CWD Hydrology & Soils 2 — Questions and Answers
Question 1: Which soil horizon is most likely to exhibit redoximorphic features in a seasonally saturated wetland?
- O horizon
- A horizon
- B horizon (Correct answer)
- C horizon
Correct answer: B horizon
The B horizon typically shows redoximorphic features because it experiences prolonged saturation and reducing conditions during wet seasons.
Question 2: What is the primary mechanism by which iron concentrations (mottles) form in wetland soils?
- Oxidation of ferrous iron upon re-aeration (Correct answer)
- Leaching of ferric iron downward
- Biological uptake of iron by plant roots
- Precipitation of iron from groundwater
Correct answer: Oxidation of ferrous iron upon re-aeration
Iron concentrations form when ferrous iron (Fe2+), mobilized under anaerobic conditions, is oxidized to ferric iron (Fe3+) and precipitates upon re-aeration.
Question 3: A wetland site shows a water table consistently at 15 cm below the surface for 8 consecutive weeks during the growing season. Does this meet the USACE inundation/saturation threshold for wetland hydrology?
- Yes, because saturation within 30 cm for 5% of the growing season is sufficient
- No, because the water table must be at the surface
- Yes, because saturation within 30 cm for 14+ consecutive days during the growing season qualifies (Correct answer)
- No, because only inundation (not saturation) counts
Correct answer: Yes, because saturation within 30 cm for 14+ consecutive days during the growing season qualifies
The USACE threshold requires saturation within 30 cm of the surface for 14 or more consecutive days during the growing season.
Question 4: Which of the following is a primary indicator of wetland hydrology under the 1987 USACE Wetland Delineation Manual?
- Presence of hydric soil map units on county soil surveys
- Surface water present for more than 5 days
- Recorded inundation or saturation data from a staff gauge or well (Correct answer)
- Occurrence of obligate wetland plants alone
Correct answer: Recorded inundation or saturation data from a staff gauge or well
Recorded inundation or saturation data from a staff gauge or monitoring well is a primary hydrologic indicator under the 1987 manual.
Question 5: What does a gleyed soil matrix (low chroma, bluish-gray color) indicate about soil conditions?
- Prolonged aerobic conditions with high organic matter
- Prolonged anaerobic reducing conditions causing iron reduction (Correct answer)
- Rapid drainage and oxidation cycles
- High calcium carbonate content
Correct answer: Prolonged anaerobic reducing conditions causing iron reduction
A gleyed matrix results from prolonged anaerobic conditions where iron is reduced to its ferrous form, producing bluish-gray or greenish-gray colors.
Question 6: Which hydrologic feature is considered a secondary (not primary) indicator of wetland hydrology?
- Inundation visible in aerial photographs
- A staff gauge reading showing surface water
- Oxidized rhizospheres around live roots (Correct answer)
- A continuous water-level record from a monitoring well
Correct answer: Oxidized rhizospheres around live roots
Oxidized rhizospheres are secondary indicators because they reflect indirect evidence of hydrology rather than direct measurements.
Question 7: In a wetland with a fluctuating water table, what causes iron depletions (low-chroma zones) to form adjacent to iron concentrations?
- Iron is leached downward by percolating rainwater
- Iron migrates from reduced zones and precipitates in aerated microsites, leaving depleted areas behind (Correct answer)
- Plant roots actively exclude iron from the rhizosphere
- Iron is converted to carbonates and precipitates as calcite
Correct answer: Iron migrates from reduced zones and precipitates in aerated microsites, leaving depleted areas behind
Under fluctuating redox conditions, iron mobilized in reduced zones migrates and oxidizes in aerated microsites, leaving behind low-chroma (depleted) zones.
CWD Hydrology & Soils 2
Which soil horizon is most likely to exhibit redoximorphic features in a seasonally saturated wetland?