Geoscience Reference
In-Depth Information
3.4.1.4 Urbanization
also appears to apply to organic contaminants, as
shown by Lau et al. (1989), who estimated that
chlorinated organic contaminants were almost
equally distributed between the soluble and par-
ticulate phases in the Detroit River, USA. The
'dissolved' portion encompasses contaminants
that are truly dissolved, colloids or sulphide
'clusters'. The latter were described by Rozan
et al. (2000), who showed that Fe, Cu and Zn
were bound to multinuclear sulphide clusters
in oxic rivers in Connecticut and Maryland,
USA, which received substantial inputs of sewage
effluent. The 'particulate' portion of the con-
taminant load comprises contaminant-rich grains
(e.g. metal sulphide grains from tailings effluent)
or contaminant-bearing Fe and Mn oxide coat-
ings on other particles.
In rivers subject to acid-mine or acid-rock
drainage, enormous quantities of metals can be
transported downstream in the solute phase
(Filipek et al. 1987). In general, however, this
solute phase is considerably reduced downstream
of the source as pH rises to neutral (usually as a
result of tributary or ground-water inputs), and
the dissolved metals are precipitated or adsorbed
onto sediments (Davis & Leckie 1978). In rare
cases, such as the Río Tinto in south-west Spain,
acid pH is maintained along the whole course of
the river and significant quantities of dissolved
metals are transported to estuaries where they
precipitate or are sorbed to other particulates
(Hudson-Edwards et al. 1999b). In rivers of
neutral or higher pH, metals and other sub-
stances are largely transported downstream in
the particulate load, as discussed above (Gibbs
1973; Benjamin & Leckie 1982; Horowitz &
Elrick 1987).
The partitioning of contaminants between
the dissolved and particulate load in fluvial sys-
tems depends on both physical and chemical
factors. The chemical factors include variations
in amounts of suspended and deposited sedi-
ments (Gibbs 1977; Kratzer 1999), adsorption
onto fine-grained material, co-precipitation with
or sorption on hydrous Fe-Mn oxyhydroxides
and carbonates, association with organic matter
(Karickhoff 1981), incorporation in crystal lattices
of minerals, acidification, salinity, complexing
Urban expansion can have a major effect on
channel morphology and sediment supply and
deposition. Activities such as building on
floodplains, increased ground-water extraction,
channelization, flood embankment and urban-
drainage-system construction, sewage disposal
and (potentially contaminated) road runoff are
among those that have major effects. These effects
include floods and slope failure, particularly in
tropical areas (Gupta & Ahmad 1999). Often
the maintenance of river channels and flood
alleviation systems is extremely costly.
Sediment discharge is often highest during
the initial stages of urban development, when
stripping of soil and vegetation to make way
for built structures is at a peak. After urbaniza-
tion is complete, there is a period of adjustment
as rivers respond to decreased sediment supply.
The combination of the removal of soil and
vegetation, creation of impermeable surfaces and
installation of drainage networks in urban areas
causes increased runoff during storms, with rela-
tively short lag times and high peak discharges
(Knighton 1998). More detailed information on
urbanized rivers can be found in Chapter 6.
3.4.2 Sediments as sinks for contaminants:
transport, deposition and remobilization
3.4.2.1 Sediment contaminants
As discussed in section 3.2.1.1, rivers are subject
to contamination by a large number of metal,
nutrient, organic and radionuclide elements and
compounds. A large proportion of the contam-
inant load in fluvial systems is transported by
particulate matter. Trefry & Presley (1976) and
Gibbs (1977) suggested that up to 90% of the
metal load is transported by sediments in many
rivers, but warned that this varied from metal
to metal. This was borne out by Goldstone et al.
(1990), who showed that the majority of Zn
released from effluent in Norwich, England,
existed principally as dissolved species (95%),
whereas Pb was partitioned equally between dis-
solved and particulate phases. The partitioning
