start to sublimate as void pathways through the
SUMMARY
soil structure become available. Variations in the
The retention basin hydraulic conductivity has
availability of pathways through the soil account
increased to the point where it will not be useable
for the wide variability in the amount of WP mass
another season without recompaction, and it
remaining. Another set of plugs will be analyzed
probably should not have been used during the
in September of 1997 to indicate the results of a
1996 season. The probable cause of deterioration is
full year of drying. Preliminary results indicate
freezethaw cycling over the winter, caused by the
that the contaminant has naturally attenuated
retention of water in the highly organic liner mate-
and is no longer present.
rial. Low salinities continue to plague flocculation
and settlement of the suspended solids and negate
Basin contamination
the usefulness of the filter fabric in the drop inlet
Prior to dredging in 1996, a series of samples
structure. The low salinities result in a lack of par-
was collected from the retention basin for analy-
ticulate flocculation, which in turn leads to pro-
sis for white phosphorus. The results would be an
longed settling times, with the result that continu-
important indicator for estimating how well the
ous dredging is not practical if TSS is to be below
natural attenuation of the contaminant was pro-
levels found in the Flats during flooding tides.
gressing. Sample points corresponded with the
Basin sedimentation rates should allow for at least
locations chosen for the soil moisture samples
two more dredging seasons before it will be neces-
(see Table E1). As can be seen from Table 11, no
sary to remove the sediments.
white phosphorus was detected at any of the
Using the instrumentation located on the four
sample points. This does not prove that the con-
stations in the basin to monitor basin use is a good
taminant has completely disappeared from the
way to record contractor performance at the Flats.
Table 11. Results of limited
It is also useful in determining rough percolation
basin sampling (1996).
rates and thus basin performance. Soil tempera-
ture and moisture sensors should be useful if the
Site
WP mass found
(g)*
attenuation study is continued. No analysis on
ID
this has been done to date.
5m
Not detectable
Spoils line sample analyses indicate that con-
10 m
Not detectable
taminant rates are comparable to those found in
15 m
Not detectable
20 m
Not detectable
previous years. Presence of contaminant in the
25 m
Not detectable
basin from current year dredge spoils confirms
30 m
Not detectable
transfer of contaminant to the basin. Some low-
Station 1
Not detectable
level amounts of WP contaminant are still being
Station 3
Not detectable
Fence corner
Not detectable
rereleased from the decantation of supernatant
Inflow pipe
Not detectable
from the basin into the Flats. However, this is most
* Samples analyzed using SPME.
probably colloidal, not particulate, in form and
should quickly be sorbed onto the vegetation in
basin, only that the contaminant is not wide-
the area.
spread and that, in areas where sampled, it has
Composite sampling techniques made it much
either disappeared through natural attenuation
easier to sample the area dredged in 1996. They
or was never deposited there. More intensive
may be magnifying the problem of residual
sampling of the basin will be necessary to obtain
contamination, however, by consolidating the
more reliable results.
contaminant from a large number of samples
Within the retention basin, the locations of the
along a sampling line into a single sample, thereby
planted WP particle plugs and sediment moisture
overemphasizing isolated hits and exaggerating
sample points were surveyed (see App. E). A pro-
concentrations. Post-dredge sampling indicates
file of the sediment delta was surveyed to deter-
that some contaminant is still being left behind af-
mine the distribution and depths of sediment
ter dredging, albeit at significantly lower occur-
built up in the retention basin (Table E5) for the
rences than before dredging (< 7% hits vs. ≈100 %).
1996 season's dredging effort. Although survey
Causes of the contamination may be as follows:
data indicate the capacity of the basin has not
Contamination may be coming from slumped
been fully utilized, removal of the sediment will
banks on the edge of the dredged area. This is
be necessary to compact the liner before the struc-
the most likely cause.
ture can be reused.
17
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