous Phase Liquids, (Washington DC: American Geophysical Union).

EXERCISES
1. Explain briefly:
a. why chloride concentrations in groundwater are generally lower below heather than
below forests;
b. why dissolved iron disappears in the sulphate reduction zone in groundwater;
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c. why the Na : K ratio increases from the centre to the edge of a landfill leachate
plume;
d. why the alkalinity of groundwater increases sharply at a certain depth in a sandy
aquifer;
e. why groundwater in peatlands contains no nitrate.
2. Name three fundamental causes of the temporal changes in groundwater composition
observed in a groundwater monitoring well.

3. Explain why in some regions, the mean nitrate concentartions in groundwater do not
show a decreasing trend in repsonse to the reduction in fertiliser application rates
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4. The concentrations of Ca measured in groundwater abstracted from a well rise over
time. Give two possible causes of this.

5. From Figure 17.8 estimate the evapotranspiration rates for forest and heather, given an
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average annual precipitation of 780 mm y and a chloride concentration in rainwater of
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4 mg l (hint: see Box 5.I).
6. Sketch the isolines for the redox potential (in mV) for the same cross-section
downgradient from the Grindsted landfill site as depicted in Figure 17.22 (hint: see
Figure 4.10).

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