1 Aquifers Flashcards
The world‘s water
96.5% salt water
- 6% freshwater
- Polar ice 69.5%
- Groundwater 30.2% (play major role)
- Lakes, river 0.3%
Development of water consumption worldwide
- Population growth
- Growth in industry
- Level of living
Definition aquifer
aqua: water
ifer: carrier
geologic unit that can store and transmit water at rates fast enough to supply reasonable amounts to wells (e.g. unconsolidated sands and gravels, sandstones, limestones, basalt flows …)
Definition aquitard (confining layer)
geologic unit with low hydraulic conductivity (e.g. clay)
Definition aquiclude
geologic unit that has no
hydraulic conductivity.
Impossible to find in reality
Attachment of water in pores
(1) Gravitative (free) water # Te only water we can actually use
(2) Capillary bound water # Important for plants
(3) Water adsorbed to mineral surfaces (nm)
(4) Crystal water
(2) + (3) = bound water
Imagine you wet a sponge, the water that flows out naturally is (1), the water that flows out after you squeeze thoroughly is (2), the water that keeps the sponge moist no matter how strong you squeeze is (3)
Groundwater and void volume
Porosity = Vpores / Vtotal
1. Pore aquifer # We have some grains and the pores between the grains are filled with water
2. Fractured aquifer # Solid rock that has been affected by tectonic processes which led to the formation of this fracture. Water flows to this fractures. # Double porosity = porosity inside the block of rock and fractured porosity (e.g. sandstone aquifer)
3. Karst Aquifer # Special type of fractured aquifer, usually only occuring in soluble rocks (e.g. gypsum, limestone, marble) # The fracture was enlarged by water solution. Can form large caves.
Aquifers in Germany
Pore aquifers dominate in Northern Germany (glacial and fluvial)
Fractured aquifers in mountaineous areas
Karst aquifers (Swabian-Franconian Alb)
Aquitards (consolidated rocks)
→ reservoir lakes
Example for type of aquifers
Pore aquifer:
- Sandstone
- Gravel, sand
According to the sorting:
- Well sorted
# Most grains have similar size, higher porosity n=32%
- Poorly sorted
# Small grains occupy pores between bigger grains, lower porosity n=17%
# Well sorted sediment is better aquifer than poorly sorted sediment
Example for type of aquifers - GRAPH
- The max of drainable porosity are in Sand and Gravel
- Clay contains a lot of water but not useful for us because it can not be drained gravitically
Sorting and packing affects porosity
Ideal: cubically packed, n=48%
Normally: rhombohedral packing, n=26%
Mix grains like cubic arrangement, n=12,5%
Pore aquifers
Clay
- high total porosity
- low effective porosity
- narrow pore openings
- low interconnection of pores
- high surface area
Sand
- middle-high total porosity
- high effective porosity
- wide pore openings
- good interconnection of pores
- low surface area
Pore aquifers - GRAPH
Total porosity, volume occupied by water in the sediment is not important compare to effective porosity.
Effective porosity is the water can be moved by gravity; water that can be extracted to the surface by man through well.
Fractured aquifers: single and double porosity
a = single porosity, only fractures
b = double porosity, fractures and pores (e.g. sandstone)
c = fractures filled with porous material (e.g.
some types of sandstone)
Karst aquifers
• only in chemically soluble rocks, e.g.
- carbonates: limestone, dolomite, marble,
- sulphate: gypsum, anhydrite
- (chlorides: rock salt)
- typical surface phenomena (e.g. dolines)
- high subsurface groundwater flow velocities
- high yields possible
- strong springs possible
- high vulnerability, but easier to clean up than pore aquifers
