Department of Oceanography
SOES2009 Estuaries
Lecture 16
Lecture 16: Estuarine Chemical Processes
By Mr. B. Dickie
The main reasons for interest in estuarine chemical processes are:
• estuaries are important transition zones in the transfer of continentallyderived products or rock weathering to the coastal seas and the oceans;
processes in estuaries can alter the fluxes of riverine material and such
changes are potentially important in understanding oceanic budgets of
constituents;
• as in other aquatic systems, distributions of chemical species may
influence biological processes and the biological productivity of
estuaries (in turn the biology affects the distributions);
• many estuaries receive considerable anthropogenic inputs of chemical
species, either directly or through rivers; understanding chemical
processes is important in the context of water quality models for
estuaries.
Estuaries are coastal areas where river water (low salinity normally) and
seawater (high salinity) mix. Salinity then varies with position in the
estuary:
Different types due to mix-flow differences:
1) Salt Wedge, e.g. Mississippi
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2) Highly Stratified,
e.g. Vellar River (India)
e.g. Hardanger Fjord, Norway
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3) Slightly Stratified
e.g. Chesapeake Bay, USA
4) Well Mixed, e.g. Myall River channel in New South Wales, Australia
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5) Inverse Estuary, e.g. Alligator River Northern Territory, Australia
Evaporation
↓
These salinity gradients are accompanied by large changes in the
concentration of the water, in part because seawater and river water have
radically different compositions:
Comparison of the compositions of sea water and the global average
river water supply to the ocean.
River water
Constituent
Sodium
Potassium
Magnesium
Calcium
Chloride
Concentrati
on
mg l-1
5.1
1.3
3.3
13.4
5.7
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% of total
dissolved
material
5
1
3
13
6
Sea water
(salinity S=35)
% of
Concentrati
-1
salinity
on mg l
10765
399
1294
412
19353
31
1
4
1
55
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SOES2009 Estuaries
Sulphate
Bicarbonate
Silicon (as
SiO2)
a
b
8.2
52
10.4
Lecture 16
8
52
10
2712
142a
<0.1->10b
8
0.4
---
Inorganic carbon as bicarbonate.
Variable with location and depth.
The major cations and anions in (A) sea water, (B) the globally averaged
river input to the oceans
Changes in the relative proportions of major ions in estuarine waters.
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Except in inverse estuaries, the salinity simply reflects dilution with the
freshwater source(s), i.e.
Mixing of Fresh and Sea Waters
Volume of
Freshwater
S=0
Volume of
Seawater
S=34
10% SW
=> S=3.4
50% SW
=> S=17
33% SW
=> S=11.2
100% SW
=> S=34
75% SW
=> S=25.5
All the major ions (Na+, Cl-, Mg2+, SO42+, etc.) and some of the minor ions
(e.g., Br-) are also simply diluted (mixed). Therefore a plot of S versus the
concentration of that ion is a straight line; for example:
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Other minor and trace chemical species don’t plot linearly with Salinity.
Why?
- Various processes can alter their concentration, independent of S, as
they travel through the estuary.
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Processes that can effects solutes:
a) Photosynthesis – removes dissolved inorganic carbon, nutrients (P, N,
Si), and some trace metals. Particles formed.
b) Adsorption & desorption – removes and adds solutes, e.g. Na+
displaces adsorbed Ca2+.
C) Coagulation – the charge on some suspended particles in effectively
neutralized by adsorption of seawater ions. This allows van der Waal’s
forces to bind them and larger particles are formed that settle.
d) Settling – large particles can settle through the water to the sediments,
removing adsorbed species.
e) Reactions in the Sediment & Exchange – many solids are
thermodynamically unstable (e.g. organic matter, silica test of diatoms,
etc.) or become so in sediments (e.g. metal oxides, carbonate minerals,
etc.). New minerals can form in sediments (e.g. pyrite).
These reactions mean that the sediment porewaters can have very
different composition than the overlying estuarine water. These
differences in concentration (gradients) can drive solute exchanges
between sediment and overlying water, i.e. via diffusion or resuspension or
biological pumping (irrigation).
Example or Reactions in Sediments:
1) Rotting (organic matter decay)
CH2O + O2 −> CO2 + H2O
5 CH2O + 4 NO3- −> 2 N2 + 4 HCO3+ CO2 + H2O
2 CH2O + SO4= −> H2S + 2 HCO3↑
“rotten-egg smell”
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Oxygen is used first, then when that runs out, Nitrate is used, then when
that runs out, Sulfate is used. (Oxides may be used between Nitrate and
Sulfate.)
2) Oxide reduction:
+
2 FeOOH + H2S + 4 H −> 2 Fe
2+
o
+S
+ 4 H2O
+
o
MnO2 + H2S + 2 H −> Mn2+ + S
+ 2 H2O
3) Mineral formation or dissolution:
Fe
2+
2-
+ S
−> FeS(s)
+
o
MnO2 + H2S + 2 H −> Mn2+ + S
+ 2 H2O
SiO2·H2O −> H4SiO4
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Lecture 16(suppl): Estuarine Chemical Processes
The main reasons for interest in estuarine chemical processes are
•
estuaries are important transition zones in the transfer of continentally-derived
products or rock weathering to the coastal seas and the oceans; processes in
estuaries can alter the fluxes of riverine material and such changes are potentially
important in understanding oceanic budgets of constituents;
•
as in other aquatic systems, distributions of chemical species may influence
biological processes and the biological productivity of estuaries (in turn the biology
affects the distributions);
•
many estuaries receive considerable anthropogenic inputs of chemical species,
either directly or through rivers; understanding chemical processes is important in
the context of water quality models for estuaries.
There are complex interactions between the chemical, biological and physical
processes in estuaries. Since estuaries are temporally variable systems (characteristics
such as topography, circulation and residence time can change considerably), the
behaviour of chemical constituents may vary widely from estuary to estuary. The same
chemical processes potentially occur in all systems - it is the extent to which their
effects are apparent which varies.
Some important processes in estuarine chemistry are shown schematically in the figure
below:
composition
Salinity (Ionic strength)
mixing
River water
pH
Sea water
Salinity (Ionic strength)
composition pH
Dissolved material
particles
Dissolved material
particles
Isolated water
Redox boundary
(oxidizing - reducing)
Estuarine sediments
Exchange
Estuarine mixing processes create a continuous salinity variation within the estuary
which corresponds to a gradient in ionic strength, an important variable in terms of
physicochemical reactions. The major constituents in sea water have concentration
gradients in the same direction as salinity, but some important minor constituents are
typically higher in concentration in river water than in sea water. Sea water is much
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more constant in its major constituent composition than river water. The proportions of
major dissolved ions in estuarine waters largely reflect those in sea water down to a
salinity of ~5 and are similar in different estuaries except at low salinities. Total
dissolved material concentrations in river water vary widely but are mostly within 20-400
mg l-1.
River waters are highly variable in composition reflecting catchment geology,
weathering processes and other factors. This variability is reflected in variable
compositions of the more abundant dissolved constituents at low salinities, and the
direction and slope of concentration gradient for many minor constituents.
The pH of coastal sea water is usually 7.8 or slightly higher. River waters have pH
values varying from acid streams to waters of high alkalinity. While extreme variations
can occur, many river waters lie in the pH range 7.3-8.4. Changes in pH with mixing are
non-linear; additional changes arise through primary productivity (photosynthesis) and
respiration.
A further important physicochemical variable is the oxidation/reduction condition in the
water. In the presence of free oxygen at saturation concentrations, natural waters are
strongly oxidising media. Decomposition of organic material (mainly by bacteria) is an
oxidative process; at low oxygen concentrations, nitrate will start to be used as an
oxidant (terminal electron acceptor). The medium then has an oxidation/ reduction
condition, causing some elements (which can occur in different oxidation states in
aquatic systems) to undergo reduction. Following depletion of nitrate, sulphate is used
in the oxidation of organic material and reducing conditions exist which strongly favour
reduced species for the “redox-sensitive” elements. In estuarine environments, the
presence of relatively high organic matter in sediments often leads to a change in
conditions (from oxidizing to reducing just below or at the sediment/ water interface).
Where bottom waters are isolated from mixing with surface waters, they may also
become de-oxygenated and the change from more oxidising to more reducing
conditions (often termed a redox-boundary) will then occur in the water column - this
occurs commonly in fjords as a result of isolation of deep water.
The marine and riverine end-members mixing in the estuary contain suspended
particulate material (SPM) with different characteristics. These populations of particles
also mix in the estuary, but within the estuary gravitational setting, resuspension from
sediments, and biological production of particles can all greatly influence the distribution
and nature of SPM leading to variability spatially and on various time scales (diurnal
tidal, spring-neap cycle, seasonal cycle,.....). Some estuaries act as extremely effective
traps retaining in some cases not only all the particulate material delivered by rivers but
also accumulating additional sediment from the shelf. In other regimes virtually all the
sediments delivered by rivers may be exported.
In the absence of additional boundary exchanges (sediment interface; atmospheric
interface) dissolved constituents can change in concentration only by uptake into or
onto particles or release from particles. These particle/ solution interactions bring about
changes in distribution potentially and give rise to geochemical cycles of constituents
within the estuary. They also can potentially alter the flux of a dissolved constituent in
the estuary so that the export flux is no longer equal to the riverine input flux. Net
removal to particles will reduce the flux, whereas net addition from particles will
enhance (amplify) the flux. Conceptually, the particle/ solution interactions can be
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categorised, although the processes may be difficult to disentangle in the environment,
where their overall effects are observed.
Because of the complexity and variability of the behaviour of SPM, estuarine chemical
behaviour is usually viewed from the perspective of how dissolved constituents change
in concentration during estuarine mixing. This will be the case in the subsequent
lectures but the role of particles in bringing about these changes must be borne in mind.
Changes in dissolved concentrations are of course reflected in the solid phase
compositions but generally they are harder to detect by measurement, against the
background compositions - this is another reason why estuarine chemical studies often
adopt the perspective already indicated.
Particle/ solution interactions
Ion-exchange (reversible electrostatic associations of ions with charged sites on
particles).
Specific adsorption (chemical binding)/desorption reactions under changes
physicochemical conditions.
Incorporation into organic material (biologically produced)/ decomposition.
Precipitation/ dissolution (minor role in oxygenated waters).
Comparison of the compositions of sea water and the global average river water
supply to the ocean.
River water
Constituent
Sodium
Potassium
Magnesium
Calcium
Chloride
Sulphate
Bicarbonate
Silicon (as SiO2)
a
b
Concentration
mg l-1
5.1
1.3
3.3
13.4
5.7
8.2
52
10.4
% of total
dissolved material
5
1
3
13
6
8
52
10
Sea water
(salinity 35 × 10-3)
Concentration
% of
-1
mg l
salinity
10765
31
399
1
1294
4
412
1
19353
55
2712
8
a
142
0.4
<0.1->10b
---
Inorganic carbon as bicarbonate.
Variable with location and depth.
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Lecture 16
Changes in the relative proportions of major ions in estuarine waters.
The major cations and anions in (A) sea water, (B) the globally averaged river input to
the oceans
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