Environmental Engineering Reference
In-Depth Information
but there are no reports of the impact this has on metal concentrations or the
ecology within the literature.
Experiments conducted in field situations revealed that once iron-coated
rocks were placed in a clean environment desorption of sulphate occurred
from the precipitates, and this resulted in destabilisation of oxyhydroxides
enough to release them from the rock surface (DeNicol & Stapleton
2002
).
The release of fine flocculated precipitate was thought to have a much greater
impact upon the fauna than the adhering precipitate, which was not directly
toxic. The dispersal and deposition of sediments, therefore, can affect metal
toxicity temporally (Besser et al.
2009
). This has important implications in
remediation as once the source of contamination is removed the accumulated
iron precipitates on the sediments will no longer be in equilibrium with the
aqueous environment and, thus, could be a potential source of metals to the
environment (DeNicol & Stapleton
2002
).
Not all reports support the importance of sediment chemistry. In other
studies, water chemistry was found to be more important than sediments in
the toxicity to benthic communities (Van Damme et al.
2008
). This was also true
when comparing surface water to pore water quality, which is closely related to
sediment chemistry (Battaglia et al.
2005
). Al and Fe contaminated sediments
were also found not to be directly toxic and that it was the pore waters and
water column that were the cause of toxicity in invertebrates (Schmidt et al.
2002
). Again, it is likely that the toxicity is related to specific environmental
characteristics and the metals present.
If we return to the question of the Water Framework Directive, this legisla-
tion requires that surface waters should be of 'good ecological and chemical
status' by the year 2015. Although this does not directly refer to sediment
quality, it is clear from the reports above that sediments may act as a source of
metals to the environment and affect the ecological status of the system. It is,
therefore, imperative that sediment quality is also considered in the manage-
ment of river catchments. Macklin et al.(
2003
) noted that there are no specific
European standards for heavy metals in sediments and this has not changed to
date. However, a recent report by the Environment Agency in the UK (Hudson-
Edwards et al.
2008
) has highlighted the potential importance of mining sedi-
ments as a risk to ecosystem health and reported on the development of
sediment quality guidelines for the assessment of harm from such deposits.
The development of guidelines is a major step forward in tackling this poten-
tial environmental threat, although it is recognised that more toxicological
data are required for sediments.
Recovery of macroinvertebrate communities
There have been significant efforts to prevent the release of industrial dis-
charges to rivers over recent years following changes in environmental
