Environmental Engineering Reference
In-Depth Information
Characteristics of metalliferous discharges
Metalliferous releases to the environment occur in two main forms: net-acidic
and net-alkaline. Many of the sources of metals from industry and urban runoff
tend to be net-alkaline, but some, such as mining activities result in the release
of sulphuric acid and hydrogen ions due to the weathering of sulphide min-
erals (predominantly pyrite) resulting in a net-acidic discharge (e.g., Younger
et al.
2002
). The speciation of metals within aquatic systems is controlled by
many environmental factors of which pH and Eh (redox potential) are probably
the most important, although the presence of organics, temperature and
salinity can also be significant controlling factors (Stumm & Morgan
1995
).
Where discharges are acidic in nature, the speciation of metals is such that
they become bioavailable, although the point at which this occurs varies with
the metal in question. In contrast, within net-alkaline discharges metals are
more likely to be sorbed to particulate matter or present as precipitates and
unavailable for biological uptake.
Historically, many metalliferous discharges into water courses would have
been from a point source, such as the output from a processing plant. However,
today the majority of inputs are probably chronic and diffuse and derived from
urban road runoff, contaminated floodplain sediments and mine workings.
The nature of these releases means that the transfer of pollutants is strongly
influenced by local weather conditions, particularly rainfall events, and there-
fore varies temporally as well as spatially. Although traditionally the release of
metals following contact with waters (either storm waters or flooding of mine
workings) are thought the follow the 'first flush' model (e.g., Gray
1998
; Lee
et al.
2002
; Younger et al.
2002
), departures from this classic model have been
reported. For example, concentrations of metals were found to show the
weakest first flush patterns within urbanised areas, particularly in larger
catchments (Tiefenthaler et al.
2008
). In addition Lawler et al.(
2006
) found that
fine sediment (one of the main ways in which metals may be transported)
influxes into a river arrived much later due to the presence of distal sources
such as roads and mine workings. There is, however, little information on the
relative importance of acute and chronic toxicity and the associated patterns
of metal inputs into rivers on controlling macroinvertebrate community
structure which should be an important consideration both in management
and recovery of rivers following remediation efforts.
The potential impact of metals on biota therefore can vary greatly from
site to site, depending upon specific environmental conditions and the sources
of metals. However, there are some overall effects that have been reported
in response to industrial metalliferous discharges, although it should be
noted that the majority of research has focused upon the effects of mining
activities.
