WATER Flashcards
Porosity
- the amount of pore space in a rock, expressed as % of total rock
- % porosity = (vol of pore/vol of rock) × 100
- can be calculated with Eureka can
- can be calculated with % mass change (final-dry / dry) × 100
- difficult to achieve porosity >26%
- crystalline rocks have 0 porosity
Permeability
- the ability for a rock to transmit fluid, expressed as a rate of flow
- permeability = distance/time
Factors affecting permeability
- Porosity
-connectivity of pores - sorting
-poorly-sorted = small particles
between grains = not permeable - diagenisis
-cement reduces permeability - grain shape
- secondary permeability
-faults, joints, fractures
Water table
saturated - all pore spaces are occupied by water
unsaturated - not all pore spaces occupied by water
Water table separates unsaturated rock from saturated rock below
infiltration - water entering ground
percolation - water moving through ground
Head
- causes hydrostatic pressure
- height difference between top of water table and point of escape
Hydraulic gradient = head/horizontal distance between 2 points
Darcys law - Method to determine permeability and flow rates
diagram 14
Cone of depression
Created by abstracting water from a well
1) - Water will fill up to water table
2) - as water is abstracted, water enters well through the aquifer
- higher rate of flow closer to water table due to larger head so larger hydraulic gradient
3) - saturated rock becomes unsaturated as pores empty
- if rate of flow is higher than rate of abstraction, rock will become saturated
draw down - height difference between original water table and new position
diagram 15
London basin
Exposed aquifer = recharge zone
artesan fountain doesn’t require pumping due to high hydraulic gradient
-forms when a well is made
diagram 16
Kessingland
perched aquifer
- aquiclude holds water close to surface
- e.g clay lense prevents water sinking into sand
- if peirced, water will sink
Bahamas
Low rainfall, high evaporation, low lying, sub-tropical
- salty sea water is denser than fresh water so underlies
- incursion - salt water enters aquifer
- rain recharges thin soil layer
- shallow well - fresh water
- deep well - brackish water
diagram 17
Horsey mere
Low lying, North sea nearby, permeable sand and peat
- incursion of salt water through sand dune and peat
- fresh water sits on top of denser salt water
diagram 17
Sahara desert
Arid, hit, formerly wetter - fossil groundwater
- recharge zone at Atlas mountains
- groundwater flow through aquifer
- exposed aquifer form an oasis
diagram 18
Thr Great Artesian Basin - Eastern Australia
- very important for rural Australia
-European farmers used it for
cattle and irrigation - boreholes supplied water without pumping
- Birdsville geothermal energy supply
- many boreholes are abandoned - attempts to shut boreholes have been made
- slow rate of ground flow
Drinking groundwater and tap water
- naturally filtered (clear of particles)
- history of health benefits
- profit for bottled water companies
- Water isn’t pure
-contains dissolved ions
(calcium,magnesium) - expensive (transport costs)
Tap water
- regulated for quality but contains nitrates left by farm runoff (not removed by treatment)
- percolation acts as natural filtration which removes bacteria and chemical impurities
Water in South East UK
- mainly urban - high population density
- young, soft sedimentary aquifer
- lowland (hills,plains,vales)
- low rainfall, high sunshine
- Water deficit
-high population + low rainfall - high % of groundwater so high volumes abstracted
Water in North West UK
- mainly rural, low population density
- old, hard igneous/metamorphic aquiclude
- high rainfall, low sunshine
- Water surplus
-low population + high rain - mostly surface storage
-deep valley reservoirs - recharged by rain so low volumes of groundwater abstracted
