Water Composition

Question 1

What is the Hydrologic cycle?

Answer 1

Movement of water between biosphere, atmosphere, lithosphere, hydrosphere

Question 2

Rain water composition

Answer 2

evaporation from sea, evapotranspiration from land

Question 3

Bnefits of water

Answer 3

excellent solvents for ionic compounds, good for polar organic compounds

Question 4

3 main natural resources

Answer 4

1. gases dissolve in water Kh=[gas]/partial pressure of gas 2. atmospheric aerosols dissolve, including sulphates. 3. Sea surface bubbles NaCl aerosol -> dissolves

Question 5

“Average” rainwater compositions?

Answer 5

Depends on location, very diluted seawater, human inputs cause changes

Question 6

Why acidic pH 5.5?

Answer 6

Water falls in soils, sediments, dissolved minerals via acid hydrolysis reactions

Question 7

Acid hydrolysis reaction

Answer 7

limestone+acidic natural water, clay mineral produce H+

Question 8

pH logarithmic scale

Answer 8

change in 1 unit means 10 fold change in water acidity

Question 9

“Boomerang profile” - Temperate region

Answer 9

Rich in Ca relative to Na due to hydrolysis reactions

Question 10

“Boomerang profile” - Tropical region

Answer 10

River simillar to rain, low in TDS, rich in Na, shallow soil cover and intense rainfall limits weathering

Question 11

“Boomerang profile” - Arid region

Answer 11

Evaporation of water = less soluble Ca to precipitate = Na rich brine, sea water composition end of process

Question 12

Temperate

Answer 12

TDS 10-20x than rain water, wich in Ca, Mg, hydrogen carbonate and silicates

Question 13

HCO3

Answer 13

derived from carbonic acid (CO2 in rain water( reaction with carbonaceous minerals in rocks

Question 14

Silicates

Answer 14

mix of complexed hydrated silicate chemical species

Question 15

Water residence time

Answer 15

t increases as water slows

Question 16

Spring and summer surface of heating water

Answer 16

surface water less dense than underlying water - results in stratification of water column

Question 17

Water column

Answer 17

Deeper water isolated from surface contact with atmosphere for weeks

Question 18

Late autumn and early spring

Answer 18

surface cools by lower air temperature with strong wind mixing -> stratification broken

Question 19

Broken Stratification

Answer 19

leads to water turnover periods - water surface becomes cool and dense to sink to be replaced - water formerly held at depth

Question 20

Winter

Answer 20

strong surface cooling leads to ice formation & weak water stratification

Question 21

What does water stratification and stability allow photosynthetic plankton do?

Answer 21

Grow and thrive in the light and nutrient rich surface of water

Question 22

What happens when plankton grow?

Answer 22

Surface water nutrients become depleted - deep water nutrients unavailable

Question 23

Deep water

Answer 23

cannot mix upward -> leads to crash of plankton bloom

Question 24

Dead plankton cells

Answer 24

gravitationally sink, decompose in deep water

Question 25

What nutrients are re-released?

Answer 25

N and P -> leads to elevated nutrient concentration in hypoliminion in contrast to depleted low conc. in epiliminion

Question 26

Decomposition

Answer 26

leads to dangerously low oxygen levels in deep water -> carbon-rich biogenic particles oxidise

Question 27

Seawater composition

Answer 27

Mostly Na+Cl then Mg, S, Ca, K. Rest = ions of elements in periodic table. No units "PSU"

Question 28

Coastal waters

Answer 28

sea water influence by fresh water, simillar chemistry to lakes : stratification (thermal & salinity) and nutrient supply

Question 29

Groundwater

Answer 29

impermeable -> (2 years) -> AQUIFER - (decades) -> Centuries -> impermeable

Question 30

Oxidising and solution reactions

Answer 30

mildly acidic rainwater infiltrates: acid hydrolysis reactions. Filtration process means pathogen content low. Water: Ca, Mg, HCO3 (hard water)

Question 31

Ion exchange (sorption) reaction

Answer 31

Clay minerals sorb higher charged ions (Ca2+, Mg2+, SO4 2-), groundwater enriched in monovalent ions (Na+, K+, Cl-), change to "soft water" - low pathogens

Question 32

Saltiness of water

Answer 32

becomes salty with age - restricts usefulness for abstraction

Question 33

Reducing reactions

Answer 33

Oxygen used in decomposition & reduction reactions, others: NO3- -> N2 and SO4 2- -> S2 (transition metals precipitate)

Question 34

Methanogenesis

Answer 34

Alternative, less energetically favourable, CO2 -> CH4 (bacterial mediation)