Environmental Engineering Reference
In-Depth Information
Figure 4.1.
Acid mine drainage from Parys Mountain, Anglesey, UK, showing the
characteristic orange colour of iron hydroxide deposits coating the riverbed.
Photograph taken 3 km downstream of the main discharge.
See colour plate section
.
Investigations in a catchment receiving mine effluents revealed that all of
the rivers were impacted in terms of macroinvertebrate assemblages when Fe
was present at concentrations
0.44mg L
1
(Freund & Petty
2007
). This is
because of the highly reactive nature of iron under oxidising conditions. The
water quality limit in the United States is set at 0.5mg L
1
, because above this
concentration iron hydroxide precipitates can coat stream beds. Within the EU,
there is currently no specific water quality standard for iron, although the
drinking water standard is set at 200
m
gL
1
. However, the Water Framework
Directive will require good ecological and chemical status for water bodies,
and therefore the potential impact of low concentrations of iron on biota
through the precipitation of hydroxide minerals may increase the importance
of iron release to the environment. Metal precipitates forming in rivers receiving
metalliferous discharges are not only iron hydroxides but can also be manganese
oxides and aluminium hydroxides (e.g., Younger et al.
2002
;Balistrieriet al.
2003
).
Al, Fe and Mn hydroxide precipitates have been observed on invertebrate species
(
Fig. 4.2
) and on the sediments within affected streams (e.g., McKnight & Feder
1984
;Gerhardt
1992
), with crevices being filled by flocculants (Schmidt et al.
2002
).
All these metal precipitates act in a similar way to any other suspended
solids within rivers and can impact macroinvertebrates in a number of ways.
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