physical properties of aquifers
DESCRIPTION
Physical Properties of Aquifers. Groundwater Hydraulics Daene C. McKinney. 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 - PowerPoint PPT PresentationTRANSCRIPT
Physical Properties of Aquifers
Groundwater Hydraulics
Daene C. McKinney
Outline• Porous Medium
– Porosity– Moisture Content– Particle Size– Distribution of water in subsurface– Capillary Pressure– Soil Moisture Characteristic Curves– Specific Yield and Retention
• Piezometric head in aquifers• Aquifer Types
– Aquifer Storage
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.
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.)
Porosity
solid
Pore with water
Soil volume V(Saturated)
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
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:
Typical Values of Porosity
8
Material Porosity (%)Peat Soil 60-80Soils 50-60Clay 45-55Silt 40-50Med. to Coarse Sand 35-40Uniform Sand 30-40Fine to Med Sand 30-35Gravel 30-40Gravel and Sand 30-35Sandstone 10-20Shale 1-10Limestone 1-10
Volumetric Water Content
Soil volume V(Unsaturated)
SaturationSoil volume V(Unsaturated)
Particle Size of Some Soils
11
Particle Size Distribution
12
Well sorted fine sand
Poorly sorted silty fine to
medium sand
• Particle size distribution curves– Relative % of grain sizes
• Soil classification standards• Soil texture
Particle Size Distribution
13
Sand 49%
Clay 40%
Soil Characteristics of Cyprus Soil Sample
Occurrence of Groundwater
14
• Ground water occurs when water recharges the subsurface through cracks and pores in soil and rock
• Shallow water level is called the water table
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
Saturation
• Saturation
• Water Content
• Water Saturation
Soil volume V(Unsaturated)
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
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
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)
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
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
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.
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
Piezometric Head• Confined aquifer
• Unconfined aquifer
zph g
zph g
0p
zh
Pressure head = 0
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)
Subsurface Pressure Distribution
Capillary pressure head in zone above water table
Hydrostatic pressure distribution exists below the water table (p = 0).
01 dP g
1d
Water table
z
0p0<p 0p
0;0 pzPressure is positive below water table
Ground surface
Unsaturated zone
Saturated zone
Pressure is negative above water table
Soil Water Characteristic Curves
• Capillary pressure head• Function of:
– Pore size distribution– Moisture content
PorosityVadose Zone
Capillary Zone
qo fIrreducible
Water contentPorosity
b
CritacalHead
(Bubbling Press.)
Capillary Rise in Soils
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
Aquifer Storage• Storativity (S) - ability of
an aquifer to store water• Change in volume of
stored water due to change in piezometric head.
• Volume of water released (taken up) from aquifer per unit decline (rise) in piezometric head.
Unit area
Unit decline in head
Released water
Aquifer Storage• Fluid Compressibility (b)• Aquifer Compressibility (a)• Confined Aquifer
– Water produced by 2 mechanisms
1. Aquifer compaction due to increasing effective stress
2. Water expansion due to decreasing pressure
• Unconfined aquifer– Water produced by draining
pores
gV ar
Unconfined Aquifer Storage
• Storativity of an unconfined aquifer (Sy, specific yield) depends on pore space drainage.
• Some water will remain in the pores - specific retention, Sr
• Sy = f – Sr
Unit area
Unit decline in head
Released water
Porosity, Specific Yield, & Specific Retentionyr SS f
Confined Aquifer Storage
• Storativity of a confined aquifer (Ss) depends on both the compressibility of the water (b) and the compressibility of the porous medium itself (a).
Unit area
Unit decline in head
Released water
Example• Storage in a sandstone aqufier• f = 0.1, a = 4x10-7 ft2/lb, b = 2.8x10-8 ft2/lb, g = 62.4 lb/ft3
• ga 2.5x10-5 ft-1 and gbf 1.4x10-7 ft-1
• Solid Fluid• 2 orders of magnitude more storage in solid• b = 100 ft, A = 10 mi2 = 279,000,000 ft2
S = Ss*b = 2.51x10-3
• If head in the aquifer is lowered 3 ft, what volume is released?V = SAh = 2.1x10-6 ft3
Summary• Porous Medium
– Porosity– Moisture Content– Particle Size– Distribution of water in subsurface– Capillary Pressure– Soil Moisture Characteristic Curves– Specific Yield and Retention
• Piezometric head in aquifers• Aquifer Types
– Aquifer Storage