Chapter 12
Water On and Under the
Ground
Water on Earth
n
Water
is not known to be present as
a liquid anywhere else except on earth.
n
On
other planets, water is in the
form of ice and/or vapor.
n
On
Mars, liquid water may have
previously been present. Current conditions, (low temps and
pressures), only
permit water in the form of vapor or ice.
n
Venus
only has water in the form of
vapor due to its very hot surface temps.
n
Many
of the Jovian planets contain
ice due to cold temperatures.
The Hydrologic Cycle
n
Also
called the water cycle, is the
movement of water from one reservoir to another in the earth system
n
It
consists of a set of
interconnected reservoirs and pathways
n
The
largest reservoir of water in
the hydrologic cycle is the ocean, holding 97.5% of all the
earth’s water.
This is salt water, however.
n
The
largest reservoir of fresh water
is the frozen polar ice sheets, which contain almost 74% of all fresh
water.
n
98.5 %
of the remaining unfrozen,
fresh water resides as groundwater.
n
Very
small amounts of water actually
reside in the atmosphere or in surface freshwater bodies.
n
Even
smaller amounts reside in the
biosphere
n
Pathways
– processes by which water
moves back and forth from one reservoir to another, both within the
hydrosphere
and among the other reservoirs (biosphere and atmosphere)
n
The
hydrologic cycle maintains a
mass balance (the total amount of water in the cycle is fixed).
n
Solar
energy causes water to
evaporate transforming it into water vapor.
n
Transpiration is the process by which
water taken
up by the roots of plants passes directly into the atmosphere.
n
The
vapor condenses
(the process by which water changes from a vapor into a liquid or
solid) to
form clouds and then precipitates as rain or snow, onto both the sea
and land
n
Precipitation on land flows back to
the sea:
–
as surface
runoff
(precipitation that drains off over the land or in stream channels)
–
or can
contribute to glaciers as
glacial ice,
–
or
evaporates back to the atmosphere
from the land surface and from plant leaves
–
by infiltrating
(the
process by which water works its way into the ground, passing through
small
openings and channels in the soil and other surface materials) into the
ground,
joining the groundwater reservoir.
Fresh Water on the Surface
n
Stream – A body of water
that flows
downslope along a clearly defined natural passageway, transporting
particles
and dissolved substances.
–
Channel – The passageway
of a stream.
n
River
– A
stream with a considerable volume
and well-defined channel.
Factors Controlling
Stream Behavior
1.
Gradient
– steepness
of a
channel
2.
Channel
Cross-Sectional Area – Stream Width x Stream
Depth
3.
Velocity
of water
flow
4.
Discharge
– the
amount of
water passing by a point on a channel’s bank during a unit of
time. (stream
width x stream depth x velocity of flow)
5.
Roughness
of the
stream
bottom
6.
Load
– the
material carried along by a
stream.
–
Includes
suspended load and
dissolved load.
Stream Landforms and
Drainage Patterns
n
Streams
create landforms through two
processes:
1.
erosion (breakdown, removal, and
transport
of materials)
2.
deposition
(lying
down of
sediment)
Types of Channels
1.
Straight
2.
Meandering
– Forms
a series of
smooth bends
3.
Braided
–
When
water
repeatedly divides and reunites as it flows through interconnected
channels
separated by bars or islands.
•
Tend
to form in
streams with a highly variable discharge and easily eroded banks that
can
supply abundant sediment to the system.
Stream Deposits
1.
Alluvium
– Sediment
that has
been deposited by a stream in fairly recent times
2.
Point
bar
– formed
along the
inner side of a meander where water is shallow and velocity is low
3.
Floodplains
– The
relatively
flat valley floor adjacent to a stream channel, which is inundated when
the
stream overflows its banks
4.
Alluvial
fans – formed where
a stream flowing through a steep upland valley suddenly emerges onto
the floor
of a much broader valley
•
A
decrease in
slope, drop in velocity, and decrease in ability to carry sediment
creates this
feature
5.
Deltas
– A
deposit that forms where a stream
flows into standing water
Drainage Basins and
Divides
n
Drainage
Basins – The total area from which
water
flows into a stream.
–
The
drainage basin of the Mississippi encompasses more than 40% of the
total area of the U.S.
n
Divide
– The
topographic high that separates
adjacent drainage basins.
Lakes and Ponds
n
Lakes - A body of water that
has an upper
surface exposed to the atmosphere and has no appreciable
gradient.
•
The
scientific
study of lakes and other inland bodies of water is called limnology.
n
Ponds
– small,
shallow lakes
Wetlands
n
Areas
of poor surface drainage, such
as marshes and swamps, which may contain standing water
Flood
n
An
event in which a water body
overflows its banks
Fresh Water Underground
-Terminology
n
Groundwater
– Subsurface
water - the water
contained in spaces within bedrock and regolith
n
Water
table – The top surface of the
saturated
zone.
•
The
water table
tends to imitate the shape of the land.
•
In
other words,
the water table is high beneath hills and low beneath valleys.
n
Saturated
Zone – The zone where all
openings are
filled with water.
•
Movement
of
water in the saturated zone tends to be from where the water table is
high
toward areas where it is lower.
•
Water
generally
flows toward surface streams or lakes.
n
Unsaturated
Zone – The zone where water may
be
present, but it does not completely saturate the ground
n
Groundwater
– moves
very slowly and is measured in
cm/dy or m/y.
•
The
rate of flow
depends on the rock or sediment type, its porosity and
permeability.
n
Porosity
–
is the percentage of total volume
of a body of rock or regolith that consists of open spaces, or
pores.
•
Porosity
determines the amount of fluid a sediment or rock can hold.
n
Permeability
– A
measure of how easily a solid allows
fluids to pass through it; the interconnectedness of the pores.
n
Percolation
–
The process by which groundwater
seeps downward under the influence of gravity.
n
Recharge
–
Replenishment of groundwater occurs
when rainfall and snowmelt infiltrate the ground and percolate downward
to the
saturated zone.
n
Discharge
–
The process by which subsurface
water leaves the saturated zone and becomes surface water.
•
In
discharge
zones, subsurface water either flows out onto the ground surface as a
spring or
joins bodies of water such as streams, lakes, ponds, etc.
n
Caverns
and Karst – A type of terrain
existing In
regions underlain by rocks that are highly susceptible to chemical
weathering
(such as limestone/dolostone).
•
Groundwater
creates extensive systems of underground caverns.
Cavern
n
A
large cave or system of
interconnected cave chambers.
Karst Topography
n
Many
small, closed basins and a
disrupted drainage pattern characterize karst topography.
Can be
seen on topographic maps quite distinctively.
n
Karst
is typical of areas underlain
by limestone or dolostone where dissolution of minerals and rock
materials
occur in the groundwater and is carried away in solution.
Caves
n
Underground
open spaces form when
circulating groundwater dissolves carbonate rock
n
Carbonate
formations are chemically
precipitated drop by drop on the cave walls and ceiling, while a stream
occupies the floor.
n
After
the stream stops flowing,
similar formations are deposited on the floor.
n
These
carbonate deposits are called stalactites
(ceiling) and stalagmites (projecting upward from the
floor)
Sinkhole
n
A
dissolution
cavity that is open to the sky.
Groundwater as a Resource
n
Aquifer
– A
body of water-saturated, porous
and permeable rock or regolith.
•
When a
person
wants to find a reliable supply of groundwater, he/she searches
for an
aquifer.
•
Gravel,
Sand,
Sandstone – make
good aquifers
Types of Aquifers
1.
Unconfined
Aquifer – the water
is free to rise to its natural level.
•
In a
well
drilled into an unconfined aquifer, the water will rise to the level of
the
surrounding water table.
2.
Confined
Aquifer – is
overlain by impermeable rock units called aquicludes.
•
When a
well is
drilled into a confined aquifer, the pressure below will cause the
water to rise
in the well.
•
If the
ground
surface is lower than the level of the water table in the recharge
zone, water
may flow out of the well without pumping. This is called an
artesian well.
Springs
n
The
simplest kind of spring is one
that emerges from a place where the land surface intersects the water
table.
n
A
change in permeability can give
rise to a spring or line of springs.
n
Springs
can also come from lava
flows.
Cone of Depression
n
Forms
when water is pumped from a
well
Contamination
n
Most
common contaminants in springs
and wells are:
1.
untreated
sewage
2.
pesticides
3.
fertilizers
4.
leaking
underground storage tanks
5.
refineries
6.
Other
industrial
chemicals.