Module 7

Question 1

Describe the 7 “weird” properties of water and explain why they occur and their relevance to
geochemistry and/or earth sciences

Answer 1

good solvent
-Ability to break ionic bonds, liberating
ions
-strong polar charges - can attract an ion and break bonds

High Heat capacity
-energy needed to raise T
-Strong bonds, energy used to break them before T rises

All 3 phases at Earth’s surface
-Only natural substance that does this
-high boiling and melting temperature
-due to H bonds, tough to break

Densest as liquid, not solid
- other liquids opposite
-Molecular structure
-Liquid: clumpy
-Ice: longer H bonds in the lattice

High enthalpy of fusion (s to l), sublimation (s to g), and vaporization (l to g)
 Takes a lot of energy to change phase
-H bonds require energy input to break

All those properties can be explained by:
* H-O bonds strongly polar covalent and hard to break

Question 2

List the 7 major ions in freshwater and explain why they are common. Infer the geology water has come into contact with based on the concentrations of these ions

High/med/low tds relationships

Answer 2

cations: Na+, Mg2+, Ca2+, K+
anions: HCO3– , Cl–, SO42–

Low TDS:
Ca2+, Mg2+, HCO3-

dissolution of carbonates or silicates
Calcium bicarbonate waters
-mg silicate weathering
Sodium bicarbonate waters
-Na silicate weathering
-acid dissolution & hydrolysis

Medium TDS:
Ca2+, Mg2+, SO42-
-highly soluble gypsum or sulfide minerals
calcium sulfate waters
-gypsum dissolution
Calcium bicarbonate waters
-carbonate dissolution(med/high tds)

High TDS:
Na+, Cl-
-highly soluble NaCl
sodium chloride waters
-halite dissolution (sea/road salt)
Calcium bicarbonate waters
-carbonate dissolution(med/high tds)

Question 3

weaknesses of piper diagrams

Answer 3

Only take 7 major ions into account
-Must ignore any other ions so the sum is 100%

Best for uncontaminated water

Also doesn’t work for anoxic or low-O2 waters, where reduced ions like Fe2+ and S2– can dominate

Not all groundwaters follow the chemical evolution sequence

leaves out nitrate
-not plotting everything, not using everything

Question 4

charge balance and 7 ions

Answer 4

Dissolved constituents in water must always sum to a net charge of zero
(electroneutrality)
 assume 7 major ions in freshwater make up >99% of charged species

 Check to see if we’re missing any major ions in our analysis
 Solve for the concentration of a missing ion (e.g., we messed up that analysis)

Charge balance:
measure concentration of all ions in milliequivalents/L (meq/L), or the Normality (N)
 Absolute value of charge is used for both anions and cations
 they should equal each other (or
balance to zero when anions are subtracted from cations)

Question 5

Define water hardness
How to calculate it
three ways to treat it

What is the assumption when determining hardness

Answer 5

ability of relatively insoluble minerals to precipitate out of water
 Sum of Ca2+, Mg2+, any other 2+ ions (e.g., Sr, Ba, Fe, Al, Mn)
 Form CaCO3, CaMg(CO3)2, CaSO4 precipitates
 Can clog pipes etc.

General approach:
Sum Ca and Mg in meq/L
Convert to CaCO3 using molar mass
-assumes any hardness will eventually form CaCO3

Treat hardness:

Boiling water:
 Ksp (solubility) of CaCO3 decreases with increasing temperature
 HCO3- becomes CO32- that binds with Ca2+ and CaCO3 precipitates
-produces flakes, remove with a filter

Water softener
 cation-exchange resin to replace Ca2+, Mg2+ with cations that are very soluble (Na+)
 Charge balance must be maintained, so 2Na+ have to replace one Ca2+
-Ca2+ goes in, Na+ goes out
binding affinity of 2+ > 1+

Water filters with activated carbon
-remove some Ca2+ and Mg2+ (not all)

Question 6

Explain which nutrients most often limit algal growth in freshwater and in oceans. Determine
which nutrient is likely to be limiting given relevant information

Answer 6

N and P

Concentration
-very low in unimpacted systems.
-higher agricultural areas and urban streams/lakes
-fertilizer, manure, sewage

aquatic life limited by P or N
generally:
 Freshwater: P limited
 Nearshore marine: N limited
 Some areas co-limited
 Some algae may be limited by other nutrients

Which Nutrient is Limiting:
-Redfield Ratio
 If N:P is > 16, more N than necessary, so P limited
 If N:P < 16, more P than necessary, so N limited

limitation:
-rough guide
 Not all algae are the same!
 Freshwater algae may have more variability

Question 7

Differentiate TSS, turbidity, TDS and specific conductivity and explain how they impact water
quality

Answer 7

Total Dissolved Solids (TDS)
 The sum of all dissolved species in water (Mostly ions)
 Measure all dissolved species separately, sum
OR
 Weigh a filtered water sample – any extra density is due to dissolved species

Electrical Conductivity:
- A cheaper way to get a TDS
 Measures how well water conducts an electrical current, in μSiemens/cm
 Higher conductivity = More ions (or TDS)
 increases with T so we measure at standard state (25º)

Total Suspended Solids (TSS)
- tss particles bigger than tds particles (debated boundary)
 soil, minerals, algae, dead plant bits
 sign that water is not potable, correlated to pathogens (E. coli,
Giardia, and other bacteria)
measure by:
-pre-weigh filter, filter known volume of water, dry filter and weigh it.
 The difference in mass is your TSS per volume filtered

Turbidity
water clarity,
-measured by light penetration
 cheaper way to measure TSS
Secchi disc depth
-The maximum depth disc is seen in water
 Approximates turbidity or TSS

Question 8

Explain how lake stratification occurs in summer and winter and the potential consequences for
dissolved O2

Answer 8

-large temp difference between surface and water in summer and winter
-Dense, cold water sinks to bottom of lakes and ocean
-Ice formation insulates lake in winter
-Water becomes stratified in layers (doesn’t mix)
-mixes in spring and autumn - no large T difference

Dissolved Oxygen (DO)
-O2 solubility
-more soluble in cold water
-higher in winter at top of lake than in summer

Higher DO better for aquatic life