Groundwater Flashcards
What is groundwater?
Water beneath the ground surface contained in void spaces
What void spaces can be found under surface?
Pore spaces between rock and soil particles or in bedrock fractures
What is an aquifer?
Underground layer of water-bearing permeable rock or unconsolidated material (gravel, sand, silt, clay) from which groundwater can be extracted using a water well
What is a water table?
The level at which the groundwater pressure is equal to the atmospheric pressure
What is an aquitard?
Zone within the earth that restricts the flow of groundwater from one aquifer to another
What do aquitards comprise of?
Layers of either clay or non-porous rock with low hydraulic conductivity (bedrock)
What is an unconfined aquifer?
An aquifer with the water table as its upper boundary and lower boundary is an aquitard
In an unconfined aquifer what and why is the groundwater pressure equal to?
= atmospheric pressure because aquifer is not under pressure
What is a confined aquifer?
An aquifer found between 2 relatively impermeable layers
What is a water well?
An excavation or structure created in the ground to access groundwater in underground aquifers
What is an artesian aquifer/well?
Confined aquifer containing groundwater that flows upwards through a well without the need for pumping
What is a flowing artesian well?
When water reaches ground surface when natural pressure is high enough in a confined aquifer
Why is there no need for pumping in an artesian well?
Confined aquifer water is trapped and aquifer is full hence the water is under hydrostatic pressure which pushes the water up when a well is drilled into it
What is a piezometric surface?
Water pressure surface that everywhere coincides with the piezometric head of the water in the aquifer.
Where is the piezometric surface for artesian ground water areas?
Above land surface
What is a borehole and what is it used to measure?
Narrow shaft drilled in the ground as part of a groundwater site assessment used to measure how deep groundwater layer is
What is base flow and what is a requirement for this to occur?
Portion of stream flow that comes from groundwater - water table must be high enough
Why is base flow important for rivers?
Sustains river flows during dry periods between rainstorms
What is groundwater recharge?
Natural or intentional infiltration of surface water into the groundwater system
How can groundwater recharge occur
a) naturally
b) intentionally
a) rainfall increases water table
b) adding water to an aquifer using a well
What is fossil water and how does it occur?
Groundwater that has remained in an aquifer for thousands of years when geological changes seal the aquifer off from further recharging
Explain the characteristics of unconfined groundwater
- Free water table
- Hydraulic gradient can change rapidly since water table can rise and fall with rainfall
- Great fluctuations to seasons ^
- Drill to water table
- No flowing wells since water table is at atmospheric pressure
- Recharge area is around borehole
- On pumping, aquifer is dewatered (water table reduces)
- Must be below ground surface
- Complex maths since water can move
Explain the characteristics of confined groundwater
- No water table only a piezometric surface since water is confined
- Hydraulic gradient is more uniform with less seasonal fluctuations since water table isn’t affected as much by rainfall
- Drill to aquifer
- May get flowing wells since aquifer is confined the pressure could exceed ground level pressure and flow up
- Recharge area is away from borehole
- On pumping there is no dewatering, only a decrease in pressure over a large area
- May be above ground surface
- Simpler maths since water doesn’t fluctuate as much
When is Darcy’s Law valid (in terms of flow requirements)?
Slow flow and laminar flow state with a Reynolds number less than 1
