33
only from strongly supersaturated porewater (Postma 1981).in addition, the complexation of Fe with organic colloids (e.g. humic substances) may lead to a
supersaturation of siderite (Emerson 1976; Aller 1980b; Elderfield 1981), which
unfortunately is not considered in the calculation.
4.1.5 Sulfate reduction
SO42" profiles in porewater show a depletion in SO42" below the sediment-water
interface. In the Neckar River, SO4 ~" concentrations in the overlying water varied
from 0.79 to 1.26 mM. They decreased rapidly in the sediments at depths between 2
and 40 cm (Fig. 4.10a). Below 40 cm, the SO42- concentrations were less than 0.05
mM.
The SO42" concentrations in the overlying water of the tributaries were lower than
those in the Neckar River. In the Enz River water 0.50-0,65 roam of SO42- were
measured. A decrease of SO42" in the porewater began at 22 cm depth (Fig. 4.10b). In
the Enz River, the SO42- concentrations decreased rapidly from 0.43 rmM in the
bottom water to about 0.01 mM at 20 cm depth. In the sediments of the Schwarzbach
River, SO42- was consumed between 4 and 20 cm depth. Below this zone, the SO42concentrations were lower than 0.01 mM. SO42- profiles in the sediments at Lauffen
show no significant seasonal variation (Fig. 4.10c).
Numerous studies have attested that the availabilit3,' of SO42- and organic matter in
sediments control SO42" reduction (Mountfort and Asher 1981; Billen 1982;
Jergensen and Sorensen 1985). Freshwater has usually low concentrations of SO42"
(0.1-0.2 mM) compared to sea water (20-30 mM, Capone and Kiene 1988). A large
2
difference in SOa " concentrations between freshwater and marine sediments results
in a different speciation of sulfur in the sediments. Generally, SO,: 2. reduction leads
to HS" production in the sediments:
(CH20 h 13(NH3)IsH3PO4 + 56.5 SO42---~113 HCO 3" + 15 NH 3 + H3PO 4 + 56.5 HS- + 56.5 H +
HS- may react with Fe 2+, yielding Fe monosulfides and pyrite (FeS2). Therefore,
strong SO42" reduction accounts for the accumulation of inorganic reduced sulfur in
marine sediments. Since freshwater has low SO42" concentrations (0.2 raM, Capone
and Kiene 1988) and a high input of organic matter, organic sulfur, instead of
inorganic sulfur, is the main fraction of sulfur in the sediments. As a result of human
activities (e.g. agricultural runoff, industrial and communal discharge), the concentrations of SOa 2 in the Neckar River and its tributaries were 0.40-1.26 raM. The
strong SOa 2- reduction in the sediments of the study area is clearly reflected by the
sharp decrease in SO42" concentrations. In addition, the black color of the sediment
cores is an indicator of amorphous FeS nI-120 and makinawite (FeS). As heavy metals
can be precipitated with HS- to form highly insoluble metal-sulfides or be
only from strongly supersaturated porewater (Postma 1981).in addition, the complexation of Fe with organic colloids (e.g. humic substances) may lead to a
supersaturation of siderite (Emerson 1976; Aller 1980b; Elderfield 1981), which
unfortunately is not considered in the calculation.
4.1.5 Sulfate reduction
SO42" profiles in porewater show a depletion in SO42" below the sediment-water
interface. In the Neckar River, SO4 ~" concentrations in the overlying water varied
from 0.79 to 1.26 mM. They decreased rapidly in the sediments at depths between 2
and 40 cm (Fig. 4.10a). Below 40 cm, the SO42- concentrations were less than 0.05
mM.
The SO42" concentrations in the overlying water of the tributaries were lower than
those in the Neckar River. In the Enz River water 0.50-0,65 roam of SO42- were
measured. A decrease of SO42" in the porewater began at 22 cm depth (Fig. 4.10b). In
the Enz River, the SO42- concentrations decreased rapidly from 0.43 rmM in the
bottom water to about 0.01 mM at 20 cm depth. In the sediments of the Schwarzbach
River, SO42- was consumed between 4 and 20 cm depth. Below this zone, the SO42concentrations were lower than 0.01 mM. SO42- profiles in the sediments at Lauffen
show no significant seasonal variation (Fig. 4.10c).
Numerous studies have attested that the availabilit3,' of SO42- and organic matter in
sediments control SO42" reduction (Mountfort and Asher 1981; Billen 1982;
Jergensen and Sorensen 1985). Freshwater has usually low concentrations of SO42"
(0.1-0.2 mM) compared to sea water (20-30 mM, Capone and Kiene 1988). A large
2
difference in SOa " concentrations between freshwater and marine sediments results
in a different speciation of sulfur in the sediments. Generally, SO,: 2. reduction leads
to HS" production in the sediments:
(CH20 h 13(NH3)IsH3PO4 + 56.5 SO42---~113 HCO 3" + 15 NH 3 + H3PO 4 + 56.5 HS- + 56.5 H +
HS- may react with Fe 2+, yielding Fe monosulfides and pyrite (FeS2). Therefore,
strong SO42" reduction accounts for the accumulation of inorganic reduced sulfur in
marine sediments. Since freshwater has low SO42" concentrations (0.2 raM, Capone
and Kiene 1988) and a high input of organic matter, organic sulfur, instead of
inorganic sulfur, is the main fraction of sulfur in the sediments. As a result of human
activities (e.g. agricultural runoff, industrial and communal discharge), the concentrations of SOa 2 in the Neckar River and its tributaries were 0.40-1.26 raM. The
strong SOa 2- reduction in the sediments of the study area is clearly reflected by the
sharp decrease in SO42" concentrations. In addition, the black color of the sediment
cores is an indicator of amorphous FeS nI-120 and makinawite (FeS). As heavy metals
can be precipitated with HS- to form highly insoluble metal-sulfides or be