Redox, Saturation, and Soil Indicators

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Redox, Saturation, and Soil Indicators
David Lindbo Michael Vepraskas Chris Stall
Aziz Amoozegar Alex Graves NCSU Soil Science
Dept. Diana Rashash, Onslow County NCSU-CE
Erik Severson, VT/VDH
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Download presentation at

• www.soil.ncsu.edu/lockers/lindbo
• In folder
• Alabama PSCAA

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What we know about hydrology/morphology

• Redoximorphic features are formed under saturated
  and reduced conditions

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Redox Transformations
Coating of Fe2O3
Remove Fe
Fe2
Gray Soil
Brown Soil Fe3
2e- 6H Fe2O3 ? 2Fe(II) 3H2O
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Redox features are like MMs

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Redox Concentration (Pore lining)
1. Plant root grows into soil
5. Reduced Fe moves away from decomposing
root Reduced Fe oxidizes, soil turns red
2. Root dies and starts to decompose
3. Water table rises
4. Bacteria continue to decompose root Oxygen
reduced Nitrate reduced Fe reduced and removed,
soil turns gray
6. Water drains from root channel
7. Root completely decomposed
8. Water table drops
Redox depletion
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Redox concentrations
WT 4
4 chroma depletion
WT 3
3 chroma depletion
WT 2
WT 1
lt 2 chroma depletion
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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What else do we know about hydrology/morphology?

• 2 chroma colors are interpreted as an indicator
  water table or wetness conditions in several
  states
• 2 chroma colors are not exclusively used to
  determine water table or saturation by USDA-NRCS
• From previous work (He, 2002)
• 21 days required to created reducing conditions
• Strong correlation between percent low chroma
  color and saturation

NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Interpreting Redoximorphic Features 7 Rules

• Adapted from
• Dr. Michael Vepraskas

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Reduction, oxidation and translocation
saturation
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The Rules require

• Five conditions to be met
• Organic matter is present
• Bacteria are active
• Soil is saturated (no air filled pores)
• Water is stagnant (anaerobic), i.e. not flowing,
  low dissolved oxygen
• Presence of Fe

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Interpretation Rule 1

• Redox depletions form
• where roots grow
• i.e. around root channels or cracks.

Remember all 5 conditions need to be met
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Five conditions met here
Not met here
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Comments on Rule 1

• If depletions occur where roots could not grow,
  then features may be relict

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Interpretation Rule 2

• Redox concentrations form
• where oxygen is present

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Pore lining formed when oxygen was in root
channel and matrix was reduced
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Redox feature formation
O2
O2
O2
O2
O2
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Redox concentrations
4 chroma depletion
3 chroma depletion
lt 2 chroma depletion
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Interpretation Rule 3

• Redox depletions form when the soil is saturated
  and reduced

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Example of Hydrograph
0
-10
-20
-30
Depth
-40
-50
-60
-70
-80
1/1
4/8
5/6
1/15
1/29
2/12
2/26
3/11
3/25
4/22
5/20
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Question

• How do you find the water table?

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Determining Depth to Soil Wetness by Code

• Find depth to 2 chroma depletions (by code)
• These colors mark the top of the seasonal high
  water table (SHWT) or wetness condtions

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Depth of water table may rise to
Depth of seasonal high water table by code
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Question

• Do redox depletions form after soil becomes
  saturated or after it becomes reduced?

Answer after it becomes reduced
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Critical duration of saturation (days) needed for
Fe-reduction to occur
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Question

• If a soil needs 21 d of saturation before it gets
  reduced,
• then
• what is the shortest time a soil can be saturated
  before redox depletions form?

Answer 21 days
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Example of Hydrograph
0
Find the 21-day saturation depth
-10
25 inches is equivalent to depth to low chroma
-20
-30
Depth
-40
-50
Where do low chroma colors occur?
-60
-70
Depth to wetness is 25 inches
-80
1/1
4/8
5/6
1/15
1/29
2/12
2/26
3/11
3/25
4/22
5/20
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Defining an Equivalent Standard

• Can saturation be used in place of morphology?
• If, 21 days of saturation corresponds to 2 chroma
  features and
• If, 2 chroma colors are used to determine the
  water table
• Then, the water table can be defined by 21 days
  of saturation
• However, 14 days of saturation has been chosen as
  the critical duration of saturation

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Comments on Rule 3

• Redox depletions form when the soil is saturated
  and reduced
• Water tables rise above depth of redox depletions

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Interpretation Rule 4

• Depletions deeper in the soil require longer
  durations of saturation

Because it is all about reduction
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When saturated becomes reduced in 4 days
When saturated becomes reduced in 50 days
WHY?
CARBON
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Relationship between saturation index and lt 2
chroma at different depths
18 in.
24 in.
30 in.
36 in.
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What about other Redoximorphic Features?

• Concentrations, and
• gt 2 Chroma Depletions

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Duration of saturation and depth to feature
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10
12
10
14
14
14
17
20
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Stallings
Wet-Foreston
Foreston
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12
10
12
12
10
14
14
14
12
17
20
27
Stallings
Wet-Foreston
Foreston
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Interpretation Rule 5

• Longer periods of saturation are needed to form
  redox features in coarser textured soils

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Duration of saturation and texture
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5
12
14
18
17
15
27
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Clayey
Coarse-loamy
80
Sandy
40
12
12
18
Coarse-loamy
80
Sandy
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Interpretation Rule 6

• Stratified soils (fine over sand) can
• form redoximorphic features
• when unsaturated.

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Fe pore linings and masses
A
Ag
C1
Depletion
C2
Oxidation of mineral grains
C3
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Interpretation Rule 7

• If you need to know how long a
• horizon is saturated in a year,
• monitor water tables with a well.

You cant tell how long a horizon is saturated
just by looking at color
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Question

• What if you think the colors are relict or
  lithochromic?

Answer Monitor the site
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Question

• Can relict features and/or lithochromic mottles
  occur in the same horizon as redox features?

Answer Yes
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Redox depletion
Lithochromic mottle
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Summary

• Redoximorphic features are used to identify soils
  that are saturated and reduced.
• The features dont tell us how long a soil is
  saturated unless they have been calibrated using
  water table measurements.

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Summary

• The features need five conditions to form
  organic matter, active bacteria, saturation,
  stagnant water, and Fe.
• Water tables rise above depth at which low chroma
  redox features occur.

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Morphology
Mottles
Redoximorphic features
3 4 chroma depletions
lt 2 chroma depletions
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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So who cares about duration and separation
distance?

• What may it mean to treatment and risk?

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12
12
12
Stallings
Wet-Foreston
Foreston
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Experiment

• 17 soil columns
• Saturation at 12, 18, and 24 inches
• Simulated wastewater applied
• LTAR 0.3 gpd/ft2
• Water sample at top of saturated zone analyzed
  for fecal coliform

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Soil Surface
12 Sampling port
Saturation
18 Sampling port
24 Sampling port
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Question 1

• Is 12 inches (30 cm) an adequate separation
  distance?

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Question 2

• Is 24 inches (60 cm) an adequate separation
  distance?

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Question 3

• If saturation was at 12 inches and fecal coliform
  did survive will increasing the separation to 24
  inches improve treatment?

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Conclusions

• Redoximorphic features indicate a water table
  shallower than 2 chroma features
• 2 chroma depletions correspond to different
  cumulative saturation depending on depth and
  texture
• As depth increases, 2 chroma depletions
  represent longer cumulative duration of
  saturation
• 2 chroma depletions in sandy soils represent
  longer cumulative duration of saturation
• 12 inches of separation may not be sufficient to
  remove fecal coliform

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WE JUST NEED MORE DATA