Environmental Engineering Reference
In-Depth Information
(Stolt
2005
) as evidenced by a dark gray to black coating. Rabenhorst et al. (
2010
)
modified the IRIS tube technology to produce IRIS panels for assessing soluble
S
2
in brackish marshes. The technology can be used for a qualitative assay to
detect the presence of soluble S
2
, or a quantitative assay to determine
S
2
concentrations. A simple color change indicates the presence of soluble S
2
,
and the tube shape is sufficient. A quantitative assay requires a set of standards and
image analysis capabilities. The standards correlate the intensity of the color
change to a known S
2
concentration. Image analysis is used to quantify the surface
area of the tube or panel that represents an individual intensity of color change.
Rabenhorst et al. (
2010
) decided to use large flat panels rather than the cylindrical
tubes to facilitate quick recording of the images with a flatbed scanner. In contrast
to the pore water sampling approach (sippers and peepers), this new technology
shows the potential for generating quantitative information on S
2
concentrations
with millimeter-scale spatial resolution. An additional benefit is the time required
to obtain data is restricted to a couple of hours.
Using specially prepared Fe oxide paint (Rabenhorst and Burch
2006
), the lower
50 cm of PVC panels or tubes (usually 60 cm in length) are painted. These are then
inserted into the marsh soil for periods of 5 or 60 min. The FeS phase is unstable
under oxidizing conditions, and the dark color fades over a period of minutes to
hours when exposed to the air. Therefore, collection of the images (either by
scanning or by photography) must be done quickly. Images are then compared
with standard images prepared from painted PVC chips placed into Na
2
S solutions
(adjusted to pH 7.5) of known concentration (range: 3-300 mg/L S
2
) for set
periods of time (usually 5 or 60 min.) For more detail, the reader is referred to
Rabenhorst et al. (
2010
).
7.8 Oxidation-Reduction Processes in Soils
7.8.1 Overview
The formal definition of wetlands explicitly refers to the prevalence of “saturated
soil conditions” (U.S. Army Corps of Engineers Environmental Laboratory
1987
).
These “saturated soil conditions” of wetlands are specifically known as hydric soils,
which are defined as having “formed under conditions of saturation, flooding, or
ponding long enough during the growing season to develop anaerobic conditions in
the upper part” (Federal Register
1994
). Organic materials in soils (mostly plant
remains) are routinely decomposed by heterotrophic microorganisms as an energy
source, and during this oxidation reaction where electrons are lost from the
oxidized C, some other compound must serve as an electron acceptor and receive
or “gain” the electrons (thus, being electrochemically reduced). When available in
the soil solution, O
2
is usually the preferred electron acceptor. In saturated soils,
however, dissolved O
2
can become depleted during microbial oxidation of organic
