Water as a Resource

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Transcript Water as a Resource

Water as a Resource
1
The Global Water Budget
• Fresh water is limited on the earth
– Mostly Polar ice and in the ground
• Water is regionally a renewable resource
– Locally water may not be renewable
• The geologic condition affects the quality
and quality of water in a region
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• Groundwater is the water in the saturated zone
• Recharge is the water entering the saturated zone
• 30% of freshwater on Earth trapped below the
surface
• In many parts of the world, groundwater is the
only source of fresh water
• Water may stay in the groundwater reservoir
between several days and thousands of years.
• Management of catchment areas requires
understanding of groundwater flow
• Many environmental issues involve groundwater
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Fluid Storage and Mobility
• Porosity (‫ –)المساميه‬portion of void space in
material (soil or rock) where fluid can be
stored
• n= 100 Vvoid / Vtotal
– Usually expressed as a percent (1.5%) or a
decimal (0.015)
– Pore space can be occupied by fluid or gas,
or a combination of the two
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Factors affecting porosity
)‫(العوامل التي تؤثر على المسامية‬
• Grain size )‫ (حجم الحبيبات‬:Well rounded sediments that
are packed into the same arrangement generally have
porosities from 26% to 48% depending on the
packing.
• Sorting (‫)الفرز‬: Well sorted sediments generally have
higher porosities than poorly sorted sediments for the
simple reason that if a sediment is a range of particle
sizes then the smaller particles may fill in the voids
between the larger particles.
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Porosity and permeability vary with grain shapes and the way grains fits together
Low porosity in an igneous rock
Poorly sorted sediment: fine grains fill
pores between coarse ones
A sandstone made of rounded grains similar in size
has more pore space
Packing of plates of clay in a shale may result
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in high porosity but low permeability
Fluid Storage and Mobility
• Permeability )‫ –(النفاذية‬measures how
readily a fluid passes through a material
– Measures degree of interconnection between
pores and cracks in rocks and soil
– Grain shape and size are factors
– How grains fit together influences permeability
also
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Fluid Storage and Mobility
• Rock type will impact porosity and permeability
– Igneous, metamorphic, and chemical sedimentary
rocks have crystals that are tightly interlocked and
low permeability and porosity
– Weathering, dissolution, and fracturing of these rocks will
increase porosity and permeability
• Clastic sediments have more porosity and permeability
– Sandstones are generally very porous
– Clay minerals or micas, may be porous, but because the
flat grains can be packed closely together parallel to the
plates, these sediments may not be very permeable.
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Fluid Storage and Mobility
Low porosity in an igneous rock
Poorly sorted sediment: fine grains fill
pores between coarse ones
A sandstone made of rounded grains similar in size
has more pore space
Packing of plates of clay in a shale may result
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in high porosity but low permeability
Subsurface Waters
• Soils which are permeable will allow excess
precipitation to infiltrate
– Gravity will draw water downward until an impermeable
rock or soil layer (Aquitard) is reached
– Above this layer ground water will accumulate – infilling
pore spaces
• The saturated zone (or phreatic zone) will fill with
water
– Ground water is stored
• The unsaturated zone (or vadose zone) lies
above the saturated zone and pore spaces are
filled partially with water, partly with air
• The water in unsaturated soil is soil moisture, and
is often an important factor in agricultural
productivity
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Subsurface Waters
• All of the water occupying pore space below the ground surface is
called subsurface water
• Subsurface water includes ground water, soil moisture, and water
in unsaturated rocks
• The water table separates the two the saturated and unsaturated
zones
• The water table is defined as the top of the saturated zone, where
the saturated zone is not confined by overlying impermeable rocks
– The height of water table varies
– It is highest when the ratio of input water to water removed is
greatest, typically in the spring, when the rain is heavy or snow
and ice accumulations melt
– In dry season, or when local human use of ground water is
extensive, the water table drops
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Subsurface Waters
• The water entering the saturated zone is called recharge.
• Water moving into an aquifer by the processes of
infiltration and migration, or percolation
• A rock that stores enough water and transmits it rapidly at
rates sufficient enough to be useful is called an aquifer
– The best aquifers are sandstones or other coarse
clastic sedimentary rocks
– Other examples include porous limestone, fractured
basalt, or weathered granite
• Rocks of very low and low permeability are called
aquicludes or aquitards
– Aquitards have low permeability regardless of its
porosity (They store considerable amount of water)
– Shales are common aquitards
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Nomenclature of surface and subsurface water. Ground
water is water in the saturated zone. Below the water table.
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Aquifer Geometry and Groundwater Flow
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Geology and geometry of rocks and sediments will control the behavior
of ground water
– An aquifer without an aquitard (low permeability rocks) above it is an
unconfined aquifer
• An unconfined aquifer is directly overlain only by permeable
rocks
• An unconfined aquifer is recharged by infiltration over the whole
area
• If a well is drilled into an unconfined aquifer, the water will rise in
the well to the same height as the water table in the aquifer
• Not under pressure, usually shallow aquifers
– An aquifer with an aquitard above and below is a confined aquifer
– Under pressure, usually deep aquifers associated with artesian wells
– A confined aquifer may see hydrostatic water pressure increase and
form an artesian system
• Drilling into a confined aquifer under pressure will see the water
rise above the aquifer
• The water in an artesian system may or may not rise all the way
to the ground surface
• In this system the potentiometric surface is the height to which
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the water would rise
An unconfined aquifer. Water rises in an unpumped well just to the
height of the water table.
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Natural internal pressure in a confined aquifer system creates artesian
conditions, in which water may rise above the apparent (confined) local
water table. The aquifer here is a sandstone; the confining layers, shale.
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Darcy’s Law
• Darcy’s Law is Q = K∙A(Δh/Δl)
– Q = discharge
– K = hydraulic conductivity
– A = cross-sectional area
- Dh = difference in hydraulic head
– Dl = distance between well heads
• Discharge is the amount of water flowing past a
point over a period of time
– It is influenced by the porosity and permeability of the
rock or sediment of the aquifer
– Hydraulic conductivity is ability of particular material
to allow water to pass through it
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Darcy’s Law relates groundwater flow rate, and thus discharge, to
hydraulic gradient. Height of water in wells reflects relative hydraulic
head.
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Groundwater Flow
• Hydraulic head is potential energy in an
aquifer
– The height of water in an unconfined aquifer
reflects the hydraulic head
• The higher the water table the higher the head
– The higher the potentiometric surface in a
confined aquifer will equate to higher hydraulic
head
– Ground water flows spontaneously from areas
of high hydraulic head (high elevation) to areas
with low hydraulic head (low elevation)
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Hydraulic head and Hydraulic gradients
Land surface
Water table
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Hydraulic head (h)=(land surface elevation – depth to water)
Hydraulic gradient (dh/dl): Is simply the slope of water table or
potentiometeric surface
Dh/dl= difference in head/horizontal distance between wells=(h2h1)/L
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Other Factors in Water Availability
• Geometry of host rock units
– The host rock is defined as the type of rock in
which the mineralization occurs
• Lenses of low-permeability rocks within
permeable ones may result in the formation of
perched water tables
• Local precipitation patterns and fluctuations
• Minerals in host rock
• Location of wells relative to recharge zones and
discharge points
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A perched water table may create the illusion of a shallow regional
water table.
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Consequences of ground water withdrawal
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Lowering of the water table
Compaction of aquifer rocks
Ground subsidence
Sinkhole formation
Saltwater intrusion
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Consequences of Groundwater Withdrawal
• Whenever a well is sunk into an aquifer and begins to
pump two things happen:
1.The water table is lowered into a cone of depression
2. The direction of groundwater flow changes locally
around the well
• The lowering of the water table around a pumped well in
an unconfined aquifer is called cone of depression
– This may impact local and regional ground water
availability
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The formation and effect of cones of depression. (A) Lowering of the water
table in a cone of depression around a pumped well in an unconfined
aquifer.
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The formation and effect of cones of depression
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The cones of depression of adjacent wells may overlap, further
lower the water table between wells
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Potentiometric surface of the principal aquifer system in northern Illinois.
Contours are in feet above sea level. The shaded area denotes where the
potentiometric surface is below sea level.
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Areas of the arid
Western United States in
which ground water is
clearly being “mined,”
with withdrawal rates
exceeding recharge.
Mined ground water:
Where ground water is
being depleted by too
much withdrawal too fast
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Consequences of Groundwater Withdrawal
• Compaction of aquifer rocks and surface subsidence
– The aquifer rocks, no longer saturated with water, may
become compacted from the overlying rocks (which
decreases porosity and permeability)
– As the rocks below compact and settle, the ground
surface itself may subside
– Subsidence is the very slow to rapid sinking or settling
of the land surface
– Lowering of the water table/potentiometeric surface
also may contribute to sinkhole formation
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Consequences of Groundwater
Withdrawal
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Consequences of Groundwater
Withdrawal
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Surface subsidence near Galveston Bay, Texas. Faulting is a consequence
of the subsidence. The coastline has shifted, and some coastal wetlands
have been lost.
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Subsidence of as much as 9 meters
occurred between 1925 and 1977 in
the San Joaquin Valley, California, as
a consequence of groundwater
withdrawal. Signs indicate former
ground surface elevation.
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Saltwater intrusion
• Saltwater intrusion can occur in coastal
regions where ground water withdrawal
exceeds recharge (Over pumping in coastal
areas results in salt water intrusion)
• It results in the replacement of fresh ground
water with saline ground water
– Freshwater is less dense than saltwater
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Saltwater intrusion in a coastal zone. If groundwater
withdrawal exceeds recharge, the lens of fresh water thins,
and salt water flows into more of the aquifer system from
below.
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Other Impacts of Urbanization on
Groundwater Systems
• An increase in people in an area may cause
ground water supplies to be exhausted or loss
of recharge to occur
– Pavement and parking lots reduce or prevent water
infiltrating into ground water
– Building on wetlands reduces recharge, water
storage, and water quality
• Ground water recharge can be enhanced by
incorporating various artificial recharge
strategies
– Build artificial recharge basins
– Employ any method to slow down run off and
increases surface water infiltration
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Recharge to a confined aquifer. (A) The recharge area of this confined
aquifer is limited to the area where permeable rocks intersect the
surface. (B) Recharge to the confined aquifer may be reduced by
placement of impermable cover over the limited recharge area.
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Artificial recharge basins can aid recharge by slowing
surface runoff
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Other Features Involving Subsurface
Water
• Ground water may dissolve large volumes of
rock (soluble rock)
– Collapse of the surface rock may result in natural
depressions, known as sinkholes
– Caverns may also be enlarged
• Karst is a type of land form associated with
many sink holes in soluble bedrocks such as
limestone, dolomite, or gypsum
– Ground water flow rates will increase in karst areas
• Ground water flows faster without sediments and rock in the
flow path
• Pollutants move faster through ground water systems in karst
areas
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Highly soluble rocks underlie more than half of the contiguous
United States.
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(A) Sinkhole at Winter Park, Florida. The sinkhole was over 100 meters
across and 30 meters deep
(B) Development of a new irrigation well in west-central Florida triggered
dozens of sinkholes like this one over a 20-acre area.
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(A) Typical rolling karst terrain, developed on dolomite rock, near
Radford, VA.
(B) Satellite view- Karst topography in eastern Florida, showing many
round lakes formed in sinkholes.
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• The first midterm exam will be on 28/10/2006
• The exam will include the following:
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Rocks and minerals
Plate tectonics
Earthquakes
Volcanoes
Streams and flooding
Mass wasting
Water as a resource
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Groundwater Quality
• Measures for expressing Water Quality
– Concentrations of specific constituents
• Parts per million (ppm)
• Parts per billion (ppb)
– Total Dissolved Solids (TDS)
• The sum of dissolved solid chemicals in the water
• It is important to know what chemicals are dissolved!
– Hardness: Water contains substantial amounts of calcium and
magnesium
• Greater than 80 to 100 ppm
• Its major common drawback is that is prevents soap from lathering
properly
– pH (A measure of acidity)
– Identification of impurities
– Presence of naturally occurring radioactive elements
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Water Use and Water Supply
• In U.S. the east is generally humid
– More dependence is on surface water
• In the west more arid condition are found
– More dependence is on ground water and
impounding surface water for storage
• Global water usage
– Too many people
– Too much demand
– Not many places to find more water
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Water Use and Water Supply- U.S. water
budget
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Surface water versus ground water as
supply-Average annual precipitation in USA.
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Surface water versus ground water as supplyAverage U.S. variations in water withdrawals
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Surface water versus ground water as supplyAverage Water withdrawal by sector
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Sources and disposition of
water withdrawn/diverted
for the four major uses.
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Water use for irrigation, by state
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European Internal and external
renewable water resources
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Extending the Water Supply
• Conservation – a must do strategy in U.S.
• Interbasin Water Transfer: Moving surface
waters from one stream system's drainage
basin to another where demand is higher
• Desalination of seawater
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Filtration and Distillation as
Desalination Methods
• Filtration is fast and efficient if used
on not-too-impure water. But it will not
work on very saline water because
the filters clog.
• Distillation will work on any salinity of
water, but it requires energy. Potential
of desalination is presently limited by
the factor of cost
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Methods of desalination
Membrane is a thin material with very small pores
Filtration: dissolved and suspended material is
Screen out by very fine filters
Distillation: As water is heated, pure water is evaporated,
then recondensed for use. Dissolved and suspended remains61
stay behind