Water Composition Flashcards
What is the Hydrologic cycle?
Movement of water between biosphere, atmosphere, lithosphere, hydrosphere
Rain water composition
evaporation from sea, evapotranspiration from land
Bnefits of water
excellent solvents for ionic compounds, good for polar organic compounds
3 main natural resources
- gases dissolve in water Kh=[gas]/partial pressure of gas 2. atmospheric aerosols dissolve, including sulphates. 3. Sea surface bubbles NaCl aerosol -> dissolves
“Average” rainwater compositions?
Depends on location, very diluted seawater, human inputs cause changes
Why acidic pH 5.5?
Water falls in soils, sediments, dissolved minerals via acid hydrolysis reactions
Acid hydrolysis reaction
limestone+acidic natural water, clay mineral produce H+
pH logarithmic scale
change in 1 unit means 10 fold change in water acidity
“Boomerang profile” - Temperate region
Rich in Ca relative to Na due to hydrolysis reactions
“Boomerang profile” - Tropical region
River simillar to rain, low in TDS, rich in Na, shallow soil cover and intense rainfall limits weathering
“Boomerang profile” - Arid region
Evaporation of water = less soluble Ca to precipitate = Na rich brine, sea water composition end of process
Temperate
TDS 10-20x than rain water, wich in Ca, Mg, hydrogen carbonate and silicates
HCO3
derived from carbonic acid (CO2 in rain water( reaction with carbonaceous minerals in rocks
Silicates
mix of complexed hydrated silicate chemical species
Water residence time
t increases as water slows
Spring and summer surface of heating water
surface water less dense than underlying water - results in stratification of water column
Water column
Deeper water isolated from surface contact with atmosphere for weeks
Late autumn and early spring
surface cools by lower air temperature with strong wind mixing -> stratification broken
Broken Stratification
leads to water turnover periods - water surface becomes cool and dense to sink to be replaced - water formerly held at depth
Winter
strong surface cooling leads to ice formation & weak water stratification
What does water stratification and stability allow photosynthetic plankton do?
Grow and thrive in the light and nutrient rich surface of water
What happens when plankton grow?
Surface water nutrients become depleted - deep water nutrients unavailable
Deep water
cannot mix upward -> leads to crash of plankton bloom
Dead plankton cells
gravitationally sink, decompose in deep water
What nutrients are re-released?
N and P -> leads to elevated nutrient concentration in hypoliminion in contrast to depleted low conc. in epiliminion
Decomposition
leads to dangerously low oxygen levels in deep water -> carbon-rich biogenic particles oxidise
Seawater composition
Mostly Na+Cl then Mg, S, Ca, K. Rest = ions of elements in periodic table. No units “PSU”
Coastal waters
sea water influence by fresh water, simillar chemistry to lakes : stratification (thermal & salinity) and nutrient supply
Groundwater
impermeable -> (2 years) -> AQUIFER - (decades) -> Centuries -> impermeable
Oxidising and solution reactions
mildly acidic rainwater infiltrates: acid hydrolysis reactions. Filtration process means pathogen content low. Water: Ca, Mg, HCO3 (hard water)
Ion exchange (sorption) reaction
Clay minerals sorb higher charged ions (Ca2+, Mg2+, SO4 2-), groundwater enriched in monovalent ions (Na+, K+, Cl-), change to “soft water” - low pathogens
Saltiness of water
becomes salty with age - restricts usefulness for abstraction
Reducing reactions
Oxygen used in decomposition & reduction reactions, others: NO3- -> N2 and SO4 2- -> S2 (transition metals precipitate)
Methanogenesis
Alternative, less energetically favourable, CO2 -> CH4 (bacterial mediation)
