Page 138 - Soil and water contamination, 2nd edition

Nutrients 125

table , phosphate breakthrough to the groundwater may occur. In anaerobic groundwater,
phosphate is much more mobile, because of the absence of ferric hydroxides.
Another phosphorus sink on agricultural land is soil loss due to soil erosion ; this may be
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up to 60 kg P ha y . Soil erosion is greater on arable land than on grassland and forested
land and increases with increasing slope gradient. Most of the eroded soil particles are
redeposited at the foot of slopes and may ultimately enter a river network.
Besides inputs of eroded soil, the principal external sources of phosphorus in surface
water consist of effluents from wastewater treatment plants, untreated wastewater discharges,
and industrial releases. Phosphates, primarily sodium tripolyphosphate, are used as a laundry
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detergent additive to eliminate free Ca during the washing process. Phosphate detergents

may contribute up to one third of the total phosphorus load of effluent from wastewater
treatment plants. The rest is primarily from human excrement and wasted foodstuffs. Since
the introduction of phosphate -free detergents during the 1980s and the implementation
of tertiary wastewater treatment (see Section 3.4.3 and Box 3.I), the phosphorus loads
from wastewater treatment plants have been reduced substantially. In surface water, too,
phosphates are mainly associated with particulate matter , and the deposition of these
suspended particles promotes the removal of phosphates from the water column. As a
consequence, phosphorus may build up in the bottom sediments of rivers and lakes . If, as
is generally the case, the redox potential in bed sediments is low, the iron hydroxides that
bond phosphate dissolve. Therefore, the phosphate concentrations in the pore water of
bed sediments are much larger than the concentrations in the overlying water column. The
resulting concentration gradient between the pore water of bed sediments and surface water
may cause a net phosphorus flux from the bed sediment to the surface water. This process is
called internal loading , as it refers to the recycling of previously present phosphorus in the
water system. Internal loading may continue to cause phosphorus releases into surface waters
many years after the direct releases have ceased. The effect of internal loading is greater in
shallow lakes than in rivers or deep lakes. This is because the length of the water column over
which the released phosphorus is distributed is greater and the residence time is longer in
deep lakes than in shallow lakes and rivers. The process of internal phosphorus loading may
seriously hamper the ecological rehabilitation of shallow lakes that have had a long history of
external phosphorus loading (Van der Molen and Boers, 1994).

EXERCISES

1. Define the following terms
a. Micronutrients
b. Eutrophication
c. Nitrogen immobilisation
d. Nitrogen fixation
e. Ammonium fixation
f. Phosphate fixation
g. Ammonification
h. Nitrification
i. Denitrification
j. Internal loading

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2. The water in a lake contains 0.63 mg N l and 0.041 mg P l . Is this lake N or P limited?
3. Name four consequences of eutrophication of aquatic ecosystems.

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