Hydric Soils Technical Note 6
Field Indicators for Discharge and Recharge Hydric Soils
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Discharge hydric soils release ground water to the land surface through springs, seeps, and other discharge zones. Recharge hydric soils transmit water to the ground water/aquifer and to discharge hydric soils. Hydric soils in the humid southeastern and eastern United States generally are discharge hydric soils; however, they may function as season dependent recharge systems. Both recharge and discharge hydric soils exist in the sub humid midwest, southwest, and west parts of the United States. The significance to hydric soils is that discharge systems generally have different morphological indicators than recharge systems.
Discharge Hydric Soil Indicators
Classic discharge hydric soils have morphologies that reflect water moving to the soil's surface. This water carries materials, such as reduced Fe, and these materials become part of the soil. Discharge hydric soils below a depth of about 0.5 m often lack morphological evidence of saturation; most often because of additions of Fe and low available organic matter needed for microbial activity. The following are examples of discharge hydric soil indicators: A5 (Stratified Layers), TA1 (Playa Rim Stratified Layers), F3 (Depleted Matrix) where the depleted matrix is the near surface, F8 (Redox Depressions), F9 (Vernal Pools), F12 (Iron/Manganese Masses), F16 (High Plains Depressions), TF2 (Red Parent Material), and TF8 (Redox Spring Seeps).
Recharge Hydric Soil Indicators
Recharge hydric soils are wettest at the surface and remain wet there longer than discharge hydric soils. The amount of organic matter and microbial activity is very high and these hydric soils have maximum expressions of anaerobiosis. Recharge activities often leach soils, creating acidity. The acidity may be reflected in plants that produce tannin. Tannins in turn create organic surfaces that aid in holding water for anaerobiosis. Classic recharge indicators include A1 (Histosols), A3 (Black Histic), S2 (Sandy Gleyed Matrix), F2 (Loamy Gleyed Matric), F3 (Depleted Matrix) where the depleted matric is not the near surface and is continuous, F5 (Thick Dark Surface), and TF7 (Thick Dark Surface 2/1).
Seasonally Dependant Hydric Soil Indicators
Indicators not specified as one of the discharge or recharge hydric
soil indicators above have either discharge/recharge dependent
morphologies or they are for hydric soils that function as season
dependent discharge and recharge hydric soils. For example, indicator F6
(Redox Dark Surface) occurs in both discharge and recharge hydric soils.
In recharge hydric soils, the layer immediately below the dark surface
should have a depleted or gleyed matrix. In discharge hydric soils, the
depleted/gleyed matrix may be absent below the dark surface. Another
example is F3 (Depleted Matrix). Where F3 is a thin near surface
phenomena, the wetland most often functions as a discharge system. Where
the indicator is thick, highly leached, and continuous the wetland most
often functions as a recharge system. It is recommended that delineators
evaluate the hydrologic source and examine soils accordingly.