Environmental Engineering Reference
In-Depth Information
potential less than
100 mV) conditions. Methane is oxidized to CO
2
by a group of
aerobic bacteria (methanotrophs). Although the two competing microbial processes
require extremely different redox potentials, both can occur simultaneously in the
same soil. For example, CH
4
produced in an anaerobic subsurface horizon can
diffuse into an aerobic surface horizon where it is converted to CO
2
. Therefore, field
attempts to quantify CH
4
emissions in actuality measures net CH
4
fluxes.
Many of the techniques for assessing gaseous emissions from wetlands work
equally well for CO
2
as for CH
4
. Most of the techniques depend on chamber-based
approaches introduced in
Soil Respiration
above. One major difference is in the
options for measuring the concentrations of the respective gases in the sample.
Portable infrared gas analyzers are commercially available for the instantaneous
measurement of CO
2
in the field. Equivalent technology is not readily available for
CH
4
, so gas samples must be stored and transported to the lab for analysis of the gas
through gas chromatography.
7.4.6.2 Static Closed Chambers
The method presented here employs a static-chamber approach to measure CH
4
evolution from soils. It is discussed in detail by Weishampel and Kolka (
2008
). This
approach may be used for other gases (e.g., CO
2
) evolving from soil. However,
there are subtle differences in the design, construction and deployment of chambers
depending on the target gas. The reader is referred to Livingston and Hutchinson
(
1995
) for a discussion of these factors. It is conducive to spatially intensive
sampling exercises in wetlands or uplands. This method employs a static enclosure
system comprised of collars that are permanently installed in the ground and
portable chambers designed for syringe sampling that fit over the collars. The
permanent nature of the installed collars maintains a tight seal with the soil surface
and minimizes disturbance effects associated with collar installation during periods
of gas flux measurement. Both collars and chambers are made of 25 cm diameter,
schedule 40 PVC pipe. Methane flux is calculated from the change in concentration
during the incubation period (period of chamber deployment in the field). The
required calculations are presented by Weishampel and Kolka (
2008
).
It is critical that sediments are not disturbed during instrumentation or sampling
as it can impact gas fluxes so wooden pallets may be needed to accommodate foot
traffic. This is especially a concern for saturated or organic soils. In some instances,
the collars are hammered partially into the ground. However, hammering can cause
significant compaction to organic soils or very wet soils and should be avoided in
those instances. Collars should also be installed at least 1 week prior to sampling to
minimize any impact from soil disturbance.
Altor and Mitsch (
2008
) described a static chamber design for measuring CH
4
and CO
2
emissions from freshwater marshes. Chambers were constructed of PVC
chamber frames and circular, high density polyethylene (HDPE) bases (0.27 m
2
),
and transparent 4-mil (0.1 mm thick) polyethylene bags. The frame consisted of
a circular top and three legs. Frame heights were 50 cm for sampling points
