Geology and Soils in Realtion to Vadose Zone Hydrology - PowerPoint PPT Presentation

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Geology and Soils in Realtion to Vadose Zone Hydrology

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narrow continuous banding of alternating high and low permeability ... Typical Geologic Configurations: Karst. Karst is water eroded limestone. ... – PowerPoint PPT presentation

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Title: Geology and Soils in Realtion to Vadose Zone Hydrology


1
Geology and Soils in Realtion to Vadose Zone
Hydrology
2
Typical Geologic Configurations floodplains
  • Key points
  • narrow continuous banding of alternating high and
    low permeability
  • not necessarily oriented down stream

3
Typical Geologic Configurations floodplains
  • Terraced stream channel with likely ephemeral
    perched water.

4
Typical Geologic Configurations Karst
Karst is water eroded limestone. This creates
subsurface channels, some large enough to survey
by boat. Equivalent structures (macropores) are
also critical in vadose environments.
5
Geologic Configurations beach deposits
  • Beach deposits, although similar to river
    deposits in texture, have unique structure
  • Generally (not always!) fining upward
  • Laterally extensive
  • Lower variation in energy (more uniform)

6
Typical Geologic Configurations Lava
  • Lava flows may have alternating porous,
    fractured,
  • and low permeability regions with sedimentary
  • deposits between flows

7
Typical Geologic Configurations
Fractures, Dikes, Fill
8
Geologic Configurations various aquifers
  • Whats an aquifer? Water that will flow into a
    well

9
Water Tables (continued)
  • Many aquifer systems have perched water tables
    that can be productive

10
A Primer on Properties and Description of Natural
Media
  • Particle Size Distribution
  • Soil Classification
  • Clay mineralogy

11
Hey, like, why do we care?
  • Transport through natural porous media cannot be
    understood from mathematical notation and
    boundary conditions alone.
  • The structure, setting, history and chemistry of
    the mineral system in the vadose zone all play
    central roles in transport.

12
The ultra-basics
  • Particle size distribution is plotted as the mass
    which is made up of particles smaller than a
    given size.
  • Very useful in estimating the soils hydraulic
    properties such as the water retention
    characteristics and the hydraulic conductivity.

13
  • Standard
  • sieve
  • sizes

14
Typical Particle Size Plot
15
Summary statistics for particle size distribution
  • d50, d10, d80 etc.
  • Uniformity coefficient, U
  • U d60 /d10 1.1
  • U between 2 and 10 for well sorted and poorly
    sorted materials

16
Dependence of bulk density on particle size
distribution
  • Uniform particle size distribution
    gives uniform density
  • increasing the range of particle sizes gives
    rise to greater bulk density.

17
(No Transcript)
18
What are is the basis of size classes?
  • Clay wont settle (lt2m doesnt feel gritty
    between your teeth).
  • Silt settles freely, but cannot be
    discriminated by eye (isnt slippery between your
    fingers doesnt make strong ribbons goes
    through a number 300 sieve 2mltsiltlt0.05mm).
  • Sand you can see (gt0.05 mm), but is smaller
    than pebbles (lt2mm).

19
Systems of soil textural classification
  • (The USDA is standard in the US)

20
Sand, Silt, Clay Textural Triangle
  • Standard textural triangle for mixed grain-size
    materials

21
Soil Classification
  • Based on present features and formative
    processes
  • Soil is geologic material which has been altered
    by weathering an biological activity. Typically
    extends 1-2 meters deep below soil is parent
    material
  • Soil development makes sequence of bands, or
    horizons.

22
Eluvial processes
  • Clay is carried with water in eluviation and
    deposited in illuviation in sheets (lamellae)
    making an argillic horizon.
  • Soluble minerals may be carried upward through a
    soil profile driven by evaporation giving rise to
    concentrated bands of minerals at particular
    elevations.

23
Vertical Variations in Soils
  • Banding also arises from the depositional
    processes (parent material).
  • The scale of variation shorter in the vertical
    than horizontal.
  • Layers may be very distinct, or almost
    indistinguishable.

24
System of designations
  • Three symbol designation e.g. Ap1
  • A here is what is referred to as the
    designation of master horizon
  • There are six master horizon designations O, A,
    E, B, C, and R.

25
Master Horizon Designations
  • O dominated by organic matter
  • A first mineral horizon in a soil with either
    enriched humic material or having properties
    altered by agricultural activities (e.g.,
    plowing, grazing).
  • E loss of a combination of clay, iron and
    aluminum only resistant materials. Lighter in
    color than the A horizon above it (due to a
    paucity of coatings of organic matter and iron
    oxides)

26
Master Horizon Designations (cont.)
  • B below A or E, enriched in colorants (iron and
    clays), or having significant block structure
  • C soil material which is not bedrock, but shows
    little evidence of alteration from the parent
    material.
  • R too tough to penetrate with hand operated
    equipment.
  • For complete definitions, see the SCS Soil
    Taxonomy (Soil Conservation Service, 1994).

27
Master Horizon Designations (cont.)
  • Major designations may be combined as either AB
    or A/B if the horizon has some properties of the
    second designation

28
Subordinate classifications
  • Lower case letter indicates master horizon
    features.
  • There are 22. e.g.
  • k accumulation of carbonates
  • p plowing
  • n accumulation of sodium
  • May be used in multiple

29
Final notes on designations
  • Arabic numerals allow description of sequences
    with the same master, but with differing
    subordinate (e.g., Bk1 followed by Bn2).
  • Whenever a horizon is designated, its vertical
    extent must also be reported.

30
Color and Structure tell genetic and
biogeochemical history
  • Dark colors are indicative of high organic
    content
  • Grayish coloration indicates reducing (oxygen
    stripping) conditions
  • Reddish color indicates oxidizing (oxygen
    supplying) conditions.
  • Relates closely to hydraulic conditions of site
  • Often of greater use than a slew of lab analysis
    of soil cores.

31
Quantification of Color
  • Munsell Color chart by hue, value and chroma
    summarized in an alpha-numerical coding
    shorthand.
  • Pattern of coloration is informative. Mottling,
    where color varies between grayish to reddish
    over a few cm, most important.
  • Intermittent saturation oxidizing then reducing
  • Precise terminology for mottle description (e.g.,
    Vepraskas, M.J. 1992).

32
Structure
  • Must identify the smallest repeated element which
    makes up the soil ped Include details of the
    size, strength, shape, and distinctness of the
    constituent peds.

33
Climate
  • Six major climatic categories employed in soil
    classification useful in groundwater recharge
    and vadose zone transport.
  • Aquic precipitation always exceeds
    evapotransiration (ET), yielding continuous net
    percolation.
  • Xeric recharge occurs during the wet cool
    season, while the soil profile is depleted of
    water in the hot season.
  • Identifying the seasonality of the local water
    balance is fundamental to understanding the
    vadose zone hydrology.

34
  • Six categories of climates

35
High Points of Clay Mineralogy
  • General
  • Clay constituents dominate hydraulic chemical
    behavior
  • Two basic building blocks of clays
  • silica centered tetrahedra
  • variously centered octahedra

36
Basic Formations
  • chain structures (e.g., asbestos)
  • amorphous structures (glasses)
  • sheet structure (phyllosilicates clay!)

37
Unit-cells octa- and tetrahedral units
38
Isomorphic Substitution
  • Silica tetrahedron four oxygen surrounding one
    silica atom
  • Space filled by the silica can accommodate atoms
    up to 0.414 times O2 radius (5.8 x 10-9 m)
    includes silica and aluminum.
  • Balanced charge if the central atom has charge
    4, negative charge if the central atom has a
    less positive charge (oxygen is shared by two
    tetrahedra in crystal so contributes -1 to each
    cell).
  • Same for the octahedra 0.732 times O2 radius
    (1.02 x 10-8 m) iron, magnesium, aluminum,
    manganese, titanium, sodium or calcium, (sodium
    and calcium generate cubic lattice rather than
    octahedra)

39
Ionic radii dictate isomorphic substitution
Fit
into
Tetrahedron

(radius lt0.41

t
imes that of
oxygen


Fit
into

Octahedron


(radius lt0.732
Na

0.097

0.693

2
Ca

0.099

0.707

t
imes that of

K

0.133

0.950

oxygen)

2
Ba

0.13
4

0.957



Rb
0.147

1.050


 
 
Ca2
40
Surface Functional Groups
  • Clay minerals surfaces made up of hexagonal rings
    of tetrahedra or octahedra.
  • The group of atoms in these rings act as a
    delocalized source of negative charge surface
    functional group (a.k.a. SFG).
  • Cations attracted to center of SFGs above
    surface of the sheet.
  • Some (e.g., K and NH4) dehydrated and attached
    to the SFG inner sphere complex with the SFG
  • Cations bound to the SFG by water outer sphere
    complex
  • Inner and outer sphere ion/clay complexes are the
    Stern layer.

41
Details of Stearn Layer
  • Anions will be repelled from clay surfaces.
  • Zig-zag arrangement of negative and positively
    charged elements in the clay generates a dipole
    moment which attracts charged particles.
  • Diffuse attraction results in increased ionic
    concentration Gouy layer (Gouy, 1910).
  • Dipole-dipole attraction also holds water to the
    clay surfaces, in addition to osmotic force from
    cation concentration near the clay surfaces.

42
Hydration of Cations
43
Cation Exchange
  • The degree to which soil cations may be swapped
    for other cations is quantified as the cation
    exchange capacity (CEC) which is measured as
  • CEC cmol of positive charge/kg cmol() is
    equal to 10 Milliequivilents (meq)
  • 1 CEC 1 meq per 100 grams of soil.
  • Typical values of CEC are less than 10 for
    Kaolinite, between 15 and 40 for illite, and
    between 80 and 150 for montmorilonite.

44
Swelling of Clays
45
Distinguishing features between clays
  • Order of layering of tetra and octa sheets
  • Isomorphic substitutions
  • Cations which are bound to the surface
    functional groups

46
Examples Kaolinite
  • 11 alternating octatetra sheets
  • Little isomorphic substitution. Thus...
  • Very stable thicker stacks
  • Relatively low surface area 7-30 m2/gr
  • Do not swell much

47
Examples Montmorilonite (smectite family)
  • 21 octa sandwiched in 2 tetra sheets.
  • Lots of isomorphic substitution Mg2, Fe2,
    Fe3 for Al3 in octa. Since the octa is
    between tetras, cations in outer sphere
    complexes with hydrated SFGs. Thus
  • High surface area (600-800 m2/gr)
  • Lots of swelling
  • Big CEC.

48
Examples Illite
  • 21 octa sandwiched in 2 tetra sheets.
  • Lots of isomorphic substitution Al3 for the
    Si4 in the tetra. Generates charged SFGs
    binding potassium ionically between the
    successive 21 units. Thus
  • Moderate surface area 65-120 m2/g)
  • Little swelling
  • moderate CEC.

49
Summary of Clays
  • Clays are 10s atomic radii thick and thousands
    of atomic radii in horizontal extent
  • high surface to weight area plate structure.
  • Hold both water and cations
  • Highly reactive.
  • Swell wetted state due to hydration.
  • Dissociate if cations which glue layers together
    are depleted
  • Paths tortuous high resistance to flow of water
    impermeable Careful in the vadose zone
    shrinkage voids
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