Ground Water: Water Flowing Underground

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Ground Water Water Flowing Underground
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A Spring Ground Water Becomes Surface Water
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Why study ground water?
Remember Only about 3 of Earths total water
budget is fresh water. And Ground water is
about 98 of Earths unfrozen fresh water.
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Why study ground water?

• Where is ground water found?
• How does ground water flow?
• How fast does ground water move?
• What is the composition of ground water?
• How does ground water shape the landscape?

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Why study ground water?

• In the U.S. we use 314 billion liters (83
  billion gallons) of ground water every day.
• About 40 of our drinking water in the U.S. comes
  from ground sources.
• About 66 of our ground water is used for
  agriculture.
• It represents 98 of Earths unfrozen fresh
  water.
• It influences the shape of surface landscapes.
• It is a critical resource for sustaining human
  life.

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Why study ground water?

• A hamburger requires water to raise wheat for
  the bun, to grow hay and corn to feed the cattle
  and to process the bread and beef. Together with
  french fries and a soft drink, this all-American
  meal uses about 1,500 gallons of waterenough to
  fill a small swimming pool. To grow cotton for a
  pair of jeans takes about 400 gallons. A shirt
  requires about 400 gallons. To produce the amount
  of finished steel in a car has in the past
  required about 32,000 gallons of water.

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Where is ground water found?

• Ground water is water that soaks into soil and
  rock at the surface, moves through fractures and
  pores to some depth below the surface, and
  resides there for periods of up to thousands of
  years.
• Ground water may reside in alluvium (loose
  sediment) or rocks which have porosity (e.g.,
  clastic sedimentary rocks).
• Porosity - pores, fractures, joints, and other
  open spaces in rock or alluvium. It is simply
  the percentage of open space.
• Permeability a measure of the
  interconnectedness of pore space. High
  permeability equals ability of water to flow
  through the pore spaces of the rock or alluvium.

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Where is ground water found?
Porosity varies according to the type of pore
space and material. Generally, pores are small (a
few mm or less) and found in between
unconsolidated grains, and as fractures in rock.
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Where is ground water found?
Calculation of porosity Two containers are
filled to an equal level, one with sediment, the
other with water. The water is poured in until it
reaches the top of the sediment. The amount of
water that fits into the spaces within the
sediment is the amount of porosity. If the
volume of water poured in is 50 of the sediment
volume, then the porosity of the sediment is 50.
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Where is ground water found?
The Concept of a Water Table
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The water table is simply the top surface of the
saturated zone. The water table is a surface that
generally follows the topography of the ground
surface above.
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Where is ground water found?
Where the water table intersects the ground
surface there will be a body of water. It may be
a stream or other body of water such as a lake.
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Where is ground water found?
Periods of high rainfall ground water may move
into streams and lakes. The water table is high.
Periods of low rainfall water from streams and
lakes may move into the ground water. The water
table has lowered.
Periods of drought water from streams and lakes
may move into the ground water. The water table
has dramatically lowered and may be disconnected
from surface water.
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Where is ground water found?
Ground water flows through the pore space. Its
ability to flow is a measure of the permeability.
Permeability is literally the ability for water
to flow through a porous material. The
connectedness of the pore space is the primary
control, and to a lesser extent the size of the
pore spaces.
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Where is ground water found?
The water table separates the saturated zone from
the unsaturated zone (or the zone of
aeration) The saturated zone is the region below
the water table where all pores are completely
filled with water. The unsaturated zone, above
the water table, contains some air in the pores.
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Where is ground water found?
A perched water table is formed where an
impermeable layer of rock (an aquitard, e.g.
shale or mudstone) stops the downward flow of
water to the deeper, main water table. Springs
at the surface are often connected to perched
water tables. An aquifer is simply an underground
body of water.
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Where is ground water found?
Some aquifers are very large. Others are quite
small by comparison. For example, the aquifers
beneath the Las Vegas valley are only 10 miles
in lateral extent.
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Where is ground water found?
Aquitards may control and confine ground water
and form aquifers. Water moves downhill under the
influence of gravity, often from higher elevation
recharge areas in mountainous regions.
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How the water table may change over time.
(A) Change in ground water level with rainfall
for a shallow well in NV. (B) Ground water
depletion caused by increased withdrawal over
time for wells in NM.
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17.2 Why and how does ground water flow?
Subsidence caused by ground water withdrawal.
Compaction of pore space when water is removed
causes surface elevation decrease and fissure
formation.
Fig 11.12
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How does ground water flow?
Ground water flows downwards under the influence
of gravity from higher areas of recharge to lower
areas, where it may be either stored in aquifers,
or discharged into streams. In ground water
systems deeper slower movement longer
residence time.
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How does ground water flow?
Downward moving ground water may discharge into
streams, enter and reside in unconfined aquifers,
or become trapped in confined aquifers.
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How does ground water flow?
Pollution in ground water Once contaminants
enter the ground water they may remain there for
thousands of years, or be discharged into streams.
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How fast does ground water move?
Can ground-water flow velocity be measured? We
can measure the transit time of water using a
tracer chemical. In this experiment on Cape
Cod, a bromide chemical was injected into a well
in a moving aquifer and monitored over the course
of 18 months until it reached another test well
and was detected.
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How fast does ground water move?
656 detection wells and 3 injection wells were
used to determine flow rate and direction.
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How fast does ground water move?
Experiments and isotopic dating give us
information on movement rates of ground waters
and on how long they remain in aquifers. Generally
movement of ground water is a few centimeters
per day. Water in some aquifers may be as old as
10,000 years or more. Combined, this information
tells us that if we deplete aquifers by pumping
they will not recharge naturally in our
lifetimes. In other words, they are in some
cases a non-renewable resource like crude oil.
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What is the composition of ground water?

• Water-mineral reactions
• Occur constantly in the saturated zone.
• Slow-moving water, long travel paths, and
  increased temperature at depth all increase
  reactions.
• Cementation
• Occurs where mineral precipitation takes place.
• Slows water down even more, so more reaction
  potential.

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What is the composition of ground water?
Hard water high concentration of dissolved
material Minerals from ground water precipitate
under the right conditions. This may be in
between mineral grains, or in
your water pipes. These are usually Ca or Mg
salts from calcite or dolomite. They decrease
effectiveness of soaps and clog showerheads,
irons, and coffee makers. Minerals can be removed
using water softeners, but soft water leaches
ions from metal pipes.
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What is the composition of ground water?
Some economic minerals are produced by hot,
circulating ground waterCu, Pb, and Zn ores.
These (and other metals) dissolve easily in warm
water. They precipitate as sulfide minerals such
as
galena, chalcopyrite, and sphalerite.
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How does ground water shape the landscape?

• Soluble minerals such as calcite in limestone
  dissolve in ground water.
• Dissolved material leaves open spaces behind.
  Caves are formed this way.
• These spaces can become large enough to be
  expressed on the surface as
• Subsidence
• Sinkholes
• Regions exhibiting such features are said to have
  karst topography.

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How does ground water shape the landscape?
Development of karst features in a landscape.
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The formation of caves involves both erosion and
redeposition of soluble minerals.