Environmental Engineering Reference
In-Depth Information
the Water Framework Directive (EU/2000/60/E). However, although the
presence of metals within river waters has been shown to be important in
responses of macroinvertebrate communities, in many cases concentrations of
metals are actually below toxicity thresholds or water quality objectives. This
does not necessarily result in a 'clean' environment, because high concentra-
tions of metals may be present in sediments and particularly those that are fine
grained (e.g., Petersen et al.
1998
; Brumbaugh et al.
2007
). For example, sedi-
ments within the Britannia Creek in Vancouver were found to be significantly
enriched with copper, although active mining ceased in 1974 (Levings et al.
2004
). Structural and functional changes have been reported in rivers that have
received metals historically but no longer have a direct metals source due to
the presence of metals within sediments (Besser et al.
2009
). The extent to which
this may affect communities is unclear as there is little information on the
ranges and degree of contamination in sediments, particularly in relation to
urban pollutants (Beasley & Kneale
2003
).
Metal sludge deposition was reported to cause significant toxicity to amphi-
pods related to Cu, Zn and Cd. Zn in sediment was found to be particularly
important in toxicity to cladocerans (Finlayson et al.
2000
). Zn accumulation is
found to be in direct proportion to sediment levels, and therefore, the aqueous
and particulate sources are important in controlling the concentrations of Zn
found in macroinvertebrates (Goodyear & McNeill
1999
). Sediment Fe and Mn
concentrations were found to be closely related to the growth and reproduc-
tion of amphipods (Besser et al.
2007
); however, pore water concentrations of
Ni, Zn, Cd and Pb were also found to be important in amphipod survival. The
process by which organisms are affected by sediment metal concentrations is
not clear (e.g., Milani et al.
2003
), although it is possible that direct toxicity may
be due to ingestion (see earlier section). Where Fe is present then it is more
likely that invertebrate response is affected by the presence of iron hydroxide
precipitates (see earlier section).
It is thought that continued remobilisation of metals within the hyporheic
zone may be the cause of changes in the hyporheic community, which has been
reported to occur even 7 years after the source of metals was removed (Nelson &
Roline
1999
). This was attributed to changes in the chemical characteristics in
the hyporheic zone from influxes of water at lower pH which would alter the
form in which the metal was present. In floodplain sediments, changes in the
water table can cause fluctuations in the oxygen status of the deposits and,
therefore, release metals from their insoluble form, but this is less likely to
occur within river channel sediments (Hudson-Edwards et al.
1998
). When
metals are present in the more mobile fractions and, therefore, have higher
bioavailable concentrations, then they can cause toxic responses in sediment
associated organisms (Riba et al.
2006
). Scouring and dispersal of river sedi-
ments may also occur during periods of high flow, potentially releasing metals,
