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Aquatic Geochemistry:
Introduction
Chapter 12
Also Chapters 13, 14, 16
Thought for today: Water is H2O: 2 parts hydrogen and one part oxygen: but there’s also a third thing that makes it water and no one knows what it is.—D. H. Lawrence, novelist, poet, playwright (1885-1930)
Week 8
Lab: Lake conditions (first)
– Stratification
– Turnover
– Assessment of conditions (last time)
EOW Ch 11-12
Lecture material
Objectives of Lectures on
Aquatic Geochemistry
Blackwater River/Beaver Creek
Acid Neutralization Station
Some Questions to Address
What’s in water and how do we characterize its
chemical content?
– Example: What makes water “hard” and why is pure
water a poor conductor of electricity?
How does the chemistry of natural waters come
to be?
Why are some streams naturally acidic?
Why are some streams resistant to acid rain?
Why can’t you judge AMD by the color of the
water?
Chemical Content:
What is water?
H2O—know the properties of water!
–Hydrogen
–Oxygen
“Water is H2O: 2 parts hydrogen and one part oxygen: but there’s also a third thing that makes it water and no one knows what it is.” —D. H. Lawrence)
Classification of Materials in Water
Dissolved vs.
Particulate
Organic vs.
Inorganic
Dissolved vs.
Colloidal vs.
Gravitoidial
Figure 11.2
Figure
11.2
What is in water?
Suspended inorganic matter
Dissolved major ions– Ca2+ > Mg2+ > Na+ > K+
– HCO3- > CO3
- > SO4- > Cl-
Dissolved nutrients
Suspended (particulate—POM) and Dissolved Organic Matter (DOM)
Gases
DIC
Metals
Descriptive Terms
Total Suspended Solids
(TSS)
Total Dissolved Solids
(TDS)
Total Solids
Salinity
Conductivity
Hardness
pH
Alkalinity
Buffering Capacity
Acidity
Redox Potential
Concentration
Descriptive Terms
Total Suspended Solids (TSS)
– Organic and inorganic particulates
Total Dissolved Solids (TDS)
– Ca2+ > Mg2+ > Na+ > K+
– HCO3- > CO3
2- > SO42- > Cl-
Total solids—a measure of salinity
Salinity
Total ionic composition
– Anions and cations
– Controlled by:
Rock
Precipitation
Evaporation and precipitation rates
– Essentially TDS
Hardness
Ca2+ and Mg2+ salts in water
– Carbonate hardness = Ca and Mg associated
with carbonates and bicarbonates
– Non-carbonate hardness = Ca and Mg associated
with other anions (sulfates, Cl)
Soft water = low salinity
Hard water = high salinity
Conductivity
Measures reciprocal of
resistance to electrical
flow
– Increases with ionic
concentration, which
lowers resistance
– Indirect measure of TDS
pH
Water is amphiprotic (both an acid and a
base)
Concentration
mg/L
ppm
Equivalent Weight
Redox Potential
Oxidation and Reduction Reactions
– Transfer of electrons
– Just like acid-base chemistry is the transfer of
H+ ions
Oxidation
Reactions with oxygen
More generally:
– loss of electrons
Reduction
Removal of oxygen
More generally,
– Reduction is gain
– OIL RIG
– Examples
Redox Potential
Tendency to transfer electrons
Dictates what reactions will occur
Controlled by molecular oxygen (high
electro-affinity)
– High redox potential = oxidizing conditions
(>200 mV)
Low electron availability for chem reactions
– Low redox = reducing condition
High electron activity
FIGURE 12.4
Redox
potential and
activation
energy
Redox Reaction Example
Fe2+ = Fe3+ + e-
Anoxic
lake
bottom
Oxic
waters
Examples
Oxidizing conditions
Reducing conditions– IRON...
Fig. 12.5: Energy yields from redox reactions
– Organic matter oxidation—molecular oxygen reduction couple yields most energy
– Often mediated by bacteria
FIGURE 12.5
H2O
Mn (IV)
CO2
Fe (III)
Take home
Reduced forms dominate in mud
– But oxidized forms can exist (why?)
Oxidized forms dominates in O2 rich env
– But reduced forms can exists (why?)
Redox Reaction Example
Respiration, controlled burning of O.M.
– Organic C
– O2
Oxygen
Controlled by temperature and pressure
– Oxic = NPP>0
– Hypoxic NPP ~ 0 (1 – 2 ppm DO)
– Anoxic NPP <= 0
NPP = GPP - Rs
Ps and Light Intensity
Trade off for Ps organisms
Low light adapted
– Steep P-I curve, low compensation point,
photo-inhibited at low irradiance
High light adapted
– Shallow P-I curve, higher compensation point,
higher peak Ps
FIGURE 12.8
More on Oxygen
Controls and controlled by Ps and Rs
FIGURE 12.11 - explain
Groundwater
Lake
Aquarium periphyton
Stream
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