Lecture Notes (PDF) - Aqueous and Environmental Geochemistry

Estuaries
Environmental Geochemistry
DM Sherman, University of Bristol
Significance of Estuaries
Estuaries are major
habitats for wildlife.
Many estuaries are
greatly impacted by
urban and agricultural
runoff.
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Significance of Estuaries (UK)
Geochemical Significance of Estuaries
Interface between Rivers and Seawaters:
determines flux of chemical species into the ocean.
Riverine pollutants react with seawater and may be
accumulated in estuarine sediments.
Chemical Processes:
• Mixing between River water and Seawater
• Change in ionic strength
• Change in pH
• Colloid flocculation
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Rivers vs Seawater
Concentration
in Average
River (mmol/
kg)
Seawater
Concentration
(mmol/kg)
(M/Cl)sw
(M/Cl)riv
pH 5 - 7 8.3
Cl - 0.16 54.6 1
Na + 0.224 48.0 0.62
Mg +2 0.138 5.4 0.11
SO
-2
4 0.068 2.9 0.12
Ca +2 0.334 1.0 0.009
K + 0.033 1.0 0.09
HCO 3 0.852 0.211 0.0007
Br - -- 0.087
H 4 SiO 4 0.173 0.10 0.002
Colloids
Colloids are very small particles that remain
suspended in aqueous solutions. Much of the
chemical flux in rivers in in the colloidal form.
Primary colloidal phases in Rivers are:
• Clay minerals (kaolinite, montmorillonite)
• Iron (hydr)oxides (goethite, hematite, ferrihydrite).
• Humic/Fulvic acids associated with FeOOH.
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Colloidal, particulate and dissolved
In practice, everything that passes through a 0.2 µm
filter is classed as dissolved. However, this includes
much of the colloidal fraction.
Surface Charge of Colloids
+1/2 -1/2
-1/2
Surface charge is a function of
pH, reflecting the variable
protonation of surface oxygens.
At the pH pzc , the surface charge
is zero.
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Sorption of Metals on Colloids
Sorption of Cu +2 , Ni +2 on Goethite
Recall that sorption onto
FeOOH minerals is a major
control on the dissolved
concentrations of metals.
Colloid Flocculation
Colloids will flocculate when their
surface charge = 0. This will
occur at the pH pzc and with
increasing ionic strength.
Flocculation of colloids in
estuaries will incorporate
them (and their sorbed
metals) into estuarine
sediments.
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Behavior during mixing River and
Seawater
Mechanisms for Removal from Solution
• Increased sorption of cations with increasing pH
• Flocculation of < 0.2 µm colloid particles to decrease
apparent dissolved concentration of Fe
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Mechanisms for Adding to Solution
Complexation by Cl or HCO 3
-
from seawater causes
desorption from colloidal
FeOOH.
But increasing pH (with
salinity) causes readsorption.
Estuarine Sediments
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Anoxic Conditions in Estuarine Sediments
As organic matter is
consumed by respiration, O 2
is depleted and new electron
acceptors are used.
Anoxic mud
Marine Evaporite Basins
Restricted circulation and evaporation leads to increased salinity.
Evaporite mineral sequence reflects relative solubility of CaCO 3
(calcite), Gypsum (CaSO 4. 2H 2 O) and Halite (NaCl)
Thermodynamic modelling of evaporite sequences is complicated
by the highly non-ideal activities of ions at high ionic strength.
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Pollution of Estuaries
Case Study: Hg pollution in California
Hg was used to extract gold
during the California gold
rush. Hg deposits are found
in the Coast Ranges.
Erosion of mine tailings
has led to Hg pollution
of San Francisco Bay
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The Wheal Jane Spill (Falmouth)..
In 1993 a major spill of mine
waste released serious
amounts of acid mine waste
and heavy metals into
Falmouth Bay.
Tamar Estuary (Pb)
Tamar Valley/Buttspiill Mine
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Depth (cm)
30
50
70
90
2 4 6 8 10
pH
0 2 4 6
Porewater Pb (ppm)
0 1000 2000
Tot. Pb (ppm)
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Tamar Estuary (Zn)
Depth (cm)
10
30
50
70
90
2 4 6 8 10
pH
0 20 40 60
Porewater Zn (ppm)
0
200
400
600
800
Tot. Zn (ppm)
1000
Summary
• Much of the chemistry of estuaries is controlled by
the flocculation of colloids which may remove some
dissolved ions from solution.
• Some sorbed species (anions) should be released
by increase in pH.
• Sorption at increased pH may incorporate trace
heavy metals into estuarine sediments.
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