Environmental Engineering Reference
In-Depth Information
soils will have consistently low chromas with little change with depth. In an
endosaturated soil that is rarely inundated, matrix chromas gradually decrease to
2 directly above a horizon with high
chromas indicates episaturation. In most cases, a physical distinction (change in
texture or structure) between the two adjacent horizons will be apparent.
2 with depth. A horizon with chromas
4.13.1.2 Monitoring Well Installation
A soil description should accompany the installation of monitoring wells or
piezometers. Soil characteristics can impact the proper depth of well installation
and may be needed to interpret the well data. For these purposes, the most important
characteristics are color, texture, and structure. The identification of soil horizons
that may restrict water movement is critical, and episaturation should be distin-
guished from endosaturation. A common scenario is a precipitation driven
depressional wetland which maintains wetland hydrology through episaturation.
At times, there will be two water tables-a perched water table and the deeper
apparent water table. The water table may drop significantly during the growing
season but the soil close to the surface may remain saturated. Installation of a well
to a depth below the perched zone will result in misleading data as the wetland will
appear to have a dry hydroperiod. If episaturation is suspected, it is best to install
two wells, one above and one below the horizon that is restricting water flow. Water
may perch directly above an aquitard; a soil layer that transmits water very slowly
or not at all. Aquitards can often be identified by high bulk densities or by platy or
prismatic structure. Perching can also be due to relatively small differences in
texture in adjacent horizons. Free water below the aquitard may have positive
pressure. If a well is installed through the aquitard, water will rise in the well to
an elevation above the water table, again resulting in misleading data.
4.13.2 Assessing Changes in Wetland Hydrology
4.13.2.1 Field Indicators of Hydric Soils
As stated previously, soil morphology typically reflects long term hydrologic
conditions. Draining a wetland will not cause rapid changes in morphology.
Some organic matter decomposition will occur and subtle changes in redoxi-
morphic features can occur. However, it is difficult to distinguish between drained
and undrained versions of the same soil. This is a limitation to the Field Indicators.
However, morphological stability of hydric soils can be used to determine if the
hydrology of a wetland has been altered when used in conjunction with direct
assessment of hydrology such as monitoring well data. For example, consider a
