29
The computer program MINTEQA2 was used to solve a set of simultaneous nonlinear equations. Parameters for these equations are set by the thermodynamic
equilibrium and mass balance. The principal advantage of this model is to consider
the effect of complexation by ligands and competition of other minerals.
The saturation calculation indicates that the porewater is slightly supersaturated (SI
2) in respect to rhodochrosite in the sediments of the study area (Fig. 4.6, 4.7).
Using X-ray diffraction analysis for Baltic Sea sediment samples, Suess (1979)
identified that the dominant crystalline phase was a mixed Mn-carbonate:
(Mno.85CaloMg0.05)CO3, rather than pure MnCO3. This is in agreement with the
results from Aller (1980b), Pedersen and Price (1982). As no thermodynamic
parameters are available for the mixed Mn-carbonate, solid solution of other
components such as Ca and Mg considerably complicates the calculation of
saturation. It is reasonable to assume ideal behavior (pure mineral) for the component
which has a mole fraction N > 0.9 (Pedersen and Price 1982). The rhodochrosite
2+
and/or Mn-Ca-carbonate appear to control the dissolved Mn
in the anoxic
sediments. Suess (1979) found that Mn-carbonate in the Baltic Sea sediments
primarily occurs within thin layers consisting of a porous, spongy framework of
amorphous silica. It seems that precipitation of the manganous carbonate is possibly
enhanced by the presence of gram surfaces which encourage seed mineral formation
or nucleation. The porewater was strongly undersaturated in respect to alabandite
(MnS; Fig. 4.6 and 4.7), so control by alabandite is not possible.
4.1.4 Cycling of iron in the sediments
2+
At Lauffen, dissolved Fe
increased until 0.20 rmM between 10 and 14 cm depth.
Below this layer, it varied between 0.16 and 0.30 mM. At Kocherkdorf, Fe z+
9
.
.
,Z+
increased slowly between 8 and 50 cm depth. Likewise, an increase of Fe
was also
found m the sediments at Wieblingen.
~-4..
.
.
.
I~ow Fe ' concentrations were measured in the Elsenz River sediments. Dissolved
/.'4.
.
.
.
Fe
mcre~,sed with depth to 0.03 rmM at 48 cm depth (Fig. 4.9a). The maxtmum
2+
value of Fe
was 0.53 mM at 32 cm depth m the Enz Rwer sediments (Fig. 4.9b). In
the
2"+
"
Schwarzbach River sediments, Fe
cortcentrations increased continuously to
09
2+
"
9 0 mM at 42 cm depth. Below this depth, Fe
decreased with depth (Fig. 4.9c).
No significant difference of particulate Fe with depth was found at all sites. The
average values were the same at Lauffen, Kochendorf, and at Wieblmgen (3.0 %; Fig.
4.8). Higher contents of particulate Fe were measured in the sediments of the
tributaries: 4.5 % m the Elsenz River and 3.4 % m the Enz River.
Similar to Mn cycling, the behavior of Fe is controlled by redox reactions. Ferric Fe
is present as oxyhydroxides in the oxic waters. It is reduced to Fe 2+ in the anoxic
sediments. Unlike other electron acceptors such as 0 2, NO3", and SO42-, Mn and Fe
oxides remain high concentrations through anoxic sediments. This is probably due to
their extremely low solubility, which may make them less biologically available than
other oxidants.
The computer program MINTEQA2 was used to solve a set of simultaneous nonlinear equations. Parameters for these equations are set by the thermodynamic
equilibrium and mass balance. The principal advantage of this model is to consider
the effect of complexation by ligands and competition of other minerals.
The saturation calculation indicates that the porewater is slightly supersaturated (SI
2) in respect to rhodochrosite in the sediments of the study area (Fig. 4.6, 4.7).
Using X-ray diffraction analysis for Baltic Sea sediment samples, Suess (1979)
identified that the dominant crystalline phase was a mixed Mn-carbonate:
(Mno.85CaloMg0.05)CO3, rather than pure MnCO3. This is in agreement with the
results from Aller (1980b), Pedersen and Price (1982). As no thermodynamic
parameters are available for the mixed Mn-carbonate, solid solution of other
components such as Ca and Mg considerably complicates the calculation of
saturation. It is reasonable to assume ideal behavior (pure mineral) for the component
which has a mole fraction N > 0.9 (Pedersen and Price 1982). The rhodochrosite
2+
and/or Mn-Ca-carbonate appear to control the dissolved Mn
in the anoxic
sediments. Suess (1979) found that Mn-carbonate in the Baltic Sea sediments
primarily occurs within thin layers consisting of a porous, spongy framework of
amorphous silica. It seems that precipitation of the manganous carbonate is possibly
enhanced by the presence of gram surfaces which encourage seed mineral formation
or nucleation. The porewater was strongly undersaturated in respect to alabandite
(MnS; Fig. 4.6 and 4.7), so control by alabandite is not possible.
4.1.4 Cycling of iron in the sediments
2+
At Lauffen, dissolved Fe
increased until 0.20 rmM between 10 and 14 cm depth.
Below this layer, it varied between 0.16 and 0.30 mM. At Kocherkdorf, Fe z+
9
.
.
,Z+
increased slowly between 8 and 50 cm depth. Likewise, an increase of Fe
was also
found m the sediments at Wieblingen.
~-4..
.
.
.
I~ow Fe ' concentrations were measured in the Elsenz River sediments. Dissolved
/.'4.
.
.
.
Fe
mcre~,sed with depth to 0.03 rmM at 48 cm depth (Fig. 4.9a). The maxtmum
2+
value of Fe
was 0.53 mM at 32 cm depth m the Enz Rwer sediments (Fig. 4.9b). In
the
2"+
"
Schwarzbach River sediments, Fe
cortcentrations increased continuously to
09
2+
"
9 0 mM at 42 cm depth. Below this depth, Fe
decreased with depth (Fig. 4.9c).
No significant difference of particulate Fe with depth was found at all sites. The
average values were the same at Lauffen, Kochendorf, and at Wieblmgen (3.0 %; Fig.
4.8). Higher contents of particulate Fe were measured in the sediments of the
tributaries: 4.5 % m the Elsenz River and 3.4 % m the Enz River.
Similar to Mn cycling, the behavior of Fe is controlled by redox reactions. Ferric Fe
is present as oxyhydroxides in the oxic waters. It is reduced to Fe 2+ in the anoxic
sediments. Unlike other electron acceptors such as 0 2, NO3", and SO42-, Mn and Fe
oxides remain high concentrations through anoxic sediments. This is probably due to
their extremely low solubility, which may make them less biologically available than
other oxidants.