Laboratory Methods

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Laboratory Methods
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Laboratory Methods

• Direct methods ()
• XRD (all crystalline minerals)
• Infrared spectroscopy (mainly kaolinite)
• Thermal analysis (DTA) mainly kaolinite,
  gibbsite)
• Indirect methods
• Atterberg Tests
• COLE
• Bulk Density/Ksat
• CEC

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DIRECT METHODS
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XRD

• Describe the test
• Define the results
• Picture of Diffractogram
• Mineral ID
• Quantify results
• Pros and Cons
• Direct measurement of minerals
• Cost
• Time
• Sensitivity (approx 10) exactly where cut is
  in taxonomy
• Semi-quantitative (approx. 10-20)

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X-ray Diffraction (XRD)

• Identifies minerals based on their crystal
  structure (repeating planes of atoms), and
  expansion and contraction of structure following
  chemical and heat treatments.

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X-ray diffractometer - for crystalline minerals
when greater than 5 to 10 concentration
100,000
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soil
X-ray diffraction pattern
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X-ray Diffraction (XRD)

• Pros and Cons
• Direct measurement of minerals
• Cost (175 - 300 for XRD cost for
  time-consuming clay separation)
• Time (2-3 day turnaround for XRD at commercial
  lab, excluding separation time)
• Detection limit (approx 5-10) exactly where
  cutoff is in taxonomy
• Semi-quantitative (approx. 10-20)

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Indirect Methods
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Atterberg Limits

• Demonstrate the test
• Define the results
• Plastic limit
• Liquid limit
• Plasticity index
• Pros and Cons

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Liquid Limit

• The liquid limit (LL) is the water content where
  a soil changes from plastic to liquid behavior.
• Uses a Casagrande device.
• Soil is placed into the cup portion of the device
  and a groove is made down its center.
• The cup is repeatedly dropped until the 13 mm (½
  inch) groove is closed.
• The moisture content at which it takes 25 drops
  of the cup to cause the groove to close is
  defined as the liquid limit.

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Casagrande Device
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Plastic Limit

• The plastic limit (PL) is the water content where
  soil starts to exhibit plastic behavior.
• A thread of soil is at its plastic limit when it
  is rolled to a diameter of 3 mm and crumbles.

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Plasticity Index

• The plasticity index (PI) is a measure of the
  plasticity of a soil.
• The plasticity index is the size of the range of
  water contents where the soil exhibits plastic
  properties.
• The PI is difference between the liquid limit and
  the plastic limit (PI LL-PL).
• Soils with a high PI tend to be clay,
• Soils with a lower PI tend to be silt,
• Soils with a PI of 0 tend to have little or no
  silt or clay.

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Atterberg Limits

• Plastic limit moisture content at which a soil
  loses plasticity and behaves as a solid
• Liquid limit moisture content at which soil
  begins to flow
• Plasticity index difference between LL and PL
  (range of moisture the soil behaves in a plastic
  state)
• If
• Liquid limit gt 50 and
• Plasticity limit gt 30
• Soil has expansive clay mineralogy

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Atterberg Limits Pros and Cons

• Pros
• Relatively cheap
• Easy to perform
• In the rules

• Cons
• Operator dependent
• Designed for geotechnical engineering
  foundations etc.
• Not designed for clay mineralogy
• Unclear relation to clay mineralogy

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COLE (Coefficient of Linear Extensibility)

• Describe the test
• Define the results
• Discuss Pros and Cons

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COLE (Coefficient of Linear Extensibility)

• Modified Bulk Density Analysis
• Volume change of clod between moist and dry
  conditions

Cole Dbd/Dbm1/3 - 1
Dbm bulk density of moist sample (_at_ 1/3 bar
or field capacity) Dbd bulk density of dry
sample
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COLE (Coefficient of Linear Extensibility)

• Field Quick Test
• Uses a rod

Cole Lm Ld/Ld
Lm length of moist sample Ld length of dry
sample
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COLE Results

• 0.00-0.03 slight
• 0.03-0.06 moderate
• 0.06-0.09 severe
• gt0.09 very severe
• What does it mean?

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Bulk Density/Ksat

• Describe the test
• Define the results
• Discuss Pros and Cons

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Bulk Density/Ksat

• Standard bulk density analysis
• Standard laboratory textural analysis
• Nomographs produced by NRCS
• Use nomographs to determine Ksat and
• maximum LTAR

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Bulk Density/KsatPros and Cons

• Pros
• Moderate cost
• Theoretically OK

• Cons
• Difficult test
• Operator error
• Based on theory not real measurements
• Relation to field and wastewater unknown
• No defined parameters in Rules

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APPARENT CEC
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Apparent CEC

• Cation Exchange Capacity (CEC) definition Sum
  of exchangeable bases (Na, K, Ca2, Mg2) plus
  acidity (H, Al3) at a specific pH.
  (from Soil Sci. Soc. Am., 1997).
• However, direct extraction of soil cations cannot
  distinguish between exchangeable cations and
  cations dissolved from soluble salts.

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Apparent CEC (contd)

• More accurate to first saturate soil with a
  single index cation (NH4, Na, or Ba2), then
  displace and measure the amount of that cation
  adsorbed).
• In practice, CEC is operationally defined by the
  procedure used and tailored to the purpose for
  which the results will be applied (measured CEC
  depends on the method used).

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CEC Method
NH4OAC Solution pH 7
clay platelet
water
NH4
NH4
NH4
Ca2
NH4
NH4
NH4
NH4
NH4
Na
NH4
NH4
NH4
NH4
NH4
Soil pH
NH4
Al3
NH4
NH4
NH4
NH4
NH4
NH4
NH4
NH4
NH4
Mg2
NH4
NH4
NH4
NH4
NH4
K
NH4
NH4
NH4
NH4
Na
NH4
NH4
NH4
NH4
H
NH4
NH4
NH4
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H2O or alcohol
water
clay platelet
NH4
NH4
Ca2
NH4
NH4
NH4
NH4
Na
NH4
NH4
NH4
NH4
NH4
Al3
pH 7
NH4
NH4
NH4
NH4
NH4
NH4
NH4
Mg2
Na
NH4
NH4
NH4
NH4
NH4
NH4
H
NH4
K
NH4
NH4
NH4
NH4
NH4
NH4
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NaOAC Solution pH 7
clay platelet
water
Na
NH4
Na
Na
Na
Na
NH4
Na
Na
Na
NH4
Na
Na
NH4
Na
Na
pH 7
Na
NH4
Na
Na
NH4
Na
Na
Na
Na
Na
NH4
Na
Na
NH4
Na
Na
Na
NH4
Na
Na
Na
Na
Na
NH4
Na
Na
Na
Na
Na
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water
clay platelet
Na
Na
Na
Na
NH4
Na
NH4
Na
Na
Na
Na
Na
Na
Na
NH4
Na
NH4
pH 7
Na
Na
Na
Na
Na
Na
NH4
NH4
Na
Na
Na
Na
Na
Na
NH4
Na
NH4
Na
NH4
Na
Na
Na
NH4
Na
Na
Na
Measure displaced ammonium in the water (e.g.,
Kjeldahl)
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Apparent CEC Methods
Soil Taxonomy Method
EPA Method 9081

• Wash soil with pH-7 NH4OAc solution to NH4
  saturate the exchanger phase.

Wash soil with pH-8.2 NaOAc solution to Na
saturate the exchanger phase.
Rinse out dissolved Na.
Rinse out dissolved NH4.
Displace (w/ NH4) and measure exchangeable Na.
Displace (w/ Na) and measure exchangeable NH4.
Divide soil CEC by clay.
Divide soil CEC by clay.
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Apparent CEC (contd)

• Because EPA Method 9081 is a pH 8.2 method
  instead of a pH 7.0 method, it is NOT appropriate
  for apparent CEC measurements tied a cutoff of
  16.3 meq/100 g as used in soil taxonomy.

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Apparent CEC Calculation

• Apparent CEC (meq CEC7/ 100g soil) / ( clay/
  100g soil)
• Apparent CEC is a weighted test based on the clay
  fraction. CEC at pH 7 and Particle Size Analysis
  must be known.
• Note Must use the CEC measured at pH 7 not 8.2.

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CEC increases with increasing pH

• (Data compiled by S. W. Buol)

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Soil minerals have a range of CEC(chemical
compositions vary)
(typical CEC range in meq / 100 g mineral)

• kaolinite 2 15
• smectite 70 120
• vermiculite 100 200
• mica 10 40
• (organic matter) 150 300
• (Sparks, 1995)

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Natural variation in chemical composition and CEC
of 47 soil smectites (Borchardt, 1989)
Increasing octahedral Al vs. Fe
CEC 110 ? 23 Range 47 162 meq / 100 g
Increasing tetrahedral charge vs. octahedral
charge
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So why 16.3?

• The AG office wants a single number!

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Basis for apparent CEC cutoff of 16.3 meq / 100 g
for 10 smectite

• Based in taxonomy to define kandic horizons
• Assumes kaolinite and smectite are dominant clay
  minerals
• Average CEC values used
• kaolinite 7 meq / 100 g
• smectite 100 meq / 100 g
• (90 kaol. x 7) (10 smect. x 100)
• 16.3 meq/100g

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Range of apparent CEC for range of mineral CEC

• Low end (kaolinite 2 smectite 70 meq/100 g)
• (90 kaol. x 2) (10 smect. x 70) 9.8
  meq/100g
• High end (kaolinite 14 smectite 120 meq/100g)
• (90 kaol. x 15) (10 smect. x 120) 25.5
  meq/100g

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Pros and Cons

• PROS
• Reproducible and precise
• Relatively cheap (50)
• Based on the actual mineral properties that
  relate to water movement.

• CONS
• Should not be used in soils lt 35 clay
• Should not be used on soils gt0.5 OC
• 16.3 meq/100g ACEC is based on ideal soil sample
  containing only smectite and kaolinite
• Most soil labs are not set up to run the correct
  test

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Summary of methods in relation to rules

• Only when a soil is determined unsuitable will
  any alternative methods be used
• No single method (field or lab) will be fool
  proof
• Of the available lab methods ACEC is the most
  accurate, cost effective and time efficient
  method for estimating mineralogy
• If the soil is still unsuitable for mineralogy
  there is always .1948d or DWQ