Water supply Flashcards
What is porosity
The volume of pore space
What is permeability
Rate at which a fluid flows through a rock
How do you calculate porosity percentage
Total volume of pore space/total volume of rock or sediment *100
How do you calculate premeability
Distance water has traveled/time taken
What are the factors affecting porosity
Rock type
The degree of sorting (well sorted rock has high porosity)
The amount of diagenesis (a loose unconsolidated rock has a much higher porosity than a rock that has undergone compaction
Grain shape (rocks containing rounded grains has a higher porosity than angular)
The packing o the grains
GRAIN SIZE IS NOT A FACTOR IN POROSITY
What are the factors affecting permeability
Grain size (course grained rocks have a higher permeability than fine grained rocks because there is less resistance to flow around coarse grains) High porosity
What is the water table
The level at which water sits within the ground
What is the height of the overlying column of water known as
The hydro static head
How do you calculate the hydraulic gradient
difference in hydro static pressure or head/distance between two points
What is an aquifer
A body of porous and permeable rock capable of storing and yielding significant amounts of water
What is a recharge zone
Area of an aquifer open to the atmosphere, allowing replenishment of water
What is an aquiclude
An impermeable rock that does not transmit water
What are the two main types of aquifers
Unconfined aquifer-open to the atmosphere, under atmospheric pressure and is recharged by rainwater from directly above. Water will need to be pumped to the surface from a well or borehole sunk into an unconfined aquifer
Confined aquifer is overlain by impermeable rocks and the groundwater held within it is under hydro static pressure. Groundwater can only be replenished in a confined aquifer if it has recharge zones that are open to the atmosphere
What is a live or fossil aquifer
Live-one that is currently being replenished by rainwater via a recharge zone on the surface
Fossil-One that is no longer being replenished and represents a relic of a past wetter climate
What is an artesian basin
A large, synclinal confined aquifer under hydrostatic pressure
What is an artesian well
Hold water under hydrostatic pressure, which rises up the well on release
How is ground water abstracted
By sinking a borehole and pumping the water to the surface, as water is pumped up the level of the water table falls, leading to a cone of depression
What is the cone of depression
Occurs where there is a lowering of the water table in the vicinity of a well
What is the draw down
Describes the height difference between water table and water level in the well
What are the main problems caused by groundwater extraction
Lowering of the water table
Subsidence at the surface resulting from the removal of water from the pore spaces of rocks
Salt water encroachment in coastal areas
What are the threats to ground water supply
Over-pumping
Pollution- groundwater is vulnerable to contamination, once polluted it is difficult to clean
What are the sources of groundwater pollution
Nitrates, pesticides and microbes from run-off
hydrocarbon and solvents from petrol stations and factories
Toxic fluids from landfill waste disposal sites
Acid mine drainage water containing toxic metals such as lead and cadmium from coal and metal mines
What are springs
Occur when the water table meets intersects the land surface and groundwater flows out onto the surface
What are the types of springs
Lithological springs-result in change of rock type, occur where porous and permeable rock overlies impermeable rock, the water table will intersect the land surface at the junction between the two rock types, there will be a spring line along the base of the permeable rock
Springs at faults-If faults have moved porous and permeable rock into contact with an impermeable rock a spring line will occur where the fault plane intersects the land surface
Springs at unconformities-If porous and permeable rock lies uncomfortably on top of impermeable older rock the water table will intersect the land surface at the junction between the two rock types, a spring line will occur where the line of the unconformity intersects the land surface
