An Explanation of the Hydric Soil Technical Standard and “Normal” Precipitation From: National Technical Committee for Hydric Soils. 2001. Hydric Soil Technical Standard (Technical Note 11). http://soils.usda.gov/soil_us e/hydric/hstn.htm
Introduction You are currently aware there are two methods that a soil can be termed a hydric soil (1) it has a hydric soil indicator and (2) it frequently floods or ponds for long or very long duration and has Anaerobic Conditions. This lecture explains the third method; meeting the Hydric Soil Technical Standard (HSTS). The concepts of Anaerobic Conditions and Saturated Conditions and the reasons in situ pH and on site precipitation data are needed to apply the HSTS. are explained. The concept of and problems with “Normal” precipitation is also explained.
NTCHS Recommendation for Application of the HSTS The NTCHS recommends that the HSTS be used to: a. evaluate the function of wetland restoration, mitigation, creation, and construction, b. evaluate onsite the current functional hydric status of a soil, and c. with appropriate regional data modify, validate, eliminate, or adopt hydric soil field indicators for the region.
Uses for HSTS: a. Evaluate the hydric status of restoration, mitigation, creation, and construction projects
Uses for HSTS: b. Site Specific: Hydric or Nonhydric c. Indicator Specific: Valid, Invalid, or Revise, or Add
Uses for Standard According to NTCHS, the Hydric Soil Technical Standard (HSTS) can be used 1.) To evaluate whether wetland restoration, mitigation, creation, and construction projects are successful or not, or 2.) Determine onsite the hydric status of a soil, or 3.) To validate, invalidate, revise, or add hydric soil indicators regionally. If the soil meets an indicator, the HSTS cannot be used to exclude the soil from being considered a hydric soil on a site specific basis on hydrologically altered sites.
Standard Requirements The standard requires that two conditions be met: 1. Anaerobic Conditions and 2. Saturated Conditions There are several sub-requirements for each.
1: Anaerobic Conditions Option 1: Confirmed by Redox Potential (Eh) data (Platinum Electrode data), or Option 2: Confirmed by IRIS tube data (PVC pipe coated with iron hydroxide paint), or Option 3: Confirmed by Reduced Iron (FE ++ ) data (alpha-alpha-Dipyridyl) In-situ pH data and on-site precipitation data are also needed.
Anaerobic Conditions; Option 1 5 electrodes are installed at 25 cm in most loamy and clayey soils, 12.5 cm in sandy soils, 10 cm in soils that inundate but do not saturate to a significant depth. Electrodes are installed at the appropriate (25, 12.5 or 10 cm) depth measured from muck or mineral surface (with some exceptions as explained later).
Anaerobic Conditions; Option 2 5 IRIS tubes are installed vertically so that at least 30 cm of the hydroxide painted portion is in contact with undisturbed soil at the appropriate depth (see below). IRIS tubes should be inspected during periods the soil is saturated, ponded, or flooded. Inspection must be frequent enough to quantify the amount of iron removal that occurred during a single continuous period of saturation, ponding, or flooding.
Anaerobic Conditions; Option 3 3 replicate samples within electrode installation depth (exact depths and thickness requirements are explained below) are tested by colorimetric dye such as Alpha-Alpha-Dipyridyl .
Interpreting for Anaerobic Conditions; Option 1 For a soil to meet the Anaerobic Conditions part of the standard, Eh measurements of < 175 mv at pH 7 must exist. Eh requirements are adjusted for pH on a line with a slope of negative 60. This Eh-pH line is used for soils with pH values of 3-9. It was not developed for any specific mineral species and is actually somewhere between Iron and Manganese reduction.
Eh/pH Line The Eh/pH Line defined in the previous slide may seem confusing to understand, however, the next two slides should made the definition clear. The first is the actual graft of the line. The second is the definition explained in table format with the minimum Eh (energy in millivolts) values requires for a soil to have anaerobic conditions at specific pHs (Hydrogen ion concentration)
Eh/pH Line for Determining Aerobic or Anaerobic Conditions 500 Aerobic Conditions Exist 400 Eh = 175 + 60 (7-pH) 300 Eh 200 100 Anaerobic Conditions Exist 0 pH3 pH4 pH5 pH6 pH7 pH8 pH9
Minimum Eh Values for Anaerobic Conditions at Specific pHs Eh (energy in mv) pHs (H ion concentration) 415 3.0 385 3.5 355 4.0 325 4.5 295 5.0 265 5.5 235 6.0 205 6.5 175 7.0 145 7.5 115 8.0 85 8.5 55 9.0
Iron and Oxygen Reduction Lines VS Anaerobic Conditions Line The next slide shows the relationships of Anaerobic Conditions to the reduction of Iron (green line) and Oxygen (red line); slide 27 of redox chemistry lecture. Anaerobic Conditions (blue line) on the reduction scale is between Iron and Oxygen.
Iron and Oxygen Reduction Lines VS Anaerobic Conditions Line Eh/pH Graph for Determing Redox State for Oxygen and Iron 600 Oxygen is not reduced 400 Oxygen is reduced Eh Iron is not reduced Anaerobic conditions Line 200 Iron is reduced 0 pH4 pH5 pH6 pH7 pH8
Aerobic or Anaerobic? 5 Electrodes installed at 12.5 cm in sands (would be at 25 cm or 10 cm for other soil conditions).
In-situ pH Data Since soils, as they become saturated, tend to have pH values that move toward neutral (pH 7), in-situ pH value are used to locate the precise point on the Eh/pH line. pH is measured on a saturated paste in-situ. Water pH can be used if it is shown similar results are obtained.
Interpreting for Anaerobic Conditions: Option 2 Saturated, ponded, or flooded soils: A soil meets the Anaerobic Conditions part of the standard when at least 3 of 5 IRIS tubes have iron removed from 30% of a zone 15 cm long. Top of zone of iron removal must be within 15 cm of the soil surface for all soils.
These IRIS tubes were removed from a upland to wetland transect 21 days after installation. The two tubes on the right meet the Anaerobic Conditions part of the HSTS based on iron removal. The two on the left fail to meet the Anaerobic Conditions part of the HSTS based on iron removal.
Interpreting for Anaerobic Conditions: Option 3 For a soil to meet the Anaerobic Conditions part of the standard a positive reaction to alpha-alpha-Dipyridyl is the dominant (60% or more) condition of a specific layer (at least10 cm of the upper 30 cm, at least 1/2 of the upper 12.5 cm, or at least 1/2 of the upper 10 cm) for at least 2 of the 3 required samples.
Positive Reaction to alpha- alpha-Dipyridyl A positive reaction to alpha-alpha-Dipyridyl is required for at least10 cm of the upper 30 cm for most loamy/clayey soil material if the material is not sandy to a depth of 12.5 cm. The positive reaction is requires for at least1/2 of the upper 12.5 cm in sandy soils. Soils that require a positive reaction for at least 1/2 of the upper 10 cm are mainly Vertisols and Vertic subgroups, occupy specific landforms such as Vernal Pools, or have a specific indicator (F8, Redox Depression).
A positive reaction to alpha-alpha- Dipyridyl is indicated by a pink/red color. See Hydric Soil Technical Note 8 for proper use of the dye.
Preferred Option for Determining Anaerobic Conditions? There is no preferred method for determining Anaerobic Conditions. The NTCHS has approved all three methods; however, local conditions may dictate which method is used. For example alpha-alpha-Dipyridyl may not work in soils low in iron such as in some sandy soils and either Option 1 or Option 2 must be used. Also, in soils shallow to hard bedrock, correct placement of Platinum Electrodes and IRIS Tubes may be impossible and, therefore Option 3 (alpha-alpha-Dipyridyl) is the only option available.
Growing Season The classical concepts of “Growing Season” is not considered (28 degrees, leaf buds, etc.). NTCHS considers that Anaerobic Conditions (as confirmed by Eh and pH data) occur only when soil microbes are active (for wet soils this is throughout the year for most of the U.S.).
2: Saturated Conditions A. Confirmed by piezometer data. B. NTCHS recommends that the piezometer data be verified by open well data. C. On-site precipitation data are needed.
2: Saturation Measurements For Vertisols in Louisiana and Texas, 3 piezometers at 25 cm and 3 piezometers at 100 cm are installed. All are measured at least weekly. For all other soils, one open well to 2 m (preferably auto-recording), 2 piezometers at 25 and, 2 piezometers at 100 cm are installed. All are measured at least weekly.
2: Interpreting for Saturated Conditions For a soil to meet the Saturated Conditions part of the standard, free water has to exist within both of the shallowest piezometers (25 cm).
Saturated or Unsaturated? 2 Piezometers at 25 cm 2 Piezometers at 100 cm Open well to a depth of 2 meters
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