Groundwater Flashcards
what is ground water
water that infiltrates into the ground
how is water available to us?
Most fresh water on Earth tied up in glacial water
Most fresh water comes from groundwater aquifers
- Underground layers of permeable soil, sediment or rock that host extractable groundwater
Most fresh water used for irrigation (in agriculture), less for residential use
what is the zone of aeration
pores contain water + air
what is the of saturation
pores contain water only
where is the water table located?
top of zone of saturation
where does groundwater occur?
in the zone of saturation (below water table)
storage capacity of an aquifer is controlled by what?
by porosity (spaces in material)
what is the ability to flow
permeability (interconnectedness of pores)
what is a runoff of water
runoff occurs when there is more water than land can absorb
What is the groundwater relationship with surface water
Addition to water from runoff, streams can also receive water from groundwater sources
This is why river level can remain high long after an event of heavy rainfall (takes time for groundwater to reach the river)
what is the flow behaviour of groundwater
Water flows very slowly downslope in generally predictable pattern (provided that the characteristics of the material is fairly uniform)
Water table contours represent line of equal elevation above sea level
what are the two main types of aquifers
Unconfined aquifer
Confined aquifer
what is a unconfined aquifer?
an aquifer that receives water directly from the land surface above
what is a confined aquifer
an aquifer that is overlain by a layer of relatively impermeable material (commonly clay or clay-rich rock) that limits groundwater movement in or out the aquifer
what is an aquitard
An impermeable unit of material
what are the important feature of aquifer
Flow direction
Recharge area
Aquifers (confined, unconfined)
Aquitards
Perched water table
Additional features:
Influent stream (loses water to groundwater system)
Effluent stream (gains water from groundwater system)
The buildup of water over a local aquitard produces a spring
What type of system “pumps themselves” and why?
Artesian Systems
Artesian systems “pump” themselves” due to water table in recharge area being at a higher elevation than land surface above a well in confined aquifer in a lower-lying area (creates “hydraulic head”)
what is a artesian system
The higher the water level, the higher the water pressure
The downward “push” of the water (hydraulic head) helps the delivery of water through pipes to homes
What are the groundwater consequences when water exceeds recharge
Local drawdown of water table
Formation of cone of depression
Cone of depression expands laterally as more water is withdrawn
what is consequences of water overdraft
Agricultural demand (irrigation)
Output»_space; input
Regional drawdown of water table
Shrinking water supplies
Consequences:
Compaction
Permanent lowering of permeability
Subsidence (sinking of land)
What are the consequences of withdrawal of groundwater
Increased flood hazards, more runoff due to decreased permeability, greater concentration of pollutants in wetlands
In coastal areas, excessive groundwater withdrawal (from freshwater lens) can lead to an inflow of salt water (from the sea)
what is a septic tank sewage disposal
Contaminated water slowly infiltrates into soil, organic contaminants naturally broken down by bacteria in soil
Under normal conditions, “beneficial” bacteria naturally occurring in the soil dominate over potentially harmful bacterial strains
what are the effects on permeability on groundwater flow?
Contaminated water may be “purified” by beneficial microbes as the groundwater is flowing relatively slowly through moderately permeable sandstone
Highly permeable limestone (w/ large holes dissolved in the rock) allows groundwater to move too quickly to allow beneficial microbes to “purify” the water of contaminants
How do we avoid polluting drinking water sources?
We can predict effects of water table drawdown + the migration of the septic plume
- Excessive pumping can alter direction of groundwater flow
Groundwater moves through high permeability materials faster than less permeable materials
- Less time for microbes to filter out contaminants
what is Karst
Dissolution of carbonate sedimentary rocks by groundwater produces a distinctive type of land topography called karst
Characterized by underground drainage systems (dissolved by groundwater) with caves/caverns + sinkholes (produced when caves collapse)
when do sinkholes form
Form when the “roofs” of caves collapse
Collapse events often spectacular
Common in temperate to tropical regions underlain by carbonate rocks
