Download 8. Groundwater

Groundwater
I.
II.
Groundwater is the water trapped between the spaces of the rocks and unconsolidated material
A. This is a major part of the hydrologic cycle and makes up 22% of the fresh water supply
1. Most groundwater is used for agriculture, industry, and domestic use
2. There has been great demand for the groundwater reserves to use for agriculture and others
that depletion of supply is becoming a major concern
3. Others concerns are contamination, ground subsidence, saltwater intrusion, and agriculture
pollutants
B. Groundwater supplies are replenished through precipitation infiltrating into the soil and pore space
of consolidated rock
1. It can also be replenished through lakes and reservoirs, streams, and recharge pools
2. Groundwater is filtered by soil and rock to remove bacteria and impurities
3. Some pollutants cannot be removed by earth and these become toxic pollutants that can move
through the groundwater system that remains for many years or longer
4. Groundwater eventually reaches low lying areas where it supplies springs, rivers, and ponds,
or, eventually, enters the ocean
C. Porosity is measured as the percentage of the material’s total volume that is open space or pore
space p.369
1. Water soaks into the ground because there is open space between the particles of soil
2. Porosity is present in soil, sediment, and sedimentary rocks but can also include fractures,
faults, and cracks in solid rock of igneous or metamorphic origin
3. Porosity is greatest in detrital sediments of coarse material like conglomerate, but can be
important in sandstones, gravels, and even limestones
D. Porosity varies among the rock types depending on the size, shape, or physical arrangement of the
material making up the rock
1. Igneous and metamorphic rocks have a low porosity as do most limestones and dolomites
because of the tight interwoven structure of the crystals
2. They can contain some groundwater if they have fractures or extensive weathering
3. This is especially apparent if fractures in the limestone has been dissolved and opened by
acids in the groundwater
4. High porosity can be developed by detrital sedimentary rocks made of well sorted, well
rounded grains because the individual grains touch in only one spot, leaving large openings or
pore spaces
5. Lower porosity forms in poorly sorted sedimentary rocks because the smaller grains fill the
spaces between larger grains
6. Also cementing agents can fill the pore spaces decreasing the porosity
E. Permeability is the ability of the rock to transmit fluids
1. This depends on the porosity but also on the interconnectedness of the fractures or pore
spaces
2. Siltstone and clay have more spaces or pore space between the grains but since the space is
so small the attraction of the grains and water is so great that the permeability is small
3. Sandstones and conglomerates have larger pores spaces and have good permeability
4. Other rocks can have high permeability depending on the interconnection of the spaces
F. An aquifer is a permeable layer of strata that readily transmits water p.371
1. The best aquifers are well sorted and well rounded sand and gravel
2. Limestones which have been enlarged by solution dissolving are good aquifers
3. Shales and metamorphic and igneous rocks are poor aquifers because they are impermeable
4. Rocks such as shales and siltstone are usually called aquicludes
The Water Table
A. Water percolates into the soil and some collects into the pores mixed with air in the zone of aeration
III.
B. The zone of saturation is below this where all pore space is filled with water
C. This zone of saturation extends to an impermeable layer, either bedrock, shale, or very compact rock
D. The capillary fringe is the zone above the saturated zone where water is pulled upwards by surface
tension much like a sponge will
E. The water table is the zone above the saturated zone and below the aeration zone
1. The water table follows the shape of the surface in that the highest points are at the hills and
the lowest point in the valleys
2. In wet periods the water table rises in the hills and flattens during dry seasons
Groundwater movement
A. Velocities vary greatly from 250 m per day in the permeable material to less than a few cm per year
in impermeable material
B. Generally it moves at a few cm per day
C. Gravity causes the downward movement of groundwater
1. As water accumulates below the water table, it moves downward from areas of high table to
areas of lower table
2. In this way it travels downward from high pressure to low pressure
3. It can follow a straight or curved pathway downward to a spring, river, or lake
D. Springs are places where the groundwater flows or seeps out of the ground p.373
1. They seem to almost magically appear out of the ground in places
2. Basically when groundwater intersects an impermeable layer it flows laterally and when this
flow intersects the surface it is developed into a spring
3. Most springs are on the sides of mountains or areas where the creeks and erosion have cut
down into or below the water table
4. They can also develop along a perched water table where an impermeable layer stops the
downward movement of water and causes it to move laterally which then intersects the surface
E. Water wells p.375
1. Made by drilling or digging down into the zone of saturation
2. Water fills up the hole to the level of the water table
3. When a well is pumped it depresses the water table forming a cone of depression
4. This cone of depression increases in size depending on the rate of draw down
5. If the water is pumped faster than it can be replaced, the well will go dry
6. If an adjacent well pumps the water too fast it can cause a depression in the water table that
causes surrounding wells to go dry
7. Or a well uphill from another well or spring can cause the lower ones to slow down or stop
producing
8. Lowering of the regional water table can cause widespread problems, lawsuits, and loss of
water
9. Water wells are difficult to predict where they will be
a) If you drill into an area of no fractures water may not be found
b) If the fractures are not connected, water may be limited or nonexistent
F. Artesian wells are those which flow without pumping p.376
1. This is a confined system that has enough hydraulic head to flow out to the surface
2. There must be three geologic conditions met to form an artesian system
a) The aquifer must be confined above and below by aquicludes
b) Tilted subsurface layers with a higher exposed area
c) Precipitation must be sufficient to keep aquifer filled with water
3. The elevation of the water table and the distance of the well from the recharge area determine
the height the water will rise
4. The artesian-pressure surface is the potentiometric surface that slopes downward due to
friction or transmissivity in the aquifer
5. The artesian well will only flow if it is below the level of the pressure surface
II.
III.
6. A nonflowing artesian well has the water rising toward the top
B. Artesian springs and systems form along fault lines or fractures
1. These occur when the fault intersects a confining layer allowing water to flow to the surface
2. An oasis in a desert is this type of system
C. There are many examples of artesian systems
1. Dakota Sandstone in North Dakota extends from the Black Hills to Texas
2. Wells in this area used to flow freely but have been depleted by irrigation and must now be
pumped
3. Florida aquifer has limestone layers that are riddled with caves interbedded with shale
aquicludes that transports water to southern Florida where artesian wells were abundant but are
now being depleted
Groundwater erosion
A. Accomplished by groundwater picking up carbon dioxide and forming carbonic acid which dissolves
limestones forming caves and karst topography
B. Karst topography is developed by the collapse of these shallow cave formations, usually in two
ways p.378
1. Soluble rock beneath the soil is dissolved by percolation of rainwater, the soil slumps and
erodes away, leaving shallow depressions
2. Sinkholes are also formed by the collapse of the roof of a cave
a) These are the serious hazards especially in populated areas
b) These areas must be mapped and exploratory drilled for cave openings
c) This type of area is typically riddled with caves, springs, sinkholes, solution valleys,
and disappearing streams
d) Must have thick bedded soluble rock near the surface and enough water for solution
activity to occur and is usually restricted to humid areas
C. Caves are the best example of weathering, erosion, and deposition by groundwater p.379-181
1. Formed by the gradual enlargement of fractures and openings that are interconnected by
underground streams
2. A cavern is a large cave or large cave system
3. The groundwater dissolves the limestone and carries it down to a stream outlet
4. The stream or outlet erodes downward over time, lowering the water table and causing the
cave to erode lower and lower, leaving the upper levels as open cavities
5. As the cave system is enlarged, water percolating down drips from the ceiling and walls
depositing calcium carbonate as dripstone
6. This dripstone forms as the water is evaporated concentrating the carbonate solution until it
precipitates onto the dripping surface as a stalactite
7. As the water hits the floor, calcite precipitates in a cone shape called a stalagmite
8. This dripping water also forms columns, curtins, travertine terraces, helictites, ribbons, and
others
Effects of overuse of the groundwater system can cause long term results that effect the future usability
A. Pumping and withdrawal of groundwater faster than the recharge area can replenish it will have
serious effects
1. The high Plains aquifer in Nebraska, Kansas, Colorado area is being pumped at 2 to 100
times the rate that it can be recharged
2. This aquifer has contributed to the high productivity of the area by abundant irrigation water
from the groundwater system
3. This causes the water table to drop, and may lead to decreased irrigation water and reduced
crops yields and higher prices in the future
4. Reduction in the pumping and use of other water systems may be a solution
B. Saltwater incursion or intrusion is a serious problem especially in coastal areas p.383
1. In permeable rocks along the coastal areas salt water migrates inland
IV.
2. Saltwater is more dense that fresh water and will work its way inland
3. The weight of the fresh groundwater flowing to the sea keeps it back, but if the rate of
recharge slows or stops, the salt water will begin to migrate inland
4. If excessive pumping of the aquifer occurs, there is a cone of depression that forms, allowing
the salt water to move up into its place
5. This forms a salt water well and will stay that way until fresh water is allowed to replace it
6. This is a problem in inland valleys that are open to the sea, like Salinas Valley
a) The valley has been pumped for many years for irrigation
b) Salt water has moved up the valley more than 11 km in the last 13 years and if
nothing is done the wells will become salty and unusable for most crops
7. Recharge wells and recharge ponds can be used to correct the problem by adding water
directly into the groundwater system
C. Subsidence can occur when water is withdrawn from unconsolidated sediments, reducing the pore
pressure in the rock allowing the material to pack closer together
1. There are many areas where over pumping of groundwater has caused considerable
subsidence p.385
2. Santa Clara and San Jose has subsided over 8 feet, the San Joaquin Valley has reported 9m,
New Orleans is subsiding, along with Houston and even Las Vegas, Nevada
3. The tilting of the Leaning Tower of Pisa is a partial result of subsidence and once it was
recognized and pumping strictly controlled
4. Mexico City is having problems from over pumping of the lake deposits and the city is
unevenly settling and sinking
5. Oil extraction can also cause subsidence as in Long Beach which experienced 9m of
subsidence in 34 years until water was pumped back into the formation
D. Contamination of the groundwater is a serious problem everywhere p.387
1. Landfills, sewage, toxic wastes, and agriculture cause pollutants to enter the groundwater
system
2. When a contaminant enters a system it moves so slowly that it takes many years to flush out
if at all
3. Septic systems are common ways to dispose of sewage, if they are not maintained they can
pose risk to the system
4. Landfills are potential toxic wastes
a) Liquid wastes move downward
b) Rainwater percolates through the dump, moving wastes into the groundwater
c) This is why the landfills are now covered by plastic layers to keep the rain off
5. Love Canal near Niagara Falls is the most famous toxic waste site
a) In the 1940's the Hooker Chemical Company dumped 19,000 tons of chemical wastes
into the canal
b) In 1953 it back filled and covered one of the dumps sites with dirt and sold it to the
Niagara Falls Board of Education, which built an elementary school and playground on
the site
c) During the winter of 1977, heavy rains and snow raised the water table and the area
turned into a muddy swamp
d) This bought out the toxic chemicals that formed puddles in the playground, peoples
yards and gardens, and into basements of houses
e) Trees, lawns, and gardens died and people suffered from illnesses
f) Costs of cleanup exceeded $100 million and the area is now abandoned
Hot Springs and Geysers p.391-393
A. Groundwater entering hot rocks from volcanic deposits heats up and returns to the surface as hot
springs and geysers
B. A hot spring is a spring where the water flows from the Earth hotter than the human body
1. All hot springs vary in temperature from body temp to boiling
2. There are over 1100 hot spring known in the US, most of which are in the western part, some
in Arkansas, Georgia, and the Black Hills of South Dakota
3. Heat for hot springs comes from cooling igneous rocks, especially volcanic deposits
4. Some hot springs develop from moving deep into the Earth where the temperature is higher
due to the increase of the geothermal gradient or mountain building processes
5. This is true in the Oauchita Mountains of Arkansas where the water moves underground into
regional thermal areas and eventually exits at the surface near Hot Springs
C. Geysers are hot springs that intermittently emit hot water and steam
1. Yellowstone has perhaps 80% of all the geysers in the world
2. Old Faithful is the most famous, but not the largest. It erupts hot water and steam every 65
minutes and has been effected by recent earthquakes and movement
3. Geysers form in an extensive network of underground fractures where the water is heated by
the hot rocks
4. Because the water at the bottom of the network is under more pressure than the top, it must
be heated to higher temperature before it boils
5. A slight lowering of the pressure by escaping gas causes the deeper superheated water to
change to steam, forcing the upper column of water up and into the air producing the geyser
6. Cool groundwater re-enters the system and the process begins again
7. Minerals, dissolved by the hot water, move up through the system and are deposited by
precipitation
a) It depends on the composition of the rocks on what gets deposited
b) Yellowstone rocks contain fair amount of calcium carbonate and travertine and
calcarous tufa is deposited, especially at Mammoth Hot Springs
c) Other types of deposits are siliceous sinter or geyserite that forms around the geyser
opening
D. Geothermal Energy harnesses the steam or hot water formed in the rocks
1. This is a cheap and clean form of heating and hot water
2. Klamath Falls has heating systems developed for heating that runs through parts of the town
3. Usually the best system is where fluids are pumped into the hot rocks and back up and the
heat is taken off in a heat exchanger to avoid the mineral deposits and acids in the water
4. In Geyserville, California electrical generating plants run off the water that rises and turns to
steam as the pressure is lowered near the surface
5. Corrosion from acids in the water destroys the turbines and pipes and they have to made of
corrosion resistant metals that are expensive and difficult to maintain
6. Political problems with the development of the fields is not easy to overcome
7. The use of the natural groundwater system can cause depletion of the water and eventual
abandonment of the wells