Water-use statistics from the U.S.
Geological Survey
comparing the percentage of ground water
versus surface water used
Water drawn from rivers, lakes,
shallow ground water, deep aquifers
Ground water as a percentage of total water usage
high percentage of
total water use in the Great Plains
(NE,
KS, OK, AR, MS, NM, SD)
It is NOT an underground river
Figure
11.1 {
*** Know the basic terms on this figure *** }
the space between the grains in
sediment or rock
Porosity – the % of open pores
(of the total
volume)
Permeability – ability of water to flow
through
sediment or rock
It is possible to have high porosity,
but low
permeability – for example, silt
Larger connections
carry more water, faster
High Moderate Low Very low
Gravel Silt Till
Sand
(clean)
Sandstone Shale
Granite
Schist
(no fractures)
And flow velocities
meters per day down to mm per year
The water level in a pond, or quarry
represents the top of the water table
The water level in a shallow well also represents the water
table
But the yield (of water) depends on the permeability Figure 11.4
Perched water table Figure
11.2
A small volume of ground water separated from the
water table by a low-permeability layer – this is important for siting wells
Can
water flow uphill?
Think about
how water gets to a faucet in the third floor of a building
Pressure gradient,
permeability, distance
Compare flow through gravel,
sand, silt, clay
High slope, low slope (equiv.
to pressure)
Henri Darcy 1856 an engineer
Experiments on flow rates through different sediments
Developed “Darcy’s Law”
Well clusters show
the pressure gradient
both laterally (parallel to
the land surface)
and vertically (water flowing
down or up)
Ground-water
flow along pressure
gradients Box 11.1
Blue lines – ground-water flow
Dark lines – lines of equal pressure
Water flows from high to low pressure
Recharge – water flowing into the aquifer Figure 11.1
Discharge
– water flowing out of the aquifer Map showing the elevation of the water table
Ground water will flow “downhill” with the
slope of the water table
Wet season – typically early spring
Dry season – typically early fall
Ground
water flows toward a gaining stream
Losing stream – water from the stream
flows down into the aquifer (and out of the stream)
Ground water flows away from a losing stream
Ground water discharging to the surface
Different settings for springs Figure 11.12
Fractures
Confining layer
Karst (limestone
with dissolution caverns)
Fault
Seeps on a cliff face Figure 11.11
Sandstone over a shale
Body of sediment or rock, saturated with water (ground water), water flows through easily
Confining layer – low
permeability material that blocks ground-water flow
Water flows easily
along the fractures, wells collect this water where they intersect the
fractures
The pressure in a confined aquifer raises water
in a well above the land surface
Pumping changes the direction of flow Figure 11.18
Effects of pumping Figure 11.8
which may lead to problems
Nebraska has the most irrigation of any state in the U.S.
Water for irrigation is drawn from surface
water and ground water
Pumping
for irrigation has caused major changes in ground-water levels – From 50 feet higher, to 30 feet lower than
ground-water levels before irrigation
Flow in a stream is maintained
by ground-water discharge during dry periods (between rain events)
Heavy pumping for
pulp mills (making paper) As much as 80 feet
of drawdown near Savannah
Contamination of ground water Figure 11.14
Different types of contaminants
Non-Aqueous Phase Liquids Figure 11.16
Septic-tank effluent Figure 11.17
long distances from the source Using ground water for drinking supply Usually don’t need to filter to remove suspended
solids
Contaminant plumes
(in
ground water) Figure 11.15