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Physical Properties of Aquifers

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

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Physical Properties of Aquifers

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  1. Physical Properties of Aquifers Groundwater Hydraulics Daene C. McKinney

  2. 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

  3. Occurrence of Groundwater • Ground water occurs when water recharges the subsurface through cracks and pores in soil and rock • Shallow water level is called the water table

  4. Distribution of Water in Subsurface Moisture Profile Soil Profile Description • 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 Field capacity - Water remaining after gravity drainage Wilting point - Water remaining after gravity drainage & evapotranspiration

  5. 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.

  6. Particle Size of Some Soils

  7. 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.)

  8. Porosity Soil volume V (Saturated) Pore with water solid

  9. Porosity Soil volume V (Saturated) • Property of the voids of the porous medium • % of total volume occupied by voids Pore with water solid Rhombo Packing Cubic Packing

  10. Porosity Porosity: total volume of soil that can be filled with water V = Total volume of element Vi = Volume of Pores Vs = Volume of solids Soil volume V (Saturated) Pore with water solid rm = particles density (grain density) rd = bulk density Void Ratio:

  11. Typical Values of Porosity

  12. 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.

  13. Saturation Soil volume V (Unsaturated) • Saturation • Water Content • Water Saturation

  14. Particle Size Distribution Poorly sorted silty fine to medium sand Well sorted fine sand • Particle size distribution curves • Relative % of grain sizes • Soil classification standards • Soil texture

  15. Particle Size Distribution Soil Characteristics of Cyprus Soil Sample

  16. air Interface water Net forceinward No net force 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

  17. Surface Tension sgl air gas liquid ssg b solid • < 90o - liquid is wetting the solid • b> 90o - liquid is non-wetting the solid Hg ssl solid b air solid b water Mercury nonwetting solid Water wetting solid

  18. 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. pnwis the pressure in the nonwetting fluid (air, say) pwis the pressure in the wetting fluid (water, say)

  19. Capillary Pressure Air Solid Solid Water r Rise of water in a capillary tube. Capillary forces must balance the weight of water Capillary pressure head

  20. Capillary Pressure Air B Negative pressure A Solid Solid Water Positive pressure r (A) Below the water level (B) Above the water level Difference in pressure across the interface is

  21. 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 solid r Pore water press. = -p Pore air press. = 0 solid 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.

  22. Energy in Flow Systems Velocity head v2/(2g) p/g Pressure head z Elevation head v2/(2g) EGL HGL p/g v2/(2g) z Hydraulic grade line (HGL) – height of water in piezometer tube datum Energy grade line (EGL) – Height of water in pitot tube

  23. Piezometric Head • Confined aquifer • Unconfined aquifer Pressure head = 0

  24. Piezometric Head in Unsaturated Flow Soil volume V (Unsaturated)

  25. Subsurface Pressure Distribution • Capillary pressure head in zone above water table • Hydrostatic pressure distribution exists below the water table (p = 0). Ground surface Pressure is negative above water table Unsaturated zone Water table Pressure is positive below water table Saturated zone

  26. Soil Water Characteristic Curves y Vadose Zone • Capillary pressure head • Function of: • Pore size distribution • Moisture content Porosity Capillary Zone yb Critacal Head (Bubbling Press.) qo f Irreducible Water content Porosity

  27. Capillary Rise in Soils

  28. 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

  29. 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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