Chapter 1 INTRODUCTION 1.1 Estuarine behaviour of heavy metals 1.2 Approaches in heavy metal speciation 1.3 Cochin backwaters 1.4 Scope of the present study Estuaries encompass river-ocean interface, a region regarded as one of the most important aquatic systems. They are highly dynamic and are subject to changes occurring over a spectrum of durations ranging from very short periods to geologic time spans. Pronounced biogeochemical reactivity, including sorption, flocculation and redox cycling of trace metals, is induced by sharp gradients in the estuarine master variables of salinity, pH, dissolved oxygen, particle character and concentration that result from the mixing of fresh and saline end members. These processes drastically modify the riverine composition and must be fully understood to accurately determine chemical fluxes and to define geochemical mass balances. Every estuary is unique. There are, however, some general trends, which make it possible to predict the nature of the estuarine environments, the circulation pattern and various processes and interactions taking place in estuaries. From pre-historic times, the banks of rivers and estuaries have been the centres of civilization, because of the favourable features such as the profuse vegetation, fertile soil, access to navigational facilities etc. that have catalysed the flourishing of
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Dynamics And Speciation Of The Heavy Metals In The Lower Reaches
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Chapter 1
INTRODUCTION
1.1 Estuarine behaviour of heavy metals
1.2 Approaches in heavy metal speciation
1.3 Cochin backwaters
1.4 Scope of the present study
Estuaries encompass river-ocean interface, a region regarded as one of the
most important aquatic systems. They are highly dynamic and are subject to
changes occurring over a spectrum of durations ranging from very short periods to
geologic time spans. Pronounced biogeochemical reactivity, including sorption,
flocculation and redox cycling of trace metals, is induced by sharp gradients in the
estuarine master variables of salinity, pH, dissolved oxygen, particle character and
concentration that result from the mixing of fresh and saline end members. These
processes drastically modify the riverine composition and must be fully understood
to accurately determine chemical fluxes and to define geochemical mass balances.
Every estuary is unique. There are, however, some general trends, which
make it possible to predict the nature of the estuarine environments, the circulation
pattern and various processes and interactions taking place in estuaries. From
pre-historic times, the banks of rivers and estuaries have been the centres of
civilization, because of the favourable features such as the profuse vegetation,
fertile soil, access to navigational facilities etc. that have catalysed the flourishing of
human habitats in those regions. The growing up of large cities nearby estuaries
has in many cases caused environmental disturbances, particularly due to the
discharge of domestic and industrial waste. Human activities to improve the
standard of living, has led to the introduction of many hazardous, non degradable
chemicals into the aquatic ecosystem, the presence of which has attracted serious
concern of the environmentalists. Organic effluents such as domestic sewage is a
serious problem - the discharge of small quantities of sewage into estuarine
systems can actually increase the productivity of ecosystem, but excessive
quantities will deplete oxygen causing severe threat to aquatic life. The alarming
rate of pollution input far exceeds that of nature's cleansing processes and has
consequently resulted in an ecological imbalance.
Once the pollutants enter the environment, they are subjected to a variety of
physical, chemical, geological and biological processes that bring about their
disintegration or sometimes, their ultimate removal. Persistent chemicals, that do
not breakdown, stand to pose serious environmental problems. Heavy metals,
because of their relatively long "half life" and biological significance, constitute one
such class among non-degradable contaminants causing great concern. The fate
and behaviour of heavy metals in estuarine environment are of extreme importance
due to their key role in the biogeochemical cycles. Consequently, cycling of
heavy metals and their inherent toxicity has formed an integral component of
estuarine water quality monitoring programmes.
The various anthropogenic activities by which heavy metals are introduced
into the aquatic systems include smelting, mining, shipping, industrial efnuent
discharge, urbanisation, application of fertilizers, algicides, automobile exhaust etc.
The natural processes that contribute metals to the aquatic environment include
weathering of rocks, leaching of ore deposits, forest fires, terrestrial and marine
volcanism etc. The above sources directly regulate the net flux of heavy metals
that interplay with natural/artificial systems and pose relevant questions on their
cycling, transport and ultimate removal.
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1.1 ESTUARINE BEHAVIOUR OF HEAVY METALS
Estuaries represent geologically ephemeral transition zone, in which many
features of geomorphology, water circulation, biogeochemistry and ecology are
varied and diverse. An estuary is a mixing zone of riverine and oceanic waters
with widely varying compositions where end members interact both physically and
chemically. The importance of estuaries lies in the fact that they act as a mediator
(filter) in the transfer of elements from continents to oceans. The estuaries thus,
can be either a source or sink for different heavy metals. Therefore, it is imperative
to study the composition of water, particulate and sediment phases in the estuaries
along with temporal fluctuations to identify different biogeochemical processes and
pathways in metal cycling.
During estuarine mixing, the trace metals in the dissolved and particulate
forms can behave either conservatively or non-conservatively depending on
physico-chemical factors such as salinity, pH, Eh and suspended solids. The
chemical behaviour of a trace metal during its transport within the estuary is
determined to a large extent by the chemical form in which it is transported by the
rivers. There are also somewhat conflicting reports on the behaviour of heavy
metals during estuarine mixing. Windom et. al. (1988) have reported conservative
behaviour of dissolved and particulate metals in Bang Pakong estuary, Thailand.
Poucot and Wollast (1997) have reported that the concentration of nickel and
chromium in the Scheldt estuary, southwest of the Netherlands decreased with
increase in salinity. He also reported that manganese exhibited a non-conservative
behaviour. Guieu et. al. (1998) reported that copper and zinc showed conservative
behaviour in the Danube river. Much of the disparity between the results of
different workers may be attributed to reasons such as decompOSition of pre
eXisting solids (which release the incorporated metals), differences in the rate of
mixing, nature of solids supplied by the end members and dependency of solids'
association of trace metals on the grain size distribution. Another important factor,
which can influence the behaviour of heavy metals in estuaries, is the hydrogenous
preCipitation of iron and manganese oxides in the low salinity region.
A study of the distribution of heavy metals in the dissolved and particulate
phases is very important to understand their role in various biogeochemical
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processes occurring in estuaries and tidal rivers. In estuarine waters, the different
processes controlling metal distribution tend to be superimposed. Inputs can be
from rivers, sediments, atmosphere and from degradation of materials formed in
situ; removal can be by biological uptake, sorption on to sedimentary particles
(both organic or inorganic) and flushing with ocean and coastal waters. Further, a
knowledge of the distribution and concentration of heavy metals in estuarine waters
would help detect the sources of pollution in the aquatic systems.
Sediments have proved to be excellent indicators of environmental pollution,
as they accumulate pollutants to levels that can be measured reliably by a variety
of analytical techniques. Heavy metals tend to be adsorbed on to suspended
particles and are scavenged from the water column into proximal sediments
(Karickhoff, 1984; Daskalakis and O'Connor, 1995; Lee et. al., 1998). To assess
the impact of contaminated sediments on the environment, information on total
concentrations of metals alone is not sufficient because heavy metals are present
in different chemical forms in sediments. Only a part of the metals present may
take part in short-term geochemical processes or may be bio-available. For this
reason, a series of different extraction procedures have been devised to gain a
more detailed insight into the distribution of metals within the various chemical
compounds and minerals. In this study, an attempt is made to differentiate the
metals in the surficial sediments into exchangeable, carbonate bound, easily
reducible, oxidisable and residual fractions.
1.2 APPROACHES IN HEAVY METAL SPECIATION
The rapid increase in the levels of environmental pollution over recent
decades has resulted in increasing concern for people's well being, and for global
ecosystems. The need to determine different species of trace metals in
environmental and biological materials is of paramount importance since the
toxicity of an element and its behaviour depend to a great extent on its chemical
form and concentration. The growing awareness of this dependence has led to an
increasing interest in the qualitative and quantitative determination of specific metal
species. Changes in environmental conditions, whether natural or anthropogenic,
can strongly influence the behaviour of both essential and toxic elements by
altering the forms in which they occur. Some of the more important controlling
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factors include pH, redox potential and availability of reactive species such as
complexing ligands.
Originally, most analytical measurements dealt with the total content of a
specific element in an analysed sample (such as lead, mercury or cadmium as
examples of toxic elements, or cobalt, selenium or magnesium as examples of
essential elements). Biochemical and toxicological investigations have shown that,
for living organisms, the chemical form of a specific element, or the oxidation state
in which that element is introduced into the environment is crucial. Therefore to get
information on the activity of specific elements in the environment, more particularly
for those in contact with living organisms, it is necessary to determine not only the
total content of the element but also its individual chemical and physical form.
Generally, the appearance of multiform is described by speciation, but the process
leading to quantitative estimation of the content of the different species is called
"Speciation Analysis".
According to the official definition speciation analysis is the process leading
to the identification and determination of the different chemical and physical forms
of an element existing in a sample (Kot and Namiesnik, 2000). Although this
definition tends to restrict the term speciation to the state of distribution of an
element among different chemical species in a sample, in practice the use of this
term is much wider, specifying either the transformation and/or the distribution of
species, or the analytical activity, to identify chemical species and measure their
distribution. For the description of these processes the terms "Species
Transformation" and "Species Distribution", respectively, are suggested. The
analytical activity involved in identifying and measuring species is hence defined as "Speciation Analysis".
The use of chemical extractants to quantify the element in a particular solid
phase was originally attempted in soil studies (Jackson, 1958; Jenne et. aI., 1986;
Bermond and Sommer, 1989) for selective sampling and determination of nutrients of
different solubilities. Because of the similarities between soil and aquatic sediments,
extraction procedures used in soil have often been adapted for sediment analyses
(Tessier et. aI., 1979). A number of extraction procedures, varying in manipulative
Complexity, have been proposed for the partitioning of metal phase. Several methods
for determination of different forms of metals in sediments are described in scientific
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literature (Kersten and Forstner, 1991). Some of these techniques (Agemian and
Chau, 1976; Loring, 1976; Malo, 1977) employ a single extractant and are designed to
effect separation between residual and non-residual metals. The most widely used
methods are based on sequential extraction procedures, whereby several reagents
with increasing power, under specified conditions are used consecutively to extract
operationally defined phases from the sediment in a set sequence (Gupta and Chen,
1975; Luoma and Jenne, 1976; Engler et. al., 1977; Kerster and Forstner, 1986;