Transcript 98b - TeacherWeb

Groundwater Depletion
https://www.youtube.com/watch?v=XQ0X_O-uRQ0
Groundwater



Water that moves downward into Earth’s
surface due to gravity.
Water stored underground in pore spaces of
rock layers.
Water in soil or rock that supplies springs
and wells.
Permeable and Impermeable Layers


permeable material allows water to pass
through (sponge, bread, sandstone)
impermeable materials do not allow water
to pass through; also called aquitards or
aquicludes
Examples of this type of material include:
clay
bedrock
shale
granite
Permeability


Permeability – ability of rock to allow water to
pass through
Depends on the extent to which pores are
connected to each other and the size of the
pores
-
Examples:
sand, gravel - permeable
silt - less permeable
clay and shale - impermeable
Permeability:
allows water to pass through
Porosity

The amount of water
that soil or rock can
hold depends on its
porosity.

Porosity is the % of a
material’s volume
that is pore space.
Porosity

Porosity = Volume of Pores
Total Volume
High Porosity
x 100
Low Porosity
Porous but not permeable …
Sometimes a material is porous but it won’t let
water pass through. Cork is an example. If it let
liquid through, it would leak out of the bottles!
Pumice is pitted with holes but can float because
water can’t get into it. In both cork and pumice,
the pores are not connected to each other.
Main Types of Porosity in Rocks
In rocks that hold groundwater, the water
fills the spaces between sand grains or in
rock crevices and in limestone openings.
Particle Shape
Rounded particles create more porosity
while angular particles have less porosity.
rounder = more porous
Particle Sorting
Well sorted (same size) particles have
greater porosity while poorly sorted
(mixed size) particles have less porosity.
uniform size = more porous
Aquifers
a geologic formation (rock layer) that stores
and/or transports water


An unconfined aquifer has a
permeable layer above
A confined aquifer has impermeable
layers both above and below it and
may be under pressure.
The water table is the top of the water surface in
the aquifer. Wells are used to get the water.
GROUNDWATER
Water table

The level of the water table can be affected
by different factors. These can include:






Amount of rainfall
Amount of time between rains
Season
Slope of the ground
Climate
Human impact
The saturated zone is the area beneath the water table where ALL
pore spaces are filled with water.
There is NO air in this zone.
Both air and water exist in the pore spaces of the
unsaturated zone above the water table.
Recharge areas are surface areas where water seeps in and flows
down to refill the aquifer. An aquifer’s recharge area may be far
away.
Contamination of aquifers occurs from the surface. Since
groundwater moves slowly, contamination continues for
many, many years.