21
have led to an accumulation of PO43- in the porewater, pH values varied between 7.3
and 8.0 in the porewater (Fig. 4.2h). Conductivity increased continuously with depth,
indicating the release of ions into the porewater during early diagenetic processes
(Fig. 4.2i).
Organic carbon (Corg.) contents in the sediments of the study area are plotted in
Fig.4.3. In the Neckar River, the Corg. contents was the lowest at Kochendorf with
values of 3.6 %, slightly higher at Lauffen (4.3 %), whereas the highest was at
Wieblingen with 4.7 %. No significant change with depth was found in the Neckar
sediments (Fig. 4.3a).
The sediments of the Elsenz River mainly accumulate from soil runoff. The average
Corg. content was low (2.3 %) at this site. In the Enz River sediments, 7.8 % of Corg.
was measured in the surface sediment layer, and the contents of Corg. decreased with
depth (Fig. 4.3b).
With the decomposition of organic matter, a decrease of Corg. with depth in the
sediments should be expected. In fact, no distinct decrease of Corg. with depth was
found in the sediments of the Neckar River. This is in contrast with the results in
marine sediments (Balzer 1989). It seems that the primary sedimentation of organic
matter rather than diagenetic processes controls the contents of Corg. in the
sediments. In the Enz River sediments, a high input of organic matter is reflected by
the high Corg. contents in the surface sediment layer (7.8%). This may result in a
rapid depletion of 02 and an accumulation of labile organic matter in the surface
sediments. Therefore, the decrease of Corg. with depth might be a result of the
decomposition of labile organic matter within the sediments.
4.1.2 Denitrification in the sediments
At all sites, NO3- concentrations in the overlying water varied between 0.28 and 0.67
mM. They decreased rapidly with depth in the porewater until 20 cm depth (Fig. 4.4).
Below this depth, NO 3- concentrations ranged from 0.01 to 0.04 rmM. As the aerobic
oxidation of organic matter releases NO3" into porewater, the depth, where maximum
NO 3- concentration occurs, represents the depth of 02 penetration. Below this depth,
O 2 concentration is too low for an oxidation. Such a NO3" peak is reported for many
marine sediments (Froelich et al. 1979; Goloway and Bender 1982; J'ahnke et al.
1982; Bender and Heggie 1984; Reimers and Smith 1986; Jalmke et al. 1989).
However, no peak of NO3" concentrations was found below the sediment-water
interface in the study area. It seems that 02 was consumed within the first millimeters
below the sediment-water interface. The sampling interval (2 cm) may be too coarse
to observe this peak. Similar NO3" profiles were also found by Kuivila and Murray
(1984) in Lake Washington, and by Sagemann et al. (1994) in the Weser Estuary,
Germany. Such a thin oxic layer is typical of orga2ni.'c-rich sediments. This feature, in
conjunction with the data of Fe oxide and SO4 " reduction, indicates that anoxic
conditions control porewater chemistry in the sediments of the Neckar River and its
tributaries.
have led to an accumulation of PO43- in the porewater, pH values varied between 7.3
and 8.0 in the porewater (Fig. 4.2h). Conductivity increased continuously with depth,
indicating the release of ions into the porewater during early diagenetic processes
(Fig. 4.2i).
Organic carbon (Corg.) contents in the sediments of the study area are plotted in
Fig.4.3. In the Neckar River, the Corg. contents was the lowest at Kochendorf with
values of 3.6 %, slightly higher at Lauffen (4.3 %), whereas the highest was at
Wieblingen with 4.7 %. No significant change with depth was found in the Neckar
sediments (Fig. 4.3a).
The sediments of the Elsenz River mainly accumulate from soil runoff. The average
Corg. content was low (2.3 %) at this site. In the Enz River sediments, 7.8 % of Corg.
was measured in the surface sediment layer, and the contents of Corg. decreased with
depth (Fig. 4.3b).
With the decomposition of organic matter, a decrease of Corg. with depth in the
sediments should be expected. In fact, no distinct decrease of Corg. with depth was
found in the sediments of the Neckar River. This is in contrast with the results in
marine sediments (Balzer 1989). It seems that the primary sedimentation of organic
matter rather than diagenetic processes controls the contents of Corg. in the
sediments. In the Enz River sediments, a high input of organic matter is reflected by
the high Corg. contents in the surface sediment layer (7.8%). This may result in a
rapid depletion of 02 and an accumulation of labile organic matter in the surface
sediments. Therefore, the decrease of Corg. with depth might be a result of the
decomposition of labile organic matter within the sediments.
4.1.2 Denitrification in the sediments
At all sites, NO3- concentrations in the overlying water varied between 0.28 and 0.67
mM. They decreased rapidly with depth in the porewater until 20 cm depth (Fig. 4.4).
Below this depth, NO 3- concentrations ranged from 0.01 to 0.04 rmM. As the aerobic
oxidation of organic matter releases NO3" into porewater, the depth, where maximum
NO 3- concentration occurs, represents the depth of 02 penetration. Below this depth,
O 2 concentration is too low for an oxidation. Such a NO3" peak is reported for many
marine sediments (Froelich et al. 1979; Goloway and Bender 1982; J'ahnke et al.
1982; Bender and Heggie 1984; Reimers and Smith 1986; Jalmke et al. 1989).
However, no peak of NO3" concentrations was found below the sediment-water
interface in the study area. It seems that 02 was consumed within the first millimeters
below the sediment-water interface. The sampling interval (2 cm) may be too coarse
to observe this peak. Similar NO3" profiles were also found by Kuivila and Murray
(1984) in Lake Washington, and by Sagemann et al. (1994) in the Weser Estuary,
Germany. Such a thin oxic layer is typical of orga2ni.'c-rich sediments. This feature, in
conjunction with the data of Fe oxide and SO4 " reduction, indicates that anoxic
conditions control porewater chemistry in the sediments of the Neckar River and its
tributaries.