Geophysical Investigations at

a Buried Disposal Site on

Fort Richardson, Alaska

Allan J. Delaney, Jeffrey C. Strasser, Daniel E. Lawson,

Steven A. Arcone, and Edward B. Evenson September 1997

CRREL REPORT 97-4
Abstract: The Poleline Road Disposal Area, located on

Fort Richardson, Alaska, was a U.S. Army dump in the

early 1950s. In 1990 it was identified as an area

potentially contaminated with volatile organic compounds. CRREL conducted extensive geophysical investigations that delineated anomalous responses in

many areas of burial within glacial outwash deposits.

Ground penetrating radar and electromagnetic induction surveys were used prior and subsequent to excavation. Geophysical data collected on a 5-m grid defined locations for several anomalous areas containing

both dispersed and large, discrete targets. Radar defined anomalous areas by the concentration of strong

diffractions. The induction survey differentiated metallic

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Cover: Electromagnetic metal detector (inset) and high-frequency radar in action.

from nonmetallic contaminations. The interpreted maximum depth of debris was 4 m. Uncontaminated areas

were generally defined by continuous, horizontal radar

reflections, suggesting undisturbed or compacted soil

horizons. The anomaly maps produced from these

surveys guided an excavation that removed hazardous material. The removed material included munitions, mustard gas cylinders, medical waste, steel

drums, and other trash. The radar and electromagnetic

surveys were repeated using a more closely spaced

grid to verify that the excavated areas were clean and

to define more precisely anomalies in the areas not

excavated. That survey shows many targets of potential or present contamination that should be removed.
CRREL Report 97-4

Geophysical Investigations at

a Buried Disposal Site on

Fort Richardson, Alaska

Allan J. Delaney, Jeffrey C. Strasser, Daniel E. Lawson,

Steven A. Arcone, and Edward B. Evenson September 1997

Prepared for

U.S. ARMY ALASKA

DIRECTORATE OF PUBLIC WORKS

Approved for public release; distribution is unlimited.

US Army Corps

of Engineers

Cold Regions Research &

Engineering Laboratory

®®

Cold Regions Research &

Engineering Laboratory
PREFACE

This report was prepared by Allan J. Delaney, Physical Science Technician, Snow

and Ice Division, Dr. Jeffrey C. Strasser, Research Geologist, Geological Sciences

Division, Dr. Daniel E. Lawson, Research Physical Scientist, Geological Sciences

Division, Dr. Steven A. Arcone, Geophysicist, Snow and Ice Division, all of the

Research and Engineering Directorate, U.S. Army Cold Regions Research and

Engineering Laboratory, and Professor Edward B. Evenson, Lehigh University,

Bethlehem, Pennsylvania.

Funding and support for this research was provided by the U.S. Army Alaska,

Directorate of Public Works, Fort Richardson, Alaska, through the U.S. Army

Engineering District, Alaska.

Technical review was provided by M. Moran and C. Collins, both of CRREL.

The contents of this report are not to be used for advertising or promotional

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.

ii
CONTENTS

Page

Preface ................................................................................................................... ii

Introduction .......................................................................................................... 1

Geology ................................................................................................................. 2

Methods ................................................................................................................ 2

Ground-Penetrating Radar (GPR) ................................................................ 2

Electromagnetic Induction (EMI) Technique .............................................. 4

Measurements ...................................................................................................... 4

Results ................................................................................................................... 5

West of the baseline ........................................................................................ 5

East of the baseline ......................................................................................... 9

Conclusions .......................................................................................................... 13

Literature cited ..................................................................................................... 14

Abstract ................................................................................................................. 15

ILLUSTRATIONS

Figure

1. Poleline Road Disposal Area located on Fort Richardson near the

town of Eagle River, Alaska ................................................................ 1

2. GSSI ground-penetrating radar system, video display, 400-MHz

transducer and cables ........................................................................... 3

3. 100-MHz radar antenna set in tow behind a tracked vehicle ............. 3

4. Geonics EM61 electromagnetic induction unit during a survey ....... 4

5. Entire Poleline Road Disposal Area showing the CRREL baseline

and the end points of CRREL transects ............................................. 5

6. EMI and GPR anomalies west of the CRREL baseline and a

complete catalog of excavated material removed from those

sites during the 1994 excavations........................................................ 6

7. Profiles recorded along transect 02 with GPR and EMI...................... 7

8. Profiles recorded along transect 11 with GPR and EMI...................... 8

9. Anomalous zones west of the CRREL baseline as interpreted from

the June 1995 survey ............................................................................. 9

10. 400-MHz GPR profile recorded along transect 16 in June of 1995 .... 10

11. Contoured maps of the EMI anomalies in the area of T-1 and

T-2 east of the CRREL baseline ........................................................... 10

12. 400-MHz GPR profile and interpretative depth sections .................... 11

13. EMI and GPR anomalies east of the CRREL baseline as compiled

from data recorded in June 1995 ......................................................... 13

iii
INTRODUCTION

The Poleline Road Disposal Area (PRDA) is an

abandoned site containing buried hazardous

materials and volatile organic compounds in proximity to multiple groundwater horizons. Extensive environmental investigations and remedial

activities have followed their discovery. The

PRDA, on Fort Richardson near Anchorage, Alaska

(Fig. 1), is located in a low-lying wooded area,

near the origin of Fossil Creek. The site is bounded

on the west by two small hills of glacial deposits,

on the south by a marsh, and on the north and

east by hummocky terrain. Some earth material

has been removed from the south-facing hill slope

and may have been used as fill in the disposal

area. A gravel road extending from Poleline Road

crosses the center of the 300- × 500-ft (90- × 150-m)

disturbed area.

In 1990 PRDA was identified by former military personnel, and information on materials and

chemicals that may have been buried there was

also provided. The list of probable buried items

included solvents, smoke canisters, World War II

vintage munitions, and kits for personnel identification of mustard gas. Some of the munitions

were reportedly destroyed

in trenches, while others

may have been buried

directly. A 1954 Army Post

map also identified the area

as a potential dump.

In 1990 the U.S. Army

Toxic and Hazardous Materials Agency (USATHAMA,

now the Army Environmental Center, AEC) contracted Environmental Science and Engineering, Inc.

(ESE, Seattle, Washington)

to conduct a preliminary

site investigation, which

included a surface geophysical survey (using

electromagnetic conductivity, magnetic techniques,

and ground-penetrating radar), installation of five

wells to monitor groundwater contamination, and

10 shallow soil borings. The

Geophysical Investigations at a

Buried Disposal Site on Fort Richardson, Alaska

ALLAN J. DELANEY, JEFFREY C. STRASSER, DANIEL E. LAWSON,

STEVEN A. ARCONE, AND EDWARD B. EVENSON

70°

65°

60°

55°

180°

160°

140°

ALASKA

Arctic Circle

Site

Location

Poleline

A

Site l a s k a

R a i l R o a d

G l e n n

H i g

h w a y

E a g l e Ri v e r

0 0.5 mi.

0 0.5 1.0 km

Eagle River

Figure 1. Poleline Road Disposal Area located on Fort Richardson near the town of

Eagle River, Alaska. The site is approximately 1.3 km west of the Glenn Highway.

Eagle River
results indicated the presence of volatile organic

compounds in the subsurface. A subsequent

expanded site investigation, which included more

soil borings, was undertaken to determine the

extent of contamination (ESE 1991). A remedial

plan was developed from the preliminary site

investigation. In 1993 OHM Remedial Services

excavated two trenches, unearthing mustard gas

cylinders in an area where only mustard gas test

kits were anticipated. Excavation was immediately halted and a more rigorous exploration

effort begun.

In 1994, CRREL conducted Ground-Penetrating

Radar (GPR) and ElectroMagnetic Induction (EMI)

surveys of the primary area. The adjacent marsh,

considered a potential disposal site, was also

surveyed. That geophysical investigation identified four areas of distinct anomalies (Lawson et

al. 1994). A map of the suspicious areas was

created and the anomalies were interpreted as

large excavations or multiple, closely spaced

trenches. At some locations the GPR records

showed evidence of a buried soil horizon, suggesting that these areas may have been covered

by fill without significant prior excavation.

A 100-MHz GPR survey of the ice-covered

marsh revealed only one area containing suspicious anomalies. These appeared to coincide with

the water bottom at a depth of about 3 m. EMI

found no significant conductive anomalies in the

marsh area. Using the CRREL geophysical results,

the Alaska District Survey Section produced a site

map containing topography, the location of all

survey line end-points, and detected anomalies.

In response to the 1994 CRREL survey results,

the Directorate of Public Works at Fort Richardson

requested that the U.S. Army Engineer District,

Alaska, excavate some of the geophysical

anomalies to remove buried debris from the

PRDA. That request was further directed by the

Rapid Response Section, U.S. Army Engineer

District, Omaha, to OHM Remedial Services.

Following the excavation, CRREL returned to

the PRDA. The primary objective was to check

that the excavated areas were free of significant

buried refuse. The secondary objective was to map,

in greater detail, the anomalies that were not excavated. The preliminary geophysical investigations at PRDA have been discussed in contract

reports (Lawson et al. 1994, Strasser et al. 1995)

prepared directly for the Alaska District.

In this report, we compare the geophysical data

with the record of hazardous material excavated

from the mapped anomalies. We also discuss the

data collected during the 1995 survey of the

anomalies east of the baseline. The EMI survey of

the main disposal area revealed multiple anomalies that directly correlated with the GPR results.

The EMI survey aided interpretation of the GPR

survey by confirming the general shape of the

anomalous areas. Further, the metallic discrimination capability of the EMI aided the GPR in

areas where multiple reflections from nearsurface debris cluttered the GPR records.

GEOLOGY

The surficial deposits at the Poleline Road

Disposal Area are glacial sediments and tills of

Quaternary age that have been reworked by

flowing water. Rapid vertical and lateral facies

changes attest to a complex, dynamic depositional

environment, where groundwater horizons are

not well defined (Schmoll and Dobrovolny 1972).

The site lies within the Elmendorf Moraine complex and contains materials deposited by a large

piedmont glacier that extended down the Knik

Arm of Cook Inlet during the last major glaciation episode. The moraines are composed of a

variety of marginal and proximal ice deposits,

ranging from well-sorted gravels and sands to

tills. Soil horizons often dip steeply and are laterally discontinuous. The uppermost 25 m consists

of well sorted to silty or clayey gravels (Strasser

et al. 1995). Sand lenses are typically several meters

thick. Clay and silt lenses are more common at

depth (ESE 1991). The monitor-well-drilling

records confirmed the primarily poorly graded to

clayey gravel that is characteristic in the upper 5

m. A shallow intermittent aquifer exists in the

glacial deposits at a depth of between 5.5 and 10

m, while a deeper, more substantial aquifer is at

the bedrock interface at a depth of approximately

35 to 40 m (ESE 1991).

METHODS

Ground-Penetrating Radar (GPR)

The GPR system used in the CRREL surveys

consisted of the digital control unit (GSSI system

10+) and transducer (Fig. 2). This system is manufactured by Geophysical Survey Systems Incorporated (GSSI) of North Salem, New Hampshire.

The radar control unit triggers pulses at a selected

repetition rate (50 kHz for these investigations).

The received radar signals are sampled in pro2
gressive time steps and converted to audio frequency scans for display and storage at a rate of

32 per second. The scans were stacked to improve

signal quality and to reduce the amount of stored

information. Each recorded scan can be displayed

using an operator selected time-range to suppress

the higher amplitude early returns (especially from

the direct transducer coupling). This technique

allows enhancement of the lower amplitude later

signals that reflect at layer interfaces and material

transitions. The technique is well known and is

used extensively for shallow subsurface exploration (Morey 1974).

The radar antennas discussed in this report

were 100- and 400-MHz shielded dipole pairs,

which radiate pulses with 20- and 5-ns duration

respectively. The antenna terminals connect directly to the transmitter and receiver electronics

and an entire unit is called a transducer. When

using the 400-MHz transducer, the signals were

recorded for a time range of 100 ns. To search for

deeper horizons, profiles were recorded with the

100-MHz transducer at a time range of 400 ns.

The transducers were towed along the ground

surface by hand or tracked vehicle (Fig. 3). Survey speed was approximately 1 m/s. This speed

provided a profile data density of about 16 scans

for each meter of transducer travel. The bottom

of the transducer was maintained in close contact

with the ground or snow surface to maximize

signal transmission through the air/ground interface. The profile data were later filtered to remove noise and horizontally scaled between event

markers to compensate for uneven towing speed.

GPR events within a profile consist of bands of

reflections from continuous horizons and discrete

hyperbolic diffractions that originate from individual targets and abrupt material transitions. The

depth to a target is calculated from the apex of

each hyperbola when the material dielectric permittivity, which determines the in-situ velocity, is

known. Clusters of diffractions often point to a

target of interest. However, this is not a foolproof

interpretation because diffractions can originate

from natural inclusions, such as large buried rocks.

In addition, strong reflections can originate from

continuous fine-grained soil layers that can contain more water than the surrounding materials.

Also, horizontal scale compression can cause sloping subsurface horizons to appear artificially steep

and this may be misinterpreted.

All GPR profiles presented in this report show

distance along the transect as the horizontal axis

and the two-way travel time t, increasing with

depth down the vertical axis. Travel time may be

converted to depth d according to

 

d ct = 2 ε

where c = 30 cm/ns, the speed of radio waves in a

vacuum, and ε is the relative dielectric permitFigure 2. GSSI ground-penetrating radar system, video

display, 400-MHz transducer and cables.

Figure 3. 100-MHz radar antenna set in tow

behind a tracked vehicle.

3
tivity of the soil. The factor of 2 accounts for the

round trip propagation of the pulse, to and from

the reflecting surface.

The dielectric permittivity of soils within the

PRDA was determined from the time–distance

slope of hyperbolic diffraction asymptotes.

Several permittivity determinations were made

in March 1994 when the soils were frozen. Additional permittivity determinations were made in

June 1995 when frost was absent and a layer of

silty backfill covered the site. The mean frozen

soil permittivity was 4.4. Five determinations of ε

in June 1995 gave a range of 5.4 to 7.4, with a

mean of 6.8. All of these values are reasonable for

both frozen and unfrozen silty sand and gravel of

low moisture content (Arcone and Delaney 1985).

With use of the mean ε of 6.8, the time range

settings for 100- and 400-MHz transducers provided an approximate depth range of 20.0 and 5.2

m, respectively, in these materials. The in-situ signal wavelengths are about 30 cm at 400 MHz and

1.2 m at 100 MHz. Accurate depth calibration of

the radar records is difficult, however, owing to

the local variability within these glacial deposits.

Electromagnetic Induction

(EMI) Technique

To complement the radar data and remove some

of the ambiguity in interpretation, we used an

EM61 electromagnetic unit manufactured by

Geonics LTD, Mississauga, Ontario (Fig. 4). It is a

time-domain metal detector that can operate in

highly conductive settings where radar techniques

fail. The unit consists of a backpack-mounted

transmitter and coaxial coil antennas. The closely

spaced antennas (vertical magnetic dipoles) are

mounted on wheels and towed by the operator.

In operation the primary magnetic field induces

secondary currents in nearby metal objects, which

in turn induce a secondary magnetic field that is

sensed at the receiver. After pulsing the transmitter (the repetition rate is 75 Hz), the response

from the ground generally decays in about 200 μs.

The responses from metal may persist for 400–500

μs. These longer duration responses are detected

in time and measured in millivolts (mV) of induced coupling. This high-sensitivity receiver can

detect a drum-sized metal object at a depth of

over 3 m beneath the antenna. Digital data for up

to 10,000 stations are recorded on a hand-held

datalogger.

EMI data recordings were controlled by the

carriage survey wheel, resulting in approximately

four readings per meter. Each data point was subsequently assigned an X,Y location and the data

were contoured for comparison with the radar

profiles. The amplitude of the secondary field response can vary with the size, depth, and lateral

proximity of the buried metal. The off-line responses can complicate interpretation and careful

survey techniques are required. Readings between

100 and 1000 mV were considered moderate to

strong anomalies, while readings greater that 1000

mV were considered very strong anomalies. Readings from nearby locations without disturbance gave background

noise levels of approximately 10

mV. For these investigations,

readings greater than 100 mV are

considered potential targets, but

any persistent deviation from

background levels must be considered to signify the presence of

metal.

MEASUREMENTS

The entire PRDA was surveyed

using both GPR and EMI techniques in the late winter of 1994,

when the site was covered by 30

to 50 cm of snow. The GPR transducers were towed behind a

tracked vehicle and the EMI antenna was towed by hand along

Figure 4. Geonics EM61 electromagnetic induction unit during a survey.

The operator tows the antenna, while the backpack unit generates signals.

Data are recorded on the hand-held datalogger.

4
the same transect. Data were collected along transects 2.5 and 5 m

apart in an east–west and north–

south grid pattern. A CRREL baseline (Fig. 5) was arbitrarily established, extending south–southeast

from a birch tree adjacent to the site

access road, through monitoring

well MW-5. Each transect end point

was staked. Event markers on the

radar profile correspond with the

5-m marks along each transect. The

profiles were offset approximately

1.5 m north or east of the grid lines

to avoid driving over stations marks.

Most of the GPR surveys used the

shorter pulse 400-MHz transducer

because it provided greater vertical

resolution than the 100-MHz transducer.

Upon completion of the 1994 field

work, a level survey of the entire

area was performed and all transect

endpoints, intercepts, and additional marked stations were located.

During the late summer of 1994, the

mapped anomalies west of the

CRREL baseline were excavated by

OHM Remediation Services and all

of the removed material was cataloged (OHM 1995).

Following those excavations, the

entire PRDA was covered with a

silty gravel fill and subsequently

graded to a relatively smooth surface. This smooth surface facilitated acquiring

geophysical data and improved transducer signal

coupling into the ground in later surveys. However, in some locations, fill thickness exceeded 60

cm. Signal loss within this additional fill and

reflection at its bottom interface resulted in lower

pulse amplitudes reflected from deeper targets.

In June 1995, CRREL returned to acquire a second data set. During this survey, the transducers

were towed by hand along this smoothed ground

surface. The objectives of this second effort were

to verify that all hazardous buried material west

of the CRREL baseline had been removed and to

refine the survey east of the baseline with a more

closely spaced grid. The adjacent marsh south of

the main study area was surveyed only in 1994,

and as no significant anomalies were discovered

(Lawson et al. 1994), there is no additional discussion provided.

RESULTS

Each GPR profile in the disposal area was

examined to define horizontal layering and the

location of subsurface diffractions, which might

indicate disturbance or the presence of buried

objects, or both. These hyperbolic diffractions are

important in attempting to identify areas in which

larger metallic objects, such as cylindrical containers, may be buried. This analysis was done

independently of the EMI data analysis.

West of the baseline

The pre-excavation GPR profiles recorded along

transects west of the baseline defined two large

anomalous areas, A-3 and A-4. Those areas are

delineated by the dashed lines on Figure 5. Profiles within these areas are characterized by multiple, strong hyperbolic diffractions and sloping

Area

A-2

N

0 50 100 ft

0 10 20 30 m

CRREL Baseline

Area

A-1

End of CRREL Survey Line

Tree line

Marsh

Gravel Road

Area

A-4

Area

A-3

Figure 5. Entire Poleline Road Disposal Area showing the CRREL

baseline and the end points of CRREL transects (x’s). The dashed lines

define the extent of GPR anomalies identified from the 1994 investigations. (After Lawson et al. 1994.)

5
≥ 1000 mV

500 -999 mV

100 -499 m/V

10- 99 mV

EMI Anomalies:

Monitoring Well/Boring

End of CRREL Survey Line

Treeline

0 50 100 ft

0 10 20 30 m

N

MW - 6

MW - 4

MW - 7

MW - 5 Soil

Boring

2

Soil

Boring

3

Soil

Boring 8

Zone 7

Zone 6

Zone 2

Zone 3

Zone 4

Zone 1

Zone 9

Zone 10

Zone 5

Zone 8

Figure 6. EMI and GPR anomalies west of the CRREL baseline

(areas A-3 and A-4) and a complete catalog of excavated

material removed from those sites during the 1994 excavations. CRREL survey lines run from northwest to southeast.

 (After OHM 1995.)

Zone Items

1 none

radiation dosimeter kit

1-glass vial with stopper

1-empty CAIS unit

1 large carbon filter canisters

misc. scrap metal

3-empty CAIS units,

2-full CAIS units

2 rusted drums

wood DANC crate parts

2/3 none

3/4/5 1-rusted drum

1-drum lid

6 large quantities of scrap metal and intact rusted drums

6 white phosphorus smoke grenade

7 2-CAIS units, 1-full, 1-empty

1-lecture bottle

7 1-8”×3” diameter amber bottle with 2 inches clear liquid

large amounts of wood/debris inc. several rusted drums

2-rusted drums partially full with unknown molasses-type material

3/7 extremely large amounts of wood debris

6/7 1-full, unopened case of HC smoke canisters

several small vials of ”Eye Decon Solution”

7 9-full or partially full amber bottles labeled HD Toxic Gas Set M-1,

and 5-empty unmarked clear bottles

1-larger 24-oz. amber bottle with small amount of clear liquid

2-artillery round fuses

7 1-white phosphorus smoke grenade

100’s of small medicine vials containing aureomycin, rabies serum,

and gangrene serum

1-1 gallon size HC smoke pot

large amounts of wood from smoke grenade crates

7 300–400 medical vials

1-HC smoke grenade

7 2-CAIS kits empty by X-ray

6-lecture bottles

7 large amounts of wood and crate parts

7 large amounts of wood and crate parts

1

1

Zone Items

9 oily soil, oily pails, funnels, kits labeled “lewisite eye ointment”

9/10 oily cans and debris

unbroken, sealed glass ampoule containing yellowish liquid 5/8 full

flame thrower canisters and parts

atropine injection kits

9 1-bakelite CS grenade, empty

1-warhead from a bazooka rocket

1-artillery round fuse

10 3-empty drums

4-smoke grenade fuses

2-HC smoke grenades

10 9-M51 type artillery round fuses

1-small bottle marked CN

1-76 mm artillery round warhead

10 1-smoke grenade 4-M51 fuses

3-nose fuses, 1-HC smoke pot

8-intact rifle grenades

2-empty 105 mm casings

16-rifle grenade tail booms

27-aluminum grenade bodies

1-fire starter, 2-empty 75 mm casings

4-flame thrower canisters

10 22-M51 fuses

85-rifle smoke grenades

1-HC smoke grenade

10 1-HC smoke pot, 8-M51 type fuses

1-M18 smoke grenades, 1-rifle grenade fuse

2-105 MM artillery rounds with HE

1-nose fuse

several-CN Id set bottles

10 scrap metal, rusted buckets and cans

10 scrap metal

3-emtpy 105 mm casings

10 1-1/2 gallon intact amber jar

5-flame thrower canisters

2-lecture bottles

5-empty 55 gallon drums

misc. metal debris

10 6-flame thrower canisters

2-lecture bottles

5-cubic yards of scrap metal

1-small white jar with unknown white powder

10 2-HC smoke grenades

1-empty 105 mm round

1-empty 75 mm round

1-8 oz. small jar with unknown white material

Zone Items

4 1-5 gallon pail with water-reactive granular solid that reacts with

moisture in air

2 (hill- 2-M60 machine gun belts with unfired blanks

side)

scrap metal and respirator cartridges

2-tail fins for 250 or 500 lb bombs

1-broken glass jar with poison markings

1-white phosphorus grenade

24-small bottles marked HCL

1-wood crate marked “hydride charge”

1000’s of old mustard and lewisite detector kit tubes, predecessors

to the M-18 kit

6
horizons that ranged from 1 to over 4 m depth.

The difference in depth, shape, and intensity of

the hyperbolic returns suggested that different

types of materials were buried in different parts

of and at different depths in this excavation

(Lawson et al. 1994). Zones of intense and uniform hyperbolic diffractions were interpreted as

individually stacked and horizontally positioned

cylindrical objects, particularly when these diffractions appeared on parallel transects. The

reflections from scattered targets were more difficult to interpret and could originate from large

boulders or other buried debris. We interpreted

the linear trends (in diffractions) as trenches with

multiple buried objects.

The EMI data were recorded along each transect

to verify the location of metallic targets. These

data were then contoured according to the strength

of the readings relative to the general level of the

background noise. Contours of 100, 500 and 1000

mV correspond with and refine the interpretation

of the radar data (Fig. 6).

The excavation of anomalous areas (A-3 and

A-4) resulted in the removal and cataloging of a

huge volume of material (Fig. 6).

Items recovered, which were presumably buried during the 1950s,

included: 55-gal. (208-L) drums,

miscellaneous metal containers,

wooden crates, scrap metal, glass

vials and bottles, exploded and

unexploded ordnance, chemical

agent test kits, and steel cylinders

containing chemical agents. The

record of excavation and careful

cataloging of debris allows comparison of our pre-excavation

survey interpretation with the

actual targets. The post-excavation

survey was conducted to verify the

performance of the excavation

contractor.

The geophysical data recorded

along two transects (before and

after excavation) are shown in

Figures 7 and 8, which compare

records of GPR reflectors and

diffractions with the plotted EMI

response.

Figure 7 shows GPR and EMI

data recorded along transect 02.

The pre-excavation profiles (top

and middle) show two large areas

of disturbance starting at about

1–2 m depth. Many hyperbolic

diffractions extend both to the surface and to a sloping subsurface

horizon. These events are interpreted as metallic, because of their

relatively high amplitude, the

presence of diffractions, and the

polarity of the reflections, which

indicate a high dielectric permittivity (which can also be associated with high conductivity). The

pre-excavation EMI profile conPre-excavation EMI

Post-excavation EMI

Distance (m)

30 25 20 15 10 5 0

1500

1000

500

0

0

20

40

60

80

0

20

40

60

80

Pre-excavation GPR

Post-excavation GPR

Induced Coupling (mV) Time Range (ns) Time Range (ns)

Depth (m)

0

1

2

3

4

0

1

2

3

4

Depth (m)

Figure 7. Profiles recorded along transect 02 (area A-4) with GPR and

EMI. The data were recorded before (1994) and after (1995) excavation.

The excavation, based on the 1994 survey results, revealed 5 yd3

( 

3 1

2 m3) of scrap metal and other debris near the broad, low amplitude

anomaly. Several 5-gal. (19-L) pails, funnels, and oil-contaminated soil

areas were revealed near the sharp anomaly at station 28 m.

7
firms this interpretation, showing a narrow highamplitude anomaly and a broad, low-amplitude

anomaly. The narrow anomaly denotes shallow

burial of metallic items, while the broad anomaly

evidences a more substantial target that is buried

deeper. The excavation resulted in removal of

more than 5 yd3 ( 

3 1

2 m3) of scrap metal, 55-gal.

(208-L) drums, and other debris from the zone

beneath the broad anomaly. Material recovered

from the zone beneath the narrow anomaly included 5-gal. (19-L) pails, metal funnels, and

contaminated soil. The GPR profile recorded

after excavation and backfill is also shown in

Figure 7. The lack of any sustained reflections in

this profile means that all debris was removed.

The post-excavation EMI plot (top,

Fig. 7) confirms this.

Figure 8 shows GPR and EMI

data recorded along transect 11.

The pre-excavation profiles (top

and middle) show a highly disturbed area that includes intense

diffractions with nearly symmetrical hyperbolas. These diffractions

were interpreted as metallic, cylindrical objects because of the strong

secondary diffractions generated

beneath them (Lawson et al. 1994).

Secondary diffractions are characteristic responses from the intersection between closely spaced

cylinders. Similar diffractions occur

on several adjacent profiles, indicating the presence of substantial

amounts of buried metal. The preexcavation EMI plot of a broad,

high-amplitude anomaly confirms

the presence of metallic objects

along a 15-m segment of this

transect. More than fifty 55-gal.

steel barrels were removed from

this zone. Many of the barrels were

intact, which probably contributed

to the symmetrical GPR anomalies.

The post-excavation GPR profile

(bottom) still shows some artifacts where the hyperbolas were

previously located. This could

point to some small amount of

metal remaining off-line; however,

the post-excavation EMI survey

revealed no substantial anomaly.

The sloping GPR horizon seen on

the post-excavation profile may be

a soil horizon compacted by the excavation process or it may be an old soil horizon previously

obscured by the diffractions from the drums.

The results of the post-excavation GPR profiles

are summarized in Figure 9. The contoured area

corresponds to anomalies at a depth of 1 to 4 m.

Profiles recorded on transects 000 through 03

showed no detectable horizons. Most of the area

within these transects was excavated in 1994, and

we attribute the lack of strong returns in the GPR

data to signal attenuation in the silty gravels used

as backfill. The remainder of the profiles (04–21)

showed a zone with few or no signal returns near

the top of the record, which also probably

resulted from signal attenuation in the silty fill

Figure 8. Profiles recorded along transect 11 (area A-3) with GPR and

EMI. The excavation revealed fifty 55-gal. (208-L) steel barrels in the

area distinguished by the broad EMI anomaly and the distinct GPR

hyperbolic reflections.

Distance (m)

Pre-excavation EMI

Post-excavation EMI

30 25 20 15 10 5 0

1500

1000

500

0

0

20

40

60

80

0

20

40

60

80

0

1

2

3

4

0

1

2

3

4

Pre-excavation GPR

Post-excavation GPR

Induced Coupling (mV) Time Range (ns) Time Range (ns)

Depth (m) Depth (m)

8
material. Several of the profiles show dramatically the interruption of horizons and scattered

returns resulting from the 1994 excavations and

backfilling (Fig. 10). Profiles from transects 09–13

also lack returns to 5 m depth in an area that

extended from the baseline to about 15 m west of

it. West of 15–17 m, each one of these profiles

exhibits sloping horizons and diffractions at

depths of from 1 to 3.5 m.

Several of the EMI anomalies mapped prior to

excavation (Fig. 6) exceeded 1000 mV in amplitude. After excavation, the 1995 EMI survey revealed no significant anomalies in this area and

amplitudes at all stations were less than 100 mV.

Levels between 40 and 90 mV, at the east end of

transects 14 and 15, probably resulted from the

steel casing around monitoring well MW-5. This

anomaly graded into a broad, low amplitude

response several meters towards the west, which

we interpret to be a relatively small amount of

refuse buried adjacent to the monitoring well. Two

broad, low-amplitude anomalies on transects 16

and 17, between approximately 20 and 25 m west

of the baseline, suggest buried material. The

remaining low-amplitude, narrow anomalies

scattered across the area probably originate from

small surface or near-surface trash (e.g., bolts and

scraps of steel banding). Since the amplitude of

the EMI anomalies is low and their positions do

not coincide with the GPR anomalies, it is likely

that the remaining GPR anomalies result from

inhomogeneous geological materials (e.g., large

boulders within gravels and sands, typical of such

glacial deposits).

East of the baseline

Our objective in the 1995 investigation was to

define more precisely the location, extent, and

depth to targets east of the baseline, which were

not excavated. During the 1994 investigations,

GPR and EMI data were collected on a 5-m grid

while the site was snow covered. The 1995 data

were recorded on a 2.5-m grid and we focused on

two suspect areas delineated by previous investigations. The transects retain the numbering used

during the 1994 survey, and those with an “A”

extension were located between existing transects.

The EMI and GPR data gathered in 1995 are considered superior in quality and accuracy to those

gathered in 1994; therefore, only the 1995 results

are discussed.

The results of the EMI survey are shown in

contour maps in Figure 11. The two maps are

derived from the same data set but are contoured

at different intervals. The bottom map in Figure

11 locates low-amplitude EMI anomalies while

the top map locates higher amplitude ones. Areas

of intense EMI anomalies (more than 200 mV)

compare favorably with the 1994 data, although

the amplitude of the 1995 readings is lower

because of the addition of 0.5 to 1.0 m of silty fill.

Two specific areas of intense EMI anomalies,

identified as T-1 and T-2, were marked in the field

for location by a survey crew.

GPR data were collected using both the 100-

and 400-MHz transducers. The 400-MHz data

were recorded on every transect within the

defined areas of interest, and the 100-MHz data

were recorded on selected transects. Additionally,

400-MHz profiles were recorded on three transects

parallel to the long north–east axis of area T-1,

Figure 9. Anomalous zones west of the

CRREL baseline as interpreted from

the June 1995 survey. It is likely that

this zone represents reflections from

large boulders within the sands and

gravels (transects are numbered L000

to L21).

N

–40 –20 0

Distance (m)

20

0

–20

–40

–60

–80

–100

–120

Distance (m)

L000

L00

L01

L02

L03

L04

L05

L06

L07

L08

L11

L12

L13

L14

L15

L16

L17

L18

L19

L20

L21

L10

L09

C

R

REL Basel ni e

9
0

1

2

3

4

5

0

20

40

60

80

10 20 30

East Distance (m) West

Scattered Non-metallic

Objects or Rocks?

Trench Excavated

and Filled in 1994

Depth (m)

Time Range (ns)

1994 Fill 1994 Fill

Figure 10. 400-MHz GPR profile recorded along transect 16 in June of 1995. A

reflection from the original ground surface (before fill) is clearly visible from

stations 24 to 35 m. Signal attenuation resulting from trench excavation and fill is

apparent near station 20 m.

Figure 11. Contoured maps of the EMI

anomalies in the area of T-1 and T-2

east of the CRREL baseline. The heavy

black lines show the locations of GPR

profiles and interpretative crosssections. Top—0–700 mV (contour interval = 50 mV); bottom—20–120 mV

(contour interval =20 mV).

10
and one additional profile extended into wooded

terrain on both sides of the cleared area.

Two 400-MHz GPR profiles and interpretive

cross-sections, representative of most of the data,

are presented in Figure 12. Many of the GPR profiles from the areas east of the baseline exhibit one

or two irregular horizons. The uppermost horizon represents the land surface prior to filling

and grading in 1994. A second, deeper horizon

exists on some profiles and may be either a buried soil horizon that existed before burial activities in the 1950s or the base of former excavations.

Zones of intense (high-amplitude) diffractions are

present within areas T-1 and T-2 where EMI

anomalies are mapped. The areal extent of the

radar anomalies exceeds the mapped EMI features, suggesting burial of nonmetallic debris or

small metal items that are too deep for detection

with the EM-61.

The profile in Figure 12a was recorded along

transect 04A and crosses area T-2, extending east

from the CRREL baseline for 40 m (Fig. 11). The

uppermost subsurface horizon, between the

depths of 0.25 and 1.0 m, represents the ground

surface prior to backfilling in 1994. Reflections

below this horizon, from 0.25- to 2.0-m depths,

may be an older buried fill or a former ground

surface. Numerous hyperbolic returns below this

second surface likely represent large objects that

are from 1.5 to 3.0 m deep. The portion along this

transect where EMI anomalies exceed 50 mV is

shown in the schematic section and GPR diffractions within this zone are consequently

labeled as originating from metal objects; other

diffractions presumably result from rocks or nonmetallic objects.

The profile in Figure 12b was recorded along a

transect traversing northeast–southweast along

the axis of area T-1 (Fig. 11). Event marks along

the horizontal axis correspond to line intercepts

and are thus spaced approximately 6 m apart.

Numerous diffractions appear between 0 and 18

m distance below the horizon, representing the

1994 ground surface. The lower limit of these

returns lies at approximately 3 m depth. A zone of

near-horizontal reflections at approximately 1.2 m

depth between 18 and 36 m distance may represent a layer of buried debris, or possibly an abrupt

increase in water content (associated with an

horizon of fine grain soil). Small diffractions below this horizon are visible to about 2.5 m depth.

The portion along this transect where EMI anomaa. Transect 4A recorded in June 1995.

0

1

2

3

4

5

0

20

40

60

80

Old Fill?

1994 Fill

Old Fill?

Metallic Targets?

EM > 50 mV

Rocks or Nonmetallic Targets?

10 20 30 40

West Distance (m) East

Depth (m)

Time Range (ns)

Figure 12. 400-MHz GPR profile and interpretative depth sections.

11
Figure 12 (cont’d). 400-MHz GPR profile and interpretative depth sections.

b. Recorded along the axis of a

suspected trench in area T-1.

0

20

40

60

80

NE Distance (m) SW

1

2

3

4

5

0 12 24 36

EM > 50 mV

Rocks or Nonmetallic Targets?

Numerous Targets Trench with Define a Lawyer? Metallic Targets?

1994 Fill

Old Fill?

Depth (m)

Time Range (ns)

lies exceed 50 mV is shown in the schematic, and

GPR diffractions within this zone are thought to

originate from metal objects.

Nearly all of the 400-MHz GPR profile data

recorded in area T-1 show evidence of a second,

deeper subsurface horizon. In many instances

this reflection can be followed to intercept with

the base of the 1994 fill section. This horizon

may represent a former ground surface that was

excavated for burial of refuse and then subsequently filled (labeled on Fig. 12 as “old fill”).

The EMI anomalies typically fall within the

areal limits of the deeper, older horizon. This

suggests that refuse was buried in wide, tapered

depressions that might allow easy access. Nonmetallic GPR anomalies adjacent to the prominent induction (metallic) anomalies might represent miscellaneous fill material with little

metallic content (crates, building materials,

concrete, etc.). Alternatively, the nonmetallic

GPR anomalies may result from the complex

geology, in which case the metallic refuse is

indeed confined to narrow trenches.

The areal extent of significant GPR anomalies

is presented in Figure 13. Also plotted on this

figure are estimated depths to specific targets as

determined from the high-amplitude hyperbolic

diffractions on the GPR profiles. These targets

represent significant items, particularly the ones

that lie within the limits of the strong EMI

anomalies. All targets are interpreted to be less

than 2.6 m deep.

Some additional isolated EMI anomalies exist

at the site, most of which are minor (less than 100

mV) and probably result from small pieces of

metal near the surface. At the time of this writing,

none of the anomalous areas east of the baseline

have been excavated.

12
CONCLUSIONS

The 1994 investigations showed that the PRDA

contains four large areas within which geophysical methods clearly point to the presence of

metallic objects and other buried debris. The variability in their intensity, depth, and location, and

the clustering of these anomalies, suggested that

both trenches (areas A-3 and A-4) and single or

closely spaced excavations (areas A-2 and A-1)

were used for burial. Anomalous horizons in

each area begin at a depth estimated at 1 to 1.5

m below the surface and extend to a depth of

over 4 m. A linear concentration of strong hyperbolic diffractions within area A-3 suggested a

trench of about 50 m in length containing

stacked cylindrical objects. Excavation of the

A-3 area confirmed this interpretation. Prior to

backfill and leveling of the entire PRDA, areas

A-1 and A-2 both had raised surfaces, which indicated that each was filled above the original

ground surface.

GPR and EMI investigations in June 1995 show

that the 1994 excavations by OHM successfully

removed all major metallic objects. The few scattered, minor EMI anomalies that remained are

most likely attributable to the presence of small

near-surface metallic debris. The remaining, small

GPR anomalies and suspicious zones not correlated with the EMI targets probably reflect the

complex geology. The fill material used subsequent to the 1994 excavations is easily delineated

with GPR because it contrasts markedly with the

original ground surface.

Comparisons between the 1994 and 1995 GPR

records show that all major anomalies previously

N

Treeline

Area A-?

Area T-?

End of CRREL Survey Line

Monitoring Well//Boring

EM Anomalies:

500 - mV

100 - 499 mV

50- 99 mV

Anomalous Zone

Target and Depth (m)

GPR Anomalies:

MW-6

MW-4

MW-1

MW-2

Soil Boring 2

MW-5

MW-7

Soil Boring 3

Soil Boring 10

Soil Boring 8

Soil Boring 7

A-2

A-1

T-2

T-1

A-3

A-4

1.5

2.6

1.8

1.4

1.8

Soil Boring 9

Soil

Boring 11

2.0 2.3

1.4

1.6

Soil Boring 4

Soil

Boring 6

0 50 100 ft

0 10 20 30 m

Soil Boring 5

Figure 13. EMI and GPR anomalies east of the CRREL baseline as compiled

from data recorded in June 1995. Interpreted depths to targets are in meters

below the existing ground surface. CRREL survey lines run from northwest

to southeast.

13
identified are absent in the 1995 data. This means

that all foreign material has most likely been

removed. The lack of significant EMI anomalies

corroborates this conclusion.

The 1995 investigations east of the baseline

further define the depth to and extent of the

anomalous areas T-1 and T-2. Both areas contain

high-amplitude EMI anomalies that we interpret

to be significant concentrations of metallic debris.

In addition, the EMI survey indicates scattered,

small, near-surface objects across the entire site.

The GPR profiles tell us that targets lie approximately 1.5 to 3.0 m below the surface and that

some of these targets are presumably large (e.g.,

the size of a 55-gal. drum). A broad GPR horizon,

present on most profiles, may represent a former

ground surface, buried after debris disposal. This

surface may have been the floor of broad pits

within which metallic materials were concentrated. Alternatively, this surface may have been

the natural ground surface onto which material

was dumped and covered. An intense and continuous GPR horizon in the T-1 area suggests

trench burial techniques. There are no current

plans for further removal of this buried debris

from PRDA.

LITERATURE CITED

Arcone, S.A., and A.J. Delaney (1985) Dielectric

studies of permafrost using cross-borehole VHF

pulse propagation. USA Cold Regions Research

and Engineering Laboratory, MP 1951.

Environmental Science and Engineering, Inc.

(1991) Poleline Road Disposal Area remedial

investigation, Fort Richardson, Alaska. Seattle,

Washington.

Lawson, D.E., J.C. Strasser, S.A. Arcone, A.J.

Delaney, and E.B. Evenson (1994) Reconnaissance

ground-penetrating radar and electromagnetic induction surveys of the Poleline Road site, Fort

Richardson, Alaska. Contract Report to U.S. Army

Engineer District, Alaska.

Morey, R.E. (1974) Continuous subsurface profiling by impulse radar. In Proceedings of Engineering

Foundation Conference on Subsurface Exploration for

Underground Excavation and Heavy Construction.

American Society of Civil Engineers, New York,

pp. 213–232.

OHM Remediation Services Corp. (1995) Excavation of the Poleline Road Disposal Area, Fort

Richardson Army installation Anchorage, Alaska.

Phase I, II and III Final Report. Pleasanton, California.

Schmoll, H.R., and E. Dobrovolny (1972) Generalized geologic map of Anchorage and vicinity,

Alaska. U.S. Geological Survey, Washington, D.C.,

Map I-787-A.

Strasser, J.C., A.J. Delaney, D.E. Lawson, and

S.A. Arcone (1995) Geophysical investigations of

the Poleline Road Disposal Area, Fort Richardson,

Alaska. Contract Report to U.S. Army Engineer

District, Alaska.

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September 1997

Geophysical Investigations at a Buried Disposal Site on Fort Richardson,

Alaska

Allan J. Delaney, Jeffrey C. Strasser, Daniel E. Lawson,

Steven A. Arcone, and Edward B. Evenson

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 97-4

Hanover, New Hampshire 03755-1290

U.S. Army Alaska

Directorate of Public Works

Fort Richardson, Alaska

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22

Electromagnetic induction Geophysical investigations Pollution

Fort Richardson, Alaska Ground penetrating radar

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

The Poleline Road Disposal Area, located on Fort Richardson, Alaska, was a U.S. Army dump in the early 1950s.

In 1990 it was identified as an area potentially contaminated with volatile organic compounds. CRREL conducted extensive geophysical investigations that delineated anomalous responses in many areas of burial

within glacial outwash deposits. Ground penetrating radar and electromagnetic induction surveys were used

prior and subsequent to excavation. Geophysical data collected on a 5-m grid defined locations for several

anomalous areas containing both dispersed and large, discrete targets. Radar defined anomalous areas by the

concentration of strong diffractions. The induction survey differentiated metallic from nonmetallic contaminations. The interpreted maximum depth of debris was 4 m. Uncontaminated areas were generally defined by

continuous, horizontal radar reflections, suggesting undisturbed or compacted soil horizons. The anomaly

maps produced from these surveys guided an excavation that removed hazardous material. The removed

material included munitions, mustard gas cylinders, medical waste, steel drums, and other trash. The radar and

electromagnetic surveys were repeated using a more closely spaced grid to verify that the excavated areas were

clean and to define more precisely anomalies in the areas not excavated. That survey shows many targets of

potential or present contamination that should be removed.