Transcript No Slide Title

Water Under
the Ground
Chapter 12
Water On and Under
the Ground
Geology Today
Barbara W. Murck
Brian J. Skinner
N. Lindsley-Griffin, 1999
Lutgens & Tarbuck, J.R. Griffin, 1999
Carlsbad Caverns, NM
The Water Table
Water Table = Top surface
of the saturated zone
Zone of aeration (unsaturated zone) - pores filled with air
Saturated zone - pores filled with water
Fig. 12.22
p. 353
N. Lindsley-Griffin, 1999
How Groundwater Moves
Fig. 12.24, p. 355
N. Lindsley-Griffin, 1999
Groundwater
and Streams
A stream that lies above the
water table loses water to
the zone of saturation, will
dry up at times.
A stream that lies below the
water table receives water
from zone of saturation,
will flow all year.
N. Lindsley-Griffin, 1999
Porosity and Permeability
Porosity - percentage of total volume of rock that consists of
open spaces (pores). (how many pores, and how big they are)
Permeability - a measure of how easily a solid allows fluids to
pass through it. (how well connected the pores are)
Fig. 12.23
p. 354
N. Lindsley-Griffin, 1999
Low Porosity
High Porosity
Poorly sorted
Well sorted
Porosity and
Permeability
IN GENERAL Porosity and permeability
are better in:
well sorted sediments
loosely packed sediments
spherical sediments
Tightly packed
Loosely packed
Cementation of sediment
decreases both porosity and
permeability
N. Lindsley-Griffin, 1999
Nonspherical
Spherical
Aquifers
Fig. 12.26
p. 358
N. Lindsley-Griffin, 1999
Aquifer - a body of water-saturated, porous
and permeable, rock or regolith
Aquifers
Confined aquifer:
trapped between
impermeable
layers (aquicludes).
Unconfined
aquifer: top of
saturation zone is
water table
N. Lindsley-Griffin, 1999
Fig. 12.26, p. 358
Recharge
Recharge - replenishment of groundwater
Recharge zone - where groundwater is replenished by
infiltration of rainfall and snowfall, which percolates downward
to the saturated zone.
Fig. 12.26, p. 358
N. Lindsley-Griffin, 1999
Discharge
Discharge - process by which groundwater
returns to the surface.
Examples: streams, springs, wells, the ocean.
N. Lindsley-Griffin, 1999
Wells
Pumping produces a cone of depression
in the water table around the well.
Fig. 12.29, p. 360
N. Lindsley-Griffin, 1999
Artesian system: water pushed up to
ArtesianWells
water table by hydraulic pressure.
Requires a confined aquifer, and sufficient pressure.
Usually supplied by folded or faulted beds that surface at a
higher elevation in the recharge area.
Fig. 12.27
p. 359
N. Lindsley-Griffin, 1999
Sources of Groundwater Contamination
Leaching of toxic chemicals from unlined landfills.
Leaking septic tanks.
Agricultural chemicals.
LUST (Leaking Underground Storage Tanks)
Fig.12.30, p. 361
N. Lindsley-Griffin, 1999
Springs
Springs form where a change in rock permeability forces
groundwater out to the surface: a) limestone over shale.
Fig. 12.28, p. 359
N. Lindsley-Griffin, 1999
Springs
Fig. 12.28, p. 359
Springs form…
b) highly jointed permeable rock like a lava
flow over impermeable limestone, or c) along
a fault.
N. Lindsley-Griffin, 1999
Springs
Springs form…
c) along a fault,
- where a permeable
layer is blocked by
being moved against
an impermeable layer,
- or where the fault
itself is impermeable.
Fig. 9.11, p. 251
N. Lindsley-Griffin, 1999
Caves and
Caverns
Caves and caverns underground open spaces form by dissolution of
carbonate rocks near the
top of a seasonally
fluctuating water table.
They are closed to the
surface or have only a
small opening.
Fig. 12.25 B, p. 357
N. Lindsley-Griffin, 1999
Caves and Caverns
Underground cavities form when water table is high,
then fill with air when water table drops and water
drains out.
Earlier
water
table
Present
water
table
Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999
Caves and Caverns
After water table drops, percolation of acidic water
from the surface dissolves ions which precipitate as
calcite cave decorations in the open air of the cave.
Earlier
water
table
Present
water
table
Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999
Sinkholes
Winter Park
sinkhole, near
Orlando FL - May
8-9, 1981
Grew to >100 m
wide in 17 hours.
Fig. 12.25 C, p. 357
N. Lindsley-Griffin, 1999
Sinkholes are dissolution cavities
that are open to the sky. They form
by collapse of cave roofs or where
rainwater is rich in carbon dioxide
Karst
Extensive dissolution produces karst
topography, with many small closed basins
and sinkholes, and disrupted drainage.
(Fig. 12.25A, p. 356)
N. Lindsley-Griffin, 1999
Karst
Limestone
towers
Auilin, China
Houghton Mifflin, 1998; N.
Lindsley-Griffin, 1999
Prolonged development of karst results in
topography with jagged carbonate towers
that served as the inspiration for Chinese
scroll painting.
Cave - a natural
cavity below
Earth’s surface
• Caves in limestone
form by
dissolution...
• below the water
table.
N. Lindsley-Griffin, 1998
“Soda Straws”
Cave decorations form by
precipitation above the water table
Sinkholes form when a cave roof collapses
Fossilization
Petrified
Forest, AZ
Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999
Groundwater aids fossilization by
leaching away organic matter and
depositing silica or carbonate ions
Hot Springs and Geysers
Hot springs form where
groundwater heated by
subsurface magma rises to
the surface.
Geysers (1) are eruptions of
hot water caused when
groundwater confined in
subsurface fissures becomes
superheated.
Houghton Mifflin, 1998; N.
Lindsley-Griffin, 1999
Hot Springs and Geysers
Geysers (2)
Superheated water at depth
expands, causing water to
slop over the rim and
releasing the confining
pressure.
(3) Water “flashes” into steam
and the geyser erupts.
N. Lindsley-Griffin, 1999
Hot spring deposits form by
precipitation of dissolved ions as
the water cools