An Explanation of the Hydric Soil Technical Standard and Normal Precipitation

Transcription

1 An Explanation of the Hydric Soil Technical Standard and Normal Precipitation From: National Technical Committee for Hydric Soils Hydric Soil Technical Standard (Technical Note 11). e/hydric/hstn.htm

2 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.

3 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.

4 Uses for HSTS: a. Evaluate the hydric status of restoration, mitigation, creation, and construction projects

5 Uses for HSTS: b. Site Specific: Hydric or Nonhydric c. Indicator Specific: Valid, Invalid, or Revise, or Add

6 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.

7 Standard Requirements The standard requires that two conditions be met: 1. Anaerobic Conditions and 2. Saturated Conditions There are several sub-requirements for each.

8 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.

9 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).

10 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.

11 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.

12 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.

13 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)

14 Eh Eh/pH Line for Determining Aerobic or Anaerobic Conditions Aerobic Conditions Exist Eh = (7-pH) Anaerobic Conditions Exist 0 ph3 ph4 ph5 ph6 ph7 ph8 ph9

15 Minimum Eh Values for Anaerobic Conditions at Specific phs Eh (energy in mv) phs (H ion concentration)

16 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.

17 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 Eh 200 Oxygen is reduced Iron is not reduced Anaerobic conditions Line Iron is reduced 0 ph4 ph5 ph6 ph7 ph8

18 Aerobic or Anaerobic? 5 Electrodes installed at 12.5 cm in sands (would be at 25 cm or 10 cm for other soil conditions).

19 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.

20 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.

21 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.

22 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.

23 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).

24 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.

25 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.

26 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.).

27 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.

28 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.

29 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).

30 Saturated or Unsaturated? 2 Piezometers at 25 cm 2 Piezometers at 100 cm Open well to a depth of 2 meters

31 Measurement Period Recommended measurement period is one year. Minimum measurement period captures a dry (moist)-wet-dry (moist) cycle. Dry-wet-dry cycles vary. For example: For most of the southeast this would be November - June. For peninsular Florida this would be May - November. For much of California this would be December - May.

32 Duration For at least 14 consecutive days, Anaerobic Conditions (confirmed by voltage readings below the Eh/pH line or iron removal from IRIS tubes or positive Dipyridyl reaction) and Saturation Conditions must exist for a soil to be considered hydric. For Vertisols in Texas and Louisiana the minimum time period is 7 consecutive days for a total of 28 annual days.

33 Frequency Frequency must be more than 50% (more than 1 in 2 years). One method is approved to evaluate precipitation (adapted from Sprecher and Warne, 2000): Precipitation data for the three months prior to the most saturated and reduced period are evaluated as well as the month during data collection.

34 Frequency Evaluation The frequency requirement is assumed to have been met if precipitation for the three months prior to the most saturated and reduced period is normal and the precipitation during the month of data collection is within one standard deviation average precipitation. Normal is defined as the percentile probability of occurrence.

35 30-70 percentile probability data are available from the WETS web site at Select a state Select a county Select Section II from the drop down menu Open the climate tab Select AgCIS Select WETS Select a weather station

36 Normal Calculation Prior Month WETS Rainfall Percentile Measured Rainfall Condition: Dry, Wet, Normal Condition Value Month weight Multiply Previous two Name 30 th 70 th (1=dry, columns inches =normal, or 3=wet) 1 st (most July Normal recent) 2nd June Normal rd May Wet Sum 13 Rainfall of prior period was: drier than normal (sum is 6-9), normal (sum is 10-14), wetter than normal (sum is 15-18) Was the Was 16 the to requirement to 84 requirement for August for met? August If yes met? normal is met.

37 On Site Precipitation Data If one were trying to prove a site nonhydric, in order to be able to apply data collected (Eh, dye, IRIS, and saturation data) to the HSTS on-site precipitation have to fall within or above the normal range during the dry-wet-dry cycle for the area. If one were trying to prove a site hydric, in order to be able to apply data collected (Eh, dye, IRIS, and saturation data) to the HSTS on-site precipitation have to fall within or below the normal range during the dry-wet-dry cycle for the area.

38 Where in this soil would platinum electrodes be installed to determine whether Anaerobic Conditions are met or not? EH data are collected at the same depth. Would there be differences if the soil material was sandy or loamy/clayey? 12.5 cm sand loam/ clay

39 Platinum Electrodes Where? The soil in the previous slide has, at the location indicated by the upper bar, about 8 cm of mucky peat underlain by about 8 cm of muck (lower bar). As depicted in the slide, the underlying mineral soil material may be sandy or loamy/clayey. The two upper arrows on the left (beneath the scale arrow) indicate where the electrodes would be located if a person was trying to prove the hydric status of the soil and the soil material was sandy and mucky peat was a concern (upper of the two arrows) or mucky peat was not a concern (the lower of the two arrows). The two lower arrows indicate where the electrodes would be located if a person was trying to prove the hydric status of the soil and the soil material was loamy/clayey and mucky peat was a concern (upper of the two arrows) or mucky peat was not a concern (the lower of the two arrows). The location of electrodes is dependant upon the texture of the soil.

40 Where in this soil would piezometers be installed to determine whether Saturated Conditions are met or not? Would there be differences if the soil material was sandy or loamy/clayey? 25 cm

41 Piezometers Where? The soil in the previous slide has, at the location indicated by the bar and scale, about 15 cm of fill material all soil material is sandy or organic (mucky peat). The upper arrow on the left indicates where the piezometer would be located if a person was trying to prove the hydric status of the new soil (taking into account the fill material) the depth would be 25 cm from the surface of the new soil. The middle arrow indicates where the piezometer would be located if a person was trying to prove the hydric status of the soil and mucky peat was a concern. The depth would be 25 cm from the surface of the mucky peat. The lower arrow indicates where the piezometer would be located if a person was trying to prove the hydric status of the soil and mucky peat was not a concern (most of the US). The depth would be 25 cm from the surface of the mineral soil. The location of piezometers is not dependant upon the texture of the soil.

42 IRIS Tubes Availability IRIS tubes available from: InMass Technologies,

43 Summary You are now aware there are three 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, and (3) it meets the Hydric Soil Technical Standard (HSTS). Although the low probability of any given site have normal climatic conditions limits the applicability of the HSTS, a technically correct method to update field indicators of hydric soils in the United States had to be developed This method is available for use.

44 Literature Cited Vasilas, Hurt, and Noble Field indicators of hydric soils in the United States (Version 7.0), USDA, NRCS, Fort Worth, TX. National Technical Committee for Hydric Soils Hydric Soil Technical Standard (Technical Note 11).