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Lecture 13 Groundwater

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Lecture 13 Groundwater Where does subsurface water come from? The water table Storage and movement of groundwater Springs groundwater pollution Formation of caves and ... – PowerPoint PPT presentation

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Title: Lecture 13 Groundwater


1
Lecture 13 Groundwater
  • Where does subsurface water come from?
  • The water table
  • Storage and movement of groundwater
  • Springs
  • groundwater pollution
  • Formation of caves and karst topography
  • Subsidence

2
  • Where does subsurface water come from?
  • When rain falls, some of the water runs off, some
    evaporates, and the remainder soaks into the
    ground -- infiltration -- as part of the
    hydrologic cycle.
  • Thus, like surface water, subsurface water has
    been supplied primarily from precipitation.
  • Subsurface water resides and moves in pore
    spaces.

3
  • Balance of water in the hydrologic cycle.

4
  • How is the water that generates a spring like
    this stored underground? How does it move?

5
  • Storage and movement of groundwater
  • Porosity
  • Porosity is the amount of pore space in a soil
    or rock as a percentage of the total volume.
    Porosity determines the amount of groundwater
    that can be stored. (Table 15.1 p.329 lists
    porosities of soil and rock.)

6
Porosity in various kinds of rocks. (W.W. Norton)
7
  • Permeability
  • Permeability measures the ability of soil or
    rock to allow the passage of fluids, e.g. the
    permeability of sands and gravels is much higher
    than that of clay.
  • Impermeable layers that hinder water movement are
    called aquitards (such as clay layers). Permeable
    rock layers that transmit groundwater freely are
    called aquifers (such as layers of sands and
    gravels).

8
Impermeable and permeable materials. (W.W.Norton)
9
Aquifer and aquitard. (W.W. Norton)
10
  • Darcy's law
  • provides a basic governing equation for flow
    through a porous medium. It states that the rate
    of flow per unit area is proportional to the
    hydraulic gradient.
  • QkiA
  • Qvolumetric flow rate (or discharge, volume per
    unit time)
  • khydraulic conductivity (coefficient of
    permeability)
  • ihydraulic gradient
  • Across-sectional area
  • (Table 5.2 lists hydraulic conductivities, P.331)

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12
  • The water table
  • Part of the water that soaks into the ground is
    held by plants near the surface. Water that is
    not held in this zone of soil moisture can go
    downward until it reaches a zone of saturation
    where all the open pore spaces are filled with
    water. Water within the zone of saturation is
    called groundwater. The upper surface of the
    groundwater is known as water table.
  • When a well is drilled a short distance into the
    saturated zone, the level of water in the well
    coincides with the water table.

13
  • The water table is rarely level its shape is
    usually a subdued replica of the surface
    topography.
  • The most important reason for this is that
    groundwater moves very slowly, thus frequent
    enough rain falls keep water piled up even in the
    high areas.

14
  • The water table is the upper surface of the zone
    of saturation. (Hamblin and Christiansen)

15
  • Distribution of subsurface water. The shape of
    the water table is usually a subdued replica of
    the surface topography. During periods of
    drought, the water table falls, reducing
    streamflow and drying up some wells. (Tarbuck and
    Lutgents)

16
  • Springs
  • Springs are mysterious to many people they
    flow constantly freely from the ground in
    seemingly inexhaustible supply but with no
    obvious source.

17
  • Thousand Springs along the Snake River canyon,
    Idaho. The springs on the canyon wall are fed by
    underground water through the high porous and
    permeable basaltic bedrock. (Hamblin and
    Christiansen)

18
  • Springs form
  • (1) whenever the water table intersects Earth's
    surface so a natural outflow of groundwater
    results
  • (2) when an aquitard blocks the downward movement
    of subsurface water and forces it to move
    laterally.

19
  • Spring may form when an aquitard blocks the
    downward movement of subsurface water and forces
    it to move laterally. (Tarbuck and Lutgens)

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21
  • Groundwater pollution
  • Groundwater pollution is a serious matter in
    areas where groundwater is used as water supply.
  • Septic tanks
  • Septic tanks are widely used by households
    in some area that lack full sewer networks.
    Sewage water is discharged into the soil to be
    filtered out. The drainage of septic tanks should
    be placed at sufficient distance from water wells
    in shallow aquifers.

22
  • Septic tanks. Sewage water passes through septic
    tanks and is discharged into the soil to be
    filtered out. A) contaminated water moves rapidly
    through the cavernous limestone and has traveled
    more than 100 meters to reach Well 1. B) Water is
    purified in a relatively short distance through
    permeable sandstone.(Tarbuck and Lutgents)

23
Sanitary landfills
  • Materials leached from landfills may find their
    way into the groundwater, contaminating water
    supply.

24
  • Sanitary landfills
  • Fine-grained soils such as clay provide a
    more desirable landfill site than coarser-grained
    soils.
  • 1) Finer soils have a lower permeability.
  • 2) Finer soils are able to exchange heavy toxic
    cations in the wastes (such as lead, zinc,
    chromium, and mercury) with Na, Ca, and Mg
    cations.

25
Saltwater encroachment
  • A) Fresh water floats on the denser salt water
    and forms a lens-shaped body that may extend
    considerable distance below sea level. B) When
    excessive pumping lowers the water table, the
    base of the freshwater zone will rise by 33 times
    that amount. This may result saltwater
    contamination of wells (called saltwater
    encroachment). (Tarbuck and Lutgens)

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27
  • Formation of caves and karst topography
  • Groundwater dissolves rock. Limestone and
    dolomite are quite soluble in carbonic acid,
    which forms because rainwater dissolves CO2 from
    the air and from decaying plants.
  • The most spectacular results of groundwater's
    erosion are limestone caverns.

28
  • A soda straw forest in Carlsbad Caverns, New
    Mexico. When a water drop reaches air in the
    cave, some of the dissolved carbon dioxide
    escapes from the drop and calcite precipitates.

29
  • Karst topography (after Kras Plateau in Slovenia)
    is another result of the dissolving power of
    groundwater in a limestone or dolomite region.
  • Karst areas have irregular terrain sinkholes.
  • Sinkholes can form gradually as the limestone
    below the soil is dissolved and the surface is
    gradually lowered. They can also form abruptly
    when the roof of a cavern collapses.
  • Karst areas in the U.S. appear in limestone
    regions in SW Illinois, southern Indiana, central
    Tennessee, Kentucky, Alabama, Florida, and Texas.

30
KARST LANDSCAPES OF ILLINOIS
31
Collapsing Cavities
Introduction
Carbonate karst landscapes comprise about 40
percent of the United States east of Tulsa, OK
32
How do sinkholes form?
Initially, a sinkhole forms as soil collapses
into a crevice and is carried away through a
conduit by water.
Further collapse of the soil cover from below
causes circular cracks to develop at the surface.
Erosion by water flowing into this new hole
smooths the holes sharp edges to from the
typical inverted cone- or bowl-shaped depression.
Then the soil roof of the developing sinkhole
falls into the hole to form a cylindrical cavity.

33
Sinkholes can be dangerous. The "throats" at the
bottoms of some sinkholes have sharp drop-offs
that reach tens of feet deep into crevices or
caves in the bedrock below. Household pets,
people, and even livestock can fall, get trapped,
or drown in funnel-like sinkholes.
34
Roadcut exposing creviced dolomite typical of
north-central and northwestern Illinois
35
Collapsing Cavities
West-Central Florida
Mining exposed this karst limestone surface
riddled with dissolution cavities
36
  • (Left) Infrared image shows an area of karst
    topography in central Florida. The numerous lakes
    occupy sinkholes. (Right) This small sinkhole
    formed suddenly in 1991 when the roof of a cavern
    collapsed, destroying this home in Frostproof,
    Florida.

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38
  • Subsidence
  • When groundwater is withdrawn faster than natural
    recharge, significant subsidence can occur,
    damaging construction, water supply lines,
    sewers, and roads.
  • This is particularly pronounced in the areas of
    unconsolidated sediments. As the water is
    withdrawn and the water pressure drops, the
    sediments undergo additional compaction, causing
    the ground to subside.

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40
  • The marks on this utility pole (left) indicate
    the level of the surrounding land in preceding
    years. Between 1925 and 1975 this part of the San
    Joaquin Valley (right) subsided almost 9 meters
    because of the withdrawal of groundwate and the
    resulting compaction of sediments.

41
  • Subsidence of buildings in Mexico City resulted
    from compaction after groundwater was pumped from
    unconsolidated sediment beneath the city.
    Subsidence has caused this building to tilt and
    sink more than 2 m.

42
Houston-Galveston, Texas
Mining Ground Water
A road (below right) that provided access to the
San Jacinto Monument was closed due to flooding
caused by subsidence
43
Mining Ground Water
Houston-Galveston, Texas
A house near Brownwood damaged by fault creep
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