Geology and soils in relation to vadose zone hydrology
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Geology and Soils in Relation to Vadose Zone Hydrology. Typical Geologic Configurations: floodplains. Key points: narrow continuous banding of alternating high and low permeability not necessarily oriented “down stream”. Typical Geologic Configurations: floodplains.

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Geology and Soils in Relation to Vadose Zone Hydrology

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Geology and Soils in Relation to Vadose Zone Hydrology

Typical Geologic Configurations: floodplains

  • Key points:

    • narrow continuous banding of alternating high and low permeability

    • not necessarily oriented “down stream”

Typical Geologic Configurations: floodplains

  • Terraced stream channel with likely ephemeral perched water.

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.

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)

Typical Hanford Formation

Reworked by mammals?

Mt St. Helens Ash

Diagonal micro-bedding

Typical Geologic Configurations: Lava

  • Lava flows may have alternating porous, fractured,

  • and low permeability regions with sedimentary

  • deposits between flows

Typical Geologic Configurations

Fractures, Dikes, Fill

Geologic Configurations: various “aquifers”

  • What’s an aquifer? Water that will flow into a well…

Water Tables (continued)

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

A Primer on Properties and Description of Natural Media

  • Particle Size Distribution

  • Soil Classification

  • Clay mineralogy

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.

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 soil’s hydraulic properties such as the water retention characteristics and the hydraulic conductivity.

  • Standard

  • sieve

  • sizes

Typical Particle Size Plot

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

Dependence of bulk density on particle size distribution

  • Uniform particle size distribution giveslow packing density

  • increasing the range of particle sizes gives rise to greater bulk density.

What are is the basis of size classes?

  • Clay: won’t settle (<2m: doesn’t feel gritty between your teeth).

  • Silt: settles freely, but cannot be discriminated by eye (isn’t slippery between your fingers; doesn’t make strong ribbons; goes through a number 300 sieve; 2m<silt<0.05mm).

  • Sand: you can see (>0.05 mm), but is smaller than pebbles (<2mm).

Systems of soil textural classification

  • (The USDA is standard in the US)

Sand, Silt, Clay – Textural Triangle

  • Standard textural triangle for mixed grain-size materials

Clay axis

Silt axis

Sand axis

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.

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.

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.

System of designations

  • Three symbol designatione.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.

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).

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).

  • 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

    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

    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.

    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.

  • 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).


    • 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.


    • 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.

    • Six categories of climates

    High Points of Clay Mineralogy

    • General

      • Clay constituents dominate hydraulic chemical behavior

      • Two basic building blocks of clays

        • silica centered tetrahedra

        • variously centered octahedra

    Basic Formations

    • chain structures (e.g., asbestos)

    • amorphous structures (glasses)

    • sheet structure (phyllosilicates; clay!)

    Unit-cells octa- and tetrahedral units dice.html

    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




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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 SFG’s 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.

    Details of Stearn Layer

    • Anions will be repelled from clay surfaces.

    • Zig-zag negative and positively charged elements in clay generates dipole moment attracting 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.

    Hydration of Cations

    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.

    Swelling of Clays

    Distinguishing features between clays

    • Order of layering of tetra and octa sheets

    • Isomorphic substitutions

    • Cations which are bound to the surface functional groups

    Examples: Kaolinite

    • 1:1 alternating octa:tetra sheets

    • Little isomorphic substitution. Thus...

      • Very stable thicker stacks

      • Relatively low surface area: 7-30 m2/gr

      • Do not swell much

    Examples: Montmorilonite (smectite family)

    • The most common smectite is Montmorillinite, with general formula :

  • (½Ca,Na)(Al,Mg,Fe)4(Si,Al)8O20(OH)4.nH2O

    • 2:1 octa sandwiched in 2 tetra sheets.

    • Lots of isomorphic substitution:Mg+2, Fe+2, & Fe+3 for Al+3 in octa. Since the octa is between tetra’s, cations in outer sphere complexes with hydrated SFG’s. Thus:

      • High surface area (600-800 m2/gr)

      • Lots of swelling

      • Big CEC.


    Examples: Illite

    • 2:1 octa sandwiched in 2 tetra sheets.

    • Lots of isomorphic substitution:Al+3 for the Si+4 in the tetra. Generates charged SFG’s binding potassium ionically between the successive 2:1 units. Thus:

      • Moderate surface area 65-120 m2/g)

      • Little swelling

      • moderate CEC.

    Summary of Clays

    • Clays are 10’s 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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