Physical and Hydraulic Properties of Variably Saturated Media

1
Physical and Hydraulic Properties of Variably
Saturated Media

• Goal Retention and movement of fluids through
  porous media
• Will introduce the basic physical properties
• Get a feel for mathematical foundations and the
  physical processes
• Williams, 2001
  http//www.its.uidaho.edu/AgE558
• Modified after Selker, 2000
  http//bioe.orst.edu/vzp/

2
Typical compositions 3 soil phases
3
How do you quantitatively describe a chunk of wet
soil?

• Want to describe
• how it might dry
• how rain falling on it would soak in (how much
  would run off)
• what would happen to it if you spilled some
  gasoline on it?
• A set of parameters..
• selected to provide as concisely as possible
  the greatest insight into the response of the
  media to a range of physical processes.

4
Definitions

• Three constituent phases
• solid
• liquid
• gaseous
• Each phase is an admixture of compounds.

5
Gaseous phase

• Dominated by constituents of atmosphere N2, O2,
  CO2, H2O vapor etc.
• Respiration elevated CO2 and CH4
• Industrially contaminated sites organic vapors.
  
• Quantify gaseous phase by partial pressures
  (temperature dependent)
• Equilibrium with soluble liquid phases via
  Henrys law

6
Gaseous phase (cont.)

• Transport of gases dominated by diffusion (? 2
  cm/day). (Diffusion in the liquid phase is ?
  0.02 cm/day.)
• Atmospheric pumping (Buckingham, 1904)
• Driven by liquid phase movement
• Driven by wind and diurnal density
• Induced mechanically to clean a site

7
Gas phase important in remediation

• Soil Vapor Extraction (SVE)
• Gas is pumped through vadose zone stripping
  volatile fraction (Henrys law).
• Prediction of flow essential to design
  remediation
• Air Sparging
• Air is pumped into aquifers to strip contaminants
  which will be lifted to the vadose zone, and
  extracted in gas phase.
• Gas movement very complicated due to effects of
  heterogeneity and fundamental instability of
  buoyant gas movement in porous media.

8
Liquid phase

• Assume incompressible in Vadose (low pressures).
  
• Interaction other phases
• contact angle (solid and gas)
• capillary pressure (gas)
• volatility (gas)
• We have two interests
• bulk water movement (agriculture and drinking
  water)
• Transport movement of solutes (contaminants and
  nutrients solutes means dissolved also
  interested in particles)

9
Solid phase

• Surface area is critical to vadose processes
• Permeability to liquid/gas go with the square of
  pore-size
• Reactive components significant Clay, organics,
  chelates.
• What about living things?
• Microbes, plants and worms are prevalent and
  important to chemical behavior of the unsaturated
  zone

10
Dry bulk Density

• Dry mass per unit volume
• V the particular volume used
• s solid phase, without fluid
• units gr/cm3 kg/m3 (or even lb./ft3 )

11
Solid phase density

• Upper limit on ?vb is density of pure mineral
• Note ?s independent of v (?vb not so
  cooperative).
• Often reported as specific gravity s, which is
  the ratio of solid density to that of water
• s ?s/?w 2.3

12
Solid Phase Density (cont.)

• Typical values of specific gravity
• 2.65 for quartz (commonly assumed for typical
  minerals
• 2.54 for feldspar
• 2.72 for calcite
• 5.0 for pyrite.
• Not universal value (although 2.6 used a lot)

13
Porosity (AKA void fraction)

• Porosity is denoted as nv and defined

14
What is the control volume, v?

• Lets just run a quick experiment with tool
  which measures ?vs for any prescribed v
• Start with the instrument set with v smaller than
  any individual grain in the soil.
• Now, holding the instrument steady, enlarge v
  continuously, all the while recording the values
  of ?vb.
• (Figure 2.3 shows two realizations of such an
  experiment.)

15
(No Transcript)
16
Bulk density with control volume size

• As the control volume enlarges, there is no scale
  over which density is entirely constant.
• Value of all physical parameters in the vadose
  zone are a function of both position and sample
  volume.

17
We have 2 choices

• 1) throw up our arms natural systems are
  hopelessly complex
• 2) make some reasonable simplifying assumptions
• For the sake of progress...
• Assume there is some volume, much larger than the
  grains of the porous media, yet smaller than the
  distance between dissimilar regions, which
  provides a representative sample of our porous
  media.
• This volume is the systems Representative
  Elementary Volume (REV).

18
Each extensive property defined at point by
taking v 1 REV about that point.

• We can provide a pseudo-rigorous definition of
  the REV by looking at two adjacent regions, each
  of the same volume. For any given parameter
  describing these parcels, the REV is the volume
  large enough so that that parameter differs
  between the two volumes by less than some
  specified amount (with high probability).

19
Lets finish the definitions

• Total Bulk Density (solid and liquid phase)
• Void Ratio (ratio of pore volume to solid volume)
• May relate the void ratio to porosity

20
Water Content

• Either a mass, volumetric basis, or degree
  of saturation (volumetric more common)
• a. Mass basis (A.K.A. Gravimetric)
• b. Volume basis
• c. Degree of saturation.

21
More on Water Content

• Units mass per volume (e.g., gr/cm3)
• volume per volume (e.g., cm3 water/cm3 media)
• inches of water per foot of depth!
• The units indicate whether on a volume or mass
  basis.
• A few necessary details regarding ?s and ?o
• Saturation rarely achieved in the vadose zone due
  to
• Dead end pores
• Water surrounded pores
• Alternate terms for soil with standing water
• Satuated
• Satiated
• field saturated

22
About Residual water content

• Unless at gt200 oC for hours, water held in
  hydrogen bonds
• Residual water content a function of the drying
  process
• Two important drying processes for ?o
• gravity drainage Field Capacity ?fc, about
  -1/3 bar
• -1/3 bar should be by soil texture
• -1/30 bar for sands
• -1 bar for clayey soils
• plant uptake permanent wilting point, ?pwp
  taken as -15 bar

23
More on residual water content

• Measurement relies on vapor transport of water
  between pores, and so is necessarily slow in
  achieving equilibrium.
• Define specific definition and there will be a
  real and measurable residual moisture content,
  otherwise not very useful
• If unsure of the context where the parameter will
  be used, provide a range of possible values

24
Typical values of physical properties