Hydric Soils Technical Note 5 (updated 4/05)

Using Hydric Soil Indicators in Disturbed Soils.

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Introduction

Making hydric soil determinations and delineating hydric soils in areas that have been filled, dredged, land leveled, or otherwise disturbed can be a perplexing and challenging assignment.

Vegetative Disturbance

In some instances of disturbance the vegetation has been destroyed or removed; therefore, soils are the only on-site indicators of pre disturbance hydrology and the only feasible means of identifying wetlands. Where upturned soil disturbance is recent, sufficient clods of various soil horizons may remain that will aid experienced soil scientists in verifying the original soil morphology. Pre disturbance soil surveys should be consulted where available. Undisturbed areas in the vicinity may be investigated to provide information of pre disturbance soil morphology. Small areas of unaltered soil may be found at the base of remaining trees; however, most frequently, the disturbance is more extreme. Fill materials spread on disturbed sites usually compound the difficulties of making hydric soil determinations. Guidelines have been established to determine the hydric status of disturbed soils after varying amounts of fill materials have been added.

Guidelines

The guidelines established are for recently filled areas. Nonhydric sandy fill material that has been in place for more than about 3 to 5 years and nonhydric loamy/clayey fill material that has been in place for more than about 8 to 10 years should first be evaluated utilizing the protocol outlined in Field Indicators of Hydric Soils in the United States. If one or more of the Indicators is present in the resulting new soil at the required depth the soil is hydric. If no indicator is present at the required depth the guidelines outlined below should be followed.

Filled Disturbance

Hydric soil requirements are the same in disturbed areas as for areas that have not been disturbed. Most significantly, the hydric soil definition must be satisfied. This is normally exemplified by the presence of a Hydric Soil Indicator. The amount of fill that can be placed on a hydric soil and that soil still be hydric is directly related to the Hydric Soil Indicator and Hydric Soil Criteria present before filling. The criteria are important because some criteria are handled differently than others. Although the areas that meet criteria 1, 3, and 4 usually have a hydric soil indicator, this is not a requirement. According to the deliberations of the NTCHS, areas that satisfy criteria 1, 3, or 4 are considered to be hydric with or without the presence of an indicator (Federal Register, February 24, 1995). It is important to understand that the criteria must be met based on actual data or best professional judgment and not the estimated soil properties such as those found in the National Cooperative Soil Survey publications.

Filled: Criteria 1

For areas that meet Hydric Soil Criteria 1 (Histels,except Folistels, and Histosols, except Folists), fill can be placed on the soil surface to the extent that the soil, after the placement of the fill, still meets the taxonomic requirements of Histosols in Soil Taxonomy (Soil Survey Staff, 1999). Therefore, the maximum amount of fill material that can be added to a hydric Histosol and that soil retain its hydric status is 16 inches (24 inches if 3/4 or more of the organic soil material is moss fibers). This would be for hydric Histosols that have organic soil material starting at the soil surface that is 16 or more inches thick (24 or more inches thick if 3/4 or more of the organic soil material is moss fibers). For Histosols with thinner organic layers and Histels to maintain their hydric status the thickness would be less. Additional information concerning this and all other indicators developed to help identify and delineate hydric soils and approved for such use by the NTCHS are in "Field Indicators of Hydric Soils in the United States".

Filled: Criteria 3 and 4

For soils that are frequently ponded for long or very long duration during the growing season (Criteria 3), or soils that are frequently flooded for long or very long duration during the growing season (Criteria 4) to maintain their hydric status after filling the added fill may be very thick or very thin. The thickness of the fill must be slightly less than the height of frequent ponding or flooding of long duration (more than 7 days). This height may be either measured or estimated. If estimated, professional judgement that the definition is met (anaerobiosis) must be carefully exercised. Although any of the indicators may occur on inundated landforms, indicators F8 (Redox Depressions), F9 (Vernal Pools), F11 (Delta Ochric), F12 (Iron/Manganese Masses), and F16 (High Plains Depressions) are restricted to inundated landforms.

Filled: Criteria 2

For other soils that are hydric due to saturation (Criteria 2) an indicator must be present. The depth of fill that can be placed on these soil in order to maintain their hydric status is variable. The range is from slightly less than 6 inches to zero inches in soils with sandy soil materials and the range is from slightly less than 12 inches to zero inches in other soils. After fill materials are added an indicator must be present in the original soil material within the prescribed depths in order for that soil to retain its hydric status. The table below can be used to determine the depth of fill material that would adversely affect the hydric status of a soil that is hydric due to saturation. This table is not to be used for indicator A1 and the indicators restricted for use to inundated landforms (F8, F9, F11, F12, and F16).

Hydric status of soils with varying amounts of fill.

¹ See "Field Indicators of Hydric Soils in the United States" for additional information concerning use of All (A), Sandy (S), and Loamy or clayey (F) indicators.
 
² The Hydric Status given is based on the presence or absence of an indicator. Even if an indicator is absent a soil may well be hydric if according to NTCHS guidance the definition is met.

³ Depth and thickness would be 10 inches if the indicator present is F3 (Depleted Matrix).

⁴ Thickness would be 4 inches if the indicator present is F3 (Depleted Matrix).

Soils with an indicator starting depth that is intermediate to those listed in the table above can have an intermediate amount of fill without changing the hydric status of the soil. For example, a soil with a stripped matrix starting at 4 inches can have up to 2 inches of any type fill material placed on the surface without changing the hydric status of the soil. Conversely, more than 2 inches of fill would change the status of the soil to not hydric. To determine the hydric status of land leveled areas the same procedure as outlined above is used. Soils that are hydric due to criteria 1, 3, or 4 prior to land leveling are evaluated after land leveling to determine their hydric status. Soils that are hydric due to saturation prior to land leveling are evaluated by applying the guidelines outlined above to determine their hydric status.

Cautions

These guidelines are applicable to the identification of the hydric status of all disturbed soil areas regardless of whether the disturbance was intentional or unintentional. Intentional disturbance may alter the hydric states of a soil; however, the disturbance may be a violation of federal, state, and/or local rules and regulations and the area may be required to be restored to its pre disturbed condition or the disturbance mitigated. Unintentional disturbance (such as erosional deposition) also may alter the hydric status of a soil; however, the disturbed area, if it originally had hydric soils, may still be eligible for federal, state, and local programs such as USDA's Wetland Reserve Program.
 

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