Environmental Engineering Reference
In-Depth Information
of the interstitial waters within the sediment itself. Nevertheless, the
oxygenation system has been successful in elevating bottom water oxygen
levels above the 4mg L
1
target value throughout the whole Turning Basin.
A set of Helixors is used during the summer months to deliver compressed air
and thus to help dissipate the oxygen enriched water within Basin 6, which is
still connected to the MSC. The importance of a constant delivery of oxygen to
the Turning Basin was highlighted during a period of inevitable oxygenation
shutdown for maintenance work in 2003. Two days after switching the units
off, bottom water oxygen fell to below the 4mg L
1
target and within a week
was depleted still further to anoxia in some areas (
Fig. 14.9
). A similar occur-
rence happened in 2005, although for the first time the Helixor system alone
was able to maintain the minimum DO target of 4mg L
1
within Basin 6,
indicating that background conditions have improved (APEM
2006a
).
Although the primary objective is to improve the aesthetic value of the MSC,
the water quality improvements have also allowed the ecology within the
system to develop and diversify. In addition, continual improvements are
occurring in the area upstream of the artificial oxygenation, mirroring the
long-term decline in suspended solids, BOD and ammonia described earlier.
During the year of installation and commissioning (2001), similar BMWP
5
scores of around 15 were recorded in both the oxygenated and non-oxygenated
(control) regions. Recent BMWP scores for the oxygenated and comparison
regions were 28.7 and 21.6, respectively, representing increases of 91% and
44% from the initial score in 2001. Although at present statistically insignifi-
cant (t-test, p
0.09), if the trend continues it would indicate that artificial
oxygenation has allowed improvements to occur at a faster rate than would
otherwise be possible.
In relation to the actual taxa present, for many years the macroinvertebrate
diversity of the upper MSC has been very low, characterised by four or five
pollution-tolerant species typical of poor water environments. Whilst these
species remain, they are accompanied by an annually increasing number of
species indicative of cleaner water. Taxa now recorded include caddis fly
(Phryganea bipunctata and Ecnomus tenellus), freshwater shrimp (Crangonyx pseudo-
gracilis), river limpet (Ancylus fluviatilis), flatworms (e.g., Dugesia sp. and Dendro-
coelum lacteum) and damselfly (Coenagriidae). Particular increases in species
diversity were recorded following the start of the artificial oxygenation
(
Fig. 14.10
). In recent years, a peak of 41 taxa were identified in the oxygenated
area of the upper MSC.
>
5
Biological Monitoring Working Party Score: benthic macroinvertebrate families are allocated
a score based on the tolerance to (organic) pollution and the score totalled (National Water
Council
1981
).
