Aquatic Geochemistry: Introduction

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Aquatic Geochemistry Introduction

• Reading Dodds Chapter 11

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Classification of Materials in Water

• Dissolved vs. Particulate
• Organic vs. Inorganic
• Dissolved vs. Colloidal vs. Gravitoidial

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Figure 11.2
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What is in water?

• H2O
• Water is H2O 2 parts hydrogen and one part
  oxygen but theres also a third thing that makes
  it water and no one knows what it is. D. H.
  Lawrence)
• Suspended Inorganic Matter
• Dissolved Major Ions
• Dissolved Nutrients
• Suspended and Dissolved Organic Matter
• Gases
• Metals

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Descriptive Terms

• Total Suspended Solids (TSS)
• Organic and inorganic particulates
• Total Dissolved Solids (TDS)
• Ca2, Na, Mg2, K, HCO3-, SO42-, Cl-
• Salinity
• Anions and cations

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Descriptive Terms

• Alkalinity
• acid neutralization capacity, buffering capacity
• acid needed to neutralize anions

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Descriptive Terms

• Hardness
• Ca2 and Mg2
• Soft vs. Hard

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Descriptive Terms

• Conductivity
• Electrical conductance of water

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Origin of Chemical Composition of Natural Waters
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Chemistry of Natural Waters

• Rain Water vs. Surface water
• Rain is more dilute
• Physical Weathering
• Chemical Weathering

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Chemistry of Natural Waters

• Role of geology
• Parent materials
• Soils
• Solubilities
• Mineral phases and oxidation states

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Chemistry of Natural Waters

• Role of Carbonic Acid System
• CO2 H2O H2CO3 HCO3- H CO32- H
• pH natural rain water 5.6
• pCO2 soils
• Makes water aggressive

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Example Calcite Dissolution

• CaCO3 H2CO3 Ca2 2HCO3-

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Example Calcite Dissolution

• pCO2 drives calcite solubility
• Provides hardness, TDS, salinity (Ca), buffering
  capacity (bicarbonate)
• Similar process for other ions

Basin Geology Soils
Dilute, slightly acidic rain
Surface water chemistry
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Acidification Acid Rain

• Oxidation of fossil fuels
• NOX and SO2 with water in atmosphere
• Strong acids
• HNO3
• H2SO4

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Acid Rain

• Rain pH lt 4.0
• High acid loading
• Increase in H
• Decrease in pH of surface waters
• Which surface waters are most susceptible to acid
  rain?

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Acidification

• Chronic
• Episodic
• High flow events
• Rain on snow events

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Acidification

• Effects on ionic content of surface waters
• Charged, aggressive water
• Mobilizes Al
• Consumes alkalinity

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Wigington et al.1996 Ionic Controls
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Biological Effects

• Osmoregulation
• Metals toxicity
• Labile Monomeric Aluminum
• Soluble at pH lt 4.5
• Role of Ca
• CaH ratio
• Aluminum Phosphate salts

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Leuctridae
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Fish Mortality
Van Sickle et al.1996
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Effects on Fish Populations
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Acid Mine Drainage

• An acid-base, oxidation-reduction weathering
  reaction producing
• An acidic, metal and sulfate rich drainage when
  reduced mineral sulfides are exposed to oxidizing
  environments
• Not just low pH water

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Acid Mine Drainage
A
B
C
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Reactions

• Environment A
• 2FeS2 7O2 2H2O 2 Fe2 4 SO42- 4H
• Redox
• Oxidizing sulfur
• Reduced iron, sulfate
• Acid producing reaction

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Reactions

• Environment B
• 2Fe2 O2 4H 4Fe3 2H2O
• Redox
• Oxidizing iron
• Acid consuming step
• Very slow at low pH
• Accelerated by chemolithotrophic, acidophilic
  bacteria (Thiobacillus spp., T. ferrooxidans)

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Reactions

• Environment C
• 4Fe3 12H2O 4Fe(OH)3 12H
• Acid producing reaction during precipitation of
  iron hydroxide (yellow boy)

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Reactions

• 14Fe3 FeS2 8H2O 15Fe2 2SO42- 16H
• Redox
• Ferric iron oxidizing more sulfur
• Acid producing reaction
• Reduced iron and more sulfates

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Consequences

• Low pH (lt3)
• High acidity
• Mobilizes metals (Fe, Al, Mn, Cr, Cd, Zn)
• High conductivity, TDS, salinity
• Low alkalinity, hardness

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• Aggressive water
• Low pH keeps metals dissolved
• As pH increases metals sequentially precipitate
• Fe (pH4)
• Al (pH5)
• Mn (pH10)

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Fickey Run pH 2.5, 2000 mg/L acidity
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Cheat River at Elsey Run
Cheat River at Lick Run
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Changes in Water Chemistry from Acidification
Acid Mine Drainage
Alkaline Mine Drainage
No-Impact
Acid Precip
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Changes in Water Chemistry from AMD
No-Impact
Alkaline Mine Drainage
Acid Mine Drainage
Acid Precipitation
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What can be done to prevent it?