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