Environmental Engineering Reference
In-Depth Information
or more forecast complete wetland filling within decades. The outcome of this
infilling is the shallowing of one wetland near Swanport adjacent to the Lower
River Murray. Here, this wetland is celebrated as a complex and diverse mosaic
of aquatic macrophyte communities. It is evident that the pre-industrial condi-
tion was of open water and the upper 2.0m or so of sediment is post-industrial.
So the present condition is derived, and an artefact of accelerated sediment
flux. The sediment accumulation trajectory of this wetland, widely considered
to be in good ecological condition, is for complete infilling in the near future.
In contrast, Murrundi Wetland to its south attained a swamp-like condition
about 2000 years ago (Gell et al.
2005a
). To reinstate its perceived open water
condition, as recollected during the extreme floods of the 1950s, this wetland
has been dredged and, in part, is now an open water environment.
Metal pollutants
Metals are often transported with sediments and can accumulate in sediment
sequences. Fluorescence spectroscopy techniques were applied to sediment
digestates from a range of levels from ten cores from Lake Pepin to reconstruct
changes in mercury flux over time (Balogh et al.
1999
). They identified that the
natural, pre-industrial whole basin mercury accumulation rate was 3 kg yr
1
,
believed to be typical of lakes sampled across the region. From c. 1830 this
increased rapidly to a maximum of 357 kg yr
1
in the 1960s. Since the 1800s,
18.1 tons of Hg were deposited into the lake, with half contributed between
1940 and 1970 when the rate of catchment development increased well ahead
of the implementation of mitigation measures. This increase from baseline
is attributed, initially, to direct industrial uses of the metal and to landscape
disturbance enhancing the transmission of sediments and Hg. In addition, coal
combustion increased atmospheric deposition of Hg to waterways later in
settlement. The commissioning of a wastewater treatment plant in 1938
merely kept pace with accelerating development, until a secondary treatment
capability was added and industrial use of Hg declined in the 1960s. These
changes, and the advent of the Clean Water Act in 1972, have seen a 70%
decline in Hg accumulation in the uppermost sediments relative to the 1960s
peak. The successful control of releases from the treatment plant is mostly
responsible and, now, diffuse sources are again the principal source, attesting
to the relative difficulty in controlling these sources relative to the more
readily identifiable point sources. This is further reinforced by the observations
of ongoing increases in the global, atmospheric contribution of Hg and recent,
regional declines (Engstrom & Swain
1997
). The timing of this recovery is
not unlike that evident from long-term observations from another stream
subject to heavy industrialisation, the River Tame in the UK (
Chapter 13
).
The modern Hg accumulation rate of 110 kg yr
1
remains well above baseline,
but sediment-based approaches have identified that the treatment plant has
