NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- This thesis presents the results of field-based, microcosm and laboratory studies on the
impacts of heavy metals on aquatic organisms, and the mobility and bioavailability of
heavy metals in current sediment, as well as in the microcosm and anoxic experimental
sediment in West Lake (Ho Tay), Hanoi, Vietnam.
Metal analyses of aquatic plants indicated that the accumulation is strongly dependent
on the kind of metals, plant species, and plant organs. In general, roots had greater
heavy metal concentration than leaves and stems. The metal levels in the plants from
West Lake should raise concern for human health because metals in plants were very
high for human consumers. Aquatic animals (nine species of fish, snails, mussels, and
shrimp) generally showed contamination in benthic animals (snails, shrimp and
mussels). All metal concentrations were low in fish muscles and higher in whole fish,
and the observed metal levels (apart from Pb) did not exceed established quality
standards for fish. However, the metals levels were usually several times higher than
relevant standards for shrimp (Cu, Pb), snails (Cu, Zn, Pb), and mussels (Pb, Zn).
Heavy metal concentrations in the sediment from most of twenty-four samples (6 areas)
in West Lake sediments varied with sites, areas and sampling times. The means of Cr,
Cu, Pb and Zn exceeded OMESL guidelines by 1.6 to 2.6 times and were higher than in
other neighbouring lake systems in Vietnam. Levels of metals in the dry season were
significantly higher than in the wet season. The south-eastern corner of the Lake (True
Bach area) has the highest pollution load in West Lake. The pollution is mostly caused
by industrial wastewater entering the lake through drains. The level of contamination
appears to be moving from this area to south, east, and other areas of the lake to some
extent, mostly due to the wind and the wave action pattern in the shallow lake. Strong
positive and significant correlations among Pb, Zn, Cu and Cd indicate a common
source and sink in sediment or their chemical similarity. In contrast, weak or lack of
correlations between Cr and Mn with other metals indicates a contribution from a
unique industry source.
The speciation patterns of six metals indicated that only a small portion of the total
metal was easily available. The mobility of these metals is related to their solubility and
geochemical forms, and it decreases in the order: Mn>>Cd>Zn>Pb>>Cr>>>Cu.
However, it seems that metals in anoxic conditions, which are becoming more likely,
are more mobile and more potentially bioavailable than those in natural conditions
(especially Zn, Pb and Mn).
The results from the microcosm study reflected the high concentration of those metals in
the environment. Predicted metal levels in submerged plants indicated that metals (Cr,
Mn and Zn) came not only from sediment.
In general, the levels of metals.in size fractionated samples display similar trends. It
appears that speciation is independent of the percentage of fines in the· samples but
time-dependent (notably for Cu, Pb and Zn).