49
deeper sediments could result from the formation of phosphorus minerals such as
vivianite.
4. !.9 Effect of oxygen-depleting substances (Nn4 +, Fe 2+)
The oxic surface layer acts as a trap for PO43" and NH4 + released by the
mineralization of organic matter. When this oxic layer is eliminated because of 02depletion in the overlying column, or by strong resuspension of the sediments due to
high diacharge or dredging, the buffer capacity of the sediments will be destroyed,
3+
9
.
and a release of PO4
and NH4 mto the overlying water can be expected. In
extreme situations this can lead to an ecological catastrophe for the aquatic
ecosystem.
In April 1992, an extensive fish kill occurred in the Schwarzbacher River at
Neckarbischofsheim, directly near a discharge conduit of a sewage treatment plant.
To find the reason for this ecological disturbance, two "peepers" were installed in this
area. Site A was located at upstream from the discharge pipe of the treatrnent plant,
while site B was at downstream from the pipe.
The measured porewater profiles at site A and site B are shown in Fig. 4~21. At site
B, NO 3- and SO42" decreased more rapidly than at site A. Dissolved Mn 2 and Fe 2+
concentrations were about two times higher than at site A. As a product of the
decomposition of organic matter, the highest N~rI4 ~ concentration (5.2 mM) was
found at site B, which was about four times higher than at site A. The porewater
profiles measured at site A and B show different decomposition processes of organic
matter. Since the water flows slowly at site B, more suspended particles deposit at site
B compared to site A. Therefore, higher input of labile organic matter to the surface
sediments of site B may result in higher decomposition rates. This explanation is
supported by higher concentrations of NH4 + and alkalinity in the sediments at site B
compared to site A.
The mineralization of organic matter in anoxic sediments results in the formation of
NHa +, Fe 2+, and sulfide. Generally, these oxygen-depleting substances diffuse
upward and are oxidized in the oxic surface layer. They have no serious effect on the
ecosystems due to their low concentrations. If the oxic layer is destroyed (e.g.
resuspension of sediments by flood or dredging), the surface sediments may lose their
+
2+
scavenger function and release NH4 , Fe , and sulfides into the overlying water. As
a result, the oxygen demand in the overlying water would increase:
NH4 + + 2 02 --~ NO 3- + H20 + 2 H +
4 Fe 2+ + 02 + 10 H20 --). 4 Fe(OH)3 + 8 H +
4 FeS + 9 02 + 10 H20 ---9, 4 Fe(OH)3 + 4 H2SO4
.4.
1 g of NH 4 can consume 4.5 g of 02. According to Imhoff (1976), 1 liter river sludge can
consume within 24 hours an amount ofO 2 equal to the quantity of dissolved O 2 in 100 liter of
overlying water. Mailer and SchIeichert (1977) measured the amounts of suspended sediment
and dissolved 02 in the Rhine, where a fish kill occurred between 6-8 June, 1971. As a result
of heavy rains in the catchment areas of several tributaries, the water level rose unusually fast
deeper sediments could result from the formation of phosphorus minerals such as
vivianite.
4. !.9 Effect of oxygen-depleting substances (Nn4 +, Fe 2+)
The oxic surface layer acts as a trap for PO43" and NH4 + released by the
mineralization of organic matter. When this oxic layer is eliminated because of 02depletion in the overlying column, or by strong resuspension of the sediments due to
high diacharge or dredging, the buffer capacity of the sediments will be destroyed,
3+
9
.
and a release of PO4
and NH4 mto the overlying water can be expected. In
extreme situations this can lead to an ecological catastrophe for the aquatic
ecosystem.
In April 1992, an extensive fish kill occurred in the Schwarzbacher River at
Neckarbischofsheim, directly near a discharge conduit of a sewage treatment plant.
To find the reason for this ecological disturbance, two "peepers" were installed in this
area. Site A was located at upstream from the discharge pipe of the treatrnent plant,
while site B was at downstream from the pipe.
The measured porewater profiles at site A and site B are shown in Fig. 4~21. At site
B, NO 3- and SO42" decreased more rapidly than at site A. Dissolved Mn 2 and Fe 2+
concentrations were about two times higher than at site A. As a product of the
decomposition of organic matter, the highest N~rI4 ~ concentration (5.2 mM) was
found at site B, which was about four times higher than at site A. The porewater
profiles measured at site A and B show different decomposition processes of organic
matter. Since the water flows slowly at site B, more suspended particles deposit at site
B compared to site A. Therefore, higher input of labile organic matter to the surface
sediments of site B may result in higher decomposition rates. This explanation is
supported by higher concentrations of NH4 + and alkalinity in the sediments at site B
compared to site A.
The mineralization of organic matter in anoxic sediments results in the formation of
NHa +, Fe 2+, and sulfide. Generally, these oxygen-depleting substances diffuse
upward and are oxidized in the oxic surface layer. They have no serious effect on the
ecosystems due to their low concentrations. If the oxic layer is destroyed (e.g.
resuspension of sediments by flood or dredging), the surface sediments may lose their
+
2+
scavenger function and release NH4 , Fe , and sulfides into the overlying water. As
a result, the oxygen demand in the overlying water would increase:
NH4 + + 2 02 --~ NO 3- + H20 + 2 H +
4 Fe 2+ + 02 + 10 H20 --). 4 Fe(OH)3 + 8 H +
4 FeS + 9 02 + 10 H20 ---9, 4 Fe(OH)3 + 4 H2SO4
.4.
1 g of NH 4 can consume 4.5 g of 02. According to Imhoff (1976), 1 liter river sludge can
consume within 24 hours an amount ofO 2 equal to the quantity of dissolved O 2 in 100 liter of
overlying water. Mailer and SchIeichert (1977) measured the amounts of suspended sediment
and dissolved 02 in the Rhine, where a fish kill occurred between 6-8 June, 1971. As a result
of heavy rains in the catchment areas of several tributaries, the water level rose unusually fast