Geoscience Reference
In-Depth Information
result of post-depositional chemical alteration
(section 6.4; Case Study 6.3 and Case Fig. 6.3).
Very few published data are available on the
mineralogical and geochemical association of
these metal contaminants (Taylor et al. 2003).
Canal and dock sediments undergo regular
dredging to maintain water depths, and this
material is now classed as controlled waste. The
little published information on the contamin-
ant geochemistry of these sediments is based on
hazard assessment work with application to
disposal or dredged material (e.g. Bromhead &
Beckwith 1994). For example, Kelderman et al.
(2000) showed that for the canals in Delft, The
Netherlands, 95% of inner city canal sediments
were classed as highly polluted, whereas only
33% of sediments in the outer city were highly
polluted. There is a requirement for more detailed
research into the
in situ
processes operating on
such sediments, and their role in contaminant
cycling in urban aquatic systems.
cussed in section 6.3.2. This section will focus
on the early diagenetic chemical and physical
changes taking place in sediments within urban
canals and docks.
Early diagenesis is the sum of the processes
operating upon a sediment after deposition and
includes physical, chemical and biological pro-
cesses. The early diagenesis of aquatic sediments
is dominated by a series of bacterially mediated
redox reactions, which result in the oxidation of
carbon species (organic matter) and the reduc-
tion of an oxidized species (Fig. 6.10). Within
sedimentary environments that have an oxy-
genated water column, which includes virtually
all urban water bodies, the primary reaction
upon sediment deposition is aerobic oxidation,
whereby O
2
dissolved in the water is utilized
to oxidize organic matter. As O
2
is primarily
sourced from the overlying water column, this
oxygen is rapidly used up in the first few milli-
metres of sediment. The depth of O
2
penetration
into the sediment depends on organic matter con-
tent, sedimentation rate and biological activity.
In highly organic systems, such as sewage-
contaminated water bodies and lakes (e.g. Boyd
et al. 1999), the sediment oxygen demand through
aerobic oxidation may be high enough to result
in an anoxic water column, especially under
low-flow conditions (Fig. 6.11). Such low-flow
conditions are common in steep-sided canals and
docks, and the high organic matter contents of
the sediments can result in the rapid consump-
tion of oxygen in the water column, leading to
serious water quality problems.
POST
-
DEPOSITIONAL CHANGES IN URBAN SEDIMENTS
6.4
As was stated in section 6.2.2, urban sedi-
ments undergo physical and chemical changes
at many stages of the urban sediment cascade.
The two environments in which sediment
potentially undergoes major chemical changes
(that are most likely to have an impact upon
sediment reactivity and contaminant mobility),
however, are gully pots and canals, docks and
lakes. The former of these has been briefly dis-
Reaction
Products
Reactants
Aerobic
reaction
Aerobic
oxidation
HCO
3
−
+ H
+
CH
2
O + O
2
Manganese
reduction
Iron
reduction
CH
2
O + 2MnO
2
+ H
2
O 2Mn
2+
+ HCO
3
−
+ 3OH
−
CH
2
O + 2Fe
2
O
3
+
3H
2
O
4Fe
2
+
+ HCO
3
−
+ 7OH
−
2CH
2
O + SO
4
2
−
HS
−
+ 2HCO
3
−
+ H
+
Sulphate
reduction
Fig. 6.10
Early diagenetic reactions
taking place within aquatic urban
sediment in docks, canals and lakes.
(From Taylor et al. 2003.)
2CH
2
O + H
2
O CH
4
+ HCO
3
−
+
H
+
Methanogenesis
