Transcript Water Table

Chapter 15
Groundwater (地 下 水)
92.12.30
Aquiclude , Aquitard, Aquifuge
阻水層
Aquifer
含水層
Artesian Aquifer
自流含水層
Cone of Depression
洩降錐
Confined Aquifer
受壓含水層
Darcy's Law
達西定律
Discharge
流出
Dripstone
滴水石
Flowstone
流水石
Hydraulic Gradient
水力坡降
Karst Topography
石灰岩地形
Percolation
滲透
Permeability
滲透係數
Porosity
孔隙率
Recharge
補注
Replacement
置換
Saturated Zone
飽和層
Sinkhole
陷洞
Spring
泉
Unconfined Aquifer
自由含水層
Water Table
地下水面
Zone of Aeration
通氣帶
水量的分布狀況
水
源
大氣水
水 量
單位：1012噸
所佔百分比
13
0.001
1,320,000
97.2
內陸鹽湖與內陸海 (鹹水)
104
0.008
淡水湖泊
125
0.009
淡水河川
1.25
0.0001
29,000
2.15
67
0.005
750公尺深度以上
4,200
0.31
750公尺深度以下 (鹹水)
4,200
0.31
50
0.004
地面水
海洋 (鹹水)
冰冠及冰川
地下水
土壤水分
生物水分
新疆土魯番盆地之坎兒井
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最早可能建於1500年前（漢朝），推測係習自中東
之引水技術。
1845年以前土魯番僅有三十多條坎兒井，部分仍在
使用中。清朝後期林則徐及左宗棠等人曾在新疆大
力推廣，此後持續增建。
至1957年坎兒井數增為1237條，總長度達4000多公
里，1966年之年進流量為7億立方米，灌溉土魯番大
部分之農地。
隨著經濟發展環境改變，1990年出水之坎兒井只有
700條左右，年流量減為2.94億立方米；至2001年僅
剩446條，年流量已減少至1.7億立方米。
Water In The Ground
• Groundwater is defined as all the water in
the ground occupying the pore spaces within
rocks and sediments.
• Groundwater is present everywhere beneath
the land surface and the ocean bottom.
• Most ground water originates from
precipitation and surface water.
• Groundwater is always in motion.
• Russian scientists encountered water at
more than 11 km below the surface.
Water Table
• Water table slopes toward the nearest stream or
lake.
• In fine-grained sediment, a narrow fringe
immediately above the water table is kept
saturated by capillary attraction. Capillary
attraction is the adhesive force between a liquid
and a solid that causes water to be drawn into
small openings.
• In humid regions, the water table is a subdued
imitation of the land surface above it.
• It is high beneath hills and low beneath valleys.
地下水面(Water Table)
地下水面之深度可隨時間、地點而變，主要影響因素包括
－ 滲透速率
－ 蒸散速率
－ 地下水流速率
蒸散作用
14 m
地
表
8m
地下水面
未飽和帶
飽和帶
Porosity
• The porosity of a sedimentary rock is affected by
several factors:
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The sizes and shapes of the rock particles.
The compactness of their arrangement.
The weight of any overlying rock or sediment.
The extent to which the pores become filled with the
cement that holds the particles together.
• The porosity of igneous and metamorphic rocks
generally is low.
Porosity
• Porosity is the
percentage of the total
volume of a soil or rock
that consists of open
spaces, called pores.
• It is porosity that
determines the amount of
water that a given volume
of soil or rock can contain.
Recharge and Discharge of Groundwater
• The process by which groundwater is
replenished is called recharge.
• The process by which groundwater reaches and
flows from the surface is called discharge.
• An area of the landscape where precipitation
seeps downward beneath the surface and
reaches an aquifer is called a recharge area.
• The water moves slowly toward discharge
areas, where subsurface water is discharged to
streams or to lakes, ponds, swamps or ocean.
Recharge and Discharge Areas
• In humid regions, recharge areas encompass
nearly all the landscape.
• In arid regions, recharge occurs mainly:
– In mountains.
– In the alluvial fans that border them.
– Along the channels of major streams that are
underlain by permeable alluvium.
• It may take from a few days to thousands of
years to move the groundwater from a recharge
area to the nearest discharge area.
How Fast Does Groundwater Flow?
In 1856, Henri Darcy, a French engineer,
concluded the velocity of groundwater is related to:
– The hydraulic gradient: the slope of the
water table.
– The coefficient of permeability of the rock or
sediment through which the water is flowing.
* The coefficient of permeability, also called
hydraulic conductivity, is determined by
permeability, density, and viscosity of water.
Darcy’s Law
K(h1-h2)
V = ----------------
,
Q = AV
L
K is the hydraulic conductivity;
h1-h2 is the difference in altitude;
L is the horizontal distance between two points;
V is flow velocity;
Q is discharge;
A is cross-sectional are.
Spring
• A spring is a flow of groundwater emerging
naturally at the ground surface.
• Small springs are found in all kinds of
rocks, but almost all large springs issue
from lava flows,limestone, or gravel.
• A change in permeability, a body of less
permeable rock adjacent to a permeable
one, is a common explanation for the
location of springs.
Pumping Well
• When water is pumped from a new well, the
rate of withdrawal initially exceeds the rate of
local groundwater flow.
• This imbalance in flow rates creates a conical
depression in the water table immediately
surrounding the well called a cone of
depression.
• The locally steepened slope of the water table
increases the flow of water to the well.
Aquifer
• An aquifer is a body of highly permeable rock or
regolith that can store water and yield sufficient
quantities to supply wells.
• Gravel and sand generally are good aquifers.
Many sandstones and limestones are also good
aquifers.
• Aquifers are of two types:
– Confined (bounded by confining beds).
– Unconfined (an aquifer wothout overlain).
The High Plains aquifer
An unconfined aquifer which lies at shallow
depths beneath the High Plains of the United
States.
– About 30 percent of the groundwater used for
irrigation in the United States is obtained from the
High Plains aquifer.
– In parts of Kansas, New Mexico, and Texas, the
water table has dropped so much over the past
half century that the thickness of the saturated
zone has declined by more than 50 percent.
The Dakota Aquifer
• The Dakota aquifer system in South Dakota
provides a good example of a confined aquifer.
• Water that percolates into a confined aquifer
flows downward under the pull of gravity.
– As it flows to greater depths, the water is
subjected to increasing hydrostatic pressure.
Artesian Aquifer
• Potentially, the water could rise to the
same height as the water table in the
recharge area.
– Such an aquifer is called an artesian aquifer,
and the well is called an artesian well.
• A freely flowing spring supplied by an
artesian aquifer is an artesian spring;
A flowing well
Maybe you've heard advertisements by
water companies wanting to sell you
"artesian-well drinking water." Is this
water different from other bottled
water taken from springs?
The water may not be different, but it
comes to the earth's surface a bit
differently. A flowing well has water
that comes up to the surface because of
internal pressure in the underground
aquifer containing the water. An
artesian aquifer is confined by rock
layers that restrict water flow, resulting
in an aquifer that is "pressurized."
Water is virtually squeezed to the
surface by underground pressure. This
picture shows how strong artesian
pressure can be!
The Floridian Aquifer
• A complex regional aquifer system in which both
confined and unconfined units are present, and in
which water locally reaches the surface by an artesian
flow.
• The aquifer system is restricted mainly to middle and
late tertiary limestones.
• The age of groundwater in the Floridian aquifer
system has been determined by radiocarbon dating of
carbonate molecules dissolved in the water.
– Water in the well farthest from the recharge area is
calculated to have been in the ground for at least 19,000
years.
Mining Groundwater And Its Consequences
• In the dry regions, groundwater is a major
source of water for human consumption.
• In many of dry regions, withdrawal
exceeds natural recharge.
• Groundwater can be a nonrenewable
resource.
• Natural recharge takes so long to
replenish a depleted aquifer that vast
underground water supplies have been
lost to future generations.
San Joaquin Valley
地 層 下 陷
1. 長期大量抽用地下水
進行農業灌溉。
2. 區域性地層下陷，累
積下陷量高達9公尺。
3. 解決方法：引進地表
水取代地下水
Subsidence of the Land Surface
• The weight of the overlying sediments is
supported by the water pressure in the pores of
an aquifer and the intergranular effective stress.
• When groundwater is withdrawn, the water
pressure is reduced, and meanwhile, the
effective stress between mineral grains of the
aquifer is increased, causing the compression of
sediments. As a result, the land surface
subsides.
• The amount of subsidence depends on:
– How much the water pressure is reduced.
– The thickness and compressibility of the aquifer.
抽取地下水導致的地層下陷
墨西哥市：近百年來的下陷量超過10 m。
美國聖瓦金谷：從1925至1977年下陷將近9 m。
日本：曾在40多處發生地層下陷。
中國大陸：天津市及上海市是新興要角。
台灣：台北盆地及西南海岸平原。
台北盆地的地層下陷
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十九世紀末期開始逐步開發地下水源，1895年之
水位仍高於地面。
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民國四、五十年代大量抽水，導致快速的地層下
陷，至65年地下水位降低多達四十多公尺。
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民國71年累積下陷2.2公尺之後，下陷逐漸停止。
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民國65年以後，深處含水層地下水位回升30至40
公尺，地層出現微幅回升現象。
The dissolved constituents in groundwater
Major constituents (greater than 5 mg/L)
Bicarbonate
Silicon
Calcium
Sodium
Chloride
Sulfate
Magnesium
Carbonic acid
Minor constituents (0.01-10.0 mg/L)
Boron
Nitrate
Carbonate
Potassium
Fluoride
Strontium
Iron
Water Quality
• The composition of groundwater varies from place to
place according to the composition of rock in which
the water occurs.
• In much of the central United States, the water is rich
in calcium and magnesium bicarbonates dissolved
from carbonate bedrock (called hard water).
• In the northern United States, where the bedrock is
often volcanic or graywacke sandstones, little
dissolved matter and no appreciable calcium found in
the groundwater (called soft water).
Groundwater Contamination (1)
• Water circulating through sulfur-rich rocks
may contain dissolved hydrogen sulfide
(H2S) that has the disagreeable odor of
rotten eggs.
• The most common source of water
pollution in wells and springs is sewage
draining from septic tanks, privies, and
barnyards.
Groundwater Contamination (2)
• If sewage contaminated with bacteria passes
through sediment or rock with large pores, such as
coarse gravel or cavernous limestone it may
remain polluted in long distances..
• If the contaminated water percolates through sand
or permeable sandstone, it become purified within
short distances. Sand promotes purification by:
– Mechanically filtering out bacteria.
– Oxidizing bacteria so they are rendered
harmless.
– Placing bacteria in contact with other organisms
that consume them.
Toxic Wastes
• Municipal and industrial wastes.
• Landfills and underground storage
tanks.
• Agricultural poisons: pesticides and
herbicides.
• The pollutants often are toxic to
humans as well as to plants and
animals.
Toxic Wastes and Agricultural
Poisons (2)
• Pesticides and herbicides are sprayed
over agricultural fields and suburban
gardens to help improve quality and
productivity.
• Some of these chemicals have been
linked with cancer and birth defects in
humans, and some have led to disastrous
population declines of wild animals.
Geologic Activity of
Groundwater (1)
• Of all the rocks in the Earth’s crust, the
carbonate rocks are among the most
readily attacked by the dissolution and
hydrolysis.
• Limestone, dolostone, and marble are the
most common carbonate rocks and
underlie millions of square kilometers of
the Earth’s surface.
• Carbonate minerals are readily dissolved
by weak carbonic acid.
Geologic Activity of
Groundwater (2)
• The weathering attack occurs mainly along
joints and other partings in the carbonate
bedrock.
• In temperate regions with high rainfall, a
high water table, and a nearly continuous
cover of vegetation, carbonate landscapes
are being lowered at average rates of up
to 10 cm/1000 years.
• In dry regions with scanty rainfall, rates
are far lower.
Chemical Cementation and Replacement
• The conversion of sediment into sedimentary
rock is primarily the work of groundwater.
• Substances in groundwater are precipitated as
cement between mineral particles of sediments.
• Calcite, quartz, and iron hydroxides (mainly
limonite) are the chief cementing substances.
• Replacement is the process by which a fluid
dissolves matter already present and at the
same time deposits from solution an equal
volume of a different substance
Cave Formation
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Caves are resulted from the dissolution of carbonate
rock by circulating groundwater.
– Limestone caves are generally believed to result
from dissolution by carbonic acid.
– Some caves, like Carlsbad, may have resulted from
dissolution by sulfuric acid.
The rate of cave formation is related to the rate of
dissolution.
A fully developed cave system may take 10,000 to 1
million years to produce.
Sequence of Cave Development
1. Initial dissolution along a system of interconnected open
joints or bedding planes by percolating groundwater.
2. Enlargement of a cave passage along the most
favorable flow route.
3. Deposition of carbonate formations on the cave walls
while a stream occupies the cave floor.
4. Continued deposition of carbonate on the walls and
floor of the cave after the stream has stopped flowing.
Cave Deposits
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clay and silt, originally present as impurities in
limestone, gradually concentrated as the
limestone was dissolved.
Flowstone is precipitated by flowing water.
Dripstone is precipitated from dripping water.
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Stalactites are icicle-like forms of dripstone hanging
from the the ceilings of caves.
Stalagmites are blunt mounds projecting upward
from cave floor.
Columns are stalactites joined with stalagmites.
Sinkholes: large dissolution cavities
–
Some sinkholes are formed when caves have
collapsed, others are formed from dissolution.
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Many sinkholes are located at the intersection of joints.
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New sinkholes are forming because of the lowering of
the water table due to excessive pumping.
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The floors of sinkholes lie below the water table.
Karst Topography
• Karst topography is a landscape in which
caves and sinkholes are so numerous that they
form a peculiar topography characterized by:
– Many small, closed basins.
– Disrupted drainage pattern.
– Streams disappearing into the ground.
– Streams reappearing as large springs.
• Karst topography was first described in the
Karst region of the former Yugoslavia,
extending from Slovenia to Montenegro.
Distinctive Karst Landscape
sinkhole karst is a landscape with dotted
sinkholes. Examples occur in Indiana, south
central Kentucky, central Tennessee, and
Jamaica.
Cone karst and tower karst occur in thick, welljointed limestone that separates into isolated
blocks as it weathers. Examples occur in part
of Mexico, Central America, the Caribbean
islands of Cuba and Puerto Rico, the South
Pacific, and southeastern China.
Pavement karst consists of broad areas of bare
limestone in which joints and bedding planes
have been etched and widened by dissolution,
creating a distinctive land surface. Examples
occur in Spitzbergen, Greenland, and the
Burren region of western Ireland.
Homework: 解釋名詞
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Darcy’s Law
Karst topography
Cone of depression
Artesian aquifer
Zone of aeration
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