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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

Geology and Soils in Relation to Vadose Zone Hydrology

Typical geologic configurations floodplains

Typical Geologic Configurations: floodplains

  • Key points:

    • narrow continuous banding of alternating high and low permeability

    • not necessarily oriented “down stream”

Geology and soils in relation to vadose zone hydrology

Typical Geologic Configurations: floodplains

  • Terraced stream channel with likely ephemeral perched water.

Typical geologic configurations karst

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

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)

Geology and soils in relation to vadose zone hydrology

Typical Hanford Formation

Reworked by mammals?

Mt St. Helens Ash

Diagonal micro-bedding

Typical geologic configurations lava

Typical Geologic Configurations: Lava

  • Lava flows may have alternating porous, fractured,

  • and low permeability regions with sedimentary

  • deposits between flows

Typical geologic configurations

Typical Geologic Configurations

Fractures, Dikes, Fill

Geologic configurations various aquifers

Geologic Configurations: various “aquifers”

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

Water tables continued

Water Tables (continued)

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

A primer on properties and description of natural media

A Primer on Properties and Description of Natural Media

  • Particle Size Distribution

  • Soil Classification

  • Clay mineralogy

Hey like why do we care

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

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.

Geology and soils in relation to vadose zone hydrology

  • Standard

  • sieve

  • sizes

Typical particle size plot

Typical Particle Size Plot

Summary statistics for particle size distribution

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

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

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

Systems of soil textural classification

  • (The USDA is standard in the US)

Sand silt clay textural triangle

Sand, Silt, Clay – Textural Triangle

  • Standard textural triangle for mixed grain-size materials

Clay axis

Silt axis

Sand axis

Soil classification

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

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

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

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

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

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 cont1

    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

    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

    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

    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

    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.

    Geology and soils in relation to vadose zone hydrology

    • Six categories of climates

    High points of clay mineralogy

    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

    Basic Formations

    • chain structures (e.g., asbestos)

    • amorphous structures (glasses)

    • sheet structure (phyllosilicates; clay!)

    Unit cells octa and tetrahedral units

    Unit-cells octa- and tetrahedral units dice.html

    Isomorphic substitution

    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)

    Ionic radii dictate isomorphic substitution




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    Ionic radii dictate isomorphic substitution




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    Surface functional groups

    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

    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

    Hydration of Cations

    Cation exchange

    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

    Swelling of Clays

    Distinguishing features between 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

    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

    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

    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

    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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