Environmental Engineering Reference
In-Depth Information
4.12.4 Hydric Soils Technical Standard
For a problematic site that requires monitoring to determine the presence of a hydric
soil, the Hydric Soils Technical Standard (NTCHS
2007
) is used as guidance for
what data must be collected to satisfy the requirements of a hydric soil. In addition,
the Hydric Soils Technical Standard can be used to:
1. evaluate the function of wetland restoration, mitigation, creation, and
construction,
2. evaluate onsite the current functional hydric status of a soil, and
3. with appropriate regional data, modify, validate, eliminate, or adopt Field
Indicators for the region.
The Hydric Soils Technical Standard includes requirements to determine that the
soils are saturated, ponded, or flooded through water table monitoring and proof
that the soils are anaerobic and reducing. Saturation (or inundation) and anaerobic
conditions must be present for at least 14 consecutive days. It should be noted that
the growing season is assumed to have started when the soil goes anaerobic since
the conditions occur when soil microbes are active. Saturation is confirmed by the
presence of free water in a piezometer installed to a soil depth of 25 cm (10 in.).
Anaerobic conditions are confirmed by direct measurement of Eh, alpha, alpha-
dipyridyl dye, or IRIS tubes. Refer to Chap.
7
,
Wetland Biogeochemistry
Techniques
, for more details on confirmation of anaerobic conditions. For more
information on the use of the Hydric Soil Technical Standard see the NTCHS
Technical Note 11 at
ftp://ftp-fc.sc.egov.usda.gov/NSSC/Hydric_Soils/note11.pdf
.
4.12.5 Normal Rainfall
Any data collected to evaluate hydric soils should be correlated to rainfall. Normal
rainfall data, for wetland purposes, are available in NRCS National Weather and
Climate Center WETS (wetlands determination) tables. WETS tables are produced
for local weather stations throughout the United States. They can be accessed at
http://efotg.sc.egov.usda.gov//efotg_locator.aspx
. Pick your state and then county
of interest. The Field Office Tech Guide menu tree will appear. Pick Section II from
the drop down menu, then open the climate tab, and select AgCIS (Climate
Information System). Select WETS as the product and then it will give you a list
of weather stations that are available for that area. Select the weather station most
appropriate for your location and then go and the WETS table will be generated.
To evaluate if a given year has had normal precipitation, local rainfall data
(either from a local weather station or from an onsite rain gauge) are compared to
data in the geographically appropriate WETS table. Rainfall is normal for any given
month if the amount of rain falls between the values for that month in the columns
“30 percent chance will have less than” and “30 percent chance will have more
