alkalinity profiles can be separated into two zones: the SO42" reduction zone and the
CH4 f~ma.entation zone.
PO4 " is released into porewater not only by ~ degradation of organic matter, but
also ~.y the reduction of Fe oxides, at which PO4 - is adsorbed. In anoxic sediments,
PO4" concentrations appear to be controlled by the formation of vivianite
(Fe3[PO4] 2 " 8H20). The porewater is supersaturated in respect to vivianite,
suggesting the slow kinetics of vivianite precipitation.
The concentrations of Br" increase with depth. The very high positive correlations
between Br, NH4*, and alkalinity reflect that bromine, originally a constituent of the
organic matter in the sediments, is released as Br during the decomposition of
organic matter. Therefore, the sediments act both as sinks and sources for bromine in
aquatic systems.
The high concentrations of NH4 + and PO43- in the porewater result from the
mineralization of organic matter. However, the flux of NH4' and PO4 from the
sediments into overlying water is low3This can be explained by the existence of an
oxic surface layer. In this layer, PO4 " diffused from the deeper sediments can be
adsorbed on the freshly formed Fe oxides, whde NH4 wdl be oxidized to NO3 9
9
.
.
-~
Whel~ the oxlc ~ayer is destroyed (e.g. by flood or dredging), a release of Ntt 4 ,
PO4 , and Fe
from the sediments Into the overlying water may occur. The
9
9
+
2+
spontaneous oxldatmn of NH4 and Fe
can cause a severe O.~-depletion in the
overlying water, which might lead to fish killss 9 In addition, NH4- is toxic for fish,
too.
As compared with the supemating water, the concentrations of the metals Cu, Pb,
Zn, and Cd in the porewaters of the anoxic sediments are considerably lower. This
can be explained by the formation of highly insoluble metal-sulfides. In contrast, Cr
concentrations in the anoxic porewater are generally higher than in the supernating
waterr 9 The explanation lies in the fact, that Cr does not form Cr-sulfide and therefore
its solubility is not determined by the HS" concentration of the porewater.
The higher concentrations of Co in the porewater f'md their explanation in the
relatively higher solubility of CoS as compared with other heavy metals. With the
reduction and dissolution of Fe and Mn oxides, Cr and Co are simultaneously
released into the porewater.
In the Neckar River sediments, the Acid Neutralizing Capacity (ANC) of the
sediments is much higher than the Acid Producing Capacity (APC). This is due to the
high carbonate content (6.5 % - 25 %) of the sediments. Therefore, an oxidation of
the anoxic sediments must not necessarily lead to an acidification. In this case, a
significant release of heavy metals by the resuspension of the sediments cannot be
expected. Considering seasonal variations of the porewater profiles, the peaks of
dissolved Cd, Zn, Pb, and Cu at the sediment-water interface may not be caused by
sediment leaching, but may result from the decomposition of organic matter
containing these metals.
In summary, the porewater profiles show that heavy metals are not leached from
but rather diffuse into the sediments, The sediments therefore act as a sink rather than
a source.
CH4 f~ma.entation zone.
PO4 " is released into porewater not only by ~ degradation of organic matter, but
also ~.y the reduction of Fe oxides, at which PO4 - is adsorbed. In anoxic sediments,
PO4" concentrations appear to be controlled by the formation of vivianite
(Fe3[PO4] 2 " 8H20). The porewater is supersaturated in respect to vivianite,
suggesting the slow kinetics of vivianite precipitation.
The concentrations of Br" increase with depth. The very high positive correlations
between Br, NH4*, and alkalinity reflect that bromine, originally a constituent of the
organic matter in the sediments, is released as Br during the decomposition of
organic matter. Therefore, the sediments act both as sinks and sources for bromine in
aquatic systems.
The high concentrations of NH4 + and PO43- in the porewater result from the
mineralization of organic matter. However, the flux of NH4' and PO4 from the
sediments into overlying water is low3This can be explained by the existence of an
oxic surface layer. In this layer, PO4 " diffused from the deeper sediments can be
adsorbed on the freshly formed Fe oxides, whde NH4 wdl be oxidized to NO3 9
9
.
.
-~
Whel~ the oxlc ~ayer is destroyed (e.g. by flood or dredging), a release of Ntt 4 ,
PO4 , and Fe
from the sediments Into the overlying water may occur. The
9
9
+
2+
spontaneous oxldatmn of NH4 and Fe
can cause a severe O.~-depletion in the
overlying water, which might lead to fish killss 9 In addition, NH4- is toxic for fish,
too.
As compared with the supemating water, the concentrations of the metals Cu, Pb,
Zn, and Cd in the porewaters of the anoxic sediments are considerably lower. This
can be explained by the formation of highly insoluble metal-sulfides. In contrast, Cr
concentrations in the anoxic porewater are generally higher than in the supernating
waterr 9 The explanation lies in the fact, that Cr does not form Cr-sulfide and therefore
its solubility is not determined by the HS" concentration of the porewater.
The higher concentrations of Co in the porewater f'md their explanation in the
relatively higher solubility of CoS as compared with other heavy metals. With the
reduction and dissolution of Fe and Mn oxides, Cr and Co are simultaneously
released into the porewater.
In the Neckar River sediments, the Acid Neutralizing Capacity (ANC) of the
sediments is much higher than the Acid Producing Capacity (APC). This is due to the
high carbonate content (6.5 % - 25 %) of the sediments. Therefore, an oxidation of
the anoxic sediments must not necessarily lead to an acidification. In this case, a
significant release of heavy metals by the resuspension of the sediments cannot be
expected. Considering seasonal variations of the porewater profiles, the peaks of
dissolved Cd, Zn, Pb, and Cu at the sediment-water interface may not be caused by
sediment leaching, but may result from the decomposition of organic matter
containing these metals.
In summary, the porewater profiles show that heavy metals are not leached from
but rather diffuse into the sediments, The sediments therefore act as a sink rather than
a source.