Urban environments - Environmental Sedimentology - page 213

Upon the consumption of O 2 , a series of
anerobic bacterial reactions are favoured, utlizing
oxygen in species such as NO 3 2− , Fe 2 O 3 , MnO 2 ,
SO 4 2− (Figs 6.10 & 6.11). Classically these were
seen as taking place through a sequential set of
reactions, giving rise to a series of diagenetic
zones (Froelich et al. 1979; Coleman 1985). Such
observations were based on thermodynamic con-
siderations and observations from hemipelagic
recent sediments. Such discrete zones are now
recognized to be the case for slow sedimentation
rate, low productivity environments. In urban
aquatic environments the organic-rich nature of
the sediment results in many of these reactions
taking place simultaneously. These anaerobic
early diagenetic reactions are many and complex,
and a complete description of them is beyond
the scope of this work. The major reactions are
nitrate reduction, Mn(IV) reduction, Fe(III) reduc-
tion, sulphate reduction and methanogenesis
(Fig. 6.11). All of these reactions break down
organic matter and, therefore, lead to an overall
decrease in organic matter content as sediments
are buried. They also tend to result in the decrease
in reactivity of organic matter with depth, having
implications for dredging remediation.
These early diagenetic reactions have an
impact upon the short- and long-term fate of
contaminants in sediments through two prin-
cipal mechanisms: release of contaminants into
sediment porewater; and the uptake of con-
taminants into authigenic mineral precipitates.
Within contaminated sediments, metal contam-
inants commonly co-precipitate with iron and
manganese oxides. The chemical reduction of
these oxides (FeR and MnR) results in the release
of these adsorbed contaminants to sediment pore-
waters (Dodd et al. 2003; Taylor et al. 2003).
These contaminants then become free to be
moved into the overlying water column, through
the process of molecular diffusion (Fig. 6.11).
This process of contaminant movement from
sediments into overlying water is commonly
termed a benthic flux . This benthic flux has been
recognized to be the most significant non-point
source of pollution to water bodies. This release
of chemical species into porewater during early
diagenesis is not restricted to contaminants, it

can also be a major pathway for nutrient release
from sediments. One example is that of ammo-
nium, which is released in the process of organic
matter oxidation. In sewage-contaminated urban
water bodies, large amounts of ammonium can
be released into sediment porewaters, and then
oxidized to nitrate in the water column. Gases
may also be generated from sediment during
early diagenesis. In freshwater organic-rich sedi-
ments, methane gas (CH 4 ) is released from sedi-
ments through the reaction of methanogenesis
(Fig. 6.11). Methane is a flammable and noxious
gas, and so can have major negative impacts
upon water quality in urban canals and docks,
both aesthetically and chemically.
The build up of chemical species in sediment
porewater also leads to the precipitation of
authigenic minerals in the sediment. Within
marine and brackish sediments the predominant
mineral formed in this way is pyrite (FeS 2 ). Pyrite
has been observed in canal sediments (Large et
al. 2002; Taylor et al. 2003) but the absence of
sulphate in freshwater leads to this being a rare
mineral in urban sediments. The limited studies
of the diagenesis of urban sediments have shown
the iron phosphate mineral vivianite (Fe 3 (PO 4 ) 2 )
to be the most common mineral (Fig. 6.12; Dodd
et al. 2003, Taylor et al. 2003). The importance
of these minerals for contaminant mobility is that
metals can be taken up by these minerals as they
precipitate, thereby locking up contaminants
in the sediment (Large et al. 2002; Taylor et al.
2003).

6.5 TEMPORAL CHANGES IN URBAN SEDIMENTS :
NATURAL AND ANTHROPOGENIC IMPACTS

The recent nature of the data on urban environ-
ments is a limiting factor on the identification of
temporal changes in response to internal and
external factors. Indeed, in most urban environ-
ments there is an urgent need for the collection
of baseline data to enable future changes to be
determined. As is evident from other chapters
in this topic there has been extensive use of
sediment records to gain insights into historical
changes in sediment input, land use, climate and