Groundwater Intro Flashcards
What is groundwater?
Groundwater is water beneath the surface that fully saturates the pores in the ground
What is the water table?
The level that the saturated zone begins at, anything below it is groundwater
What is the capillary fringe?
A thin layer below the unsaturated zone and above the water table, water moves up into the unsaturated zone through capillary action
Is the capillary fringe groundwater?
No
What is porosity?
Porosity is the fraction of rock/soil which is composed of “void space
What are pores?
Pores are void space between grains of sand and rock (what material that is underlying the surface)
How do you calculate porosity?
Porosity= volume of void space/total volume there, is n= Vs/Vt
What terms is porosity split into?
Primary porosity and secondary porosity
What is primary porosity?
porosity developed during the formation of the rock
What is secondary porosity?
larger scale porosity that is formed after the formation of the rock by other processes such as fracturing, cracking, or dissolution
How do we measure the porosity in a soil core sample?
Measure the saturated sample, then dry it out and measure again, the difference in weight is where water could fill it up, it’s void space
What is permeability?
Permeability the ability of rocks/soil to transmit water (i.e. allow flow), connectivity of pore spaces
What if effective porosity?
is the volume fraction of a rock, sediment, or soil that participates in water flow, denotes as Ne or Ve
How do we measure effective porosity?
You take the amount of pores that are interconnected over the total volume of the sample, do this by placing sample on a wire mesh above a jar and letting it drain, that volume is effective porosity
What is an aquifer?
An aquifer is an underground layer of water-bearing rock, or unconsolidated material (soil, sediment)
What is an unconfined aquifer?
An unconfined aquifer is an aquifer with no overlying confining layer. It can establish its water table freely, and it can be recharged directly from above (water can trickle in)
What is a confined aquifer?
A confined aquifer has some sort of impermeable layer over top. This means that it cannot be recharged directly from above
What is an aquitard?
A layer that stops water from percolating through and recharging the aquifer, is impermeable
What is an artesian well?
When you drill into a confined aquifer and the amount of pressure in it wants to be at the level of whatever it’s charging it so it shoots right up.
How do we use drills to find ground water?
We drill down until we hit the water table
How do we use piezometers to find the groundwater?
You drill a well, case the piezometer with something impermeable, and at the bottom you make the surface permeable, allows you to measure height of water table, can also tells us about pressure in aquifer
How does piezometer tells us about pressure and direction of flow?
You can find two points and drill, see the water table and higher water table flows to lower water table
What causes subsidence?
With an unconfined aquifer, in Jakarta they have 500 m of unconsolidated sediment before you hit the bed rock, the pores are filled with water, can draw water out of it, but in some sediments once you draw water out of it the pores actually collapse (ie clay), this is what causes the sinking. The compaction of the clay layers makes the land scape sink.
Can’t undo compaction, the sinking in Jakarta is irreversible.
