Environmental Engineering Reference
In-Depth Information
effects can be observed at low pH (3.3) with decreased locomotion and
increased diurnal activity (Gerhardt et al.
2005
). It should be noted that, when
metals were also present, this caused more significant effects in terms of
locomotion and ventilation than acidity alone (Gerhardt et al.
2005
).
The presence of adapted populations demonstrates the importance of the
history of contaminated environments as it is fluctuations away from the
'norm' that cause impacts upon communities (Courtney & Clements
1998
),
and this can be significant even in very short episodes (Felten & Gu´rold
2006
). However, as noted earlier there is little information on the importance
of short episodes of acidification and/or metal contamination on organisms in
the environment, particularly in the process of recovery.
The effects of acidity and metals are extremely difficult to separate out, and
within the literature there are no satisfactory reports that effectively do this. If
we are to fully appreciate and understand the effects of different types of
metalliferous discharge then this should be a priority within the ecotoxicolo-
gical community. Recent advances in metabolomic studies provide an oppor-
tunity to separate out these two stresses using experimental approaches. Some
reports have shown that there may be potential biomarkers that can be used to
monitor responses to metals (see Morgan et al.
2007
for a comprehensive review
of this area), but further work is needed before metals and acid toxicity are
fully understood.
Iron hydroxides
Iron is an essential element to both faunal and floral growth and therefore is
not often examined in toxicological studies, despite often being present at
high concentrations within industrial discharges (Nordstrom et al.
2000
;
Younger et al.
2002
). In the majority of waters, iron is transported in the
particulate form (e.g., Forstner & Wittman
1979
); however, under acidic
conditions iron can be present as Fe(II), which is the most toxic form (Ger-
hardt
1992
). There is little information on the direct toxicity of Fe to aquatic
invertebrates, but it is possible that mechanisms could include its role in DNA
and membrane damage as observed in vertebrates (Vuori
1995
). However,
although direct toxicity is unlikely, except in very extreme environments iron
poses another problem to aquatic communities. Iron can react with oxygen
within receiving waters to produce iron hydroxides which are present as
precipitates within the water column and quickly settle out onto the riverbed
producing the characteristic red/orange colouration of receiving water
courses (e.g., Younger et al.
2002
)(
Fig. 4.1
). The chemical reactions involved
in the production of iron hydroxides also generate acidity within the environ-
ment (
equations 1
and
2
)
Fe
2
þ
þ
=
2
H
2
O
!
Fe
ð
OH
Þ
3
ð
s
Þ
þ
2H
þ
=
4
O
2
ð
aq
Þ
þ
2
1
ð
1
Þ
1
Fe
3
þ
þ
3H
2
O
$
Fe
ð
OH
Þ
3
ð
s
Þ
þ
3H
þ
ð
2
Þ
