physical properties of aquifers groundwater hydraulics daene c. mckinney
TRANSCRIPT
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Physical Properties of Aquifers
Groundwater Hydraulics
Daene C. McKinney
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Summary
• Occurrence of Groundwater– Distribution of water in subsurface
• Porous Medium– Porosity– Moisture Content– Particle Size– Capillary Pressure– Soil Moisture Characteristic Curves– Specific Yield and Retention
• Aquifer Types– Aqufier Storage
• Piezometric head
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Occurrence of Groundwater
3
• Ground water occurs when water recharges the subsurface through cracks and pores in soil and rock
• Shallow water level is called the water table
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Distribution of Water in Subsurface
• Different zones– depend on % of pore
space filled with water• Unsaturated Zone
– Water held by capillary forces, water content near field capacity except during infiltration
• Soil zone– Water moves down
(up) during infiltration (evaporation)
• Capillary fringe– Saturated ar base– Field capacity at top
• Saturated Zone– Fully saturated pores
Soil Profile DescriptionMoisture Profile
Field capacity - Water remaining after gravity drainageWilting point - Water remaining after gravity drainage & evapotranspiration
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Porous Medium• Groundwater
– All waters found beneath the ground surface
– Occupies pores (void space space not occupied by solid matter)
• Porous media – Numerous pores of small size– Pores contain fluids (e.g., water
and air) – Pores act as conduits for flow of
fluids• Type of rocks and their
– Number, size, and arrangement of pores
– Affect the storage and flow through a formation.
• Pores shapes are irregular– Differences in the minerals
making up the rocks – Geologic processes experienced
by them.
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Particle Size of Some Soils
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Continuum Approach to Porous Media
• Pressure, density etc. apply to fluid elements that are large relative to molecular dimensions, but small relative to the size of the flow problem
• We adopt a Representative Elementary Volume (REV) approach
• REV must be large enough to contain enough pores to define the average value of the variable in the fluid phase and to ensure that the pore-to-pore fluctuations are smoothed out
• REV must be small enough that larger scale heterogeneities do not get averaged out (layering, etc.)
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Porosity
solid
Pore with water
Soil volume V(Saturated)
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Porosity• Property of the voids of
the porous medium• % of total volume
occupied by voids solid
Pore with water
Soil volume V(Saturated)
Cubic Packing
RhomboPacking
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Porosity
solid
Pore with water
Soil volume V(Saturated)
Porosity: total volume of soil that can be filled with water
V = Total volume of elementVi = Volume of PoresVs = Volume of solids
rm = particles density (grain density)rd = bulk density
Void Ratio:
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Typical Values of Porosity
11
Material Porosity (%)
Peat Soil 60-80
Soils 50-60
Clay 45-55
Silt 40-50
Med. to Coarse Sand 35-40
Uniform Sand 30-40
Fine to Med Sand 30-35
Gravel 30-40
Gravel and Sand 30-35
Sandstone 10-20
Shale 1-10
Limestone 1-10
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Flow of Immiscible Fluids
• Miscible displacement - fluids are completely soluble in each other, the interfacial tension between the fluids is zero, the fluids dissolve in each other, and a distinct fluid-fluid interface does not exist
• Immiscible displacement - simultaneous flow of immiscible fluids or phases in the porous medium. The interfacial tension between the fluids is not aero, distinct fluid-fluid interfaces exist and separate the phases in each pore.
• Unsaturated flow - flow of two immiscible fluids (water and air), except that the air is practically immobile.
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Saturation
• Saturation
• Water Content
• Water Saturation
Soil volume V(Unsaturated)
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Particle Size Distribution
14
Well sorted fine sand
Poorly sorted silty fine to
medium sand
• Particle size distribution curves– Relative % of grain sizes
• Soil classification standards• Soil texture
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Particle Size Distribution
15
Sand 49%
Clay 40%
Soil Characteristics of Cyprus Soil Sample
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Surface Tension• Below interface
– Forces act equally in all directions• At interface
– Some forces are missing– Pulls molecules down and together– Like membrane exerting tension on
the surface• Curved interface
– Higher pressure on concave side • Pressure increase is balanced by
surface tension– s = 0.073 N/m (@ 20oC)
• Capillary pressure– Relates pressure on both sides of interface
water
air
No net force
Net forceinward
Interface
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Surface Tension
gas
solid
ssg
sgl
ssl
bliquid
Hg
solid
air
bwater
solid
air
b
Mercury nonwetting solid Water wetting solid
b < 90o - liquid is wetting the solidb > 90o - liquid is non-wetting the solid
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Capillary Pressure
• Two immiscible fluids in contact exhibit a discontinuity in pressure across the interface separating them.
• This pressure difference is capillary pressure pc • It depends on the curvature of the interface.
pnw is the pressure in the nonwetting fluid (air, say) pw is the pressure in the wetting fluid (water, say)
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Solid
Solid
Water
Air
r
Capillary Pressure
Rise of water in a capillary tube. Capillary forces must balance the weight of water
Capillary pressure head
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Solid Solid
Water
Air
r
Negativepressure
Positivepressure
Capillary Pressure
A
B
(A) Below the water level
(B) Above the water level
Difference in pressure across the interface is
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Drainage• Drainage occurs when the water pressure in
the pores becomes less than the air pressure• Interfacial tension prevents displacement of
water in the left pore
r
solid
solid
Pore water press. = -p
Pore air press. = 0
If pc increases, radius must decrease, or water occupies smaller pores. Water recedes into pores small enough to support the interface with a radius required to balance the capillary force. Water drains from the large pores first.
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Energy in Flow Systems
EGL
HGL
v2/(2g)
v2/(2g)
p/g
z
datumHydraulic grade line (HGL) – height of water in piezometer tube
Energy grade line (EGL) – Height of water in pitot tube
Velocity headPressure headElevation head
v2/(2g)
p/gz
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Piezometric Head• Confined aquifer
• Unconfined aquifer
zp
h
zp
h
0p
zh
Pressure head = 0
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Piezometric Head in Unsaturated Flow
Saturated Zone Water Table Unsaturated Zone = q f < q f < 0 = 0 > 0
pw > 0 pw = 0 pw < 0
Soil volume V(Unsaturated)
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Subsurface Pressure Distribution
Capillary pressure head in zone above water table
Hydrostatic pressure distribution exists below the water table (p = 0).
01 dP
1d
Water table
z
0p0p 0p
0;0 pzPressure is positive below water table
Ground surface
Unsaturated zone
Saturated zone
Pressure is negative above water table
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Soil Water Characteristic Curves
• Capillary pressure head• Function of:
– Pore size distribution– Moisture content
PorosityVadose Zone
Capillary Zone
qo fIrreducible
Water contentPorosity
y
ybCritacalHead
(Bubbling Press.)
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Capillary Rise in Soils
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Aquifer Types
• Confined aquifer – Under pressure– Bounded by impervious layers
• Unconfined aquifer – Phreatic or water table– Bounded by a water table
• Aquifer – Store & transmit water– Unconsolidated deposits sand and gravel,
sandstones etc.• Aquitard
– Transmit don’t store water– Shales and clay
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Summary
• Occurrence of Groundwater– Distribution of water in subsurface
• Porous Medium– Porosity– Moisture Content– Particle Size– Capillary Pressure– Soil Moisture Characteristic Curves– Specific Yield and Retention
• Aquifer Types– Aqufier Storage
• Piezometric head