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)

11
(No Transcript)
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)

20
(No Transcript)
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)

26
(No Transcript)
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.

37
(No Transcript)
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.

39
(No Transcript)
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