Geoscience Reference
In-Depth Information
reactions are nitrate reduction, Mn(IV) reduction,
Fe(III) reduction, sulphate reduction and meth-
anogenesis. All of these reactions break down
organic matter and, therefore, lead to an overall
decrease in organic matter content as sediments
are buried. Many of these reactions can only
utilize simple organic molecules, such as acetate
and hydrogen, as the reductant. However, some
bacterial communities, particularly iron-reducing
bacteria, have been shown to possess the ability
to utilize complex organic molecules (Lovley
& Anderson 2000). Therefore, such diagenetic
reactions may act to break down persistent
organic contaminants in aquatic sediments.
Bacteria can also directly mediate the reduction
of some contaminant metals, for example Cr, U,
Se, Hg and Tc (e.g. Lovley 1993).
Early diagenetic reactions have an impact upon
the short- and long-term fate of contaminants in
sediments through two principal mechanisms:
release of contaminants into sediment porewaters;
and the uptake of contaminants into authigenic
mineral precipitates. The oxidation of organic
matter and the reduction of iron and manganese
oxides result in the release of contaminants asso-
ciated with these mineral phases to sediment
porewaters (Rae & Allen 1993). These increased
porewater contaminant concentrations can result
in the molecular diffusion of contaminants into
the overlying water column (commonly termed
a 'benthic flux'). There is a growing awareness
that benthic contaminant fluxes to intertidal
environments can be as significant as riverine
input and may act as a major long-term input
of contamination into water bodies. Rivera-
Duarte & Flegal (1997a; b) documented that
benthic fluxes of Co and Zn from sediments in
the San Francisco Bay were of the same magni-
tude as riverine inputs. Similarly, Shine et al.
(1998) showed that the flux of Cd and Zn from
coastal sediments in Massachusetts, USA was
of a similar magnitude to that within the water
column itself.
In marine and brackish intertidal sediment-
ary environments, sulphate reduction is a major
pathway for organic-matter oxidation and as a
result sulphide is released into porewaters. Sul-
phide forms a highly stable complex with most
metals (Cooper & Morse 1998) and consequently
metals released by Fe(III) and Mn(IV) reduction
will be precipitated out as sulphides. These pre-
cipitates are predominantly in the form of iron
monosulphides, and metals may be adsorbed onto
the sulphide surfaces, or incorporated into the
sulphide structure (Parkman et al. 1996). Early
diagenetic metal sulphides have also been docu-
mented in mining-impacted estuarine sediments,
acting as a long-term sink for contaminants in
these sediments (Pirrie et al. 2000; see Chapter 7).
In contrast to natural sediments, early diagenetic
mineral precipitates within contaminated sedi-
ments can be varied and unique. For example,
Pirrie et al. (2000) described the occurrence of
early diagenetic simonkolleite (a Zn-Cl mineral)
from metal-contaminated estuarine sediments.
Early diagenetic minerals (e.g. vivianite - iron
phosphate) can also be important in non-marine
sediments (see Chapter 6).
1.5.2 Remobilization from floodplains
Contaminants may also be remobilized from
river floodplains. Floodplains are sites of sedi-
ment accumulation within river basins and,
therefore, are classically considered to be con-
taminant sinks, thereby preserving good tem-
poral records of contaminant input (e.g. Smol
2002; Chapter 3). These sinks of contaminants,
however, can also become sources as a result of
post-depositional processes, both chemical and
physical. For example, Hudson-Edwards et al.
(1998) demonstrated that remobilization of Pb,
Zn, Cd and Cu within overbank sediments of
the River Tyne, England, occurred as a result
of changes in water-table levels and the break-
down of organic matter above the water table
(see Chapter 3).
Contaminants stored on floodplains also may
be remobilized through physical erosion, and
this may take place long after the primary con-
taminating activity (e.g. mining) has ceased. For
example, Macklin (1992) showed that the prim-
ary source of Pb and Zn to the contemporary
River Tyne, northern England, was remobilized
floodplain alluvium originally deposited during
eighteenth and nineteenth century metal mining.
