Environmental Engineering Reference
In-Depth Information
In the majority of studies, the authors have assessed impacts in terms of
metrics such as species richness and abundance with some summarisation
achieved through the use of biological indices. National monitoring of riverine
systems also utilise these assessment techniques, the most commonly used
being the Biological Monitoring Working Party (BMWP) score (see
Chapter 6
).
However, this system was based upon organic pollution rather than acidity and
metals contamination and authors have questioned the use of this for indus-
trial pollution (e.g., Metcalfe-Smith
1994
). Grey and Delaney (
2008
) compared
scoring systems in the assessment of acid mine drainage and found that the
BMWP score was reasonably well correlated with abiotic factors such as pH,
sulphate and Zn. However, a similar study carried out in Spain found that
BMWP score was not entirely appropriate because some families with the
highest BMWP scores were found at both polluted and unpolluted sites in
rivers impacted by mine drainage (Garc´a-Criado et al.
1999
). Alternative bio-
logical indices such as the Acid Water Indicator Community (AWIC) index has
been proposed, which can distinguish between acid and neutral sites (e.g.,
Wade et al.
1989
; Davy-Bowker et al.
2005
; Riipinen et al.
2008
), but these have
not been proven to be appropriate for discharges that involve significant
concentrations of metals. Given the previously highlighted need to identify
organisms to species level combined with the complicated interaction between
metals and pH, simple biological indices may not be appropriate for assessment
of the ecological impact of metalliferous contamination.
Direct toxicity of metals
Toxicity within organisms only occurs when the rate of metal uptake exceeds
the combined rate of excretion and detoxification of the metabolically avail-
able form of the metal (Rainbow
2002
). The effects of metals on organisms can
range from those which are undetectable, through sub-lethal to lethal. Mech-
anisms of metal toxicity are mainly linked to biochemical reactions involving
(a) competitive blockage of a functional group or macromolecule at the cell
membrane, this can disrupt transport and membrane stability (Gerhardt
1993
);
(b) displacement of essential cations by toxic metals, zinc can cause cross-
linking of DNA-molecule inhibiting transcription process, whereas copper
can depress the nervous system (Gerhardt
1993
); and (c) conformational change
in proteins, copper can bind to certain enzymes and inhibit their action
(Flemming & Trevors
1989
). Heavy-metal physiological effects are manifested
mainly as hypoxia (deficiency of oxygen reaching the body tissues), caused by a
reduction in gas exchange due to coagulation and precipitation of mucus or
cytological damage (Koryak et al.
1972
; Sridhar et al.
2001
; Niyogi et al.
2002
).
This information has mainly been obtained through the use of laboratory
based toxicity tests. The main problem with toxicity tests is that they are rather
simplistic given that the majority use single species and/or single contaminants
