Figure 13. Aerial view of dredged area.
were located every 2 m along each transect,
nel. Two out of 30 samples were contaminated, for
where possible. Subsamples were taken in at least
a contamination rate of less than 7%. This com-
three locations along each transect line except
pares with discrete sample contamination rates of
along the channel dredged out to Pond 183 (Fig.
between 50% and 80% and composite sample con-
13). Each location was sampled once with a spe-
tamination rates approaching 100% for areas
cially designed 250-mL long-handled scoop. Sub-
known to be contaminated, based on past investi-
samples were deposited in the bucket, which was
gations at the Flats (Racine 1995).
hung over the edge of the canoe from which sam-
The concentration and number of contaminated
pling was conducted. At the end of the transect,
samples for the compositing technique cannot be
the bucket was agitated and the fines allowed to
compared directly with data obtained from dis-
wash out. Clumps were broken up with a spoon,
crete sampling because the composite samples are
and the material was resieved. When the material
preconcentrated by sieving. This will overstate the
was fully sieved, a 500-mL sample was taken of
presence of WP in comparison to a series of dis-
the remains. On long sample transects, two sam-
crete samples. For instance, in this study, over 150
ples were taken from the bucket for analysis.
total discrete subsamples went into the 28 compos-
These are denoted by the "a" and "b" suffixes on
ite samples analyzed. The two composite samples
the line numbers in the table. The exception is line
testing positive each contained at least 10 subsam-
11, where two separate samples were taken along
ples. The small number of detections and the wide
the line: one from the east edge to the middle (11a
variation in the concentration (two orders of mag-
in Table C2) and one from the middle to the west
nitude) makes any additional statistical analysis of
edge (11b in Table C2).
the data difficult, if not meaningless. It should be
Composite sampling was used for post-
used for comparison with other areas of known
dredging sampling for several reasons. The pri-
contamination which, as stated above, normally
mary reason was that a large area can be sampled
have detection rates approaching 100% using the
quickly and the samples analyzed more economi-
sieved composite method.
cally. Because of the heterogeneous nature of the
In addition to the spoils line and dredged area
contaminant, it is difficult to determine if an area
sampling, two sediment samples were analyzed
is clean. Composite sampling allows better cover-
from the retention basin. These samples were
age because many points are sampled. Discrete
obtained near the spoils outfall pad where sedi-
sampling is much more likely to miss possible
mentation is thickest and drying time is longest
particulate contamination due to the lower num-
(see Attenuation study). Both 500-mL samples,
ber of samples taken when compared with com-
taken from material from the same 20-cm depth,
posite sampling. Discrete sampling is thus more
were contaminated. Results are shown in Table C2
likely to result in a false negative for a given num-
as "Basin."
ber of analyzed samples.
Analysis of the dredged area sampling indi-
Survey methods
cates that a small area is still slightly contaminat-
Detailed surveying was conducted to determine
ed. The likely source of the contamination is ma-
horizontal coordinates and elevations of sample
terial slumped from the edges of the dredge chan-
locations and boundaries in the dredged area dur-
14
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