1

Geology and Soils Geology and Soils in Relation to in Relation to

Vadose Zone HydrologyVadose Zone Hydrology

Williams, 2002 http://www.its.uidaho.edu/AgE558Modified after Selker, 2000 http://bioe.orst.edu/vzp/Williams, 2002 http://www.its.uidaho.edu/AgE558Modified after Selker, 2000 http://bioe.orst.edu/vzp/

2

Open water surfaceOpen water surface

For gage pressure, p=0 at open For gage pressure, p=0 at open water surfacewater surface

In a porous medium, pressure is In a porous medium, pressure is negative, or zero when saturatednegative, or zero when saturated

For gage pressure, p=0 at open For gage pressure, p=0 at open water surfacewater surface

In a porous medium, pressure is In a porous medium, pressure is negative, or zero when saturatednegative, or zero when saturated

3

Consider the following slidesConsider the following slides

Does the formation have:Does the formation have:• Small pores (surface tension forces and Small pores (surface tension forces and

capillaritycapillarity

• Large fractures, or tunnels, flowing like Large fractures, or tunnels, flowing like an open channel?an open channel?

Does the formation have:Does the formation have:• Small pores (surface tension forces and Small pores (surface tension forces and

capillaritycapillarity

• Large fractures, or tunnels, flowing like Large fractures, or tunnels, flowing like an open channel?an open channel?

4

Typical Geologic Configurations: floodplainsTypical Geologic Configurations: floodplains

Key points:Key points:narrow continuous banding of narrow continuous banding of

alternating high and low permeabilityalternating high and low permeabilitynot necessarily oriented “down stream”not necessarily oriented “down stream”

Key points:Key points:narrow continuous banding of narrow continuous banding of

alternating high and low permeabilityalternating high and low permeabilitynot necessarily oriented “down stream”not necessarily oriented “down stream”

Point Bar Floodplain deposits

BedrockBedload deposits

Meanderingdirection

5

Terraced stream channel with likely Terraced stream channel with likely ephemeral perched water. ephemeral perched water. Terraced stream channel with likely Terraced stream channel with likely ephemeral perched water. ephemeral perched water.

Typical Geologic Configurations: floodplainsTypical Geologic Configurations: floodplains

BedrockRecent floodplain

6

Typical Geologic Configurations: KarstTypical 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.

Sink Local artesianpressure rises above this surface

Crevices saturatedto this level

Soil and clay

7

Geologic Configurations: beach depositsGeologic Configurations: beach deposits

Beach deposits, although similar to river deposits in Beach deposits, although similar to river deposits in texture, have unique structuretexture, have unique structure

Generally (not always!) fining upwardGenerally (not always!) fining upwardLaterally extensiveLaterally extensive

Lower variation in energy (more uniform)Lower variation in energy (more uniform)

Beach deposits, although similar to river deposits in Beach deposits, although similar to river deposits in texture, have unique structuretexture, have unique structure

Generally (not always!) fining upwardGenerally (not always!) fining upwardLaterally extensiveLaterally extensive

Lower variation in energy (more uniform)Lower variation in energy (more uniform)

8

Typical Geologic Configurations: BasaltTypical Geologic Configurations: Basalt

Basalt flows may have alternating porous, fractured, Basalt flows may have alternating porous, fractured, and low permeability regions with sedimentary and low permeability regions with sedimentary deposits between flowsdeposits between flows

Basalt flows may have alternating porous, fractured, Basalt flows may have alternating porous, fractured, and low permeability regions with sedimentary and low permeability regions with sedimentary deposits between flowsdeposits between flows

Vesicular structure near top of each flow

Alluvial till btwn flows

Open lava tube – high K

Alluvial fill

9

Typical Geologic ConfigurationsTypical Geologic Configurations Fractures, Dikes, Fill

Soil zone

Undifferentiatedglacial till

Clays

Fracture

Unalteredbedrock

Joint

10

Geologic Configurations: various “aquifers”Geologic Configurations: various “aquifers”

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

Surficial unconfined aquifers

Interfill blanket aquifer

Bedrock valley aquifer

Bedrock surface

Interfill valley aquifer

11

Water Tables (continued)Water Tables (continued)

Many aquifer systems have perched Many aquifer systems have perched water tables that can be productivewater tables that can be productiveMany aquifer systems have perched Many aquifer systems have perched water tables that can be productivewater tables that can be productive

12

A Primer on Properties and A Primer on Properties and Description of Natural MediaDescription of Natural Media

Particle Size DistributionParticle Size Distribution

Soil ClassificationSoil Classification

Clay mineralogyClay mineralogy

Particle Size DistributionParticle Size Distribution

Soil ClassificationSoil Classification

Clay mineralogyClay mineralogy

13

Why do we care?Why do we care?

Transport through natural porous Transport through natural porous media cannot be understood from media cannot be understood from mathematical notation and mathematical notation and boundary conditions alone.boundary conditions alone.

The structure, setting, history and The structure, setting, history and chemistry of the mineral system in chemistry of the mineral system in the vz all play central roles in the vz all play central roles in transport (e.g. in flow + transport)transport (e.g. in flow + transport)

Transport through natural porous Transport through natural porous media cannot be understood from media cannot be understood from mathematical notation and mathematical notation and boundary conditions alone.boundary conditions alone.

The structure, setting, history and The structure, setting, history and chemistry of the mineral system in chemistry of the mineral system in the vz all play central roles in the vz all play central roles in transport (e.g. in flow + transport)transport (e.g. in flow + transport)

14

The ultra-basicsThe ultra-basics

Particle size distribution is plotted Particle size distribution is plotted as the mass which is made up of as the mass which is made up of particles smaller than a given size.particles smaller than a given size.

Very useful in estimating the soil’s Very useful in estimating the soil’s hydraulic properties such as the hydraulic properties such as the water retention characteristics and water retention characteristics and the hydraulic conductivity. the hydraulic conductivity.

Particle size distribution is plotted Particle size distribution is plotted as the mass which is made up of as the mass which is made up of particles smaller than a given size.particles smaller than a given size.

Very useful in estimating the soil’s Very useful in estimating the soil’s hydraulic properties such as the hydraulic properties such as the water retention characteristics and water retention characteristics and the hydraulic conductivity. the hydraulic conductivity.

15

StandardStandard

sievesieve

sizessizes

StandardStandard

sievesieve

sizessizes

16

Typical Particle Size PlotTypical Particle Size Plot

Particle Size, d (mm)

% M

ass

< d

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10

17

Moisture Content Moisture Content Characteristic CurveCharacteristic Curve

• Grain size, or particle-size distribution is Grain size, or particle-size distribution is not enough.not enough.

• We need to show how moisture content We need to show how moisture content correlates to capillary pressures. correlates to capillary pressures.

•Also called matric pressures. Also called matric pressures.

•Negative pressures.Negative pressures.

• Grain size, or particle-size distribution is Grain size, or particle-size distribution is not enough.not enough.

• We need to show how moisture content We need to show how moisture content correlates to capillary pressures. correlates to capillary pressures.

•Also called matric pressures. Also called matric pressures.

•Negative pressures.Negative pressures.

18

Capillary rise from water table Capillary rise from water table into a porous soilinto a porous soil

19

Particle size to Characteristic CurvesParticle size to Characteristic Curves((a) Particles distributed a) Particles distributed between dbetween dminmin and d and dmaxmax

(b) Pore size distribution (b) Pore size distribution similar: similar: The ordinate goes from The ordinate goes from mass of particles, to mass of particles, to volume of pores. volume of pores.

(c) Laplace’s eq. relate (c) Laplace’s eq. relate pore size filling pressure pore size filling pressure of each pore. of each pore. Plot becomes filling Plot becomes filling pressure vs. volume of pressure vs. volume of pores.pores.

(d) Finally note volume (d) Finally note volume of pores = degree of of pores = degree of saturation.saturation.

((a) Particles distributed a) Particles distributed between dbetween dminmin and d and dmaxmax

(b) Pore size distribution (b) Pore size distribution similar: similar: The ordinate goes from The ordinate goes from mass of particles, to mass of particles, to volume of pores. volume of pores.

(c) Laplace’s eq. relate (c) Laplace’s eq. relate pore size filling pressure pore size filling pressure of each pore. of each pore. Plot becomes filling Plot becomes filling pressure vs. volume of pressure vs. volume of pores.pores.

(d) Finally note volume (d) Finally note volume of pores = degree of of pores = degree of saturation.saturation.

Particle Size

d min

d max

0% 100% Percent Mass < d

Pore Size

Size

r min

r max

0% 100% Percent Volume < r

h min

h max

0% 100% Percent Volume

h min

h max

0 Moisture Content sat

(a) (b)

(c) (d)

r d

= nV

20

Summary statistics for particle Summary statistics for particle size distributionsize distribution

dd5050, d, d1010, d, d80 etc.80 etc.

Uniformity coefficient, UUniformity coefficient, U

U = U = d d6060 /d /d1010 [1.1][1.1]

U between 2 and 10 for “well sorted” and U between 2 and 10 for “well sorted” and “poorly sorted” materials“poorly sorted” materials

dd5050, d, d1010, d, d80 etc.80 etc.

Uniformity coefficient, UUniformity coefficient, U

U = U = d d6060 /d /d1010 [1.1][1.1]

U between 2 and 10 for “well sorted” and U between 2 and 10 for “well sorted” and “poorly sorted” materials“poorly sorted” materials

21

Dependence of bulk density on Dependence of bulk density on particle size distributionparticle size distribution

Uniform particle size distribution givesUniform particle size distribution givesuniform densityuniform density

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

Uniform particle size distribution givesUniform particle size distribution givesuniform densityuniform density

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

22

% S

mal

ler

0

100

Size

% S

mal

ler

0

100

Size

% S

mal

ler

0

100

Size

"Well Sorted" "Poorly Sorted"

% S

mal

ler

0

100

Size

% S

mal

ler

0

100

Size

% S

mal

ler

0

100

Size

(a)

(b)

"Poorly Graded" "Well Graded"

23

What are is the basis ofWhat are is the basis ofsize classes?size classes?

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

SiltSilt: : 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; 2<silt<0.05mm).

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

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

SiltSilt: : 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; 2<silt<0.05mm).

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

24

Systems of soil textural classificationSystems of soil textural classification

(The USDA is standard in the US)(The USDA is standard in the US)(The USDA is standard in the US)(The USDA is standard in the US)

25

Sand, Silt, Clay – Textural TriangleSand, Silt, Clay – Textural Triangle

Standard textural triangle for mixed grain-size materials

Standard textural triangle for mixed grain-size materials

26

What can we use from the soil What can we use from the soil type?type?

After soil size, what else can we get After soil size, what else can we get from the USDA soils map, or a soil from the USDA soils map, or a soil investigation, to inform us re: flow investigation, to inform us re: flow and transport?and transport?

History (soil formation)History (soil formation) Chemistry and MineralogyChemistry and Mineralogy BiologyBiology

After soil size, what else can we get After soil size, what else can we get from the USDA soils map, or a soil from the USDA soils map, or a soil investigation, to inform us re: flow investigation, to inform us re: flow and transport?and transport?

History (soil formation)History (soil formation) Chemistry and MineralogyChemistry and Mineralogy BiologyBiology

27

Soil ClassificationSoil 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.

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.

28

Eluvial processesEluvial 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.

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.

29

Vertical Variations in SoilsVertical 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.

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.

30

System of designationsSystem 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.

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.

31

Master Horizon DesignationsMaster 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)

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)

32

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).

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).

33

Master Horizon Designations (cont.)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

Major designations may be combined as either AB or A/B if the horizon has some properties of the second designation

34

Subordinate classificationsSubordinate classifications

Lower case letter indicates master Lower case letter indicates master horizon features. horizon features. There are 22. e.g.There are 22. e.g.k = accumulation of carbonatesk = accumulation of carbonatesp = plowingp = plowingn = accumulation of sodiumn = accumulation of sodium

May be used in multipleMay be used in multiple

Lower case letter indicates master Lower case letter indicates master horizon features. horizon features. There are 22. e.g.There are 22. e.g.k = accumulation of carbonatesk = accumulation of carbonatesp = plowingp = plowingn = accumulation of sodiumn = accumulation of sodium

May be used in multipleMay be used in multiple

35

Designations (last note)Designations (last note)

Arabic numerals allow description of Arabic numerals allow description of sequences with the same master, but sequences with the same master, but with differing subordinate (e.g., Bk1 with differing subordinate (e.g., Bk1 followed by Bn2). followed by Bn2).

Whenever a horizon is designated, its Whenever a horizon is designated, its vertical extent must also be reported.vertical extent must also be reported.

Arabic numerals allow description of Arabic numerals allow description of sequences with the same master, but sequences with the same master, but with differing subordinate (e.g., Bk1 with differing subordinate (e.g., Bk1 followed by Bn2). followed by Bn2).

Whenever a horizon is designated, its Whenever a horizon is designated, its vertical extent must also be reported.vertical extent must also be reported.

36

Color and Structure tell genetic Color and Structure tell genetic and biogeochemical history and biogeochemical history Dark colors are indicative of high organic

contentGrayish coloration indicates reducing (oxygen

stripping) conditionsReddish color indicates oxidizing (oxygen

supplying) conditions. Relates closely to hydraulic conditions of site

VERY USEFUL !!

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

VERY USEFUL !!

37

Munsell Color ChartMunsell Color Chart

http://en.wikipedia.org/wiki/Munsell_color_systemhttp://en.wikipedia.org/wiki/Munsell_color_system

““Munsell system is still widely used, by, among others, ANSI to Munsell system is still widely used, by, among others, ANSI to define skin and hair colors for forensic pathology, the USGS define skin and hair colors for forensic pathology, the USGS for matching soil colors, and breweries for matching beer for matching soil colors, and breweries for matching beer colors”colors”

http://www.sbg.ac.at/ipk/avstudio/pierofun/protocol/soilchart.pdfhttp://www.sbg.ac.at/ipk/avstudio/pierofun/protocol/soilchart.pdf

Question: Question: • Any experience in lab, field, where color was important?Any experience in lab, field, where color was important?• Distant students, feel free to send in any interesting Distant students, feel free to send in any interesting

examples, ag or environmental contextexamples, ag or environmental context

http://en.wikipedia.org/wiki/Munsell_color_systemhttp://en.wikipedia.org/wiki/Munsell_color_system

““Munsell system is still widely used, by, among others, ANSI to Munsell system is still widely used, by, among others, ANSI to define skin and hair colors for forensic pathology, the USGS define skin and hair colors for forensic pathology, the USGS for matching soil colors, and breweries for matching beer for matching soil colors, and breweries for matching beer colors”colors”

http://www.sbg.ac.at/ipk/avstudio/pierofun/protocol/soilchart.pdfhttp://www.sbg.ac.at/ipk/avstudio/pierofun/protocol/soilchart.pdf

Question: Question: • Any experience in lab, field, where color was important?Any experience in lab, field, where color was important?• Distant students, feel free to send in any interesting Distant students, feel free to send in any interesting

examples, ag or environmental contextexamples, ag or environmental context

38

Quantification of ColorQuantification of Color

Munsell Color chart by hue, value and Munsell Color chart by hue, value and chroma; summarized in an alpha-numerical chroma; summarized in an alpha-numerical coding shorthand. coding shorthand.

Pattern of coloration is informative. Mottling, Pattern of coloration is informative. Mottling, where color varies between grayish to reddish where color varies between grayish to reddish over a few cm, most important. over a few cm, most important.

Intermittent saturation; oxidizing then reducingIntermittent saturation; oxidizing then reducingPrecise terminology for mottle description (e.g., Precise terminology for mottle description (e.g.,

Vepraskas, M.J. 1992).Vepraskas, M.J. 1992).

Munsell Color chart by hue, value and Munsell Color chart by hue, value and chroma; summarized in an alpha-numerical chroma; summarized in an alpha-numerical coding shorthand. coding shorthand.

Pattern of coloration is informative. Mottling, Pattern of coloration is informative. Mottling, where color varies between grayish to reddish where color varies between grayish to reddish over a few cm, most important. over a few cm, most important.

Intermittent saturation; oxidizing then reducingIntermittent saturation; oxidizing then reducingPrecise terminology for mottle description (e.g., Precise terminology for mottle description (e.g.,

Vepraskas, M.J. 1992).Vepraskas, M.J. 1992).

39

StructureStructure

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

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

40

Climate Climate Six major climatic categories employed in soil Six major climatic categories employed in soil classification; useful in groundwater recharge and classification; useful in groundwater recharge and vadose zone transport.vadose zone transport. Aquic: precipitation always exceeds evapotransiration (ET), Aquic: precipitation always exceeds evapotransiration (ET),

yielding continuous net percolation.yielding continuous net percolation.Xeric: recharge occurs during the wet cool season, while Xeric: recharge occurs during the wet cool season, while

the soil profile is depleted of water in the hot season. the soil profile is depleted of water in the hot season.

Identifying the seasonality of the local water balance is Identifying the seasonality of the local water balance is fundamental to understanding the vadose zone fundamental to understanding the vadose zone

hydrology.hydrology.

Six major climatic categories employed in soil Six major climatic categories employed in soil classification; useful in groundwater recharge and classification; useful in groundwater recharge and vadose zone transport.vadose zone transport. Aquic: precipitation always exceeds evapotransiration (ET), Aquic: precipitation always exceeds evapotransiration (ET),

yielding continuous net percolation.yielding continuous net percolation.Xeric: recharge occurs during the wet cool season, while Xeric: recharge occurs during the wet cool season, while

the soil profile is depleted of water in the hot season. the soil profile is depleted of water in the hot season.

Identifying the seasonality of the local water balance is Identifying the seasonality of the local water balance is fundamental to understanding the vadose zone fundamental to understanding the vadose zone

hydrology.hydrology.

41

Six categories of climatesSix categories of climatesSix categories of climatesSix categories of climates

42

High Points of Clay MineralogyHigh Points of Clay Mineralogy

GeneralGeneralhttp://en.wikipedia.org/wiki/Clayhttp://en.wikipedia.org/wiki/Clay

Clay constituents dominate hydraulic Clay constituents dominate hydraulic chemical behavior chemical behavior

They are REACTIVEThey are REACTIVETwo basic building blocks of claysTwo basic building blocks of clays

Silica-centered tetrahedra Silica-centered tetrahedra Variously-centered octahedra Variously-centered octahedra

GeneralGeneralhttp://en.wikipedia.org/wiki/Clayhttp://en.wikipedia.org/wiki/Clay

Clay constituents dominate hydraulic Clay constituents dominate hydraulic chemical behavior chemical behavior

They are REACTIVEThey are REACTIVETwo basic building blocks of claysTwo basic building blocks of clays

Silica-centered tetrahedra Silica-centered tetrahedra Variously-centered octahedra Variously-centered octahedra

43

Clay: fn of size and mineralogyClay: fn of size and mineralogyColloidsColloidshttp://en.wikipedia.org/wiki/Colloidhttp://en.wikipedia.org/wiki/Colloid

Colloids* in soil water systemsColloids* in soil water systems• Clay particlesClay particles

• Viruses and bacteriaViruses and bacteria

• Humic macromoleculesHumic macromolecules

• Other organicsOther organics

* * All would classify as clay if size were only metricAll would classify as clay if size were only metric

ColloidsColloidshttp://en.wikipedia.org/wiki/Colloidhttp://en.wikipedia.org/wiki/Colloid

Colloids* in soil water systemsColloids* in soil water systems• Clay particlesClay particles

• Viruses and bacteriaViruses and bacteria

• Humic macromoleculesHumic macromolecules

• Other organicsOther organics

* * All would classify as clay if size were only metricAll would classify as clay if size were only metric

44

Basic FormationsBasic Formations

chain structures (e.g., asbestos)chain structures (e.g., asbestos)

amorphous structures (glasses)amorphous structures (glasses)

sheet structure (phyllosilicates; sheet structure (phyllosilicates; includes clay) includes clay)

chain structures (e.g., asbestos)chain structures (e.g., asbestos)

amorphous structures (glasses)amorphous structures (glasses)

sheet structure (phyllosilicates; sheet structure (phyllosilicates; includes clay) includes clay)

45

Unit-cells octa- and tetrahedral unitsUnit-cells octa- and tetrahedral units

46

Isomorphic SubstitutionIsomorphic SubstitutionSilica tetrahedron: four oxygen surrounding one silica atomSpace 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)

Silica tetrahedron: four oxygen surrounding one silica atomSpace 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)

47

Ionic radii dictate isomorphic substitutionIonic radii dictate isomorphic substitutionIon Ionic

radius. nm

Rx: Ro

O2- 0.140 -- F- 0.133 -- Cl- 0.181 -- Si4+ 0.039 0.278 Al3+ 0.051 0.364 Fe3+ 0.064 0.457 Mg2+ 0.066 0.471 Ti4+ 0.068 0.486 Fe2+ 0.074 0.529 Mn2+ 0.080 0.571 Na+ 0.097 0.693 Ca2+ 0.099 0.707 K+ 0.133 0.950

Ba2+ 0.134 0.957 Rb+ 0.147 1.050

Fit into Tetrahedron (radius <0.41 times that of oxygen Fit into Octahedron (radius <0.732 times that of oxygen)

48

Surface Functional GroupsSurface Functional GroupsClay 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.

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.

49

Details of Stearn LayerDetails of Stearn LayerAnions 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.

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.

50

Hydration of CationsHydration of Cations

Su r face Fu n c t i on a l Gr ou p

O

H

HO

HH

O

H

HCat io n

(a ) Inne r s phe r e c om pl e x

Su r face Fu n c t i on a l Gr ou p

O

H

HO

HH

O

H

HCat io n

(b) ���Out e r s phe r e ������c om pl e x

O

H

H OH

H

51

Cation ExchangeCation ExchangeThe 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.

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.

52

Swelling of ClaysSwelling of Clays

(a) Low moisture: clays bound (b) High moisture: clays dissociated

= H O2

clay plate

clay plate clay plate

clay plate

53

Distinguishing features between claysDistinguishing features between clays

Order of layering of tetra and octa Order of layering of tetra and octa sheetssheets

Isomorphic substitutionsIsomorphic substitutions

Cations which are bound to the Cations which are bound to the surface functional groups surface functional groups

Order of layering of tetra and octa Order of layering of tetra and octa sheetssheets

Isomorphic substitutionsIsomorphic substitutions

Cations which are bound to the Cations which are bound to the surface functional groups surface functional groups

54

Examples: KaoliniteExamples: Kaolinite

1:1 alternating octa:tetra sheets1:1 alternating octa:tetra sheets

Little isomorphic substitution. Little isomorphic substitution.

Thus...Thus...Very stable thicker stacksVery stable thicker stacksRelatively low surface area: 7-30 mRelatively low surface area: 7-30 m22/gr/grDo not swell muchDo not swell much

1:1 alternating octa:tetra sheets1:1 alternating octa:tetra sheets

Little isomorphic substitution. Little isomorphic substitution.

Thus...Thus...Very stable thicker stacksVery stable thicker stacksRelatively low surface area: 7-30 mRelatively low surface area: 7-30 m22/gr/grDo not swell muchDo not swell much

55

Examples: Examples:

Montmorilonite (smectite family)Montmorilonite (smectite family) 2:1 octa sandwiched in 2 tetra sheets. 2:1 octa sandwiched in 2 tetra sheets.

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

Thus:Thus:High surface area (600-800 mHigh surface area (600-800 m22/gr)/gr)Lots of Lots of swelling (change in permeability)swelling (change in permeability)Big CEC. Big CEC.

2:1 octa sandwiched in 2 tetra sheets. 2:1 octa sandwiched in 2 tetra sheets.

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

Thus:Thus:High surface area (600-800 mHigh surface area (600-800 m22/gr)/gr)Lots of Lots of swelling (change in permeability)swelling (change in permeability)Big CEC. Big CEC.

56

Examples: IlliteExamples: Illite

2:1 octa sandwiched in 2 tetra sheets. 2:1 octa sandwiched in 2 tetra sheets.

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

Thus:Thus:Moderate surface area 65-120 mModerate surface area 65-120 m22/g)/g)Little swelling Little swelling moderate CEC. moderate CEC.

2:1 octa sandwiched in 2 tetra sheets. 2:1 octa sandwiched in 2 tetra sheets.

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

Thus:Thus:Moderate surface area 65-120 mModerate surface area 65-120 m22/g)/g)Little swelling Little swelling moderate CEC. moderate CEC.

57

Summary of ClaysSummary of ClaysClays 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 cationsHighly reactive. Swell wetted state due to hydration. Dissociate if cations which glue layers together are

depletedPaths tortuous: high resistance to flow of water

“impermeable”Careful in the vadose zone: shrinkage voids

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 cationsHighly reactive. Swell wetted state due to hydration. Dissociate if cations which glue layers together are

depletedPaths tortuous: high resistance to flow of water

“impermeable”Careful in the vadose zone: shrinkage voids