Natural Disasters Groundwater and Subsidence PresentationPresentation Transcript
Groundwater and Subsidence
Sinkholes, Land Subsidence, and Swelling Soils.
SUBSIDENCE-- the sinking downward of the earth’s
• Subsidence is typically not dangerous, but it does
cause major economic problems in the form damage to
structures, pipelines, drainage systems, and sewer
• Subsidence can be caused by natural processes, but
most problems involving subsidence are caused by
human activities. Some of the most severe subsidence
problems in the U.S.A. occur in agricultural areas with
intensive groundwater pumping.
This map shows
by state. California
and Texas are #1
and #2 users of
Both states have
The Water Table
Going into the ground, one passes through the zone of
aeration - the region where open spaces in the rock are
filled mostly with air - and passes into the zone of
saturation, where the open spaces are filled with water.
The contact between these zones is the water table.
Ground water occurs in
AQUIFERS -- large bodies of
rock that have both high
porosity and high permeability.
Porosity refers to the percentage
of the rock made up of open
spaces that can hold water.
Pore spaces can be the spaces
between sedimentary grains (a, b),
gaps made when rock dissolves
(c ), or cracks and fractures (d ).
Permeability refers to the ease
with which water flows through
Water underground obeys gravity, just like water above
ground. Both above and below ground, water flows
downhill! The hydraulic gradient is the slope of the water
table or aquifer .
If the water table is flat there is no hydraulic gradient,
and the water won’t flow. But if pressure is applied at the
pump shown here….(next slide).
…the removal of water near the pump causes the water
table to lower, forming a cone of depression in the water
table. This creates a hydraulic gradient, and water flows
“downhill” toward the well.
Cone of Depression
This figure illustrates nicely in 3-D how cones of
depression form in the water table around pumping wells.
An aquifer that gets
recharged regularly with
new water by nature can
sustain a certain amount
of pumping indefinitely:
If an aquifer is pumped
at rates that exceed the
sustained yield, we have
an overdraft situation.
In this case the water
table will fall from year
to year, as shown in this
Subsidence is caused by pumping of water, as well as
other materials such as crude oil, or natural gas from
natural gas) is
pumped out of
pressure on the
and the ground
Long Beach, CA, has experienced as much as 30 vertical feet of
subsidence as oil has been pumped from deep underground. The
building and parking lot at the left are several feet below sea level.
Walls hold back the ocean, and boat owners walk uphill from the
parking lot to get onto their boats!
This dog is thinking “How can
I use that hydrant way up
The hydrant was at ground
level when it was installed.
The hydrant was held in
place by the piping system as
the ground subsided around
it. (This is in Long Beach, CA;
same as previous slide).
Areas of Mexico City
have subsided as ground
water has been pumped
out from the
beneath the city. This
church’s foundation was
built half on firm
bedrock and half on
sedimentary layers that
subsided as water was
withdrawn. Its pretty
easy to tell here which
side is which!
Salt Water Encroachment
In coastal areas
near the ocean,
water “floats” on
Ground water pumping that lowers the fresh water table
too much allows sea water to rise up into wells, creating
salty drinking water. This problem of salt water
encroachment occurs in many heavily populated coastal
areas, such as Long Island, NY (see figure on the next
Water is a renewable resource, but all too often it is
used faster by people than it is naturally replenished.
What are the main ways water can be conserved?
1. Recycling of waste water. Treatment of waste
water can be done to different levels of purity. Water
does not have to be potable to be useful. So called “gray
water” can be used to irrigate public parks, roadsides,
and certain crops.
3. Artificial recharge of aquifers. During wet years,
water that would run off down rivers can be trapped
and allowed to sink into the ground to recharge local
4. Personal voluntary conservation.
Ground water is
and over time it
can dissolve away
large amount of
This forms caves
ground. This type
of landscape is
known as karst
Land that has been completely perforated with sinkholes
has the rugged and otherworldly appearance shown here.
This region of southern China is classic karst terrain.
Solution of limestone rock by ground water formed this
cave, and precipitation of calcium carbonate by ground
water created the dramatic cave features shown here:
stalactites, stalagmites, and columns.
When an underground cavern collapses it forms a
sinkhole, such as the one shown here in Winter Park, FL.
Several homes and cars were destroyed (“swallowed”
really!) by this sinkhole.
The U.S. Public Heath Service has defined the maximum
amount of dissolved materials public water supplies can
contain to be considered potable (safe and drinkable).
Some materials dissolved in water are important for
health: calcium, magnesium, fluoride, and other
Some materials, such as arsenic or lead, can be hazardous
even at low concentrations.
Some materials, while not necessarily hazardous in small
amounts, can give water an unpleasant taste or smell
(iron, zinc, copper. sulfur), or make it less able to do
useful things like lather soap and remove dirt (calcium,
A pollutant is any kind of chemical, physical or biological
substance that negatively affects water’s safety and