estuaries.pdfenv

7/30/2019 Estuaries.pdfenv

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EstuariesEnvironmental Geochemistry

DM Sherman, University of Bristol

Significance of Estuaries

Many estuaries aregreatly impacted by

urban and agricultural

runoff.

Estuaries are majorhabitats for wildlife.


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Significance of Estuaries (UK)

Geochemical Significance of EstuariesInterface between Rivers and Seawaters:determines flux of chemical species into the ocean.

Riverine pollutants react with seawater and may beaccumulated in estuarine sediments.

Mixing between River water and SeawaterChange in ionic strengthChange in pHColloid flocculation

Chemical Processes:


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Rivers vs SeawaterConcentrationin 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

SO4-2

0.068 2.9 0.12Ca+2 0.334 1.0 0.009

K+ 0.033 1.0 0.09

HCO3 0.852 0.211 0.0007

Br- -- 0.087

H4SiO4 0.173 0.10 0.002

ColloidsColloids are very small particles that remainsuspended 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 dissolvedIn practice, everything that passes through a 0.2 mfilter is classed as dissolved. However, this includes

much of the colloidal fraction.

Surface Charge of ColloidsSurface charge is a function of

pH, reflecting the variable

protonation of surface oxygens.

At the pHpzc, the surface charge

is zero.

+1/2 -1/2-1/2


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Sorption of Metals on ColloidsSorption of Cu+2, Ni+2 on Goethite

Recall that sorption onto

FeOOH minerals is a major

control on the dissolved

concentrations of metals.

Colloid FlocculationColloids will flocculate when their

surface charge = 0. This will

occur at the pHpzc 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 andSeawater

Mechanisms for Removal from SolutionIncreased sorption of cations with increasing pHFlocculation of < 0.2 m colloid particles to decreaseapparent dissolved concentration of Fe


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Mechanisms for Adding to SolutionComplexation by Cl or HCO3

-from seawater causes

desorption from colloidal

FeOOH.

But increasing pH (withsalinity) causes readsorption.

Estuarine Sediments


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Anoxic Conditions in Estuarine Sediments

Anoxic mud

As organic matter isconsumed by respiration, O2

is depleted and new electron

acceptors are used.

Marine Evaporite Basins

Evaporite mineral sequence reflects relative solubility of CaCO3(calcite), Gypsum (CaSO4

.2H2O) and Halite (NaCl)

Restricted circulation and evaporation leads to increased salinity.

Thermodynamic modelling of evaporite sequences is complicatedby the highly non-ideal activities of ions at high ionic strength.


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Pollution of Estuaries

Case Study: Hg pollution in California

Erosion of mine tailings

has led to Hg pollution

of San Francisco Bay

Hg was used to extract gold

during the California gold

rush. Hg deposits are found

in the Coast Ranges.


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The Wheal Jane Spill (Falmouth)..

In 1993 a major spill of minewaste released serious

amounts of acid mine waste

and heavy metals into

Falmouth Bay.

Tamar Estuary (Pb)

90

70

50

30

10

2 4 6 8 10

De

pth(cm)

pH

0 2 4 6

Porewater Pb (ppm)

0 1000 2000

Tot. Pb (ppm)

Tamar Valley/Buttspiill Mine


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Tamar Estuary (Zn)

90

70

50

30

10

2 4 6 8 10

Depth(cm)

pH

0 20 40 60

Porewater Zn (ppm)

0

200

400

600

800

1000

Tot. Zn (ppm)

SummaryMuch of the chemistry of estuaries is controlled bythe flocculation of colloids which may remove some

dissolved ions from solution.

Some sorbed species (anions) should be releasedby increase in pH.

Sorption at increased pH may incorporate traceheavy metals into estuarine sediments.

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