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Physical Properties of Aquifers Groundwater Hydraulics Daene C. McKinney.

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Presentation on theme: "Physical Properties of Aquifers Groundwater Hydraulics Daene C. McKinney."— Presentation transcript:

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 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

4 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 ProfileDescriptionMoisture Profile 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 6

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 solid Pore with water Soil volume V (Saturated)

9 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 Rhombo Packing

10 Porosity solid Pore with water Soil volume V (Saturated) Porosity: total volume of soil that can be filled with water V = Total volume of element V i = Volume of Pores V s = Volume of solids m = particles density (grain density) d = bulk density Void Ratio:

11 Typical Values of Porosity 11 MaterialPorosity (%) Peat Soil60-80 Soils50-60 Clay45-55 Silt40-50 Med. to Coarse Sand35-40 Uniform Sand30-40 Fine to Med Sand30-35 Gravel30-40 Gravel and Sand30-35 Sandstone10-20 Shale1-10 Limestone1-10

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 Water Content Water Saturation Soil volume V (Unsaturated)

14 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

15 Particle Size Distribution 15 Sand49% Clay40% Soil Characteristics of Cyprus Soil Sample

16 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 – = N/m 20 o C) Capillary pressure – Relates pressure on both sides of interface water air No net force Net force inward Interface

17 Surface Tension gas solid sg gl sl liquid Hg solid air water solid air Mercury nonwetting solidWater wetting solid < 90 o - liquid is wetting the solid > 90 o - liquid is non-wetting the 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 p c It depends on the curvature of the interface. p nw is the pressure in the nonwetting fluid (air, say) p w is the pressure in the wetting fluid (water, say)

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

20 Solid Water Air r Negative pressure Positive pressure Capillary Pressure A B (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 r solid Pore water press. = -p Pore air press. = 0 If p c 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 EGL HGL v 2 /(2g) p/ z datum Hydraulic grade line (HGL) – height of water in piezometer tube Energy grade line (EGL) – Height of water in pitot tube Velocity head Pressure head Elevation head v 2 /(2g) p/ z

23 Piezometric Head Confined aquifer Unconfined aquifer Pressure head = 0

24 Piezometric Head in Unsaturated Flow Saturated ZoneWater TableUnsaturated Zone < 0 = 0 > 0 p w > 0p w = 0p w < 0 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). Water table Pressure is positive below water table Ground surface Unsaturated zone Saturated zone Pressure is negative above water table

26 Soil Water Characteristic Curves Capillary pressure head Function of: – Pore size distribution – Moisture content Porosity Vadose Zone Capillary Zone o Irreducible Water content Porosity b Critacal Head (Bubbling Press.)

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 dont 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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