TSSH Part 617
Water Table Determinations
“Water table” refers to a saturated zone in the soil. Estimates of the average highest water table for a soil are based mainly on observations of the water table at selected sites or on physical characteristics of the soil that are considered to be evidence of a saturated zone (typically, gray redoximorphic depletions). It may be important to establish whether a water table is perched or whether the soil is saturated throughout the profile. A perched water table is water standing above an unsaturated zone. In places an upper, or perched, water table is separated from a lower one by a dry zone.
Water table determinations may be needed for a variety of reasons. In some cases, the presence of a water table might indicate that drainage is needed in order to implement a farm practice or that a different design could be needed to compensate for the high water table. The location of a seasonal high water table may also be important in the design of cut and fill depths for some farm practices, such as wetland creation and shallow water development and management. In addition, water table depth and duration can have an effect on nutrient management plans and crop management. Water table determinations are also important for any practices that may impact wetlands in or near the project area and for compliance with Swampbuster provisions of the FSA.
NRCS programs, practices, and external customers for which water table determinations may be needed (617.01)
NRCS technical soil services for seasonal high water table determinations are done for USDA farm programs or should be performed through Federal, State, or local forms of government with which there is a memorandum of understanding or a cooperative agreement.
The most common situations for which a resource soil scientist may be called upon to do water table determinations involve the design of waste management systems and certain conservation practices. Also, if agreements are in place with local health departments, assistance may be required to identify the location of a high water table for septic system design. The design of many conservation practices may be affected by the presence of a high water table, especially when a practice requires excavation. For example, if a water table exists at a depth of 50 cm and the design of a conservation practice requires the excavation of 35 cm of soil, a wetland could be created and eventually be regulated for purposes of the Clean Water Act.
A high water table could affect practices that involve both excavation and the manipulation or storage of water, such as design for aquaculture ponds, irrigation ponds, and conveyances, such as ditches and canals; drainage water management; constructed or restored wetlands; grassed waterways; and hillside ditches. In some cases a high water table may increase the capacity needed for water storage (e.g., pond size). In other cases drainage systems may be needed to compensate for the high water table (e.g., an agricultural waste management system). Any potential drainage plans should include consideration of impacts to wetlands in or around the project area to ensure that NRCS technical or financial assistance is not used to adversely impact wetland areas.
Resource soil scientists may be asked to assist in the training of Federal, State, and local government personnel in the identification of soils that have a high water table. Personnel who may require training include those involved in wetland identification, restoration, and creation; pond or waste storage design; community planning; and evaluations for waste treatment systems.
Considerations and data collection for water table determinations (617.02)
Field determinations of water table depth are typically made through the identification of redoximorphic features in the soil profile. However, local water table monitoring should be used to calibrate these features.
Observation in a backhoe pit is recommended in cases where the water table depth is critical. It is important to remember that it may become more difficult to evaluate water table depth by use of redoximorphic features as depth increases. Lower amounts of organic matter and microbial activity lead to more subtle changes in these features. Oxyaquic conditions may occur that leave little or no indication in the soil that a water table is present. Also, as depth increases, redoximorphic features are more likely to be relict features. In some cases, wet-season observations may be needed when soil morphology is difficult to interpret.
Monitoring wells and piezometers are used to determine actual water table depths, which are then used for calibrating field indicators of a high water table. A tech note on methodology for the installation of wells and piezometers is available at the USDA NRCS Soil Science Division Web site under Technical References. Scroll to “Installing Monitoring Wells in Soils.” Shallow wells can be used in soils with no confining layers. Piezometers should be used in soils with confining layers to establish perched water table levels.
A full pedon description should be prepared when water table depths are documented. This description should include a detailed description of redoximorphic features. In some cases, parent material or environmental conditions may hinder or mask redoximorphic features that are used to identify a high water table. In some cases, soils may exhibit features, such as lithochromic mottles and white or gray parent material, that may be mistaken for redoximorphic features. Organic matter color can mask redoximorphic features and make them difficult to identify. Redoximorphic features may also be relict features that do not reflect the current soil water table. Redoximorphic features should be considered contemporary unless there is strong evidence that they reflect relict conditions.
The NRCS (Sprecher) protocol for water table monitoring should be used if monitoring is needed for:
determining the applicability of redoximorphic features for the identification of a high water table; or
verifying the presence of a high water table when environmental conditions or parent material makes it difficult to identify a high water table
Practices that may require the identification of a seasonal high water table include constructed wetlands, shallow water development and management for wildlife, wetland creation, wetland restoration, surface and subsurface drainage, agricultural waste treatment design, aquaculture ponds, drainage water management, filter strips, grassed waterways, hillside ditches, irrigation systems and water conveyance and management, land reclamation, land smoothing, lined waterways or outlets, nutrient management, open channels, ponds, precision land forming, prescribed forestry and grazing, land grading and smoothing in recreational areas, tillage management, sediment basins, spoil spreading, spring development, and water- and sediment-control basins.
One should consider the effects of any land shaping practice in areas where a water table is somewhere in the profile. It is important to ensure that any elevation reductions do not create a wetness limitation for plant growth or result in the creation of wetland hydrology.
Other information relevant to water table determinations (617.03)
The depth and duration of a water table can generally be correlated to the location and abundance of redoximorphic features in the soil profile. However, some conditions may alter the usual correspondence between soil morphology and hydrology. For example, soils or hydrology may not have been in place long enough for the development of characteristic redoximorphic features. Other problems include lithochromic red or gray parent materials; problematic environmental conditions, such as cold climates, high iron content, high pH or salinity, or oxyaquic conditions; or young soils, such as those on flood plains or those that have been recently disturbed by anthropogenic activities.
When such problems are suspected, it may be useful to calibrate redoximorphic features and other features in the soil. The studies involved may include not only a water table monitoring component but also a component to evaluate the presence of reducing conditions. Reducing conditions are an indication that the water has been there long enough for redoximorphic features to develop in the soil. These determinations can be made through direct measurements of redox potential or the use of alpha-alpha dipyridyl dye or Indicator of Reduction in Soils (IRIS) tubes. Information on measuring reducing conditions is available at here.
Understanding whether or not reducing conditions are present can be helpful in evaluating situations where expected redoximorphic features do not occur, and the identification of morphologies may be used in atypical situations.
Overall, planning activities should take into account the presence of a water table. Planning alternatives should consider impacts on water quality, effects and suitability for a land use or practice, and compliance with FSA and swamp busting.
Sprecher, S.W. 2008. Installing monitoring wells in soils. U.S. Department of Agriculture, Natural Resources Conservation Service.
U.S. Department of Agriculture, Natural Resources Conservation Service. Hydric soils.
Other useful information (617.05)
National Water and Climate Center. WETS tables.
Noble, C. 2006. Water table monitoring project design. ERDC TN-WRAP-06-02, U.S. Army Engineer Research and Development Center, Vicksburg, MS.