18
Fig. 4.1 illustrates the transport processes near the surface sediment layer. The
benthic boundary layer at the sediment surface can be described as a buffer zone
between sediments and water. Its thickness is related to hydrodynamic conditions and
biological activities. At the sediment surface, organic matter such as algae and plant
detritus is rapidly oxidized by 0 2 in overlying water. The depth of 02 penetration
depends on its rate of downward diffusion, which is influenced by biological
activities. In the Neckar River, minimum 02 concentrations of the overlying water
varied between 6.2-8.9 mg/l in summer, and 8.6-11.4 mg/l in winter (Deutsche
Kommission zur Reinhaltung des Rheins 1991). Some aquatic fauna such as mollusks
and worms were found at the surface sediments, giving the evidence of bioturbation
in the surface sediments. Although 02 penetration could extend to a few centimeters
as a result of bioturbation, it was probably too low to drive an oxidation of organic
matter. Therefore, a release of NO3" due to the oxidation of organic matter was not
found in this layerr 9
Below the surface layer, the organic matter was subsequently oxidized by Mn
oxide, NO3", Fe oxide, SO42-, and CO2. Dissolved Mn 2+ (Fig. 4.2a) was negligible
2+
to the depth of 4 cm, where Mn
was precipitated as MnO~. Mn oxide was reduced
2+
"and released Mn
into the porewater below this depth. NO3 values varied between
0.42 and 00 9 mM in the overlying water and decreased rapidly to about 0.03 mM at
20 cm depth (Fig. 4.2b). This suggests that denitrification has occurred near the
.
2+
sediment-water interfacee 9 Fe
was released below 8 cm depth as a result of the
9
9
9
2+
2+
9
9
.
reductton of Fe oxide (Fig. 4.2c). Fe
and Mn
m the deeper sedtments diffused
into the surface layer and were reoxidized to Fe/Mn oxides by 0 2. Similar porewater
profiles of Fe and Mn are reported from many marine and freshwater sediments
(Froelich et al. 1979; Carignan and Nriagu 1985; Eck and Smits 1986; Carignan and
Lean 1991), indicating redox cycling of Fe and Mn in sediments during
mineralization of organic matter.
The SO4 ~'- concentrations ranged from 0.90 to 1.02 mM in the bottom water a~d
decreased rapidly between 8 and 20 cm depth (Fig. 4.2d). Below this layer, SO4
+
was less than 0.I0 mM. The Nil 4 values in the overlying water were 0.01-0.O4 mM,
but increased rapidly with depth, the maximal value was 15 mM at 78 cm depth.
Alkalinity increased also with depth (Fig. 4.2+ef).
This reflects that the formation of NH4 and alkalinity is associated with the
mineralization of organic matter in anoxic sediments (Froelich et al. 1979; Kuivila
and ~urray 1984; Carignan 1985; Kling et al. 1991; Sigg et al. 1991). In this process,
SO4
was the dommati2ng oxidant. An increase of NH4 + and alkalinity was
identified below the SO4 " reduction zone, which can only be explained by CH4
fermentation. Jorgense~ (1983) reported that the methanogenic bacteria cannot
compete with the SO4 - reducing bacteria, thus the CH4 fermentation occurs only
after the consumption of SO4Z" in the sediments. This was conftrmed from the
sediments at Lauffen. Another evidence of CH 4 production are the gas bubbles
escaping from the sediments during sample collecti~n. These gas bubbles most
probably are from CH 4 production. Fe oxide and SO4 " reduction occurred near the
sediment-water interface (8-20 cm depth), suggesting strong anoxic conditions in the
sediments.
Fig. 4.1 illustrates the transport processes near the surface sediment layer. The
benthic boundary layer at the sediment surface can be described as a buffer zone
between sediments and water. Its thickness is related to hydrodynamic conditions and
biological activities. At the sediment surface, organic matter such as algae and plant
detritus is rapidly oxidized by 0 2 in overlying water. The depth of 02 penetration
depends on its rate of downward diffusion, which is influenced by biological
activities. In the Neckar River, minimum 02 concentrations of the overlying water
varied between 6.2-8.9 mg/l in summer, and 8.6-11.4 mg/l in winter (Deutsche
Kommission zur Reinhaltung des Rheins 1991). Some aquatic fauna such as mollusks
and worms were found at the surface sediments, giving the evidence of bioturbation
in the surface sediments. Although 02 penetration could extend to a few centimeters
as a result of bioturbation, it was probably too low to drive an oxidation of organic
matter. Therefore, a release of NO3" due to the oxidation of organic matter was not
found in this layerr 9
Below the surface layer, the organic matter was subsequently oxidized by Mn
oxide, NO3", Fe oxide, SO42-, and CO2. Dissolved Mn 2+ (Fig. 4.2a) was negligible
2+
to the depth of 4 cm, where Mn
was precipitated as MnO~. Mn oxide was reduced
2+
"and released Mn
into the porewater below this depth. NO3 values varied between
0.42 and 00 9 mM in the overlying water and decreased rapidly to about 0.03 mM at
20 cm depth (Fig. 4.2b). This suggests that denitrification has occurred near the
.
2+
sediment-water interfacee 9 Fe
was released below 8 cm depth as a result of the
9
9
9
2+
2+
9
9
.
reductton of Fe oxide (Fig. 4.2c). Fe
and Mn
m the deeper sedtments diffused
into the surface layer and were reoxidized to Fe/Mn oxides by 0 2. Similar porewater
profiles of Fe and Mn are reported from many marine and freshwater sediments
(Froelich et al. 1979; Carignan and Nriagu 1985; Eck and Smits 1986; Carignan and
Lean 1991), indicating redox cycling of Fe and Mn in sediments during
mineralization of organic matter.
The SO4 ~'- concentrations ranged from 0.90 to 1.02 mM in the bottom water a~d
decreased rapidly between 8 and 20 cm depth (Fig. 4.2d). Below this layer, SO4
+
was less than 0.I0 mM. The Nil 4 values in the overlying water were 0.01-0.O4 mM,
but increased rapidly with depth, the maximal value was 15 mM at 78 cm depth.
Alkalinity increased also with depth (Fig. 4.2+ef).
This reflects that the formation of NH4 and alkalinity is associated with the
mineralization of organic matter in anoxic sediments (Froelich et al. 1979; Kuivila
and ~urray 1984; Carignan 1985; Kling et al. 1991; Sigg et al. 1991). In this process,
SO4
was the dommati2ng oxidant. An increase of NH4 + and alkalinity was
identified below the SO4 " reduction zone, which can only be explained by CH4
fermentation. Jorgense~ (1983) reported that the methanogenic bacteria cannot
compete with the SO4 - reducing bacteria, thus the CH4 fermentation occurs only
after the consumption of SO4Z" in the sediments. This was conftrmed from the
sediments at Lauffen. Another evidence of CH 4 production are the gas bubbles
escaping from the sediments during sample collecti~n. These gas bubbles most
probably are from CH 4 production. Fe oxide and SO4 " reduction occurred near the
sediment-water interface (8-20 cm depth), suggesting strong anoxic conditions in the
sediments.