results indicated the presence of volatile organic
data collected during the 1995 survey of the
compounds in the subsurface. A subsequent
anomalies east of the baseline. The EMI survey of
expanded site investigation, which included more
the main disposal area revealed multiple anoma-
soil borings, was undertaken to determine the
lies that directly correlated with the GPR results.
extent of contamination (ESE 1991). A remedial
The EMI survey aided interpretation of the GPR
plan was developed from the preliminary site
survey by confirming the general shape of the
investigation. In 1993 OHM Remedial Services
anomalous areas. Further, the metallic discrimi-
excavated two trenches, unearthing mustard gas
nation capability of the EMI aided the GPR in
cylinders in an area where only mustard gas test
areas where multiple reflections from near-
kits were anticipated. Excavation was immedi-
surface debris cluttered the GPR records.
ately halted and a more rigorous exploration
effort begun.
In 1994, CRREL conducted Ground-Penetrating
GEOLOGY
Radar (GPR) and ElectroMagnetic Induction (EMI)
surveys of the primary area. The adjacent marsh,
The surficial deposits at the Poleline Road
considered a potential disposal site, was also
Disposal Area are glacial sediments and tills of
surveyed. That geophysical investigation identi-
Quaternary age that have been reworked by
fied four areas of distinct anomalies (Lawson et
flowing water. Rapid vertical and lateral facies
al. 1994). A map of the suspicious areas was
changes attest to a complex, dynamic depositional
created and the anomalies were interpreted as
environment, where groundwater horizons are
large excavations or multiple, closely spaced
not well defined (Schmoll and Dobrovolny 1972).
trenches. At some locations the GPR records
The site lies within the Elmendorf Moraine com-
showed evidence of a buried soil horizon, sug-
plex and contains materials deposited by a large
gesting that these areas may have been covered
piedmont glacier that extended down the Knik
by fill without significant prior excavation.
Arm of Cook Inlet during the last major glacia-
A 100-MHz GPR survey of the ice-covered
tion episode. The moraines are composed of a
marsh revealed only one area containing suspi-
variety of marginal and proximal ice deposits,
cious anomalies. These appeared to coincide with
ranging from well-sorted gravels and sands to
the water bottom at a depth of about 3 m. EMI
tills. Soil horizons often dip steeply and are later-
found no significant conductive anomalies in the
ally discontinuous. The uppermost 25 m consists
marsh area. Using the CRREL geophysical results,
of well sorted to silty or clayey gravels (Strasser
the Alaska District Survey Section produced a site
et al. 1995). Sand lenses are typically several meters
map containing topography, the location of all
thick. Clay and silt lenses are more common at
survey line end-points, and detected anomalies.
depth (ESE 1991). The monitor-well-drilling
In response to the 1994 CRREL survey results,
records confirmed the primarily poorly graded to
the Directorate of Public Works at Fort Richardson
clayey gravel that is characteristic in the upper 5
requested that the U.S. Army Engineer District,
m. A shallow intermittent aquifer exists in the
Alaska, excavate some of the geophysical
glacial deposits at a depth of between 5.5 and 10
anomalies to remove buried debris from the
m, while a deeper, more substantial aquifer is at
PRDA. That request was further directed by the
the bedrock interface at a depth of approximately
Rapid Response Section, U.S. Army Engineer
35 to 40 m (ESE 1991).
District, Omaha, to OHM Remedial Services.
Following the excavation, CRREL returned to
the PRDA. The primary objective was to check
METHODS
that the excavated areas were free of significant
buried refuse. The secondary objective was to map,
Ground-Penetrating Radar (GPR)
in greater detail, the anomalies that were not ex-
The GPR system used in the CRREL surveys
cavated. The preliminary geophysical investiga-
consisted of the digital control unit (GSSI system
tions at PRDA have been discussed in contract
10+) and transducer (Fig. 2). This system is manu-
reports (Lawson et al. 1994, Strasser et al. 1995)
factured by Geophysical Survey Systems Incor-
prepared directly for the Alaska District.
porated (GSSI) of North Salem, New Hampshire.
In this report, we compare the geophysical data
The radar control unit triggers pulses at a selected
with the record of hazardous material excavated
repetition rate (50 kHz for these investigations).
from the mapped anomalies. We also discuss the
The received radar signals are sampled in pro-
2
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