intake line without clogging the grate with vege-
Eagle River Flats is an estuarine salt marsh
tation.
located on Ft. Richardson, Alaska. For nearly 50
The retention basin is a 0.8-ha structure built
years, it has been an impact area for the Army and
Air Force. In 1982, large, unexplained die-offs of
into the EOD Pad. Some existing material was
waterfowl were documented, leading to several
used, but the presence of UXOs within the pad
years of research into the cause of this recurring
precluded complete construction with native
problem. In 1990, researchers from CRREL, in
materials. The base structure is constructed of
conjunction with the Ft. Richardson Environmen-
consolidated gravel with 2-m-high berms. The
tal Resources Branch, Directorate of Public Works
interior is lined with a peaty-silt material to
reduce the hydraulic conductivity to below 105
(DPW), discovered the cause of mortality: white
phosphorus. After extensive study of the charac-
cm/s, acceptable for capping hazardous waste
teristics of white phosphorus (P4) and the envi-
sites. Extensive testing by CRREL verified the liner
ronment of the Flats, three remediation strategies
performance. Two 3-m-square concrete splash
were earmarked for further study: Covering con-
pads were installed in the basin to check erosion
taminated areas with either geotextiles or bento-
from incoming spoils, and a drop inlet structure
nite; draining and exposing contaminated areas,
and weir was installed in one corner for con-
allowing natural remediation; and dredging con-
trolled supernatant decantation. Instrumentation
taminated sediments for later treatment. This pa-
was installed to monitor sediment characteristics
per discusses the work done to date on dredging.
for remediation performance. Computer models
The presence of unexploded ordnance (UXOs)
indicated acceptable system performance.
at Eagle River Flats (ERF) and the necessity to dis-
System performance indicates that dredging is
rupt the environment at a minimal level posed
a viable option for consideration as a remediation
serious dredge system design problems. Addi-
strategy for Eagle River Flats. The grate system
tionally, the only easily accessible storage and
has allowed dredging to occur while minimizing
treatment area for the dredge spoils is located on
the problems associated with UXO ingestion.
the explosive ordnance disposal (EOD) pad, a
Treatment studies of the P4 contaminated reten-
RCRA (Resource Recovery and Conservation Act)
tion basin sediments have begun, with definitive
site. The project was therefore initially divided
results anticipated for next season.
into two design efforts: the spoils retention and
treatment basin and the design of an appropriate
Walsh, M.R. (1997) Dredge removal of phos-
dredging system. This work was accomplished in
phorus-contaminated sediments at Eagle River
cooperation with the Alaska District, Corps of
Flats, Alaska. In Proceedings, 5th International Sym-
Engineers, and the Ft. Richardson DPW, with
posium on Cold Region Development, American Soci-
input from the Waterways Experiment Station.
ety of Civil Engineers, 410 May, Anchorage, Alaska,
The dredging system utilizes a small, remotely
p. 139142.
controlled augerhead dredge. The minimum dis-
Eagle River Flats is an estuarine salt marsh
located on Ft. Richardson near Anchorage, Alaska.
tance from the dredge to the control cab, a hard-
For 50 years, it has been used as an impact area by
ened, mobile structure, is 40 m. The genset used
both the Army and Air Force. During the 1980s,
to power the dredge is shore based, thus mini-
mizing impact in case of a UXO detonation. Other
hundreds of dead and dying waterfowl were
design features include the use of a biodegrad-
found in this area during the spring and fall
able vegetable-based hydraulic oil; locating pri-
migrations. For 5 years, various state and Federal
mary systems, such as the slurry pump and pow-
agencies tried without success to unravel the
er pack, as far from the dredgehead as feasible;
mystery of this high mortality. Finally, in the
and a cable traverse system to reduce penetrating
spring of 1990, a group of researchers from
UXO-laden sediments. An RF transmitter/receiver
CRREL, working in conjunction with the Ft. Rich-
system transmits video information and sensor
ardson Directorate of Public Works, Environmen-
data to the shore-based operator. Several strate-
tal Branch (DPW-EV), discovered the cause of
gies were tried for dealing with the UXOs, all
these massive die-offs: white phosphorus in the
with the common goal of excluding the objects
sediments of ponded areas.
from the slurry pump and leaving them behind.
Dredging is one of several remediation options
The most effective method was a cutter-and-grate
currently being considered for the Flats. The
system incorporated into the dredgehead, which
dredge is best suited for areas that are perma-
excludes the UXOs from entering the pump
nently ponded and thus not conducive to the nat-
34
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