Environmental Engineering Reference
In-Depth Information
7.5.5.4 Comments
Core dimensions, soil sample size, and sampling times are given as guidelines only
and can be modified to meet specific needs and situations. Acetylene used for the
C
2
H
2
block should not be obtained from commercial (e.g., welding) tanks, as there
are reports that these can contain acetone (Jordan et al.
2007
). Analytical grade
C
2
H
2
and C
2
H
2
produced by reacting CaC
2
with water have been used successfully.
Regarding measurement of DEA specifically, some investigators have chosen to
agitate the assay flasks during incubation rather than using static flasks (e.g., Hunt
et al.
2007
). Additionally, some investigators (Hunt et al.
2003
,
2007
) used DEA
approaches under both optimized (added NO
3
and glucose) and non-optimized
conditions and with and without an C
2
H
2
block, in an effort to obtain estimates of
denitrification having varying degrees of relatedness to true denitrification rates; the
authors conceded that the values they obtained even under non-optimized
conditions may still have been overestimates because an O
2
-free atmosphere was
used during the assay incubations (Hunt et al.
2007
).
7.5.5.5 Water Column Analysis
Critical to this approach is the assessment of the hydraulic gradient to identify
hydrologic input sources and outputs for sampling. For constructed wetlands, identi-
fication of hydrologic inputs and outputs is simple and straightforward. For natural
wetlands, the identification of hydrologic input and output sources is more problem-
atic and may require the expertise of a soil scientist. For groundwater driven slope
wetlands or depressions, a hydraulic gradient is easily characterized. Slope inputs are
easily identifiable as seeps (side slope or toe slope). For depressions, a soil investiga-
tion is required to identify the dominant hydrologic source. Often a gravel or sand
lens representing a zone of high hydraulic conductivity will extend from the upland
into the depression. Slope wetlands usually have apparent outlets; depressions may or
may not have outlets. For mineral soil flats, elevations should be shot with the
assumption that surface elevations correspond to water table elevations. For surface
water driven systems, a flume is needed to collect overland flow.
Inflow points and outflow points are the water sampling points. At each point, a
water sampling well is installed. Slotted PVC pipe (i.d. 5 cm) is inserted into a bore
hole which is then backfilled with coarse sand or pea gravel and capped with
bentonite (see Chap.
3
). In some cases, it may be desirable to separate surface
water from sub-surface water, or to separate two discontinuous water columns
(i.e., episaturation). In those cases, two sampling wells must be installed at each
sampling point.
Chemistry of water in the well casing may not be identical to that of the water
column. Prior to sampling water, the wells are purged to remove stagnant water.
Wells are emptied and the water discarded prior to sampling the wells after they
refill. This is known as “purging the wells”. As a general guideline, 3-5 casing
