Geology and Soils in Realtion to Vadose Zone Hydrology

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Geology and Soils in Realtion to Vadose Zone
Hydrology
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Typical Geologic Configurations floodplains

• Key points
• narrow continuous banding of alternating high and
  low permeability
• not necessarily oriented down stream

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Typical Geologic Configurations floodplains

• Terraced stream channel with likely ephemeral
  perched water.

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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.
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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)

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Typical Geologic Configurations Lava

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

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Typical Geologic Configurations
Fractures, Dikes, Fill
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Geologic Configurations various aquifers

• Whats an aquifer? Water that will flow into a
  well

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Water Tables (continued)

• Many aquifer systems have perched water tables
  that can be productive

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A Primer on Properties and Description of Natural
Media

• Particle Size Distribution
• Soil Classification
• Clay mineralogy

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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.

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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.

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• Standard
• sieve
• sizes

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Typical Particle Size Plot
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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

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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.

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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).

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Systems of soil textural classification

• (The USDA is standard in the US)

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Sand, Silt, Clay Textural Triangle

• Standard textural triangle for mixed grain-size
  materials

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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.

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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.

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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.

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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.

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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)

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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).

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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

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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

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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.

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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.

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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).

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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.

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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.

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• Six categories of climates

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High Points of Clay Mineralogy

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

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Basic Formations

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

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Unit-cells octa- and tetrahedral units
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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)

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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
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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.

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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.

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Hydration of Cations
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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/kgcmol() 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.

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Swelling of Clays
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Distinguishing features between clays

• Order of layering of tetra and octa sheets
• Isomorphic substitutions
• Cations which are bound to the surface
  functional groups

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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

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Examples Montmorilonite (smectite family)

• 21 octa sandwiched in 2 tetra sheets.
• Lots of isomorphic substitutionMg2, 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.

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Examples Illite

• 21 octa sandwiched in 2 tetra sheets.
• Lots of isomorphic substitutionAl3 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.

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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
  impermeableCareful in the vadose zone
  shrinkage voids