Environmental Engineering Reference
In-Depth Information
with a water-repellent substance (e.g., Saran solution) and weighed twice, first in air
and then in water. For a complete description of this procedure, the reader is referred
to Blake and Hartge (
1986
). A major limitation to this approach is that typically the
clods are extracted from a pit face which is not always feasible in wetland settings.
7.10 Sediment Gains and Losses
7.10.1 Overview
Because of ambiguity in terminology, it is important to preface this section with
working definitions for the processes addressed. Accretion is the accumulation of
sediment at a particular point; it could be sediment transported from an upslope point
or sediment derived at the point itself. The accumulation of detrital material is
commonly referred to as organic matter accretion, while sediment accretion refers
to the accumulation of both organic and inorganic materials (U.S. EPA
2008
).
Deposition refers to sediment accumulation at a particular point due to sediment
dropping out of flow when the amount of sediment in runoff exceeds the transport
capacity of runoff. Erosion is the removal of sediment from a particular point.
Subsidence is the decrease in the sediment surface elevation caused by an increase
in the bulk density of sediment or a loss of sediment mass through oxidation of
organic matter. Loss of sediment through erosion is not considered to be subsidence.
For the most part, the techniques presented in this section were developed for use
in tidal marshes where significant sediment gains and losses are inherent to ecolog-
ical functioning. However, with at most minor modification, they could be used in
less hydrodynamic wetlands. The approaches inherent to all of the techniques
presented here are based on either sampling and weighing sediment, or measuring
the deviation in elevation relative to a fixed elevation point. All of the methods
except for Cs137 techniques require multiple visits to the field sites.
7.10.2 Artificial Marker Horizons
Marker horizons provide an easy way to assess marsh accretion over a period of
months to years (Cahoon and Turner
1989
). White feldspar clay is thinly spread over
the marsh surface in replicated plots creating an artificial horizon. Over time
sediments accumulate above the artificial horizon through the process of accretion.
The depth of sediment accumulation is determined by collecting a core from the
sample plot and measuring the distance from the current marsh surface to the
artificial horizon. This method is less expensive than isotopic techniques, the cores
are simple to collect and process, and sampling success or failure is known at
collection time (Cahoon and Turner
1989
). White feldspar clay is the marker horizon
