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Cover: Photograph of Geoprobe® Model 6610DT pushing a temporary borehole for sampling at the General Communications, Inc. (GCI) compound 

site, Adak Island, Alaska. (Photograph taken by R.S. Dinicola, U.S. Geological Survey, Tacoma, Washington, 2003.)
Monitoring the Natural Attenuation of 

Petroleum in Ground Water at the Former 

Naval Complex, Operable Unit A, Adak 

Island, Alaska, May and June 2003

By R.S. Dinicola, F.W. Simonds, and Rose Defawe

Prepared in cooperation with Department of the Navy,

Engineering Field Activity, Northwest

Naval Facilities Engineering Command

Scientific Investigations Report 2005-5002

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Suggested citation:

Dinicola, R.S., Simonds, F.W., and Defawe, Rose, 2005, Monitoring the natural attenuation of petroleum in ground 

water at the former Naval complex, Operable Unit A, Adak Island, Alaska, May and June 2003: U.S. Geological 

Survey Scientific Investigations Report 2005-5002, 52 p. 
iii

Contents

Executive Summary ....................................................................................... 1

Abstract ................................................................................................... 3

Introduction ................................................................................................ 3

Purpose and Scope ................................................................................. 3

Site History .......................................................................................... 5

Environmental Restoration ........................................................................ 5

Previous USGS Evaluation of Monitored Natural Attenuation at Petroleum Sites ......... 5

Acknowledgments ................................................................................. 7

Study Methods .......................................................................................... 7

Sampling and Analysis Strategy .................................................................. 7

Monitoring Well Construction, Development, and Surveys .................................... 9

Ground-Water Level Measurements ............................................................ 10

Ground-Water Sampling and Analyses ......................................................... 10

Field Analyses of Petroleum Compounds ...................................................... 11

Diesel Range Organics, Gasoline Range Organics, and Benzene Analyses ......... 12

Quality Assurance and Quality Control of Petroleum Concentration Data ............ 13

Field Blank .............................................................................. 13

Method Blanks ........................................................................ 13

Instrument Blanks ..................................................................... 13

Field Duplicates ........................................................................ 13

Inter-Laboratory Replicate Samples ................................................ 14

Petroleum Biodegradation Processes and Geochemical Indicators .............................. 14

Monitoring for Natural Attenuation at Specific Petroleum Sites .................................... 16

Former Power Plant Building T-1451 ............................................................ 17

GCI Compound ....................................................................................... 20

Housing Area (Arctic Acres) ..................................................................... 24

ROICC Contractor’s Area (UST ROICC-7) ......................................................... 27

Runway 5-23 Avgas Valve Pit ..................................................................... 30

SA 80, Steam Plant No. 4 ........................................................................... 32

SWMU 14, Old Pesticide Storage and Disposal Area .......................................... 35

SWMU 61, Tank Farm B ........................................................................... 38

SWMU 62, New Housing Fuel Leak (Sandy Cove Housing) .................................... 41

Tanker Shed .......................................................................................... 44

Monitoring Strategy for Natural Attenuation of Petroleum Throughout the Downtown Area ... 46

Conclusions ................................................................................................ 50

References Cited .......................................................................................... 51
iv

Figures

Figure 1. Map showing location of Adak Island in the Andreanof Group of the Aleutian 

Island chain, Alaska ........................................................................ 4

Figure 2. Map showing petroleum sites investigated by the U.S. Geological Survey at 

the former Naval complex, Operable Unit A, Adak Island, Alaska, 

May and June 2003 ........................................................................ 6

Figure 3. Photograph of Geoprobe® Model 6610DT pushing a temporary borehole for 

sampling at the General Communications, Inc. (GCI) compound site, Operable 

Unit A, Adak Island, Alaska ............................................................... 9

Figure 4. Diagram showing typical monitoring well construction .............................. 10

Figure 5. Photograph of portable gas-chromatograph (GC) in the temporary laboratory at 

Adak Island, Alaska ........................................................................ 12

Figure 6. Diagram showing explanation of symbols used on site maps ........................ 16

Figure 7. Map showing diesel range organics (DRO) and carbon dioxide concentrations 

for the Former Power Plant Building T-1451 site, Operable Unit A, Adak Island, 

Alaska, May and June 2003 ............................................................... 17

Figure 8. Map showing gasoline range organics (GRO) concentrations and water levels 

for the GCI Compound site, Operable Unit A, Adak Island, Alaska, May and 

June 2003 .................................................................................... 20

Figure 9. Photograph of foreset dune about 500 ft north of the GCI Compound site (looking 

north-northwest), Operable Unit A, Adak Island, Alaska .............................. 22

Figure 10. Map showing diesel range organics (DRO) concentrations and water levels for 

the Housing Area (Arctic Acres) site, Operable Unit A, Adak Island, Alaska, 

May and June 2003 ........................................................................ 24

Figure 11. Map showing benzene, carbon dioxide, and specific conductance 

concentrations for the ROICC Contractor’s Area site, Operable Unit A, 

Adak Island, Alaska, May and June 2003 ................................................ 27

Figure 12. Map showing gasoline range organics (GRO) and carbon dioxide concentrations 

and specific conductance for the Runway 5-23 Avgas Valve Pit site, Operable 

Unit A, Adak Island, Alaska, May and June 2003 ....................................... 30

Figure 13. Map showing diesel range organics (DRO) and carbon dioxide concentrations 

for the SA 80, Steam Plant No. 4 site, Operable Unit A, Adak Island, Alaska, 

May and June 2003 ........................................................................ 32

Figure 14. Map showing diesel range organics (DRO) and ferrous iron concentrations, and 

water levels for the SWMU 14, Old Pesticide Storage and Disposal Area site, 

Operable Unit A, Adak Island, Alaska, May and June 2003 ........................... 35

Figure 15. Map showing gasoline range organics (GRO) and carbon dioxide concentrations 

for the SWMU 61, Tank Farm B site, Operable Unit A, Adak Island, Alaska, 

May and June 2003 ........................................................................ 38

Figure 16. Map showing diesel range organics (DRO) and carbon dioxide concentrations 

for the SWMU 62, New Housing Fuel Leak (Sandy Cove Housing) site, 

Operable Unit A, Adak Island, Alaska, May and June 2003 ........................... 41

Figure 17. Map showing diesel range organics (DRO) and carbon dioxide concentrations 

for the Tanker Shed site, Operable Unit A, Adak Island, Alaska, 

May and June 2003 ........................................................................ 44
v

Tables

Table 1. Petroleum sites investigated by the U.S. Geological Survey at the former Naval 

complex, Operable Unit A, Adak Island, Alaska, May and June 2003 ............... 8

Table 2. Summary of field measurement methods, ranges, detection limits, and 

accuracies used at the former Naval complex, Operable Unit A, Adak Island, 

Alaska, May and June 2003 ............................................................... 11

Table 3. Selected diesel range organics, geochemical, and water-level data from 

boreholes and wells at the Former Power Plant Building T-1451 site, 

Operable Unit A, Adak Island, Alaska, May and June 2003 ........................... 18

Table 4. Selected gasoline range organics, benzene, geochemical, and water-level data 

from boreholes and wells at the GCI Compound site, Operable Unit A, 

Adak Island, Alaska, May and June 2003 ................................................ 21

Table 5. Selected diesel range organics, geochemical, and water-level data from 

boreholes and wells at the Housing Area (Arctic Acres) site, Operable Unit A, 

Adak Island, Alaska, May and June 2003 ................................................ 25

Table 6. Selected gasoline range organics, benzene, geochemical, and water-level data 

from boreholes and wells at the ROICC Contractor’s Area site, Operable Unit A, 

Adak Island, Alaska, May and June 2003 ................................................ 28

Table 7. Selected gasoline range organics, benzene, geochemical, and water-level data 

from boreholes and wells at the Runway 5-23 Avgas Valve Pit site, Operable 

Unit A, Adak Island, Alaska, May and June 2003 ....................................... 31

Table 8. Selected diesel range organics, geochemical, and water-level data from 

boreholes and wells at the SA 80, Steam Plant No. 4 site, Operable Unit A, 

Adak Island, Alaska, May and June 2003 ................................................ 33

Table 9. Selected diesel range organics, gasoline range organics, geochemical, and 

water-level data from boreholes and wells at the SWMU 14, Old Pesticide 

Storage and Disposal Area site, Operable Unit A, Adak Island, Alaska, 

May and June 2003 ........................................................................ 36

Table 10. Selected gasoline range organics, benzene, geochemical, and water-level 

data from boreholes and wells at the SWMU 61, Tank Farm B site, Operable 

Unit A, Adak Island, Alaska, May and June 2003 ....................................... 39

Table 11. Selected diesel range organics, geochemical, and water-level data from 

boreholes and wells at the SWMU 62, New Housing Fuel Leak (Sandy Cove 

Housing) site, Operable Unit A, Adak Island, Alaska, May and June 2003 ............ 42

Table 12. Selected diesel range organics, geochemical, and water-level data from 

boreholes and wells at the Tanker Shed site, Operable Unit A, Adak Island, 

Alaska, May and June 2003 ............................................................... 45

Table 13. Wells proposed for monitoring natural attenuation at investigated petroleum 

sites, Operable Unit A, Adak Island, Alaska ............................................. 46

Figure 18. Graph showing relation between diesel range organics (DRO) and carbon 

dioxide concentrations from all petroleum sites where DRO was a contaminant 

of concern, Operable Unit A, Adak Island, Alaska ....................................... 49

Figure 19. Graph showing relation between gasoline range organics (GRO) and carbon 

dioxide concentrations from all sites investigated where GRO was a 

contaminant of concern, Operable Unit A, Adak Island, Alaska ..................... 49
vi

Conversion Factors, Datums, Abbreviations, 

and Acronyms

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Volume

gallon (gal) 3.785 liter (L)

pound, avoirdupois (lb) 0.4536 kilogram (kg)

Flow rate

foot per year (ft/yr) 0.3048 meter per year (m/yr)

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25 °C). 

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD88).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD27).

Altitude, as used in this report, refers to distance above the vertical datum.
vii

Abbreviations

Abbreviations Meaning

CaCO3 calcium carbonate

g gram

μm micrometer

mL/min milliliter per minute

mg/L milligram per liter

Acronyms

Acronym Meaning

ADEC Alaska Department of Environmental Conservation

ANC Acid-neutralizing capacity

AST above ground storage tank

BRAC Base Closure and Realignment Commission

BTEX Benzene, toluene, ethylbenzene, xylene

DRO diesel range organics

EPA U.S. Environmental Protection Agency

FID flame ionization detector

GC gas chromatograph

GCI General Communications, Inc.

GRO gasoline range organics

JP jet petroleum

MNA monitored natural attenuation

NaCl sodium chloride

NAP natural attenuation parameter

NAPL non-aqueous phase liquid

ORP oxygen-reduction potential

OU operable unit

PDS polydimethylsiloxane

PVC polyvinylchloride

ROD Record of Decision

ROICC Resident Officer in Charge of Construction

SAERA State-Adak Environmental Restoration Agreement

SPME solid phase microextraction

SWMU solid waste management unit

TFB Tank farm B

TS Tanker shed

USFWS U.S. Fish and Wildlife Service

USGS U.S. Geological Survey

UST underground storage tank

VOC volatile organic compound
viii

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Executive Summary

During May and June 2003, the U.S. Geological Survey 

(USGS) installed monitoring wells and collected data to 

characterize the effectiveness of natural attenuation processes 

for remediating petroleum-contaminated ground water at 

Operable Unit A of the former Naval complex on Adak Island, 

Alaska. Evidence for petroleum biodegradation in ground 

water was evaluated at 10 sites, plans for future natural 

attenuation monitoring were suggested for those sites, and 

the natural attenuation monitoring strategy for all petroleum 

sites in the Downtown area of Adak Island was reviewed and 

refinements were suggested.

USGS personnel measured water levels and collected 

ground-water samples from about 100 temporary boreholes 

and 50 monitoring wells. Most samples were analyzed on-site 

for concentrations of selected petroleum compounds and 

natural attenuation parameters (NAPs) such as dissolved 

oxygen, ferrous iron, and carbon dioxide. The USGS evaluated 

the data onsite, selected new monitoring well locations, 

and installed, developed, and sampled 10 monitoring wells. 

This data collection approach facilitated accurate and rapid 

delineation of the downgradient margin of contaminant 

plumes, and the new wells generally were installed at 

downgradient locations where petroleum compounds were 

detected at concentrations less than the specified cleanup 

standards.

The review and suggestions for the natural attenuation 

monitoring strategy focused on how to better achieve 

monitoring objectives specified in the Record of Decision 

(ROD) for Adak Island petroleum sites. Specific refinements 

in the monitoring strategy were proposed with regard to the 

newly available sampling locations and the additional data 

collected during this investigation.

To achieve the monitoring objective of verifying 

that natural attenuation is occurring, monitoring plans for 

each monitored natural attenuation (MNA) site need to 

include sampling at least one strategically placed well at the 

downgradient margin of the contaminant plume, preferably 

where contaminant concentrations are detectable but less than 

the cleanup level. Wells installed during this investigation can 

be used for additional sampling at the 10 investigated sites. An 

ancillary benefit of sampling properly placed downgradient 

wells is that the new downgradient wells also can be used 

as sentinel monitoring wells for the MNA sites. Collection 

of NAP data and sampling background wells is no longer 

needed to achieve the monitoring objective of demonstrating 

the occurrence of natural attenuation. Collecting and 

analyzing contaminant concentration data from downgradient 

monitoring wells better demonstrates the overall effectiveness 

of MNA. In addition, the NAP data collected to date 

are adequate to demonstrate that biodegradation plays a 

significant role in natural attenuation. A preponderance of 

existing data demonstrates on-going petroleum biodegradation 

in the Downtown area of Adak Island, and a sound scientific 

basis exists for when to expect (and not expect) biodegradation 

to occur.

Monitoring the Natural Attenuation of Petroleum 

in Ground Water at the Former Naval Complex, 

Operable Unit A, Adak Island, Alaska, 

May and June 2003

By R.S. Dinicola, F.W. Simonds, and Rose Defawe
To achieve the objective of monitoring areas where 

chemical concentrations exceed specified cleanup levels, 

at least one natural attenuation well within or immediately 

downgradient from the contaminant source area at each site 

should be monitored. Most existing natural attenuation wells 

are in appropriate locations. Contaminant concentration 

data from those near source area wells are useful to clearly 

demonstrate natural attenuation (or in a few cases, the lack of 

natural attenuation) over distance, which is more important 

than demonstrating natural attenuation over time at a single 

well when protection of downgradient receptors is a primary 

remediation goal. Data from upgradient and downgradient 

wells also would be beneficial for any fate and transport 

modeling at MNA sites.

Achieving the ROD specified final monitoring 

objective of estimating the rate of natural attenuation to 

demonstrate achievement of cleanup levels within 75 years 

will be problematic. Demonstrating (predicting) achievement 

of cleanup levels within any timeframe in a technically 

defensible manner will be difficult to achieve using any 

type of short-term monitoring and evaluation, and will 

be particularly difficult to achieve through monitoring 

and evaluation of dissolved-phase petroleum only. The 

primary reason is the likelihood that the continuing source 

of dissolved-phase petroleum in ground water is residual 

non-aqueous phase liquid (NAPL) petroleum being held by 

capillary forces in the unsaturated zone and near the top of the 

saturated zone, and that the volumes of residual NAPL at the 

Adak Island petroleum sites are poorly known. A model that 

rigorously simulates NAPL dissolution could be developed to 

derive somewhat more precise estimates of time-to-cleanup. 

However, NAPL dissolution is a complex process, particularly 

in heterogeneous environments with fluctuating water tables. 

NAPL simulation models have extensive data requirements 

and the accuracy of their results often is uncertain. 

Despite the difficulties in demonstrating (predicting) 

achievement of cleanup levels within any timeframe in a 

technically defensible manner, natural attenuation processes 

appear to have greatly limited the extent of ground-water 

contamination at most sites investigated and have limited 

the risk that petroleum contaminants pose to downgradient 

receptors. Clarification or refinement of the monitoring 

objective to demonstrate cleanup within 75 years would be 

a reasonable prelude to developing a monitoring and data 

evaluation strategy to meet the objective.

2 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
Abstract

During May and June 2003, the U.S. Geological 

Survey installed monitoring wells and collected data to 

characterize the effectiveness of natural attenuation processes 

for remediating petroleum-contaminated ground water at 

Operable Unit A of the former Naval complex on Adak Island, 

Alaska. In addition, the evidence for petroleum biodegradation 

in ground water was evaluated at selected petroleum sites, 

plans for future natural attenuation monitoring were suggested 

for the selected petroleum sites, and the natural attenuation 

monitoring strategy for the Downtown area of Adak Island 

was reviewed and refinements were suggested.

U.S. Geological Survey personnel measured water 

levels and collected ground-water samples from about 100 

temporary boreholes and 50 monitoring wells. Most samples 

were analyzed on-site for concentrations of selected petroleum 

compounds and natural attenuation parameters such as 

dissolved oxygen, ferrous iron, and carbon dioxide. The U.S. 

Geological Survey evaluated the data on-site, selected new 

monitoring well locations, and installed, developed, and 

sampled 10 monitoring wells.

The review and suggestions for the natural attenuation 

monitoring strategy focused on how to better achieve 

monitoring objectives specified in the Record of Decision 

for Adak Island petroleum sites. To achieve the monitoring 

objective of verifying that natural attenuation is occurring, 

the monitoring plans for each monitored natural attenuation 

site need to include sampling of at least one strategically 

placed well at the downgradient margin of the contaminant 

plume margin, preferably where contaminant concentrations 

are detectable but less than the cleanup level. Collection of 

natural attenuation parameter data and sampling background 

wells is no longer needed to achieve the monitoring objective 

of demonstrating the occurrence of natural attenuation. To 

achieve the objective of monitoring locations where chemical 

concentrations exceed specified cleanup levels, at least one 

natural attenuation well within or immediately downgradient 

from the contaminant source area at each site needs to be 

monitored. 

Achieving the Record of Decision-specified final 

monitoring objective of estimating the rate of natural 

attenuation to demonstrate achievement of cleanup levels 

within 75 years will be problematic. Demonstrating 

(predicting) achievement of cleanup levels within any 

timeframe in a technically defensible manner will be 

difficult to achieve using any type of short-term monitoring 

and evaluation, and will be particularly difficult to achieve 

through monitoring and evaluation of dissolved-phase 

petroleum only.

Overall, natural attenuation processes appear to have 

greatly limited the extent of ground-water contamination 

at most sites investigated and have limited the risk that 

petroleum contaminants pose to downgradient receptors. 

Clarification or refinement of the monitoring objective to 

demonstrate cleanup within 75 years would be a reasonable 

prelude to developing a monitoring and data evaluation 

strategy to meet the objective.

Introduction

The U.S. Navy is responsible for long-term monitoring 

of natural attenuation in ground water at petroleumcontaminated sites within the former Naval complex, 

Operable Unit A (OU A) at Adak Island, Alaska (fig. 1). The 

1999 Record of Decision (ROD) (U.S. Navy and others, 2000) 

for Adak Island petroleum sites specified three objectives that 

ground-water monitoring in affected areas must satisfy:

• To verify whether natural attenuation is occurring.

• To monitor locations where chemical concentrations 

exceed specified cleanup levels.

• To estimate the rate of natural attenuation to 

demonstrate achievement of cleanup levels within 

75 years.

The U.S. Geological Survey (USGS) evaluated the 

Navy’s monitoring program in 2002 to determine how well 

the program was meeting the objectives specified in the ROD. 

The Navy then asked the USGS to install new monitoring 

wells and collect data to better monitor the effectiveness of 

natural attenuation processes at the site.

Purpose and Scope

This report describes the implementation of suggestions 

made by the USGS in 2002 for refining the ground-water 

monitoring program at OU A, and proposes refinements to 

the natural attenuation monitoring strategy for the Downtown 

area of Adak Island. Specifically, this report describes:

• Installation of new monitoring wells positioned to 

better monitor the effectiveness of natural attenuation.

• Collection of new petroleum and natural attenuation 

parameter (NAP) data.

• Evaluation of the evidence for petroleum 

biodegradation. 

• Evaluation of the natural attenuation monitoring 

strategy for the Downtown area.

• Suggestions for future natural attenuation monitoring.

Introduction 3
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Figure 1. Location of Adak Island in the Andreanof Group of the Aleutian Island chain, Alaska.

4 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
Site History

The former Naval complex (referred to as the former 

base) is in the Andreanof Group of the Aleutian Island chain 

of Alaska, about 1,300 mi west-southwest of Anchorage 

(fig. 1). The former base was an important military facility 

from 1942 to 1997 (U.S. Navy and others, 2000). Constructed 

as a defensive outpost in World War II, the former base 

supported as many as 100,000 troops and 100 ships. The 

airstrip was critical for the U.S. Air Force, which maintained 

control of the island until 1950 when the Navy took control 

of all facilities. By 1953, the island’s population decreased 

to 176 military personnel, but increased steadily to about 

5,600 military and civilian personnel by 1990. In 1995, the 

base was selected for closure by the Defense Base Closure 

and Realignment Commission (BRAC). The Navy has been 

involved in environmental restoration, cleanup, and closure 

activities since the military mission ended in March 1997, 

to allow for eventual lease and transfer of land and facilities 

to non-Federal organizations. Land transfer agreements 

and associated documentation are being negotiated under 

BRAC among the Navy, the U.S. Fish and Wildlife Service 

(USFWS), and the Aleut Corporation. Aleut Corporation 

currently operates most of the infrastructure on the northern 

half of the island, and the USFWS currently manages the 

southern part of the island as part of the Alaska Maritime 

National Wildlife Refuge system. 

Environmental Restoration

Environmental restoration and cleanup activities related 

to petroleum contamination have focused on the developed 

portion of the island referred to as the “Downtown” area, 

which includes the air field, port facilities, and light industrial, 

administrative, commercial, and residential areas (fig. 2). The 

Downtown area comprises most of OU A, which is a noncontiguous area that includes all identified petroleum sites on 

Adak Island. Various petroleum products, primarily jet fuel 

(JP-5) and aviation gasoline (avgas) have leaked from former 

storage tanks and pipelines and have migrated to the shallow 

ground water. All known leaking tanks and pipelines have 

been removed or otherwise remediated, but both dissolved 

and undissolved (referred to as free product) petroleum 

compounds remain under the ground.

The State-Adak Environmental Restoration Agreement 

(SAERA) included 128 defined “petroleum sites” on the 

island. Monitored natural attenuation (MNA) was the selected 

remedy in the ROD (U.S. Navy and others, 2000) for 11 of 

the petroleum-contaminated sites at OU A (referred to as 

MNA sites). MNA also is a potential remedy for dissolvedphase ground-water contamination at 14 additional sites where 

recovery of free product was the original implemented remedy 

(referred to as free-product recovery sites). Selected remedies 

for other OU A petroleum sites were limited soil removal, 

limited ground-water monitoring, or no further action.

Previous USGS Evaluation of Monitored 

Natural Attenuation at Petroleum Sites

During 2002, the USGS evaluated the Navy’s natural 

attenuation monitoring program to determine how well the 

program was meeting the objectives specified in the 1999 

ROD (U.S. Navy and others, 2000), and made suggestions 

for refining the program. The USGS determined that 

existing data showed little direct evidence that natural 

attenuation was effective at most sites where MNA was the 

selected remedy. The downgradient extent of dissolvedphase contamination and the stability of contaminant 

plumes could not be determined using the small number 

of monitoring locations and because of recent increases in 

contaminant concentrations at the few locations that were 

monitored. Additional downgradient monitoring locations 

were proposed for at least eight of the petroleum sites 

evaluated. The USGS determined that existing data provided 

a reasonable amount of geochemical evidence to demonstrate 

that petroleum biodegradation was occurring at most MNA 

sites. It was suggested that the Navy may be able to reduce 

the frequency of sampling for NAPs such as nitrate, ferrous 

iron, and methane because the available data show reasonable 

evidence of on-going biodegradation, and because ample 

literature supports the ubiquitous occurrence of petroleum 

biodegradation. 

The USGS determined that it would be difficult 

to achieve the monitoring objective of demonstrating 

achievement of cleanup levels within 75 years in a technically 

defensible manner using the current sampling and data 

analysis strategies. Current trend-analysis strategy relies on 

two unlikely assumptions: 

1. That the change in dissolved-phase concentrations 

at a single monitoring well over time is directly 

proportional to the non-aqueous phase liquid (NAPL) 

dissolution rate, and

2. That a single monitoring well is near enough to 

the NAPL source so the measured dissolved-phase 

concentration is the maximum for a site. 

No simple method was identified to predict achievement 

of cleanup levels throughout an entire MNA site. The USGS 

proposed that NAPs should be analyzed at existing “sentinel” 

wells (wells intended to detect on-going contaminant 

migration) and at proposed new wells installed at the 

downgradient margin of a plume to determine if the wells 

actually are on the same ground-water-flow path as the 

dissolved-petroleum plumes. Existing data indicated that 

it is appropriate to characterize background ground-water 

chemistry for much, but not all, of OU A as a similar water 

type. Additional data from uncontaminated background sites, 

and the collection and analysis of higher quality field data 

would greatly reduce the uncertainty in characterization of 

background geochemistry.

Introduction 5
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East Canal

Figure 2. Petroleum sites investigated by the U.S. Geological Survey at the former Naval complex, 

Operable Unit A, Adak Island, Alaska, May and June 2003.

6 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
In addition to the evaluation of monitoring needs at 

specific petroleum sites, the Navy was interested in a review 

of their comprehensive strategy for monitoring natural 

attenuation throughout the Downtown area (fig. 2). The 

comprehensive strategy for the Downtown area, which 

includes most of the sites in the non-contiguous administrative 

unit OU A, was of particular interest. The Downtown area is 

the most developed part of the island and where the resident 

population is concentrated. It also has an accordingly high 

concentration of petroleum related infrastructure, and is 

nearly surrounded by potential surface-water receptors. One of 

the primary goals of remediating dissolved-phase petroleum 

contamination on Adak Island is the protection of lowland 

and marine surface water. Remediation for the protection of 

drinking water is a lesser concern because of an established 

(and currently greatly under-utilized) upland surface-water 

collection, treatment, and delivery system on the island. 

Ground water is not now and probably will never be used as a 

drinking-water source.

Acknowledgments

The authors thank the following individuals for their 

assistance. James Brown, Mark Wicklein, and Mark Murphy 

of the U.S. Navy’s Environmental Field Activity, Northwest 

provided guidance and funding for the entire investigation. 

Scott Vojta and Kevin Rogers of GeoTek Alaska, Inc., 

provided the direct-push equipment and expertise for the 

field investigation. Mike Allen, formerly of Westaff, provided 

valuable assistance in planning and data-collection efforts. 

Residents of Adak Island provided friendly support and 

logistical assistance. Greg Justin and Karen Payne of the U.S. 

Geological Survey, Tacoma, Washington assisted with data 

collection and analysis.

Study Methods

This investigation focused on 10 specific petroleum sites 

in OU A (table 1 and fig. 2). During May and June 2003, 

USGS personnel measured water levels and collected groundwater samples from about 100 temporary boreholes and 50 

existing monitoring wells. Most samples were analyzed using 

field test kits to determine NAP concentrations and an onsite gas chromatograph (GC) for concentrations of selected 

petroleum compounds. The USGS then evaluated the data 

on-site, selected new monitoring well locations, and installed, 

developed, and sampled 10 new monitoring wells. Data 

collection focused on delineating the downgradient margin of 

the contaminant plumes. New wells generally were installed 

downgradient from where petroleum compounds were 

positively detected at concentrations less than the specified 

cleanup standards. 

Sampling and Analysis Strategy

Expense and logistics for transporting equipment and 

people to Adak Island are formidable due to its remoteness, 

so completion of the well installation and data collection 

in a single field mobilization was considered essential. An 

expedited site-assessment strategy was used to quickly 

characterize the sites of interest, and to install the needed 

monitoring wells at optimal locations during a single 

mobilization to the island. Prior to the field work, up-to-date 

conceptual models for each site were developed, data needs 

were defined, and data quality objectives were developed 

(R.S. Dinicola, U.S. Geological Survey, written commun., 

December 23, 2002). A detailed work plan was developed, 

which included the decision criteria for collecting additional 

samples and installing new wells (R.S. Dinicola, U.S. 

Geological Survey, written commun., May 2003). 

Direct-push equipment was used in the field to allow rapid 

ground-water sampling from multiple temporary boreholes. 

Real-time analysis of ground-water samples using field 

test kits and an on-site gas chromatograph (GC) facilitated 

interpretation and decision making regarding the installation 

of new monitoring wells using the same direct-push 

equipment.

Direct-push sampling equipment was well suited to 

the Downtown area of Adak Island because of the relatively 

shallow depth to the water table and the unconsolidated 

subsurface sediments. The sampling strategy allowed rapid 

and accurate quantification of petroleum and most NAP 

concentrations. 

The primary focus of the characterization work at 

individual petroleum sites was the downgradient margin of 

the petroleum plumes, with the goal of installing new wells, 

which would allow long-term monitoring of the distal parts 

of the plumes. A specific sequence of field activities was used 

to characterize the downgradient margin of the petroleum 

plumes at each of the 10 petroleum sites:

1. Borehole and well sites were estimated where 

petroleum concentrations of concern were equal to 

compound-specific cleanup levels based on historical 

contaminant concentrations, NAP concentrations, 

and ground-water levels.

2. Direct-push equipment (Geoprobe® Model 6610DT, 

fig. 3) or a manual hammer was used to drive the 

screened portion of a ground-water sampling device 

below the water table. A Geoprobe® Screen Point 15 

hidden-screen type with a 1-in. outside diameter and 

a 41-in. long wire-wound stainless steel screen was 

used for direct-push sampling. A 0.75-in. diameter 

steel pipe with a disposable drive point was used for 

manual sampling. Initially each device was driven 

to a depth where the top of the sampling interval 

was about 1 to 2 ft below the water table where the 

highest concentrations of petroleum compounds 

historically have been detected, with a sampling 

interval about 1 to 2 ft below the water table. 

Study Methods 7
3. Water level in the sampling device was measured and 

converted to an altitude using a level referenced to a 

nearby existing well measuring-point altitude. The 

location (northing and easting) of the borehole was 

estimated by distance measurements from existing 

wells.

4. A ground-water sample was collected from the 

sampling device using a peristaltic pump. When 

feasible, the sample was analyzed immediately using 

various field kits and calibrated meters and sensors to 

determine field NAP parameters (dissolved oxygen, 

nitrate, sulfide, ferrous iron, carbon dioxide, pH, 

and specific conductance) (table 2, described in 

detail later in this section of the report). Petroleum 

concentration samples also were collected for 

analyses using a gas chromatograph at the nearby 

field laboratory. Samples with a geochemical 

signature indicative of petroleum contamination 

(elevated specific conductance, low dissolvedoxygen concentration, and elevated concentrations 

of ferrous iron and carbon dioxide relative to nearby 

uncontaminated wells) were analyzed for petroleum 

compounds.

5. An additional sampling device was installed 

nearby, about 5 ft deeper than the initial device, to 

determine if samples were from an appropriate depth. 

A new sample was then collected and analyzed 

for field parameters (and occasionally petroleum 

concentrations). If the field parameters and (or) 

petroleum concentrations indicated that petroleum 

Petroleum site name

Abbreviated 

petroleum 

site name

Primary 

contaminants of 

concern

Record of decision 

specified remedy

Former Power Plant Building T-1451 Former Power Plant DRO Monitored natural attenuation.

GCI Compound GCI GRO, benzene Free product recovery.

Housing Area (Arctic Acres) Arctic Acres DRO Monitored natural attenuation.

ROICC Contractor’s Area (UST ROICC-7) ROICC Benzene Limited ground-water monitoring.

Runway 5-23 Avgas Valve Pit Avgas Valve Pit GRO, benzene Monitored natural attenuation.

SA 80, Steam Plant No. 4 SA 80 DRO Free product recovery.

SWMU 14, Old Pesticide Storage and Disposal Area SWMU 14 GRO, DRO Monitored natural attenuation.

SWMU 61, Tank Farm B Tank Farm B GRO, benzene Monitored natural attenuation.

SWMU 62, New Housing Fuel Leak (Sandy Cove Housing) Sandy Cove Housing DRO Free product recovery.

Tanker Shed Tanker Shed DRO Free product recovery.

Table 1. Petroleum sites investigated by the U.S. Geological Survey at the former Naval complex, Operable Unit A, Adak Island, Alaska, May and June 

2003.

[Location of petroleum sites are shown in figure 2. Acronyms: GCI, General Communications, Inc.; ROICC, resident officer in charge of construction; SWMU, 

surface-water management area. Primary contaminants of concern: DRO, diesel range organics; GRO, gasoline range organics.]

probably was increasing with depth, additional 

sampling devices were driven to greater depths at 

5-ft increments (if time allowed) until the interval of 

maximum petroleum concentrations was determined. 

Once the optimal sampling depth was determined for 

a site, sampling within a local area was confined to 

that optimal depth for all remaining boreholes.

Steps 2 through 5 were repeated at additional locations 

using an iterative procedure guided by real-time mapping 

and interpretation of the water level, petroleum, and NAP 

data. Additional boreholes were driven until the distribution 

of petroleum and NAP concentrations indicated that the 

downgradient “footprints” of the contaminant plume and 

the NAPs were known. Sampling devices were left in place 

overnight at a few locations to ensure that measured water 

levels were equilibrated with aquifer conditions. Once all 

desired data were obtained from a temporary borehole, the 

sampling device was removed, the hole was filled with grout, 

and the land surface was returned to its natural condition.

Best professional judgment of the field crew and 

the senior hydrogeologist was used to determine when 

the downgradient footprint of the plume was adequately 

characterized. In addition to detectable petroleum 

concentrations, the crew identified areas with dissolvedoxygen concentrations less than 1 mg/L, sulfide greater 

than 0.01 mg/L, nitrate less than 0.5 mg/L, ferrous iron 

greater than 0.5 mg/L, and relatively elevated carbon dioxide 

concentrations. Because the NAP footprints (such as ferrous 

iron) often extended beyond the plume of detectable petroleum 

concentrations, they added confidence that the ground-waterflow path of interest had been characterized.

8 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
����������

Figure 3. Geoprobe® Model 6610DT pushing a temporary borehole for sampling at the General Communications, Inc. (GCI) compound site, 

Operable Unit A, Adak Island, Alaska. (Photograph taken by R.S. Dinicola, U.S. Geological Survey, Tacoma, Washington, 2003.)

Monitoring Well Construction, Development, 

and Surveys

Permanent monitoring wells were installed where 

long-term data were needed to meet monitoring objectives. 

Monitoring wells were installed with direct-push equipment 

and were constructed of PVC with a 11⁄2-in. inside diameter 

(fig. 4). Monitoring wells are 2.5- to 10-ft deep with 0.10 in. 

slotted PVC screens, the tops of which were from 0 to 3 ft 

above the current water table. New wells were screened at 

depths consistent with other monitoring wells in the vicinity. 

A filter pack composed of 20/40 mesh silica sand was 

installed to extend 0.5 to 2 ft above the top of the screen and 

0.5 to 1 ft below the bottom of the screen. Each well had an 

end plug at the bottom. The annular space around each well 

above the sandpack was sealed with bentonite slurry. Well 

casings extended 2.5 to 3 ft above the land surface. Aboveground casings were protected with 6 in. steel riser pipe 

surrounded by a concrete surface-seal and set about 2 ft below 

land surface. The riser pipe was set directly in a bentonite plug 

at land surface when the water table was less than 5 ft below 

land surface. Riser pipes typically extended about 6 in. above 

the top of the well casing. The top of the casing and the top of 

the protective riser pipe were equipped with caps. The aboveground space between the well casing and the riser pipe was 

filled with coarse sand to within 6 in. of the top of the casing.

New monitoring wells were developed using a dual-head 

peristaltic pump. Well development did not begin until at least 

24 hours after installation; development continued until the 

well water was visibly free of sediment. The new monitoring 

well at the Tank Farm B site was not completely developed 

and sampled before demobilizing from the island because the 

grout and concrete seal had not yet set in the saturated ground.

Altitudes, northings, and eastings of measuring points 

(the highest points of the PVC well casing) for the new 

monitoring wells were determined by completing a traverse 

survey from at least two existing monitoring wells using a 

level with a horizontal circle. Stadia or steel tapes were used 

to measure distances during the survey. The vertical datum 

for the new wells was NAVD88 and the horizontal datum was 

NAD27. Northings and eastings were for the Alaska State 

Plane, Zone 5 coordinate system.

Study Methods 9
Ground-Water Level Measurements

Ground-water levels (and free product thickness when 

encountered) were measured in existing monitoring wells 

prior to sampling using an electronic oil/water interface probe. 

Water levels were measured in temporary boreholes prior to 

sampling using a graduated steel tape because the electronic 

device did not work properly in the small-diameter steel 

drill casing. A few subsequent water levels were measured at 

selected boreholes over a 24-hour period to confirm that the 

water levels were at equilibrium. All water levels checked had 

equilibrated within the first hour.

Ground-Water Sampling and Analyses

Ground-water samples from the temporary boreholes 

and existing monitoring wells were collected with a peristaltic 

pump equipped with single-use polyurethane tubing. Existing 

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Figure 4. Typical monitoring well construction.

10 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
monitoring wells were sampled at the mid-point between the 

current water level and the bottom of the screen. Each well 

was purged at low-flow rates (less than 500 mL/min) until 

stabilization criteria measured in a flow-through cell were 

met (±0.3 mg/L for dissolved oxygen, ±0.1 unit for pH, and 

±3 percent for specific conductance). Samples for petroleum 

analyses were collected in 40-mL glass vials with septa lids 

and immediately transported for analysis to the field GC at 

the temporary field laboratory. Standard procedures for the 

collection and recording of purge-related water-quality data 

were followed as described in the USGS Techniques of WaterResources Investigations (U.S. Geological Survey, variously 

dated).

Temporary boreholes were sampled at mid-screen depth 

after only a few minutes of purging. Complete purging was 

unnecessary in the new boreholes because there was no 

stagnant well-water to evacuate. Woven stainless steel screens 

with 0.145-mL pores were fitted over the end of the sampling 

tube to minimize turbidity in the sample.

Unfiltered samples were analyzed at the wellhead for 

selected NAP analytes. A summary of field measurement 

methods, ranges, detection limits, and accuracies is shown in 

table 2. Sulfide measurements from borehole samples were 

severely affected by the visibly high turbidity of the samples 

that interfered with the colorimetric tests, so sulfide data are 

not tabulated in this report; selected sulfide concentrations for 

samples that were not visibly turbid are discussed in the text. 

Sulfate was not included because no simple and reliable field 

test is available for that compound. Nitrite was not included 

because it historically has been undetectable or detected at 

very low concentrations throughout the Downtown area. 

Methane was not included because it would have required an 

excessive length of time to switch the column of the field GC 

for methane analyses from the column needed for petroleum 

analyses. Used field kit materials were collected in sealed 

polyethylene bottles and shipped off Adak Island for proper 

disposal.

Field Analyses of Petroleum Compounds

A temporary GC laboratory was set up to rapidly 

measure concentrations of the petroleum compounds of 

concern (diesel-range organics (DRO), gasoline-range 

organics (GRO), and benzene). A Shimadzu brand GC (model 

17A) (fig. 5) was operated in a rented house where 110-volt AC 

power and heated space was available. The GC was equipped 

with a flame ionization detector and an Equity-1 capillary 

column (Supelco brand 28046-U), which was 30-m long, with 

a 0.25 mm inside diameter, and was coated with 0.25 μm of 

polydimethylsiloxane. Helium was the carrier gas at a flow 

rate of 1.2 mL/min. The GC injection port temperature was 

250 oC and the detector temperature was 325 oC. For DRO 

analyses only, the oven temperature program was isothermal 

at 40 oC. Benzene and GRO concentrations were calculated 

from the same chromatograms, and DRO concentrations 

were calculated from chromatograms created using separate 

samples.

Field measurement Measurement method Range Method detection 

limit or accuracy

Dissolved oxygen (High) (mg/L) CHEMetrics K-7512 1 to 12 1

(Low) K-7501 0 to 1 .025

Nitrate (mg/L) K-6902 0.1 to 5 .1

Sulfide (mg/L) K-9510 0.05 to 10 .05

Ferrous iron (mg/L) K-6210 0.05 to 1 .05

Carbon dioxide (Low) (mg/L) K-1910 10 to 100 10

(High) (mg/L) K-1920 100 to 1,000 100

pH (units) YSI 600XL 0 to 14 ± .2

Specific conductance (μS/cm) YSI 600XL 0 to 100 ± .5 %

Dissolved oxygen (mg/L) YSI 600XL 0 to 50 ± .2

Benzene (μg/L) GC-FID unlimited ~ 2

Diesel-range organics (DRO) (mg/L) GC-FID unlimited ~ .1

Gasoline-range organics (GRO) (mg/L) GC-FID unlimited ~ .1

Table 2. Summary of field measurement methods, ranges, detection limits, and accuracies used at the former Naval complex, Operable Unit A, Adak 

Island, Alaska, May and June 2003.

[Measurement method: GC-FID, gas chromatography with flame-ionization detector. Abbreviations: mg/L, milligram per liter; μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; %, percent; μg/L, microgram per liter. Symbols: ±, plus or minus; ~, approximately] 

Study Methods 11
Field GC methods for DRO, GRO, and benzene analyses 

approximated, but were not identical to, the established State 

of Alaska methods AK102 and AK101, and EPA method 

8021B, respectively (Alaska Department of Environmental 

Conservation, 2002; U.S. Environmental Protection Agency, 

2003). Solid phase microextraction (SPME) methods (SigmaAldrich Co., 1998) were used at the field site. These methods 

are different from the traditional liquid solvent extraction 

methods used in laboratory analyses, but they were selected 

for use in this study because they are easier and safer to use in 

a field laboratory. Subsequent identification and quantification 

of various compounds in the sample were essentially the same 

as that done in traditional laboratories.

Diesel Range Organics, Gasoline Range 

Organics, and Benzene Analyses 

Samples analyzed for petroleum compounds were 

collected in amber glass 40-mL volatile organic compound 

(VOC) vials in the field, and a 1-cm long Teflon®-coated 

magnetic stir bar was added before the cap was replaced. All 

samples were temporarily stored at about 4 °C in a samplededicated refrigerator before they were analyzed. GRO and 

Benzene samples were analyzed within 24 hours of collection. 

Most DRO samples were analyzed within 7 days of collection, 

although 11 samples were analyzed within 16 days due to 

equipment problems, which created a temporary backlog. 

����������

Figure 5. Portable gas-chromatograph (GC) in the temporary laboratory at Adak Island, Alaska. (Photograph taken by 

R.S. Dinicola, U.S. Geological Survey, Tacoma, Washington, 2003.)

12 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
For all analysis, the sample was brought to room temperature, 

and 2 mL of sample was removed from the vial. For GRO 

and benzene analysis only, 9 g of sodium chloride (NaCl) 

was added to the vial prior to removing the 2 mL of sample. 

All vials were shaken by hand for 10 seconds and placed on 

a magnetic stirrer. The septum of all vials was immediately 

pierced with the SPME needle and the sample water was 

exposed to a SPME fiber that can adsorb specific compounds 

based on the molecular weight of the analytes of interest 

to be extracted from the water sample (the 30-μm diameter 

polydimethylsiloxane (PDS) SPME fiber (Supelco, 57308-U) 

was selected for the analyses). The fiber was left immersed 

in the stirring water sample for 15 min ±30 sec. The fiber 

was retracted from the water sample and immediately 

inserted into the injection port of the GC. The extremely high 

temperature of the inlet port caused the compounds to desorb 

and flow with the carrier gas through the GC column. Specific 

petroleum compounds eluted from the column at known 

retention times, and the resulting concentration was calculated 

using calibration curves developed from the results of known 

calibration standards of DRO, GRO, and benzene. Chemicals 

used to make the calibration standards (BTEX standard, 

Product No. 31193; UST method Gasoline Range Organics 

standard, Product No. 30065-520; and UST method Diesel 

Fuel #2 composite, Product No. 30051-520) were obtained 

from Restek Corporation, Philadelphia, Pennsylvania. 

Analytical method detection limits for petroleum analyses are 

shown in table 2.

Quality Assurance and Quality Control of 

Petroleum Concentration Data

Quality assurance procedures for this investigation 

are described in a detailed work plan (R.S. Dinicola, U.S. 

Geological Survey, written commun., May 2003). The 

work plan describes procedures used for ground-water level 

measurements, ground-water sampling and analyses, field 

analyses of petroleum compounds, laboratory analytical 

procedures, and monitoring well construction, development, 

and surveying. 

Field Blank

A single field blank was analyzed for GRO and benzene 

to determine if there was any contamination from the 

sampling equipment (no field blanks were analyzed for DRO). 

To prepare the field blank, de-ionized water was pumped 

through polyethylene tubing and previously used Masterflex® 

tubing cleaned with Liquinox®. The field blank was 

transported and stored with the sample bottles until analysis. 

Analysis was identical to that of the environmental samples. 

The field blank did have a positive detection for GRO at an 

estimated concentration of 24 μg/L, which was substantially 

less than the approximate method detection limit of 100 μg/L. 

Benzene was not detected in the field blank at an approximate 

method detection limit of 2 μg/L. The slight apparent GRO 

contamination in the field blank probably was due to carryover from reusing the Masterflex® tubing in the peristaltic 

pump. Approximately 20 different pieces of the tubing were 

used during the investigation, so the magnitude of possible 

carry-over was variable. Tubing used for collection of the field 

blank previously had been used for collection of samples from 

boreholes with low estimated concentrations (less than 

94 μg/L) of GRO.

Method Blanks

Method blanks were used to determine if there was 

contamination from the analytical procedure. Nitrogen 

purged volatile grade water (EM Science, 7732-18-5) was 

used to prepare method blanks in amber glass 40-mL VOC 

vials at the field laboratory. A Teflon®-coated magnetic stir 

bar, 1 cm in length was added before the cap was replaced. 

Analysis was identical to that of the environmental samples. 

One method blank was analyzed each for DRO, GRO, and 

benzene. Concentrations in all method blanks were less than 

the method detection limits (table 2).

Instrument Blanks

Instrument blanks were used to determine if there 

was carry-over contamination from the GC or SPME after 

highly concentrated samples were analyzed. These blanks 

were essentially an indication of the elution efficiency of 

the SPME fibers. One instrument blank was used for DRO 

and six instrument blanks were used for each benzene and 

GRO. Instrument blanks were done using an SPME fiber that 

previously was used for analysis of a contaminated sample. 

Previously used SPME fiber was inserted into the injection 

port of the GC without immersing it into any environmental 

sample water, and the analysis was done using the same GC 

settings as those used for sample analysis. All instrument 

blanks were less than the method detection limits (table 2).

Field Duplicates

Field duplicates were collected and analyzed to 

determine the variation in collection of field samples and 

analysis of the samples. Field duplicates were analyzed for 

DRO in 6 samples, for GRO in 11 samples, and for benzene 

in 6 samples. Two vials were filled sequentially with sample 

water in the same manner as other samples; the first vial is 

referred to as the environmental sample and the second is 

referred to as the duplicate. Field duplicates were stored and 

processed identically to environmental samples.

Study Methods 13
Overall, the duplicate analyses were reasonably 

comparable for all compounds. Concentrations of benzene 

were <5 μg/L in the environmental and the duplicate samples 

for 4 of the 6 duplicate pairs. Interference from other 

compounds in the samples did not allow positive detection 

of benzene at the target detection level of 2 μg/L. DRO 

was detected at <100 μg/L in the environmental and the 

duplicate samples for 2 of the 6 duplicate pairs. The average 

difference in DRO concentrations between environmental 

and duplicate samples in the 4 pairs with DRO concentrations 

greater than the detection limit was 13 percent; the largest 

difference was 29 percent and the smallest difference was 

4 percent. Concentrations of GRO were detected at <100 

μg/L in the environmental and the duplicate samples for 5 

of the 11 duplicate pairs. The average difference between 

environmental and duplicate sample GRO concentrations 

in the 5 pairs that had GRO concentrations greater than the 

detection limit was 37 percent; the largest difference was 67 

percent and the smallest difference was 5 percent. GRO was 

detected at 110 and <100 μg/L in the other duplicate sample. 

Benzene was detected at 854 and 823 μg/L in one duplicate 

pair, and at <5 and 13 μg/L in the other duplicate pair. 

Inter-Laboratory Replicate Samples

Replicate samples were collected and analyzed to 

determine the accuracy of analytical field concentrations 

compared to analytical laboratory concentrations analyzed 

by standard laboratory methods. Eight replicate pairs were 

analyzed for DRO and four replicate pairs were analyzed 

for benzene and GRO. At the field location, four vials were 

filled sequentially with the same water sample. One of the 

vials was transported to the field laboratory and analyzed, 

and the other three vials were sent in a cooler with ice to 

Analytica Environmental Laboratories, Anchorage, Alaska, 

for analysis. GRO and benzene samples were analyzed in the 

field between 7 and 11 days after collection and were analyzed 

in the laboratory within 14 days. DRO samples were analyzed 

in the field between 12 to 26 days after sample collection 

and were analyzed in the laboratory within 7 days. Analytica 

Environmental Laboratories used EPA Method 8021B 

- aqueous (U.S. Environmental Protection Agency, 2003) to 

determine benzene concentrations, method ADEC AK101 

(Alaska Department of Environmental Conservation, 2000) 

to determine GRO concentrations, and method ADEC AK102 

(Alaska Department of Environmental Conservation, 2000) to 

determine DRO concentrations.

Replicate sample concentrations showed that field and 

laboratory analytical results were reasonably comparable for 

GRO and benzene analyses. GRO was not detected in either 

the field or laboratory at concentrations of <100 μg/L in the 

field and <50 μg/L in the laboratory. GRO was detected at 

<100 μg/L (field) and 65 μg/L (laboratory) in one pair, and at 

178 μg/L (field) and 310 μg/L (laboratory) in the other pair. 

In three of the four sample pairs, benzene was not detected 

in either the field or the lab at concentrations of <5 μg/L in 

the field and <2 μg/L in the laboratory. Benzene was detected 

at 13 μg/L (field) and 3.3 μg/L (laboratory) in the other pair. 

In two of the four sample pairs, GRO was not detected at 

concentrations <100 μg/L in the field and <50 μg/L in the 

laboratory. GRO was detected at <100 μg/L (field) and 

65 μg/L (laboratory) in one pair, and at 178 μg/L (field) and 

310 μg/L (laboratory) in the other pair.

Replicate sample concentrations were more variable for 

DRO analyses. In one of the eight sample pairs, DRO was not 

detected in either the field or laboratory at concentrations of 

<100 μg/L. In another pair, DRO was detected at 230 μg/L 

(field) and <100 μg/L (laboratory). The average difference 

between DRO concentrations in the remaining six sample 

pairs that had DRO concentrations greater than the reporting 

limits was 52 percent. Laboratory-determined concentrations 

were greater than the field-determined concentrations in five 

of those sample pairs, which suggests that the field analyses 

may be biased low. Laboratory-determined concentration 

was about twice the field-determined concentration in 

three of the pairs, and was about four times the fielddetermined concentrations in one of the pairs. The low result 

in the field-determined DRO concentrations is probably 

a result of difficulties encountered in getting repeatable 

calibration results when analyzing samples spiked with 

known concentrations of DRO. Although the magnitude of 

the differences between field and laboratory DRO analyses 

were greater than anticipated, the data met the data-quality 

objectives and were useful as a screening-level tool to identify 

optimal locations for new monitoring wells. 

Petroleum Biodegradation Processes 

and Geochemical Indicators

A large body of scientific research exists on the microbial 

transformation (biodegradation) of petroleum compounds in 

ground water (National Research Council, 2000). Following 

is a brief summary of that research relating to the types of 

petroleum compounds and the specific geochemical indicators 

of biodegradation that were observed at Adak. Geochemical 

indicators were relied on extensively during the USGS 

field investigation to determine ground-water-flow paths 

from contaminant source areas, and to properly locate new 

monitoring wells.

Biodegradation of petroleum hydrocarbons varies 

depending on the type of hydrocarbon. Gasoline and jet fuel 

(JP-5) were the most extensively used hydrocarbons on Adak 

Island. Gasoline is primarily composed of relatively volatile 

hydrocarbons containing 4 to 12 carbon atoms, including 

the “BTEX” compounds benzene, toluene, ethylbenzene, 

and xylene. JP-5 jet fuel is a kerosene-based fuel primarily 

14 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
composed of relatively low-volatility hydrocarbons containing 

11 to 13 carbon atoms with relatively insignificant (less than 1 

percent by weight mass fraction) BTEX content.

Biodegradation of BTEX compounds to form the final 

end-product carbon dioxide is through a process that generally 

is well understood. The efficiency of the biodegradation is 

primarily a function of the availability of the various electronacceptors that may be used in the reactions as well as the 

presence of the appropriate microorganisms and enzymes. 

Oxygen is by far the most favorable electron-acceptor, and 

BTEX degradation is relatively fast and efficient in aerobic 

ground water. When dissolved oxygen is depleted within 

a contaminant plume, nitrate is the next most favorable 

electron acceptor, followed by manganese, ferric iron, sulfate, 

and carbon dioxide. All these anaerobic biodegradation 

processes are slower than aerobic processes, and benzene 

degradation is particularly slow when carbon dioxide is the 

only available electron acceptor. Manganese is not often 

considered an important electron acceptor because it generally 

is not abundant in aquifer sediments. Biodegradation of the 

various longer chained hydrocarbons that make up JP-5 is less 

understood, although aerobic degradation generally is more 

rapid than anaerobic degradation.

Evidence for hydrocarbon degradation in ground water 

is most often evaluated by comparing the concentrations of 

various electron acceptors and end products in “background” 

ground water (located upgradient or in nearby uncontaminated 

areas) to concentrations of the same compounds within and 

downgradient from a hydrocarbon plume. Ground-water 

geochemistry in the Downtown area of Adak Island has been 

previously characterized (ICRC, 2003), so “background” data 

for comparison to data from within contaminant plumes were 

available in advance of the field investigation. Background 

ground water in the Downtown area has been characterized as 

generally aerobic (dissolved-oxygen concentrations of around 

6 mg/L), with relatively low specific conductance (generally 

less than 200 μS/cm), low nitrate concentrations (1 mg/L), 

essentially no ferrous-iron (< 0.01 mg/L), sulfide 

(< 0.01 mg/L) or methane (< 0.001 mg/L), relatively little 

sulfate (6 mg/L) and alkalinity (about 34 mg/L as CaCO3).

A universal indicator of hydrocarbon biodegradation is 

an accumulation of the end product carbon dioxide, which 

is often measured indirectly with a strong acid titration to 

determine alkalinity, but was measured more directly for 

this investigation using a strong base titration to determine 

carbon dioxide acidity. An indication of aerobic hydrocarbon 

degradation is depletion of dissolved oxygen within a plume. 

An indication of hydrocarbon degradation linked to nitrate 

reduction is the depletion of nitrate (relative to upgradient 

concentrations) and a possible accumulation of nitrite, 

although nitrite generally is short-lived in shallow ground 

water and was not analyzed for in this study. An indication 

of hydrocarbon degradation linked to iron reduction is the 

accumulation of ferrous iron. Ferrous iron is the soluble 

byproduct from iron reduction, and the oxidized ferric iron 

that serves as electron acceptors is primarily in a non-soluble 

state associated with aquifer sediments. An indication of 

hydrocarbon degradation linked to sulfate reduction is the 

depletion of sulfate and a possible accumulation of sulfide 

compound, although sulfide is generally short-lived in 

shallow ground water. Lastly, an indication of hydrocarbon 

degradation linked to carbon dioxide reduction is the 

accumulation of methane. That process, referred to as 

methanogenesis, is relatively inefficient and generally does not 

result in an observable depletion of carbon dioxide because 

of the abundance of carbon dioxide produced by the more 

efficient degradation processes.

One confounding circumstance for evaluating the 

indicators of hydrocarbon degradation is that they also are 

indicative of microbial transformation of non-petroleum 

organic compounds that may be naturally abundant in certain 

settings. In a wetland area where the water table is at or near 

the land surface, for example, the microbial degradation of 

abundant naturally occurring organic matter in saturated 

surface soils results in a geochemical signature that may be 

indistinguishable from the signature resulting from petroleumhydrocarbon degradation. The perimeter of the Downtown 

area includes such areas.

Another confounding circumstance for evaluating 

the above indicators of hydrocarbon degradation is that 

conventional 10-ft long (or even 5-ft long) monitoring well 

screens often will span a thickness of aquifer in which 

different biodegradation processes may be occurring in thin 

vertical sections. The result can be a mixture of geochemical 

indicators in a single sample (such as aerobic conditions 

with abundant ferrous iron), or widely varying indicator 

concentrations between sampling rounds (such as wells 

changing between aerobic and anaerobic conditions depending 

on the season, or annual rainfall, or sampling depth). Lowflow sampling procedures are designed to minimize this 

problem, but they do not eliminate it. If sampling protocols 

call for sampling at the same depth below land surface each 

time, samples may be from the top of the water table during 

a dry year and from deeper below the water table in a wet 

year. If sampling protocols call for sampling at the same 

depth below the water table each time, samples may be from 

different parts of the aquifer year to year. Sampling at the 

same depth below land surface is the preferred method, but 

some confounding results may still be observed.

During this investigation, the most consistent 

geochemical indicators for petroleum degradation observed 

were anaerobic conditions (dissolved-oxygen concentrations 

less than 1 mg/L), and increased ferrous iron and carbon 

dioxide concentrations compared to nearby uncontaminated 

ground water. Although nitrate concentrations were measured 

reliably in the field, the concentrations often were less than 

detection limits in uncontaminated locations. Relatively high 

measurements of specific conductance were used during this 

investigation to help identify contaminant plumes, but this is 

not a unique indicator of biodegradation.

Petroleum Biodegradation Processes and Geochemical Indicators 15
Monitoring for Natural Attenuation at 

Specific Petroleum Sites

Results of the field characterization work at each of 

the 10 selected petroleum sites are presented with a focus 

on improved monitoring for natural attenuation at OU A. 

For each site, brief background information is presented, 

followed by the presentation of the data collected, a discussion 

of the rationale used to locate new wells, an evaluation of 

the available evidence for petroleum biodegradation and 

attenuation, and suggestions for future monitoring locations. 

For most sites, additional sampling over time is needed for a 

more robust evaluation of the overall effectiveness of natural 

attenuation. 

Selected data for each petroleum site are presented 

with the discussion of the individual sites. For clarity, 

field-determined petroleum concentrations are referred to 

as “field DRO,” while laboratory-determined petroleum 

concentrations are referred to as “lab DRO.” At times there 

were substantial differences between field and laboratory 

analyses, particularly for DRO concentrations. Petroleum 

concentrations often were substantially different in a 

borehole sample and in a monitoring well sample from the 

same location. Those differences are a result of the different 

analytical methods used in the field and laboratory, the 

different screen lengths of the borehole sampler (3.5 ft) and 

the monitoring wells (generally 10 ft), and heterogeneity in the 

plumes. Selected historical data obtained from the Navy are 

included when appropriate. Each site map accompanying the 

discussion shows only the selected data that were most useful 

for identifying the site’s contaminant plumes in the field. An 

explanation of symbols used in common on the site maps is 

shown in figure 6.

Figure 6. Explanation of symbols used on site maps.

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16 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
Former Power Plant Building T-1451

Former Power Plant Building T-1451 (fig. 7) is in the 

southwest part of the Downtown area (fig. 2). Potential 

sources of petroleum contamination at the site include 

three former diesel-fuel above-ground storage tanks (ASTs) 

immediately east of the former power plant building, an 

AST for JP-5 jet fuel 50 ft south of the building, a northsouth running diesel pipeline 100 ft east of the building, 

a JP-5 pipeline along the west side of Main Road, and an 

avgas pipeline along the east side of Main Road. None of the 

pipelines are still in service. The petroleum release history is 

not well known, and no free product has been detected at the 

site. In the past, an oil sheen had been detected in East Canal 

(about 500 ft west and downgradient of the site). A section of 

the canal was filled and routed through culverts in an attempt 

to prevent the sheen. Ground water entering East Canal near 

the site flows about 1,000 ft before it is pumped over a berm 

into Sweeper Creek (fig, 2). MNA is the ROD-specified 

remedy for ground-water contamination originating at the 

Former Power Plant, although additional possible contaminant 

sources west of Main Road are not administratively included 

in the site.

The Former Power Plant site is underlain primarily by 

sands and silty sands with some gravel. Near the former power 

plant, the water table is about 15 to 20 ft below land surface, 

and near East Canal it is about 5 ft below land surface. 

Ground-water flow direction across the site is to the westsouthwest, and the previously estimated seepage velocity is 

276 ft/yr (URS Greiner, Inc., 1999). Recent (2002) monitoring 

at the site included one monitoring well (01-118), although 

nearby well E-701 was sampled as a background well. 

Historical (1996-97) ground-water data were available for nine 

abandoned wells used for previous investigations. 

Objectives at the site were to determine:

1. Downgradient extent of dissolved-phase DRO 

concentrations exceeding the 1,500 μg/L cleanup 

level that came from the suspected Former Power 

Plant source area and a suspected but unidentified 

source located west of Main Road, and

2. If concentrations of NAPs and DRO indicate 

biodegradation and attenuation of petroleum 

compounds within the DRO plume. 

Figure 7. Diesel range organics (DRO) and carbon dioxide concentrations for the Former Power Plant Building T-1451 site, 

Operable Unit A, Adak Island, Alaska, May and June 2003.

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01-148

Monitoring for Natural Attenuation at Specific Petroleum Sites 17
The field investigation to determine the lateral and 

downgradient extent of the western plume with the 

unidentified source included pushing and sampling boreholes 

at nine locations spaced 100 to 150 ft apart in a northsouth transect along the east bank of East Canal. The field 

investigation to determine the downgradient extent of the 

Former Power Plant plume itself included installing and 

sampling two boreholes spaced 75 ft apart immediately east 

and west of Main Road. Boreholes were installed along a 

ground-water-flow path downgradient from well 01-118, 

which was estimated from historical water-level data and 

confirmed by geochemical indicators. Water levels from 

boreholes (FPP-1 through FPP-9) along East Canal 

(table 3 and fig. 7) could not be interpreted directly to estimate 

flow direction because the water level in East Canal changed 

rapidly during sampling due to pumping from East Canal into 

Sweeper Creek 1,000 ft downstream (fig. 2). Borehole FPP12, about 200 ft east of the site, was installed to determine 

background (upgradient) geochemistry conditions.

Sampling results (table 3) indicate that there may be 

two overlapping DRO plumes from different sources at the 

site. One plume begins under the Former Power Plant and 

DRO concentrations exceeding the cleanup level extend to 

just west of Main Road. A second plume begins immediately 

west of the first plume and extends to East Canal (fig. 7). The 

approximate upgradient extents of the plumes shown on 

figure 7 were augmented by 1996-97 DRO concentration data 

from abandoned wells at the site. The downgradient extent 

of the Former Power Plant plume initially was indicated 

in the field by a field-DRO concentration of 1,300 μg/L 

from borehole FPP-11, although the subsequent lab-DRO 

concentration in a split sample from FPP-11 was 2,300 μg/L. 

Borehole or 

well No.

Date 

sampled

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Diesel range 

organics (μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron 

(mg/L) Field Laboratory

Boreholes

FPP-1 05-19-03 4 0.97 – – 6.2 218 20 4 0

-2 05-19-03 4 .91 – – 6.1 336 25 4 .54

-3 05-19-03 4 .79 – – 6.6 214 10 6 .06

-4 05-19-03 4 .85 630 – 6.3 365 80 <.025 1.1

-4B 05-19-03 10 – 630 – 6.5 401 150 <.025 1

-5 05-19-03 7 -.62 870 – 6.1 494 170 <.025 1.6

-6 05-19-03 8 1.45 1,400 – 6.3 618 35 .05 1.4

-7 05-19-03 8 2.94 320 – 6.6 261 35 <.025 1.3

-8 05-19-03 6 .89 <100 – 6.2 156 20 6 .15

-9 05-19-03 6 .73 – – 6.6 261 15 2 .18

-10 05-22-03 23 3.42 2,900 – 6.1 251 70 <.025 .9

-11 05-22-03 20 3.09 1,300 2,300 6.1 482 170 <.025 1.5

-12 05-22-03 20 3.35 – – 7.3 169 17 6 .1

Monitoring wells

01-118 05-08-03 22.5 4.06 970 18,700 5.7 450 200 0.05 50

-150 06-05-03 19 2.90 – 590 6.2 404 115 .9 4

-151 06-05-03 7.5 .95 – 2,000 6.6 405 120 <0.025 40

1 The laboratory diesel range organics concentration listed for well 01-118 was from October 2002; the field diesel range organics value was inconsistent 

with all historical data from the well.

Table 3. Selected diesel range organics, geochemical, and water-level data from boreholes and wells at the Former Power Plant Building T-1451 site, 

Operable Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 7. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well 

screen. Water-level altitudes are in feet above or below (-) NAD27. Diesel range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: <, actual 

value is less than value shown; –, not analyzed]

18 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
A new monitoring well (01-150) was installed at the FPP-11 

location, and a lab-DRO concentration of 590 μg/L confirmed 

that the location was beyond the downgradient margin of the 

Former Power Plant plume. We believe that the concentration 

may be much greater because the accuracy of the 970 μg/L 

field-DRO concentration for well 01-118 is suspect. DRO 

concentrations at well 01-118 from 1996 to 2002 have ranged 

from 6,400 to 10,700 μg/L.

The western plume with the unidentified source is 

presumed to begin somewhere west of Main Road (fig. 7), and 

appears to extend westward to East Canal where the labDRO concentrations in new monitoring well 01-151 

(2,000 μg/L) exceeded the 1,500 μg/L cleanup level. 

Petroleum sheen was observed in East Canal while sampling, 

and petroleum sheen was observed emerging from nearshore 

bed sediments between FPP-3 and FPP-7 when the sediments 

were stepped on. It was presumed that the East Canal area 

contamination originated somewhere west of Main Road 

and not at the Former Power Plant site because an additional 

contaminant source is the most likely explanation for 

detecting higher DRO concentrations detected along East 

Canal compared to well 01-150 location along Main Road. 

An alternative explanation is that an initial relatively large 

petroleum release at the Former Power Plant was followed in 

time by a smaller release at the same location. As previously 

mentioned, the water-level data from near East Canal were not 

reliable enough to determine the specific ground-water flow 

direction between Main Road and East Canal. Lateral limits 

of the DRO plume along East Canal (FPP-4 to FPP-7) were 

clearly indicated by anaerobic water with elevated specific 

conductance and DRO, carbon dioxide, and ferrous iron 

concentrations (table 3).

The USGS NAP data clearly indicated active 

biodegradation at the site. Upgradient shallow-ground water 

geochemistry (FPP-12) was typical for the Downtown area; 

aerobic with low specific conductance, and low concentrations 

of carbon dioxide and ferrous iron. Geochemistry within the 

plume indicated anaerobic ground water with substantially 

elevated specific conductance, and elevated concentrations 

of carbon dioxide and ferrous iron. Biodegradation endproduct carbon dioxide concentrations within the most highly 

contaminated part of each plume were about 10 times higher 

than background concentrations. Historical (2001-02) NAP 

data from well 01-118 showed very low sulfate concentrations 

of about 0.2 mg/L, and relatively high methane concentrations 

of up to 10 mg/L indicating intense biodegradation within the 

core of the DRO plume have consumed all available electron 

acceptors to create methanogenic conditions.

Lab-DRO concentrations from the Former Power Plant 

plume suggest substantial attenuation along the flow path 

between well 01-118 and the new well 01-150, although those 

data are from different years; the absolute concentrations 

from field-DRO analyses were too uncertain at this site to 

definitively demonstrate attenuation. Historical (1996-2002) 

lab-DRO concentrations from well 01-118 ranged from 6,400 

to 10,700 μg/L with no discernible trend over time, so those 

data alone do not demonstrate attenuation. DRO attenuation 

in the western plume is indicated by a decrease in DRO 

concentrations from 7,000 μg/L in 1997 in abandoned well 

01-148 to 2,000 μg/L in new well 01-151 in 2003, and by the 

clear indication of biodegradation in the NAP data collected 

along the east shore of the canal. Differences in the timing 

of data collection, well construction details, and analytical 

techniques add some uncertainty to the absolute magnitude of 

attenuation.

Overall, data from this investigation indicate 

natural attenuation processes have effectively limited the 

downgradient extent of the 1,500 μg/L DRO plume from well 

01-118 at the Former Power Plant to about 150 ft at Main 

Road. In the apparently separate western plume, natural 

attenuation processes do not appear to be wholly effective 

at preventing the discharge of ground water with DRO 

concentrations greater than 1,500 μg/L into East Canal, and a 

petroleum sheen was observed in the canal.

Suggestions for future monitoring at the Former Power 

Plant site include:

1. Sampling well 01-150 to monitor the stability of the 

downgradient margin of the Former Power Plant 

plume,

2. Sampling well 01-118 to monitor the contaminant 

source strength over time, and 

3. Sampling well 01-151 to monitor the DRO 

concentration in ground water from the apparently 

separate western plume immediately before it 

discharges to East Canal.

Monitoring for Natural Attenuation at Specific Petroleum Sites 19
GCI Compound

The GCI (General Communications, Inc.) Compound 

site, near the center of the Downtown area (fig. 8), was used 

as a gasoline station and motor pool facility in 1946. GRO 

and benzene are the primary contaminants of concern at this 

site. A 6,000 gal underground storage tank (UST) located 

40 ft southeast of the building and associated pipelines 

were removed in 1995 along with approximately 3,000 gal 

of remaining liquid. A previously unknown supply/fill pipe 

separated from the tank during removal and about 2,000 gal 

of water and petroleum residue discharged into the excavation. 

About 90 percent of the liquid was recovered. No records 

of other releases from that UST are available. The RODspecified remedy at the GCI site was free product recovery. 

Passive skimmers were used to recover free product near 

wells 04-201 and 04-202, and free product has not been 

observed in any GCI monitoring well since November 1997. 

MNA is being considered by the Navy as a follow-up remedy 

for dissolved-phase contamination in ground water at the 

site (remediation of dissolved-phase contamination was not 

addressed specifically in the ROD for sites with free product 

recovery remedies).

The GCI Compound is underlain primarily by sands and 

silty-sands with some gravel. A perched ground-water zone 

is present at a depth of about 5 ft below land surface adjacent 

to the former UST, and is present at a depth of 25 ft below 

land surface about 500 ft downgradient. Ground water from 

the GCI Compound flows in a semi-radial pattern in an arc 

extending from the south to the west. Recent (2002) waterquality monitoring included one well (04-701), although many 

additional wells are still at the site. Selected historical (1996-

2002) ground-water data were available for most additional 

wells, including NAP data from 2002.

Figure 8. Gasoline range organics (GRO) concentrations and water levels for the GCI Compound site, Operable Unit A, Adak Island, 

Alaska, May and June 2003.

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20 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
Objectives were to determine:

1. If well 04-701 is suitably placed to monitor the 

downgradient (western) extent of dissolved-phase 

benzene and GRO concentrations exceeding the 5 

μg/L or 1,300 μg/L cleanup levels, respectively,

2. Extent of the contaminant plume towards the south 

and southwest, and 

3. If concentrations of NAPs, benzene, and GRO 

indicate biodegradation and attenuation of petroleum 

compounds within the plume. 

The field investigation began with an initial west to east 

transect of three boreholes (GCI-1A and -1B, GCI-2A and 

-2B, and GCI-3A and -3C) spaced 100 ft apart and about 

300 ft south-southwest of the former UST. (A letter at the 

end of the borehole name indicates that different depths 

were sampled at the same location; A denotes the shallowest 

depth). The perched ground-water zone was detected at 

a depth of about 16 ft in the shallow boreholes (data not 

shown), but GRO concentrations were substantially lower 

than those detected in the deeper boreholes. GRO and benzene 

concentrations in the three deep boreholes (GCI-1B, GCI-2B, 

and GCI-3C, fig. 8 and table 4) greatly exceeded those at the 

GCI source area indicating a second overlapping GRO plume 

from an unidentified source for GRO somewhere south or 

southwest of the GCI Compound along Main Road. Additional 

boreholes (GCI-4 through GCI-8) were installed in an attempt 

to characterize the extent of area-wide GRO contamination, 

but that effort was terminated before the plume was fully 

characterized because it expanded well beyond the scope of 

the GCI site investigation. The investigation then refocused on 

the contaminant plume originating at the GCI Compound itself 

with two additional boreholes (GCI-10 and GCI-11), which 

were nearer the GCI source.

Borehole or 

well No.

Date 

sampled

Date of 

historical 

petroleum 

data

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Gasoline range 

organics (μg/L)

Benzene 

(μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous

iron

(mg/L) Field Laboratory Field Laboratory

Boreholes

GCI-1B 05-14-03 – 25 4.88 30,000 – 3,400 – – – – – –

-2B 05-14-03 – 25 4.34 3,000 – 0 – 6.4 161 27 0.6 30

-3C 05-23-03 – 25 5.53 – 28,000 – 6.6 5.8 262 100 <.025 14

-4 05-15-03 – 18 3.76 870 – <2 – 5.6 143 35 .2 3.6

-5 05-15-03 – 28 4.00 10,000 – 270 – 5.7 285 70 .2 2.2

-6 05-15-03 – 28 3.84 3,400 – 32 – 5.8 200 70 <.025 1.5

-7 05-16-03 – 18 4.43 15,000 – <2 – 5.7 342 180 .2 45

-8 05-16-03 – 18 3.06 370 – <2 – 6.1 135 5 4.4 1.0

-10 05-23-03 – 24 – – 3,800 – 5.8 6.2 348 130 <.025 3.3

-11 05-23-03 – 24 – – 3,100 – 8.4 6.1 310 110 <.025 9.3

Monitoring wells

03-107 05-16-03 – 28 4.60 10,000 – 30 – 6.0 266 160 <0.025 50

-502 05-16-03 – 24 3.38 2,900 – 2 – 5.8 249 100 <.025 15

04-100 06-06-03 – 26 5.25 – 850 – 1.4 6.4 306 70 .4 18

-202 05-14-03 08-01-02 22.5 7.00 – 5,100 – 8.7 – – – – –

-207 05-14-03 08-01-02 22.5 7.04 – 1,700 – 2.3 – – – – –

-210 05-14-03 08-01-02 28 5.29 – 5,000 – 12 – – – – –

-213 05-14-03 08-01-02 25 5.89 – 4,000 – 3 – – – – –

-701 05-15-03 – 13 4.67 <100 – <2 – 5.6 192 65 .1 9.2

Table 4. Selected gasoline range organics, benzene, geochemical, and water-level data from boreholes and wells at the GCI Compound site, Operable 

Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 8. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well 

screen. Water-level altitudes are in feet above or below (-) NAD27. Gasoline range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: <, actual 

value is less than value shown; –, not analyzed]

Monitoring for Natural Attenuation at Specific Petroleum Sites 21
Lab-GRO concentrations in boreholes GCI-10 and 

GCI-11 (3,800 and 3,100 μg/L, table 4) were less than those 

observed by the Navy in August 2002 in pre-existing wells 

nearer the GCI compound source area. Those results indicate 

GRO concentrations within the GCI plume itself attenuate as 

the plume moves downgradient. Attenuation continues towards 

the west as indicated by the relatively low GRO concentrations 

detected in well 04-701 (<100 μg/L for this investigation 

and 99 μg/L in October 2002). NAP data from well 04-701 

(anaerobic with relatively high ferrous iron and carbon dioxide 

concentrations) suggest that the well is on a contaminated 

flow path and that biodegradation occurs along that flow path. 

However, GRO concentrations increased substantially in many 

boreholes located south and southwest of borehole GCI-10, 

indicating an additional contaminant source in the area. A 

petroleum sheen (determined by GC analysis to be primarily 

GRO) was observed in discharging ground water at the head 

of East Canal a few hundred feet upstream from a previously 

identified DRO-related sheen located downstream of well 03-

012; our field investigation could not determine if the sheen 

was related to the GRO plume characterized with data from 

boreholes GCI-1B through GCI-8.

The discontinuous distribution of GRO concentrations in 

the area south of the GCI plume was similar to that detected 

in a DRO plume associated with a separate petroleum site 

located immediately south of the GCI Compound site. 

Subsurface hydraulic heterogeneities in this part of the 

Downtown area may be the cause of the discontinuous 

distribution. A northeast to southwest trending break in 

topographic slope west of the GCI site probably is the surficial 

expression of foreset dunes that are thought to hinder and 

channelize ground-water flow in the area. The 10,000 μg/L 

GRO contour (fig. 8) appears to follow the top of the dunes, 

suggesting a hydraulic barrier is limiting contaminant 

migration farther to the west. The barrier may be related to 

the darker, reddish band evident in a similar dune (fig. 9) 

Figure 9. Foreset dune about 500 ft north of the GCI Compound site (looking north-northwest), Operable Unit A, Adak Island, Alaska. 

(Photograph taken by R.S. Dinicola, U.S. Geological Survey, Tacoma, Washington, 2003.)

22 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
exposed about 500 ft north of the GCI Compound; the band is 

a low-permeability layer that slopes upward from east to west. 

Similar dunes likely mark the western edge of the topographic 

flat south of the GCI site, and the area east of the dunes has 

been back-filled by the military. A natural lagoon west of the 

dunes was filled to allow construction of the airfield runways.

Overall, the NAP data from the GCI site indicate 

active biodegradation beyond what was already indicated 

in background water for the site. Upgradient shallow 

ground-water geochemistry for the GCI site probably is not 

typical for uncontaminated background water because of 

upgradient petroleum contamination. The Navy detected 

petroleum compounds at concentrations less than the cleanup 

levels and indicator concentrations of NAPs in upgradient 

well MRP-MW9 in August 2002. NAP concentrations in 

that upgradient well were indicative of some petroleum 

biodegradation with no dissolved oxygen and moderately 

elevated concentrations of specific conductance (121 μS/cm), 

alkalinity (32 mg/L), ferrous iron (2.4 mg/L), and methane 

(0.8 mg/L). The Navy also sampled the GCI source area wells 

at that time and detected substantially higher concentrations 

of the NAPs, including specific conductance (336 μS/cm), 

alkalinity (85 mg/L), ferrous iron (25 mg/L), and methane 

(3 mg/L). The USGS measured similarly high specific 

conductance, carbon dioxide, and ferrous iron, concentrations 

in the anaerobic water in the nearest boreholes (GCI-10 and 

GCI-11) as well as most of the farther downgradient boreholes 

and wells (table 4). As mentioned previously, NAP data from 

well 04-701 suggest that the well is on a contaminated flow 

path, and that biodegradation is a contributor to the low GRO 

concentrations observed. Hydraulic heterogeneities in the 

area immediately east of 04-701 also may limit contaminant 

migration towards the well, although the steep slope to 

the east of the well is an excavated feature that may have 

disrupted the hydraulic-limiting structure in the pre-existing 

dune line.

GRO concentrations in the GCI plume indicate some 

attenuation along the flow path towards the southwest between 

the GCI Compound (considering the August 2002 data from 

well 04-202) and the new well 04-100. Attenuation also is 

evident along the downgradient part of the flow path to the 

west between well 04-210, borehole GCI-11, and well 

04-701, but attenuation is limited along the beginning part of 

that flow path between wells 04-202 and 04-210. Attenuation 

is not apparent along the southward flow path between 

wells 04-202 and 04-213, probably because of an additional 

petroleum source between the wells. The GRO plume at 

the GCI site appears to be stable, although that cannot be 

confirmed with the available data. Historical (1996-2002) 

GRO and benzene concentrations in all GCI source-area wells 

have decreased consistently and substantially, indicating that 

petroleum concentrations throughout the site also have been 

attenuated over time. No such consistent trends have been 

observed at downgradient well 04-701, although contaminant 

concentrations there have always been much less than the 

specified cleanup levels.

Overall, data from this and previous investigations 

indicate natural attenuation processes are limiting the extent 

of the 1,300 μg/L GRO plume and the 5 μg/L benzene plume 

that originate at the GCI Compound site to less than 150 ft 

towards the southwest, and to less than 350 ft towards the 

west. Attenuation of the plumes to the south is confounded by 

an additional source of GRO, which appears to have created 

a plume that is more extensive than the GCI plume itself. The 

newly identified GRO plume south of the GCI site was not 

characterized well enough to ascertain the effectiveness of 

attenuation. 

If MNA is selected as the remedy for this site, monitoring 

suggestions include: 

1. Sampling well 04-100 to monitor the stability of the 

downgradient margin to the southwest, 

2. Continued sampling of well 04-701 to monitor plume 

stability towards the west, and 

3. Sampling well 04-202 near the source area to 

monitor contaminant source-strength area over time. 

Monitoring for Natural Attenuation at Specific Petroleum Sites 23
Housing Area (Arctic Acres)

The housing area known as Arctic Acres was constructed 

in 1975 in the Downtown area of Adak Island (fig. 2). Each 

housing unit was furnished with a pressurized steel pipe that 

supplied heating fuel from two 27,000-gal ASTs immediately 

west of well 03-890 (fig. 10). The release history of the 

site is partially known. In 1993, a pipeline test detected 10 

leaks in a 150-ft length of pipe under Dolly Varden Drive. It 

was not known how long the pipe had been leaking, so the 

volume released is unknown. DRO-related compounds are the 

predominant contaminants and free product up to 1-ft thick 

historically has been observed in well 03-890. Monitored 

natural attenuation is the ROD-specified remedy for the site.

Arctic Acres housing area is underlain primarily by 

sands and silty-sands with some gravel. Ground water is 

20-30 ft below land surface. Ground-water flow from the 

possible source areas had been presumed to be towards the 

west-southwest, but water-level data collected monthly by 

the Navy indicate that the flow direction may be variable or 

towards the east. Recent (2002) monitoring at the site included 

nine wells (all “03” prefix wells, AA-01, AA-02, AA-05 and 

AA-06). Selected historical (1996-2002) ground-water data 

were available for most wells, although the wells with “AA” 

prefixes were not installed until 2001.

Objectives at this site were to determine: 

1. Direction of contaminant migration from the area of 

highest DRO concentrations (wells 03-890 and 

03-421), 

2. If existing wells are suitably located to monitor the 

extent of contamination and plume stability, and 

3. If concentrations of DRO and NAPs indicate 

biodegradation and natural attenuation of petroleum 

compounds within the plume. 

Data from existing wells and four temporary boreholes were 

used to meet the objectives.

Water-level data (table 5) collected for this and previous 

investigations indicates a ground-water flow divide near the 

center of the site, but the location of the divide appears to 

be variable. Eastward flow from a north-south line between 

wells 04-422 and 03-416 has been consistent, but the water 

table is nearly flat and flow directions have been variable in 

the area west of those wells. Instead of trying to determine 

flow directions from available water-level measurements 

(few of which represent winter conditions), the DRO and 

geochemistry data were interpreted to determine the 

predominant direction(s) of contaminant migration.

Figure 10. Diesel range organics (DRO) concentrations and water levels for the Housing Area (Arctic Acres) site, Operable Unit 

A, Adak Island, Alaska, May and June 2003.

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24 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
An area (or possibly two disconnected areas) with 

exceptionally high DRO concentrations is indicated by the 

distribution of DRO in wells and boreholes (fig. 10). The 

western part of the area is indicated by data from wells 03-890 

and 03-421. About 0.5 ft of free product was detected floating 

on the water table at well 03-890 during this investigation, 

and free product and high DRO concentrations have been 

regularly detected by the Navy in that well since 1998. A 

field-DRO concentration of 3,500 μg/L was detected in well 

03-421, and the USGS did not detect free product in that well. 

Less than 0.1 ft of free product was detected twice by the 

Navy during 2000 and questionably high DRO concentrations 

(26,000-81,000 μg/L) were reported during 2001 (typical 

concentrations at the well have ranged from 1,800 to 

3,500 μg/L). The eastern part of the area with high DRO 

concentrations is represented by data from well 03-416 

and boreholes AAB-01 and AAB-02. Historical DRO 

concentrations in 03-416 have ranged from 2,600 and 

3,400 μg/L. The exceptionally high lab-DRO concentrations 

in the two boreholes extended the previously identified area of 

high DRO concentrations eastward.

From that area of high concentration, DRO appears 

to have migrated predominately towards the east, with less 

consistent migration towards the west. The lack of migration 

to the north is indicated by the geochemistry in well AA04, which is representative of uncontaminated background 

conditions with a high dissolved-oxygen concentration 

(6 mg/L), low specific conductance (119 μS/cm), low carbon 

dioxide concentration (15 mg/L), and no detectable ferrous 

iron (<0.05 mg/L). Ground water in wells along the western 

margin of the site (AA-02, AA-05, and AA-06,) also was 

aerobic with no detectable ferrous iron, although DRO 

previously was reported for those wells at concentrations of 

74-455 μg/L, and specific conductance and carbon dioxide 

concentrations were elevated (286, 198, and 208 μS/cm and 

70, 40, and 40 mg/L, respectively) compared to background 

conditions. Those data indicate some DRO migration and 

biodegradation towards the west, but not enough to consume 

the dissolved oxygen replenished from ground-water recharge. 

Ground water in wells along the northern and eastern margins 

of the site (AA-03, AA-01, and 03-422) have historical DRO 

concentrations (151 and 1,190 μg/L) and elevated specific 

Borehole or 

well No.

Date 

sampled

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Diesel range organics 

(μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous iron

(mg/L)

Field Laboratory

Boreholes

AAB-01 05-21-03 22 7.47 7,700 18,000 5.9 614 400 <0.025 2.8

-02 05-21-03 22 7.43 4,200 4,400 6.2 425 240 <.025 2.4

-03 05-21-03 22 7.33 <100 – 6.2 242 60 .1 3.1

-04 05-21-03 22 6.66 460 920 6.0 340 45 <.025 1.4

Monitoring wells

03-416 05-09-03 18 7.95 1,100 – 6.1 262 50 <0.025 17

-420 05-09-03 22 8.34 300 – 6.1 265 48 <.025 8.4

-421 05-09-03 22 8.44 3,500 – 6.1 305 50 <.025 9.6

-422 05-09-03 21 8.03 <100 – 6.1 231 70 .2 13

-890 05-09-03 23 7.89 – 116,000 6.1 481 250 <.025 24

AA-01 05-09-03 22 6.86 570 – 6.2 189 34 <.025 10

-02 05-09-03 25 8.32 <100 – 6.0 286 70 6 <.05

-03 05-09-03 26 7.85 <100 – 6.1 214 37 <.025 7.9

-04 05-09-03 26 8.21 <100 – 5.7 119 15 6 <.05

-05 05-09-03 27 8.29 <100 – 5.6 198 40 4.5 <.05

-06 05-09-03 23 8.47 <100 – 5.8 208 40 5 <.05

1 The laboratory diesel range organics concentration for well 03-890 was from October 2002; the diesel range organics value was not measured in the field 

because free product was in the well.

Table 5. Selected diesel range organics, geochemical, and water-level data from boreholes and wells at the Housing Area (Arctic Acres) site, Operable 

Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 10. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well screen. 

Water-level altitudes are in feet above or below (-) NAD27. Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 

degrees Celsius; mg/L, milligram per liter. Symbols: <, actual value is less than value shown; –, not analyzed]

Monitoring for Natural Attenuation at Specific Petroleum Sites 25
conductance and carbon dioxide concentrations, but the 

wells are anaerobic with elevated ferrous iron concentrations. 

Those data indicate that petroleum migration was consistently 

towards the east-southeast with sufficient biodegradation 

to consume the dissolved oxygen replenished from groundwater recharge. Flow directions may have varied enough to 

disperse contamination under the center of the site, but the 

predominant flow path appears to have moved contaminants 

beyond the site towards the east.

Four boreholes were installed and sampled to confirm 

the assumed eastward contaminant migration direction. Three 

boreholes were spaced about 125 ft apart on a north-south 

transect about midway between wells 03-416 and AA-01. DRO 

concentrations were highest (18,000 μg/L) in borehole AAB01 about due east of well 03-890. Field-DRO concentration 

of 7,700 μg/L was roughly estimated because the highest 

concentration standard used to develop calibration curves was 

about 3,000 μg/L. A fourth borehole (AAB-04) was installed 

about 125 ft south of well AA-01 to assure that the plume 

was not passing to the southeast of well AA-01. Field-DRO 

in borehole AAB-04 (460 μg/L) was less than the DRO 

measured in AA-01 (570 μg/L). It was concluded in the field 

that well AA-01 was reasonably located to monitor the distal 

part of the contaminant plume, although an optimal location 

may be about 75 ft south of that well. The subsequent labDRO concentration was 920 μg/L for borehole AAB-04, but a 

similar replicate was not analyzed for AA-01 for comparison. 

Because the observed DRO concentrations were less than the 

cleanup level at both downgradient locations, well AA-01 is 

reasonably located for monitoring the downgradient (eastern) 

margin of the contaminant plume.

Overall, the NAP data from the Arctic Acres housing 

area indicated active petroleum biodegradation. The USGS 

measured substantially elevated carbon dioxide and ferrous 

iron concentrations in the most contaminated wells, and 

moderately elevated concentrations in other wells with current 

or historical DRO contamination. The USGS also measured 

elevated sulfide concentrations (0.16-0.20 mg/L) in nonturbid samples from wells within the core of the plume (wells 

03-890, 03-416, and 03-421), and the Navy has previously 

detected methane in multiple site wells.

DRO concentrations in the Arctic Acres housing area 

plume suggest attenuation of DRO concentrations along the 

flow path towards the east between wells 03-416 and AA01, assuming the plume is stable. However, it is too soon to 

definitively determine the stability of the DRO plume at the 

Arctic Acres housing area because the distal parts of the 

plume have been monitored only since October 2001. At 

well AA-01 to the east, DRO concentrations were below the 

cleanup level during 2001-03 and too few data are available to 

determine changes over time. At well 03-420 to the west, DRO 

was not detected during 1999-2000, but had concentrations as 

high as 12,300 μg/L during 2001-02, and was only 300 μg/L 

during this investigation. That variability may reflect the 

variable ground-water flow directions near this well. The peak 

DRO concentration of 216 μg/L at AA-05 in 2001 suggests 

that the net direction of contaminant migration from 03-420 

is to the east. Within the core of the plume (wells 03-890, 03-

416, and 03-421), no consistent trend is evident in 1996-2003 

DRO concentrations, although the 2001 concentrations were 

consistently greater than previously reported concentrations. 

Because at least one source of DRO was active until 1993 

(the leaky piping), steady-state plume conditions may not 

yet have been attained. Overall, data from this and previous 

investigations clearly indicate active petroleum biodegradation 

at the Arctic Acres housing area, but it is too soon to 

determine the extent that natural attenuation processes are 

limiting the 1,500 μg/L DRO plume.

Minimal monitoring suggestions for the Arctic Acres 

housing area site include: 

1. Sampling AA-01 to monitor the stability of the 

downgradient extent of contamination towards the 

east of the source area, and 

2. Sampling well 03-890 to monitor the strength of the 

primary contaminant source and the presence of 

NAPL over time. 

Potentially variable flow conditions at the site, combined with 

other known contamination “hot spots” such as well 03-421, 

may leave some uncertainty as to the overall performance of 

MNA with that minimal monitoring plan. For more certainty, 

sampling wells 03-420, AA-02, and AA-06 on an infrequent 

basis (perhaps every other year) would allow confirmation that 

contaminant transport towards the west remains insubstantial.

26 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
ROICC Contractor’s Area (UST ROICC-7)

The Resident Officer In Charge of Construction (ROICC) 

Contractor’s Area is north of the Downtown area in an 

unpopulated section about 0.5 mi west of Kuluk Bay 

(fig. 2). This area was used for equipment and supply storage 

for Navy contractors working on the island. Two specific 

sites with different remedies are in the vicinity. MNA was 

the ROD-specified remedy for the ROICC-8 site (near well 

08-153), but the Navy proposed no further action at this site 

because of the lack of contamination greater than cleanup 

levels. Limited monitoring (which does not include NAP 

analyses) was the ROD-specified remedy for the ROICC-7 site 

(near wells 08-200 and 08-202), but MNA is being considered 

as an alternate remedy because 2000-02 contamination levels 

did not meet the limited-monitoring criteria.

The USGS focused its efforts at the ROICC-7 site near 

wells 08-200 and 08-202 (fig. 11). The release history related 

to the former UST ROICC-7 (near well 08-201) is unknown. 

Benzene is the primary contaminant of concern at the site, 

although benzene rarely has been detected in 08-201. Benzene 

(and GRO to a lesser extent) has been consistently detected 

at concentrations greater than cleanup levels in wells 08-

200 and 08-202, north and upgradient from the former UST. 

Monitoring plans suggested for 2003 included continued 

sampling of the ROICC-8 site wells 08-153 and 08-160 for 

upgradient comparison to ROICC-7 wells, and sampling of 

wells 08-200 and 08-202 for the ROICC-7 benzene plume.

Like most of the Downtown area, the ROICC site is 

underlain primarily by sands and silty-sands with some 

gravel. However, ground water is at or within a few feet of 

land surface throughout the ROICC contractor’s area and the 

surface-soils have an abundance of natural organics, including 

plant roots and soil humus. The shallow water table is atypical 

for the Downtown area and the abundance of available 

carbon-substrate likely leads to differences in background 

water chemistry. Historical data indicate ground-water flow 

to be predominantly towards the southeast, although there 

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Figure 11. Benzene, carbon dioxide, and specific conductance concentrations for the ROICC Contractor’s Area site, Operable Unit 

A, Adak Island, Alaska, May and June 2003.

Monitoring for Natural Attenuation at Specific Petroleum Sites 27
has been seasonal flow towards the southwest. Recent (2002) 

monitoring at the site included three wells (08-200, 08-201, 

and 08-202). Historical (1999-2002) ground-water-level data 

were available for those wells, and data back to 1996 were 

available for the ROICC wells farther west. 

Objectives at the ROICC site were to determine: 

1. Direction of ground-water flow from the area of 

highest benzene concentrations near well 08-200,

2. Downgradient extent of benzene concentrations that 

exceeded the 5 μg/L cleanup level, and

3. If concentrations of NAPs indicate biodegradation of 

petroleum compounds within the plume. 

Data from 17 hand-driven, shallow boreholes and 5 monitoring 

wells (table 6) were used to achieve the objectives. Waterlevel data from the boreholes are not comparable throughout 

the site because they were not all measured simultaneously, 

Borehole or 

Well No.

Date 

sampled

Date of 

historical 

petroleum 

data

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Gasoline range 

organics (μg/L)

Benzene 

(μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron

(mg/L) Field Laboratory Field Laboratory

Boreholes

R-1 05-15-03 – 2.8 10.37 <100 – 8 – – 449 140 – –

-2 05-15-03 – 2.5 10.58 <100 – .9e – – 298 110 – –

-3 05-15-03 – 2.8 10.39 <100 – 1.1e – – 877 250 – –

-4 05-15-03 – 2.8 10.24 <100 – 1.4e – – 1,031 280 – –

-5 05-15-03 – 2.8 10.47 <100 – 1.8e – – 449 140 – –

-6 05-15-03 – 2.5 10.36 <100 – 13 – – 599 140 – –

-7a 05-15-03 – 2.8 10.28 <100 – 5.2 – – 494 190 – –

-7b 05-18-03 – 3.5 – 104 – 64 – 6.4 1,238 400 – –

-8 05-17-03 – 2.8 – 94e – 330 – – 1,210 350 <0.025 –

-9 05-17-03 – 3.2 – 433 – 2,100 – – 2,110 700 – –

-10 05-17-03 – 2.2 – 86e – 260 – – 968 450 – –

-11 05-17-03 – 3.2 – <100 – 27 – – 887 350 – –

-12 05-17-03 – 3.5 – <100 – 26 – – 415 170 – –

-13 05-18-03 – 3.7 – <100 65 <2 <1 5.9 1,189 380 1.12 –

-14 05-18-03 – 3.7 – <100 – 16 – 6.0 486 190 .72 –

-15 05-18-03 – 3.7 – <100 <50 13 3.3 6.2 817 280 1.22 –

-16 05-18-03 – 3.7 – <100 – 8.3 – 6.2 1,002 400 1.46 –

Monitoring wells

08-153 05-14-03 10-11-02 7.5 10.99 – 17.4e – <1 – – – – –

-175 06-06-03 – 5.5 10.06 – <50 – <1 6.2 582 250 <0.025 85

-160 05-14-03 10-11-02 7.5 10.85 – 12e – .6e – – – – –

-200 05-14-03 10-10-02 7.5 10.60 110 – 450 250 6.3 586 160 <.025 10

-201 05-14-03 10-10-02 7.5 10.67 – – – 1.2 6.3 289 36 <.025 11

-202 05-14-03 10-10-02 7.5 10.95 – – – 18 6.4 754 200 <.025 7.8

Table 6. Selected gasoline range organics, benzene, geochemical, and water-level data from boreholes and wells at the ROICC Contractor’s Area site, 

Operable Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 11. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well 

screen. Water-level altitudes are in feet above or below (-) NAD27. Gasoline range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: e, compound 

was detected but concentration was estimated; <, actual value is less than value shown; –, not analyzed]

28 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
and they responded quickly to rainfall, which was frequent 

during the investigation. Because the boreholes were shallow 

(less than 4 ft deep), the ground-water yield was generally 

low and the unconfined water table was at or above the land 

surface, the samples obtained from the boreholes had a 

high potential for contamination with surface water flowing 

along the temporary well casing. To minimize pumping and 

the potential for contamination, only those NAPs that were 

most indicative of petroleum contamination at the ROICC 

site (specific conductance and carbon dioxide concentration) 

were sampled at most boreholes. Dissolved oxygen was not 

measured at most boreholes because it was difficult to obtain a 

non-aerated sample; some of the reported measurements may 

be non-representative of ambient ground-water conditions. 

Specific conductance of standing surface water at the site 

(approximately 200 μS/cm) was substantially less than the 

specific conductance of shallow ground water (generally 

greater than 500 μS/cm), so exceptionally low specific 

conductance measured in a ground-water sample indicated 

contamination by surface water. When such contamination 

was detected, samples were discarded, a new borehole was 

installed, and new samples were collected.

The contaminant plume at the ROICC site that migrated 

southeast from well 08-200 was clearly identified from the 

observed benzene, carbon dioxide, and specific conductance 

data (fig. 11). Field-determined benzene data were suitable 

for locating the relatively high concentration (2,100 μg/

L) core of the contaminant plume. GRO concentrations 

were less than the cleanup level throughout the site. Fielddetermined benzene data were less suitable for determining 

the precise extent of the 5 μg/L benzene plume given how 

close that concentration is to the method detection limit of 

approximately 2 μg/L. Based on the field results, well 08-

175 was installed at a downgradient location where benzene 

concentrations were expected to be somewhat greater than 5 

μg/L but less than 30 μg/L, and where greatly elevated carbon 

dioxide concentrations indicated upgradient biodegradation. 

The new well could not be installed farther downgradient 

because the ground was too soft and saturated for Geoprobe® 

rig access.

The USGS NAP data indicated active biodegradation 

at the site primarily through elevated carbon dioxide 

concentrations. Boreholes were not installed north 

(upgradient) of the ROICC contractor’s area, but samples 

from well 08-201 and borehole R-2 are suspected to represent 

uncontaminated shallow ground water in the vicinity. These 

samples were atypical for the Downtown area because they 

were anaerobic with elevated ferrous iron concentrations. 

These conditions probably resulted from microbial oxidation 

of naturally-occurring organic carbon in the rooting zone 

linked to oxygen and iron reduction. Geochemistry data 

within the plume indicated generally anaerobic ground water 

with substantially elevated specific conductance and carbon 

dioxide concentrations. The few reported aerobic samples 

probably were influenced by surface water flowing into the 

piezometer during sample collection. Historical (1999-2002) 

NAP data from contaminated wells at the site consistently 

had elevated concentrations of ferrous iron (4-11 mg/L) and 

methane (9-22 mg/L), sulfate concentrations less than 1 mg/L, 

and detectable sulfide (data not shown). These data indicate 

that intense biodegradation within the benzene plume has 

consumed most available electron acceptors, except carbon 

dioxide creating methanogenic conditions.

Field data suggest that the center of the benzene plume is 

approximately 100 ft downgradient (east-southeast) from well 

08-200, and that concentrations decrease substantially in all 

directions from that location. Stability of the benzene plume 

at the ROICC site cannot be ascertained until additional data 

are collected from the new downgradient monitoring well. 

At wells 08-200 and 08-202, a downward temporal trend in 

benzene concentrations occurred during 1999-2002. Despite 

the field efforts to position the new well 08-175 within the 

contaminant plume, and despite the substantially elevated 

specific conductance and carbon dioxide concentration 

detected in the new well, benzene was not detected in well 08-

175 in June 2003 in either the field or laboratory GC analyses. 

However, a petroleum odor was noted while sampling and the 

well is on the overall flow path from the site, so well 08-175 is 

in a reasonable location for monitoring possible future plume 

migration. Overall, data from this and previous investigations 

clearly indicate active benzene biodegradation at the ROICC 

site, but it is too soon to determine the extent to which natural 

attenuation processes are limiting the 5 μg/L benzene plume.

Monitoring suggestions for the ROICC site include: 

1. Sampling well 08-175 to monitor the stability of the 

downgradient extent of contamination towards the 

southeast, and 

2. Sampling well 08-200 to monitor contamination 

nearer the source area over time.

Additional sampling of well 08-201 is not essential because it 

is not on the contaminant flow path, and additional sampling 

of well 08-202 is not essential because it is upgradient from 

and less contaminated than well 08-200.

Monitoring for Natural Attenuation at Specific Petroleum Sites 29
Runway 5-23 Avgas Valve Pit

Runway 5-23 Avgas Valve Pit (fig. 12) is west of Sweeper 

Creek near the end of the northernmost part of runway 5-23 

(fig. 2). About 240 to 1,600 lb of GRO and benzene leaked 

from Avgas transfer equipment into the soil at the site prior to 

1994. The extent of the contaminant plume and the potential 

discharge into an adjacent drainage ditch are the primary 

concerns at the site, and MNA is the ROD-specified remedy.

The Avgas Valve Pit site is underlain primarily by sands 

and silty-sands with some gravel. Historical data indicate 

that ground water flows eastward from the source area near 

well 14-100 at an estimated ground-water velocity of 97 ft/yr 

(URS Greiner, Inc., 1999). Depth to ground water at the site 

is relatively shallow (1−4 ft) for the Downtown area, so the 

abundance of available carbon substrate in the soil-rooting 

zone may lead to differences in background water chemistry. 

The only existing well at the site (14-100) has been monitored 

regularly since 1996. During 1996 (before abandonment), 

neither GRO nor benzene was detected in a second well at the 

site (abandoned well 14-101).

Objectives at the site were to determine:

1. Downgradient extent of benzene and GRO 

concentrations that exceed the 5 μg/L or 1,300 μg/L 

cleanup levels, respectively, and 

2. If concentrations of NAPs indicate biodegradation of 

petroleum compounds within the plume. 

Data were collected from six hand-driven boreholes spaced 

about 40 ft apart in an arc along the eastern and southern 

boundaries of the site at a distance of about 65 ft from existing 

well 14-100, and from a single upgradient borehole (AG-1) to 

the northwest of the site (fig. 12).

Field parameters and GRO concentrations (table 7) 

confirm the historical data and indicate that ground-water 

flow is from well 14-100 to the east-southeast towards the 

new well 14-110. Field-GRO concentrations decreased from 

3,200 μg/L at well 14-100 to a maximum of 250 μg/L in the 

boreholes. Water levels in general indicated flow to the eastsoutheast, although head differences in the boreholes were 

relatively small compared to possible variation due to heavy 

rainfall or non-equilibrium water levels. One new monitoring 

well (14-110) was installed on the centerline of the plume 

Figure 12. Gasoline range organics (GRO) and carbon dioxide concentrations and specific conductance for the Runway 5-23 

Avgas Valve Pit site, Operable Unit A, Adak Island, Alaska, May and June 2003.

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30 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
to monitor GRO concentrations in ground water before it 

discharges to the surface-water drainage ditch 15 ft farther 

downgradient; the field-GRO concentrations in boreholes 

nearest the new well location were 140 μg/L (AG-4) and 250 

μg/L (AG-3).

The USGS NAP data indicated active biodegradation 

at the site primarily through elevated carbon dioxide 

concentrations. Data from the upgradient borehole AG-1 

indicated that ambient ground-water redox conditions were 

anaerobic with relatively high ferrous iron concentrations, 

similar to downgradient conditions. Those conditions 

probably are related to enhanced microbial activity due to 

the availability of natural sources of organic carbon under 

the shallow water table at the site. However, biodegradation 

of petroleum compounds was still indicated at the site by 

elevated carbon dioxide concentrations in contaminated 

wells. Background borehole AG-1 had 60 mg/L of carbon 

dioxide, while the contaminated wells and boreholes (with the 

exception of AG-3) had carbon dioxide concentrations ranging 

from 100 to 160 mg/L.

There appears to be an approximate order-of-magnitude 

attenuation of GRO along the relatively short distance between 

wells 14-100 and 14-110. Although the stability of the GRO 

plume at the site cannot be ascertained until additional data 

are collected from the new downgradient monitoring well, 

equilibrium conditions probably have been established over 

the 70 ft between the two wells. The leak occurred prior to 

1996 and the estimated ground-water velocity at the site is 

97-ft/yr, so ample time has passed for the plume to achieve 

equilibrium conditions. Historical GRO concentrations at well 

14-100 have fluctuated inconsistently between 1,900 and 

4,600 μg/L, and GRO never was detected in abandoned 

well 14-101. Overall, the data indicated active GRO biodegradation at the Avgas Valve Pit site, and that natural 

attenuation is preventing discharge of ground water with GRO 

concentrations exceeding 1,300 μg/L to the nearby drainage 

ditch. Because of the shallow water table, any contaminated 

ground water from the site would likely discharge to the 

drainage ditch; however, a borehole was not installed on 

the downgradient (east) side of the ditch to confirm that 

hypothesis.

Monitoring suggestions for the Avgas Valve Pit site 

include:

1. Sampling well 14-110 to monitor the stability of the 

downgradient contamination, and 

2. Sampling well 14-100 to monitor the contaminant 

source strength over time. 

Given the relatively small size of the site, sampling only well 

14-110 would probably be suitable if contaminant discharge to 

surface water is the only concern. 

Borehole or 

well No.

Date 

sampled

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Gasoline range 

organics (μg/L)

Benzene 

(μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron

(mg/L) Field Laboratory Field Laboratory

Boreholes

AG-1 05-12-03 2.5 12.48 <100 – <2 – 6.3 217 60 <0.025 35

-2 05-12-03 2.5 11.73 <100 – <2 – 6.0 235 42 <.025 25

-3 05-12-03 2.5 11.60 250 – <2 – 6.2 156 18 <.025 30

-4 05-12-03 2.5 11.57 140 – <2 – 6.3 413 100 <.025 35

-5 05-12-03 2.5 11.55 <100 – <2 – 5.9 191 50 <.025 30

-6 05-12-03 2.5 11.48 <100 – <2 – 6.1 137 – – –

-7 05-12-03 2.5 11.51 <100 – <2 – 6.2 156 – – –

Monitoring wells

14-100 05-12-03 6.5 11.80 3,200 – – – 5.8 409 160 <0.025 40

-110 06-06-03 6.5 10.85 – 180 – 1.2 6.1 360 150 <.025 45

Table 7. Selected gasoline range organics, benzene, geochemical, and water-level data from boreholes and wells at the Runway 5-23 Avgas Valve Pit 

site, Operable Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 12. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well screen. 

Water-level altitudes are in feet above or below (-) NAD27. Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 

degrees Celsius; mg/L, milligram per liter. Symbols: <, actual value is less than value shown; –, not analyzed]

Monitoring for Natural Attenuation at Specific Petroleum Sites 31
SA 80, Steam Plant No. 4

SA 80, Steam Plant No. 4 (fig. 13) is northwest of the 

Arctic Acres housing area in the Downtown area of Adak 

Island (fig. 2). Steam Plant No. 4 was built in the late 1940’s 

and was used to supply steam heat to various buildings, 

hangars, and barracks. The plant’s boilers used jet petroleum 

(JP-5) from two 23,000 gal steel USTs located southeast of 

the main building. USTs were filled with jet petroleum from 

the Main Road pipeline. Whether or not a release occurred 

directly from either of the USTs during their use is unknown. 

A fill hose ruptured in 1991, releasing a reported 50-70 gal of 

JP-5. Trace amounts of fuel were released when the first UST 

was removed in 1993. The second UST failed a tightness test 

in 1993, and was removed in 1995 along with the associated 

piping to the steam building. No holes and little corrosion 

were observed in either UST during their removal, but the 

second tank and an adjacent utility vault did contain 4,000 gal 

of oily water. Field testing after tank removal indicated the 

presence of petroleum hydrocarbons in the soil. Sources for 

DRO contamination in ground water possibly are fuel spills 

related to overfilling, pipe releases, or direct leaks from the 

USTs. The ROD-specified remedy at the SA 80 site was free 

product recovery. Measurable free product had been observed 

in 6 of 15 wells at the site, and passive skimmers were used to 

recover free product in 5 wells. MNA is being considered as a 

follow-up remedy for contaminated ground water.

Figure 13. Diesel range organics (DRO) and carbon dioxide concentrations for the SA 80, Steam Plant No. 4 site, 

Operable Unit A, Adak Island, Alaska, May and June 2003.

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32 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
The SA 80 site is underlain primarily by sands and 

silty-sands with some gravel. It has been inferred that ground 

water flows west-southwest from the site (fig. 13). Depth to 

ground water at the site is approximately 20 ft. Since 1998, 

well 04-801 has been regularly monitored for petroleum 

and NAP concentrations. Assorted historical contaminant 

concentrations data from multiple wells near the source area 

are available for selected times between 1996 and 2002, and 

NAP concentration data are available for 2002.

Objectives at the site were to determine:

1. Downgradient extent of dissolved-phase DRO 

concentrations that exceed the 1,500 μg/L cleanup 

levels, and 

2. If concentrations of NAPs indicate biodegradation of 

petroleum compounds within the plume. 

To meet the objectives, data were collected from 4 existing 

wells, 11 boreholes, and 1 new well at the site (table 8). 

Borehole or 

well No.

Date 

sampled

Date of 

historical 

petroleum 

data

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Diesel range 

organics (μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron

Field Laboratory (mg/L)

Boreholes

SA 80-1A 05-19-03 – 20 7.18 – – 5.8 239 21 7 0.4

-1B 05-19-03 – 30 7.02 – – 6.3 191 22 8 .3

-2A 05-19-03 – 20 7.00 – – 6.3 174 12 9 .1

-2B 05-19-03 – 30 6.88 – – 6.1 163 26 9 –

-3A 05-19-03 – 20 6.89 – – 5.9 201 18 8 .6

-3B 05-19-03 – 30 6.85 – – 5.8 208 27 8 .8

-4A 05-19-03 – 20 6.69 <100 – 6.0 248 27 4 1.0

-4B 05-19-03 – 30 6.75 <100 – 5.9 263 50 .1 1.6

-5 05-19-03 – 30 6.53 <100 – 5.6 250 100 .15 1.7

-6 05-19-03 – 29 6.45 230 50e 5.9 272 150 <.025 1.8

-7 05-19-03 – 29 6.64 <100 – 6.2 239 26 3 1.5

Monitoring wells

04-103 06-05-03 – 27 6.17 – 210 6.2 290 65 <0.025 18

-159 05-08-03 08-02-02 23 7.42 – 3,300 6.1 927 190 <.025 100

-164 05-08-03 08-02-02 22 7.44 – 6,100 5.9 818 180 <.025 70

-801 05-08-03 10-14-02 22 7.19 – 80e 5.4 341 40 3 <.05

SP4-2 05-08-03 08-02-02 8 – – 9,600 5.8 939 250 <.025 400

Table 8. Selected diesel range organics, geochemical, and water-level data from boreholes and wells at the SA 80, Steam Plant No. 4 site, Operable Unit 

A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 13. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well 

screen. Water-level altitudes are in feet above or below (-) NAD27. Diesel range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: e, compound 

was detected but concentration was estimated; <, actual value is less than value shown; –, not analyzed]

Monitoring for Natural Attenuation at Specific Petroleum Sites 33
Available NAP data from the presumed downgradient well 

04-801 gave no indication of contamination (ground water was 

aerobic with little ferrous iron and low alkalinity). 

An exposed bedrock ridge is about 500 ft west-northwest 

of SA 80, so it was presumed that ground-water flow from 

SA 80 was diverted towards the southwest. Thus, the current 

investigation focused on the area south of well 04-801. 

Building 42518 (south-southwest of SA 80) is surrounded 

by a thick apron of concrete that was impenetrable by the 

Geoprobe®, so the search for the downgradient DRO plume 

was done along the western margin of the apron.

DRO and NAP data (table 8) indicated that the 

contaminant plume flows southwest from Steam Plant No. 4 

(fig. 13). Water levels somewhat suggested flow towards the 

west-southwest, but no boreholes could be installed under the 

concrete apron to obtain the widespread data needed to create 

a reliable flow map. Both shallow (20 ft) and deep (30 ft) 

boreholes (designated A and B, respectively on fig. 13) were 

installed at the first four locations (SA 80-1 through SA 80-4) 

to identify the depth of the plume. Anaerobic ground water 

and a slightly elevated carbon dioxide concentration at SA 80-

4B were interpreted to indicate the lateral margin of the 

DRO plume. DRO was positively detected at SA 80-4B 

(as well as SA 80-5), but at concentrations too low 

(<100 μg/L) to quantify in the field. The centerline of the 

plume was identified at SA 80-6 from a DRO concentration of 

230 μg/L and peak specific conductance and carbon dioxide 

concentration. The final borehole (SA 80-7) was interpreted to 

delimit the eastern boundary of the DRO plume.

The USGS NAP data indicated active biodegradation at 

the SA 80 site through elevated carbon dioxide concentrations 

throughout the contaminant plume, in addition to substantially 

elevated ferrous iron concentrations near the source area. 

Historical NAP data from near the source area (04-158, SP4-

2, and SP4-3, data not shown) showed methane concentrations up to 100 mg/L and sulfide concentrations up to 

0.3 mg/L, both indicators of substantial microbial activity and 

biodegradation. NAP data from boreholes SA 80-1 (A and B) 

through SA 80-3 (A and B) and well 04-801 are representative 

of uncontaminated background water at the site.

There appears to be nearly a two orders-of-magnitude 

attenuation of DRO (9,600 to 210 μg/L) along the flow path 

between SP4-2 and new well 04-103, although the stability 

of the DRO plume at the site cannot be ascertained until 

additional data are collected from the new downgradient 

monitoring well. Historical DRO concentrations from wells 

near the source showed no consistent trend between 1996 and 

2002, although those data were likely affected by the starting 

and cessation of free product recovery from recovery wells at 

the site. Overall, the data indicated active DRO biodegradation 

at the SA 80 site, and as of 2003 natural attenuation is 

preventing DRO concentrations that exceed 1,500 μg/L from 

extending as far as 600 ft downgradient. DRO contamination 

in ground water is detectable somewhat farther downgradient 

from the source area when compared to other Downtown area 

petroleum sites. A plausible explanation for that observation is 

that the concrete apron surrounding Building 42518 does not 

allow recharge of oxygen rich water and subsequent aerobic 

biodegradation, so petroleum biodegradation outside of the 

source area is through one of the less efficient anaerobic 

processes.

If MNA is selected as a remedy for contaminated ground 

water at the SA 80 site, monitoring suggestions include: 

1. Sampling well 04-103 to monitor the stability of the 

downgradient contamination, and 

2. Sampling well SP4-3 or a similar source area well to 

monitor the contaminant source strength over time. 

Well 04-801 also could be sampled if additional background 

water-chemistry data are desired, but the available data are 

strong indicators that the well is not on a contaminated flow 

path from Steam Plant No. 4.

34 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
SWMU 14, Old Pesticide Storage and 

Disposal Area

Solid Waste Management Unit (SWMU 14) (fig. 14) site 

is a vacant property in the southern part of the Downtown 

area near Sweeper Cove (fig. 2). From 1950 to about 1987, a 

motor vehicle gasoline filling station and a pesticide storage 

and disposal area were in a building near well MW14-5. No 

evidence of USTs has been detected at the site. The service 

station probably was supplied by pipe from an AST about 

800 ft west of the building. The release history of the site is 

not known. GRO and DRO that presumably leaked from the 

AST or associated piping are the primary contaminants of 

concern at the site. MNA is the ROD-specified remedy at the 

SWMU 14 site.

SWMU 14 is underlain primarily by sands and siltysands with some gravel. It has been inferred that ground water 

flows south from the site towards Sweeper Cove (about 

Figure 14. Diesel range organics (DRO) and ferrous iron concentrations, and water levels for the SWMU 14, Old Pesticide 

Storage and Disposal Area site, Operable Unit A, Adak Island, Alaska, May and June 2003. 

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Monitoring for Natural Attenuation at Specific Petroleum Sites 35
1,200 ft due south). Depth to ground water at the site is about 

15 ft. One well at the site (MW14-5) has been monitored 

regularly since 1998, including NAP data. A second well 

(MW14-423, not shown) 900 ft south-southwest of well 

MW14-5, has been monitored for petroleum and NAP 

concentrations regularly since 1999. Limited historical 

contaminant concentration data from abandoned upgradient 

wells also are available, but those concentrations are 

substantially less than those observed at well MW14-5.

Objectives at the site were to determine:

1. Downgradient extent beyond well MW14-5 of 

dissolved-phase DRO and GRO concentrations that 

exceed the 1,500 μg/L and 1,300 μg/L cleanup levels, 

respectively, and

2. If concentrations of NAPs indicate biodegradation of 

petroleum compounds within the contaminant plume.

To meet those objectives, data were collected from 4 existing 

wells, 14 boreholes, and 1 new well (01-153) at the site 

(table 9). The investigation started due south of the source 

Table 9. Selected diesel range organics, gasoline range organics, geochemical, and water-level data from boreholes and wells at the SWMU 14, Old 

Pesticide Storage and Disposal Area site, Operable Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 14. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well screen. 

Water-level altitudes are in feet above or below (-) NAD27. Diesel and gasoline range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: e, compound 

was detected but concentration was estimated; <, actual value is less than value shown; –, not analyzed]

Borehole or 

well No.

Date 

sampled

Date of 

historical 

petroleum data

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Diesel range 

organics (μg/L)

Gasoline range 

organics (μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron

(mg/L) Field Laboratory Field Laboratory

Boreholes

S14-1 05-17-03 – 14 6.81 – – <100 – 6.4 397 15 7 0.03

-1B 05-18-03 – 24 6.69 – – <100 – 6.8 301 11 .3 .71

-2 05-17-03 – 14 6.38 – – <100 – 6.7 430 16 7 .77

-2B 05-18-03 – 24 6.32 – – <100 – 6.2 174 20 .2 .34

-3 05-17-03 – 14 6.22 – – <100 – 6.6 579 15 7 .13

-3B 05-18-03 – 24 6.18 – – <100 – 6.0 309 30 6 .33

-4 05-18-03 – 18 6.70 – – <100 – 6.3 246 18 4.5 .33

-5 05-18-03 – 18 6.70 3,700 – 12,000 – 6.2 377 30 .05 1.5

-6 05-18-03 – 18 6.79 870 – 2,400 – 6.2 410 55 <.025 1.2

-7 05-18-03 – 18 6.06 – – <100 – 6.0 435 23 6 .34

-8 05-18-03 – 18 5.42 – – <100 – 6.5 217 18 10 .17

-9 05-19-03 – 18 6.77 – – <100 <50 5.5 368 20 7 .64

-10 05-19-03 – 18 6.95 – – <100 – 6.0 250 14 7 .18

-11 05-19-03 – 18 6.68 220 – 180 310 6.1 271 42 1.5 1.7

Monitoring wells

MW-14-5 05-07-03 10-12-02 17 7.16 – 3,000 – 12,000 5.9 258 26 0.05 6

-1(15-1) 05-07-03 08-11-98 14 7.26 – 450 – <100 6.0 340 30 5.4 3

-15-3 05-07-03 10-14-02 20 5.17 – <100 – 13e 6.4 411 33 1.7 4

01-153 06-04-03 – 19 6.48 – 550 – 63e 6.3 313 45 .9 7.5

76-148 05-07-03 – 18 7.26 – – – – 6.1 434 25 5.9 <.05

36 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
area near well MW14-5 along Public Works Road with 

boreholes S14-1 through S14-3. Both shallow (14 ft) and deep 

(24 ft) boreholes (deep boreholes designated B on fig. 14) 

were installed to identify the depth of the plume, but no 

contamination was detected and NAP data did not definitively 

indicate upgradient contamination. The investigation then 

moved closer to MW14-5 to clearly locate the contaminant 

plume and identify the southwest ground-water flow 

direction, and finally moved out farther to define the extent of 

contamination. Multiple buried utility lines are in the area, so 

boreholes could not be installed at all desired locations.

DRO, GRO, and ferrous iron data (table 9) indicated 

that the contaminant plume was flowing southwest from 

well MW14-5. Water-level altitudes consistently decreased 

towards the west in the boreholes along Public Works Road, 

also indicating that water from MW14-5 was not flowing due 

south. The centerline of the contaminant plume was identified 

at borehole S14-5, where both DRO and GRO concentrations 

exceeded the cleanup levels, and also at S14-11, where both 

DRO and GRO concentrations were less than the cleanup 

levels. New well 01-153 was installed to monitor the distal 

part of the contaminant plume near Public Works Road 

where DRO and GRO concentrations were 220 and 310 μg/L, 

respectively.

The USGS NAP data indicated some biodegradation at 

the SWMU 14 site, although carbon dioxide concentrations in 

contaminated wells were only marginally greater than those 

in cross-gradient uncontaminated wells. All contaminated 

wells were anaerobic, but deep uncontaminated boreholes 

in the area also were anaerobic. Ferrous iron concentrations 

exceeding 1 mg/L were a consistent indicator of enhanced 

microbial activity in contaminated wells. Historical NAP data 

from MW14-5 showed essentially no methane or sulfide, and 

ferrous iron concentrations of 1.8 to 3.6 mg/L. Data from well 

76-148 best represents uncontaminated background water at 

the site because it is well off the contaminated flow path, and 

outside the built-up industrial area. Past intensive land-use 

activities in the areas south of Public Works Road and near 

well MW14-5 had a high potential for multiple small-scale 

releases of petroleum compounds.

Despite the suggestion of only moderate biodegradation, 

there appears to be nearly a two orders-of-magnitude 

attenuation of GRO along the flow path between MW14-5 

and new well 01-153 (12,000 to 63e μg/L), and a one-order-ofmagnitude attenuation of DRO (3,000 to 550 μg/L). Relative 

attenuation rates are consistent with the theory that GRO has 

more readily biodegradable compounds than does DRO. As 

with most other Adak sites, the stability of the contaminant 

plume at the site cannot be determined until additional data 

are collected from the new downgradient monitoring well 

(01-153). Historical GRO concentrations from MW14-5 appear 

to have consistently decreased from a peak of 

32,000 μg/L during 1995, but DRO concentrations from 

MW14-5 showed no consistent trend between 1995 and 2002. 

Overall, the data indicate active biodegradation at the SWMU 

14 site, and that natural attenuation as of 2003 is preventing 

GRO and DRO concentrations that exceed the cleanup 

levels from extending more than about 200 ft downgradient. 

Benzene was not detected (at a detection limit of 2 μg/L) in 

the laboratory analyses of samples collected from the new 

well 01-153 or from borehole S14-11 (data not shown).

Monitoring suggestions include: 

1. Sampling well 01-153 to monitor the stability of the 

downgradient contamination, and 

2. Sampling well MW14-5 to monitor the contaminant 

source strength over time. 

Downgradient well MW14-423 (not shown) is far beyond 

the current extent of contamination from the SWMU 14 site, 

so continued monitoring of that well would not be useful for 

MNA monitoring.

Monitoring for Natural Attenuation at Specific Petroleum Sites 37
SWMU 61, Tank Farm B

SWMU 61, Tank Farm B (fig. 15) is on the hillside west 

of Sweeper Creek in the northwest part of the Downtown 

area (fig. 2). The site was a former bulk fuel storage facility 

and had multiple ASTs and associated pipelines. The release 

history and specific source of contamination is unknown. 

GRO and benzene are the contaminants of concern with an 

estimated mass of 1,300 to 4,000 lb in the soil at the site. 

Monitoring wells TFB-MW4A and B (a pair of existing wells 

adjacent to one another where the “B” well is the deeper of the 

two) are downgradient from two former storage tanks. MNA 

is the ROD-specified remedy at the site.

The geology of the Tank Farm B site is not typical for 

Downtown area sites. The upland portion of the Tank Farm B 

site is underlain primarily by dense lahar deposits consisting 

of poorly sorted, cobble and boulder gravel in a matrix of silt 

and fine sand. The lowland portion of the site is underlain 

by lagoonal deposits consisting of a mix of sand, silt, peat, 

and clay. Upland deposits are of highly variable permeability 

and water-bearing capacity, while the lowland deposits are 

porous and permeable. It has been inferred that ground 

water flows northeast from the TFB-MW4 wells beneath the 

well-defined swale, and then across the lagoonal deposits to 

Sweeper Creek. Depth to ground water in the TFB-MW4 and 

14-210 wells is 2 to 4 ft, and the water table is a few inches 

below the spongy land surface in the lowland area between 

Figure 15. Gasoline range organics (GRO) and carbon dioxide concentrations for the SWMU 61, Tank Farm B site, 

Operable Unit A, Adak Island, Alaska, May and June 2003.

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38 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
well 14-210 and North Sweeper Creek. TFB-MW4 wells have 

been monitored regularly for petroleum compounds since 

1996, and the TFB-MW4 wells and well 14-210 have been 

monitored for petroleum compounds and NAPs since 1999. 

Abandoned well TFB-MW5 was sampled once for petroleum 

compounds in 1996, and none were detected.

Objectives at the site were to determine:

1. Downgradient extent of dissolved-phase benzene 

and GRO concentrations that exceed the 5 μg/L or 

1,300 μg/L cleanup levels, respectively, and 

2. If concentrations of NAPs indicate biodegradation of 

petroleum compounds within the plume. 

Five hand-driven boreholes and three monitoring wells were 

sampled at the site to meet these objectives. Four boreholes 

spaced about 60 ft apart were installed along the west bank of 

Sweeper Creek in a transect starting near an unnamed tributary 

flowing into Sweeper Creek and continuing south about 

240 ft. A fifth borehole was installed about midway from 

existing well 14-210 and Sweeper Creek along the flow path 

centerline, which was determined from surveyed water-level 

information.

Benzene and GRO data (table 10) indicate that the 

contaminant plume flows east across the lowlands and into 

Sweeper Creek. Field-GRO concentrations ranging from 

260 to 9,700 μg/L were detected in boreholes along the 

bank of Sweeper Creek. The centerline of the contaminant 

Borehole or 

well No.

Date 

sampled

Date of 

historical 

petroleum 

data

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Gasoline range 

organics (μg/L)

Benzene 

(μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron

(mg/L) Field Laboratory Field Laboratory

Boreholes

TFB-1 05-13-03 – 3 7.19 9,600 – <2 – – 260 55 <0.025 20

-2 05-13-03 – 3 6.24 6,000 – <2 – – 300 70 <.025 35

-3 05-13-03 – 4 6.28 9,700 – 850 – – 310 45 <.025 30

-4 05-13-03 – 4 6.46 2,700 – <2 – – 307 40 <.025 12

-5 05-13-03 – 4 6.54 260 – 7 – – 220 45 .2 .6

Monitoring wells

14-113 06-07-03 – 3 6.00 – 2,000 – 15 – – – – –

-210 05-13-03 – 7.9 8.81 8,100 – <2 – 6.4 212 100 <0.025 60

TFB-MW-4A 05-13-03 – 6 33.83 <100 <2 5.9 334 140 .2 35

-4B 05-13-03 10-07-02 15 33.48 – 29,000 – 50 6.0 329 45 <.025 15

Table 10. Selected gasoline range organics, benzene, geochemical, and water-level data from boreholes and wells at the SWMU 61, Tank Farm B site, 

Operable Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 15. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well 

screen. Water-level altitudes are in feet above or below (-) NAD27. Gasoline range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: <, actual 

value is less than value shown; –, not analyzed]

Monitoring for Natural Attenuation at Specific Petroleum Sites 39
plume was identified at borehole TFB-3 where GRO and 

benzene concentrations substantially exceeded cleanup levels. 

GRO also exceeded the cleanup levels in the two boreholes 

immediately north and south of TFB-3, along the bank of 

Sweeper Creek. A new well (14-113) was installed to monitor 

discharge of the contaminant plume immediately before it 

enters Sweeper Creek.

USGS data and historical NAP data indicated some 

biodegradation of petroleum in the upland portion of the site, 

but indicated little biodegradation in the lowland portion. 

Ground water in all boreholes and wells was anaerobic 

with relatively high ferrous iron concentrations and specific 

conductance. No wells were available to sample background 

conditions for the uplands, and the direct-push rig could 

not penetrate the sediments to obtain a sample for this 

investigation. Carbon dioxide did appear to be elevated 

(140 mg/L) in the shallowest upland well TFB-MW4A, 

although that well historically has had often detectable, but 

low, GRO concentrations of less than 100 μg/L. The deeper 

well, TFB-MW4B, historically has had GRO concentrations 

up to 57,000 μg/L, but substantially less carbon dioxide 

(45 mg/L) was detected in May 2003. In contrast, six 

rounds of historical NAP data from the well pair indicate 

that both wells have similar alkalinity concentrations, with 

an average of 124 mg/L as CaCO3 in the deep well and an 

average of 112 mg/L in the shallow well. Historical methane 

concentrations ranged from 3 to 9.4 mg/L for the shallow 

well and 1.7 to 2.3 mg/L for the deep well. Overall, these 

data suggest that biodegradation may be at least partly 

responsible for the lower petroleum concentrations detected 

in the shallowest upland well. In the lowlands, however, 

carbon dioxide and other NAP concentrations showed little 

variability and gave no clear indication of active petroleum 

biodegradation. Ambient conditions for the saturated and 

organic-rich flood plain were expected to be anaerobic, so 

anaerobic and iron-rich water at the contaminated boreholes is 

not distinctly indicative of petroleum biodegradation.

Consistent with the results regarding biodegradation, 

attenuation of GRO concentrations was apparent in the upland 

portion of the site, but not apparent in the lowland portion. 

October 2002 data show an approximately one order-ofmagnitude attenuation of GRO along the upland flow path 

between TFB-MW4B and 14-210 (29,000-2,300 μg/L). 

For this investigation, a higher field-GRO concentration 

of 8,100 μg/L was detected at 14-210 in May 2003, but the 

upland well TFB-MW4B would not yield enough water to 

collect a sample for field or lab analysis. May 2003 data 

indicated essentially no attenuation along the lowland flow 

path between well 14-210 and boreholes TFB-1 and TFB-3. 

Like most other Adak sites, the stability of the contaminant 

plume at the site cannot be determined until additional data 

are collected from the new downgradient monitoring well. 

October 2002 GRO concentrations from monitoring wells 

at Tank Farm B were the lowest observed to date, but there 

was no clear decreasing trend before that date. Overall, the 

data indicated that, despite some active biodegradation at the 

upland portion of the Tank Farm B site, as of 2003 natural 

attenuation is not preventing benzene and GRO concentrations 

that exceed the cleanup levels from discharging to Sweeper 

Creek.

Monitoring suggestions include: 

1. Sampling well 14-113 to monitor the contaminant 

concentrations in ground water near Sweeper Creek, 

and 

2. Sampling well TFB-MW4B to monitor concentrations 

near the contaminant source over time. 

Intermediate well 14-210 also could be sampled to add 

duration to the historical dataset, but the contaminant 

concentrations from well 14-113 would be more indicative of 

what may be reaching surface water. Bed-sediment porewater 

under Sweeper Creek could be sampled using diffusion 

samplers, or a similar technology, to directly measure 

petroleum concentrations discharging to surface water, if that 

is considered a critical issue.

40 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
SWMU 62, New Housing Fuel Leak 

(Sandy Cove Housing)

Solid Waste Management Unit (SWMU) 62 includes 

several base housing areas in the Downtown area constructed 

for military and civilian personnel and their families (fig. 2). 

At least four distinct petroleum plumes have been identified 

in the Sandy Cove housing area of SWMU 62, but this 

investigation was focused only on the southernmost plume 

(fig. 16) suspected to originate near housing unit 134 (labeled 

as building 42234 in fig. 16). The housing unit is served by 

a JP-5 fuel oil furnace and hot water heater. Two 30,000 gal 

ASTs on raised earth pads are just south of Raven Street 

and about 300 ft east of the contamination plume of interest. 

Fuel is distributed from the ASTs to individual housing units 

via main distribution lines, lateral lines, and 0.5 in. copper 

pipes. The Navy did visual inspections and inventory record 

reviews in 1988, after occupants reported hydrocarbon odors. 

Contaminated soil was detected and removed from the crawl 

space below unit 134 in 1988, and free product was first 

detected in monitoring wells in 1989. The ROD-specified 

remedy at the Sandy Cove housing area of SWMU 62 was free 

product recovery. Between 1989 and 1990, 1,563 gal of free 

product was removed from the area. DRO is the petroleum 

contaminant of concern at the site, although chlorinated 

solvents also have been detected at concentrations exceeding 

cleanup levels in well MW-134-10. The investigation did 

not focus on the chlorinated solvents, but samples from the 

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Figure 16. Diesel range organics (DRO) and carbon dioxide concentrations for the SWMU 62, New Housing 

Fuel Leak (Sandy Cove Housing) site, Operable Unit A, Adak Island, Alaska, May and June 2003.

Monitoring for Natural Attenuation at Specific Petroleum Sites 41
newly installed monitoring well were analyzed for solvents 

using EPA method SW8260B (U.S. Environmental Protection 

Agency, 2003). MNA is being considered as a follow-up 

remedy for contaminated ground water.

The Sandy Cove housing area is underlain primarily by 

sands and silty-sands with some gravel. It has been inferred 

that ground water flows south-southwest from the site towards 

Sweeper Cove (about 1,700 ft downgradient). Depth to ground 

water at the site is about 15 to 20 ft. Abundant historical 

contaminant concentration data are available dating from 

1989, and one well (03-619) has been monitored regularly 

since 1998.

Objectives at this site were to determine:

1. Downgradient extent of DRO concentrations that 

exceed the 1,500 μg/L cleanup level, and,

2. If NAP data indicate biodegradation of petroleum 

compounds within the contamination plume. 

To meet the objectives, data were collected from five existing 

wells, eight boreholes, and one new well (03-155) (table 11). 

An initial east-west transect of four boreholes spaced about 

75 ft apart was installed 75 ft south of existing well 03-619. 

Both shallow (17 ft) and deep (22 ft) boreholes (designated 

A and B, respectively on fig. 16) were installed at the first 

Borehole or 

well No.

Date 

sampled

Date of 

historical 

petroleum 

data

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Diesel range 

organics (μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron

(mg/L) Field Laboratory

Boreholes

SCH-1A 05-22-03 – 17 8.49 730 3,200 5.9 497 170 0.05 1.6

-1B 05-22-03 – 22 8.55 80e – 5.6 267 80 .2 .9

-2 05-22-03 – 18 8.44 180 – 6.1 470 100 .9 1.7

-3 05-22-03 – 18 8.34 – – 6.4 218 16 5 .22

-4 05-22-03 – 18 8.61 <100 – 5.7 261 45 1.5 .7

-5 05-22-03 – 19 8.19 860 490 6.3 289 32 1.5 1.3

-6 05-22-03 – 19 8.26 <100 – 5.8 211 45 3 .8

-7 05-22-03 – 19 8.20 – – 6.6 175 11 8 <.05

Monitoring wells

03-155 06-04-03 – 17 8.04 – 1,200 6.0 287 100 3.5 35

-619 05-10-03 09-02-01 15 8.48 – 2,600 5.3 297 60 .4 3.0

HMW-134-2 05-10-03 09-02-01 17 8.67 – 2,870 6.1 363 170 <.025 22

MW-134-8 05-10-03 – 18 8.66 – – 5.9 291 120 <.025 15

-134-10 05-10-03 09-02-01 17 8.55 – 5,030 5.9 404 200 <.025 30

-134-11 05-10-03 09-02-01 17 8.59 – 7,450 6.0 439 210 <.025 18

Table 11. Selected diesel range organics, geochemical, and water-level data from boreholes and wells at the SWMU 62, New Housing Fuel Leak 

(Sandy Cove Housing) site, Operable Unit A, Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 16. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well 

screen. Water-level altitudes are in feet above or below (-) NAD27. Diesel range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: e, compound 

was detected but concentration was estimated; <, actual value is less than value shown; –, not analyzed]

42 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
location (SCH-1) to identify the depth of the plume. To assure 

that the distal part of the plume was sampled, a second transect 

of boreholes was installed 100 ft farther south of the first 

transect. 

Water-level, DRO, and carbon dioxide data (table 11) 

indicate that the contaminant plume flows south-southwest 

from the highest concentration source area well near the east 

side of unit 134. The centerline and depth of the contaminant 

plume was first identified by high DRO, carbon dioxide, 

and ferrous iron concentrations at the shallow borehole 

SCH-1A. The plume extent was tracked farther downgradient 

to borehole SCH-5 in the second transect, with a field-DRO 

concentration of 860 μg/L. DRO concentrations less than 

100 μg/L in boreholes SCH-4 and SCH-6 defined the lateral 

extent of the plume. New well 03-155 was installed about 

40 ft downgradient from SCH-5 to monitor the distal part 

of the DRO plume along the most contaminated flow path. 

Subsequent lab-DRO concentration in well 03-155 

(1,200 μg/L) was below the cleanup level. Trichloroethene 

was the only chlorinated solvent detected in the well at a 

concentration of 2 μg/L. At borehole SCH-5 (interpreted from 

field data to be on the centerline of the flow path immediately 

upgradient from 03-155), the relatively low lab-DRO 

(490 μg/L) and carbon dioxide concentrations, combined with 

the relatively high dissolved-oxygen concentration suggest that 

the borehole was screened on the fringe of the contaminant 

plume, while the monitoring well is screened across the core 

of the plume.

The USGS NAP data indicate biodegradation at the 

Sandy Cove housing area by a ten-fold increase above 

background levels in carbon dioxide concentrations in the 

most contaminated wells. Ferrous iron concentrations also 

were elevated in petroleum contaminated samples. Data 

from boreholes SCH-3 and SCH-7 appear to best represent 

uncontaminated background water at the site because 

the water from those sites was aerobic, with ferrous iron 

concentrations of 0.22 mg/L or less, and carbon dioxide 

concentrations of 11 and 16 mg/L, respectively. Two of the 

contaminated samples from well 03-155 and borehole SCH-5 

had dissolved-oxygen concentrations greater than 1 mg/L, but 

the elevated ferrous iron concentrations in those same samples 

suggest that they are a mix of aerobic and anaerobic water 

from different depths penetrated by the screens. 

There appears to be an approximately six-fold attenuation 

of lab-DRO along the flow path between MW134-11 and 

new well 03-155 (7,450 to 1,200 μg/L). Overall attenuation 

is approximate because the upgradient concentration is from 

2001, but a three-fold attenuation of lab-DRO (3,200 to 

1,200 μg/L) between SCH-1A and 03-155 was observed in 

2003. As with most other Downtown area sites, the stability 

of the contaminant plume at the site cannot be ascertained 

until additional data are collected from the new downgradient 

monitoring well. Historical DRO concentrations from wells 

near the source area showed no consistent trend between 1996 

and 2002, although those data were likely affected by the 

start and end of free product recovery from wells at the site. 

Overall, the data indicate active biodegradation at the Sandy 

Cove housing area, and that as of 2003 natural attenuation is 

preventing DRO concentrations that exceed the cleanup levels 

from extending more than about 200 ft downgradient.

If MNA is selected as a remedy for this site, monitoring 

suggestions include:

1. Sampling well 03-155 to monitor the stability of the 

downgradient contamination, and 

2. Sampling well MW134-11 to monitor the contaminant 

source strength over time. 

Existing well 03-619 appears to be on the west side of the 

contaminant plume and could be sampled to monitor the 

lateral extent of DRO, but the new well 03-155 appears to be 

more directly on the contaminant flow path.

Monitoring for Natural Attenuation at Specific Petroleum Sites 43
Tanker Shed

The Tanker Shed site (fig. 17) is near an airport runway 

in the Downtown area of Adak Island (fig. 2). This site was 

used for maintenance on tanker trucks that transported fuel 

and heating oil around the base. The Tanker Shed facility 

included a 6,000 gal UST, and a 12 ft by 60 ft recessed pit 

with catch basin (sump), which was used as a vehicle wash 

rack. Used oil generated from truck maintenance and waste 

fluids from an oil/water separator system associated with the 

vehicle wash rack was stored in the UST. The Tanker Shed is 

believed to have been constructed during the 1960s, but the 

UST was functional only from 1985 to 1995. When the UST 

was removed in 1995, the 6,000 gal, double-walled, fiberglass 

tank was reported to be in good condition. No record of 

releases or spills exist for the Tanker Shed site; however, 

oil stains, strong petroleum hydrocarbon odor, and heavy 

sheen on ground water in the tank excavation indicate that an 

unknown quantity of fuel and oil was released. The probable 

source of contamination was overfill or pipe leakage. DRO is 

the petroleum contaminant of concern at the site. Since 1996, 

free product has been detected in 12 monitoring wells around 

the Tanker Shed, so the ROD-specified remedy was free 

product recovery. Over 200 gal of free product were removed 

between 1997 and 1998. MNA is being considered as a followup remedy for contaminated ground water.

The Tanker Shed site is underlain primarily by sands and 

silty-sands with some gravel. It has been inferred that ground 

water flows west towards East Canal (approximately 800 ft 

downgradient). Depth to ground water at the site is about 8 to 

10 ft. Abundant historical contaminant concentration data, as 

far back as 1996, are available for the site, and well 04-601 

has been monitored regularly as a sentinel well since 1998.

Objectives at the site were to determine: 

1. If the downgradient well TS-01 is on the centerline 

of the contaminant plume flow path and is suitably 

located to monitor the extent of contamination and 

plume stability, and 

2. If NAP data indicate biodegradation of petroleum 

compounds within the plume.

To meet these objectives, data were collected from six existing 

wells and four boreholes (table 12). Two boreholes (TSB-1A 

and TSB-1B) were nested to determine the depth distribution 

of contaminants before they passed under the taxiway west of 

the site. Two other boreholes (TSB-2 and TSB-3) were placed 

north and south of the first pair to delimit the lateral extent of 

the contaminant plume. 

DRO concentrations (table 12) indicate that well TS-01 

is reasonably located on the contaminant flow path to monitor 

the distal part of the contaminant plume at the site. Data from 

the nested pair of boreholes (TSB-1A and TSB-1B) 80 ft north 

Figure 17. Diesel range organics (DRO) and carbon dioxide concentrations for the Tanker Shed site, Operable Unit A, 

Adak Island, Alaska, May and June 2003.

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44 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
of well TS-01 indicate that the contaminant plume was at the 

water table rather than at depth; the deep borehole was aerobic 

and uncontaminated, and the shallow borehole was anaerobic 

with only a trace of DRO (50e μg/L). In October 2001, the 

DRO concentration in well TS-01 was 240 μg/L. Water-level 

data from existing wells and boreholes also indicate that well 

TS-01 is directly on the contaminated flow path.

The USGS NAP data indicated biodegradation at the 

Tanker Shed by a ten- to twenty-fold increase in carbon 

dioxide concentrations above background levels in the 

most contaminated wells. Ferrous iron concentrations also 

were elevated in petroleum contaminated samples. Data 

from boreholes TSB-2 and TSB-3 appear to best represent 

uncontaminated background water at the site, which is aerobic 

with carbon dioxide concentrations of 10 to 23 mg/L. All of 

the contaminated samples had carbon dioxide concentrations 

exceeding 30 mg/L. Historical (1998-2002) NAP data 

suggest that ground water has switched between aerobic and 

anaerobic conditions at well 04-601, although the alkalinity 

concentrations have consistently been 71 to 74 mg/L as 

CaCO3. The apparent varying historical NAP data at 04-601, 

as well as the elevated ferrous iron concentrations observed 

in the aerobic May 2003 sample, suggest that the 10 ft well 

screen spans both the contaminated, anaerobic upper part of 

the aquifer, and the uncontaminated and aerobic deeper part 

of the aquifer that was characterized by nested boreholes 

TSB-1A and TSB-1B. 

There appears to be an approximately fifty-fold 

attenuation of DRO along the flow path between 04-175 and 

well TS-01 (12,400–240 μg/L). As with most other Adak 

Island sites, the stability of the contaminant plume at the 

site cannot be ascertained until additional data are collected 

from the new downgradient monitoring well. Historical DRO 

concentrations from wells near the source area showed no 

consistent trend between 1996 and 2002, although those 

data likely were affected by the start and end of free product 

recovery from wells at the site. Overall, the data indicated 

active biodegradation at the Tanker Shed site, and that, as of 

2003, natural attenuation is preventing DRO concentrations 

that exceed the cleanup levels from extending more than about 

400 ft downgradient from well 04-290 near the source area.

If MNA is selected as a remedy for this site, monitoring 

suggestions include: 

1. Sampling well TS-01 to monitor the stability of the 

downgradient contamination, and 

2. Sampling either well 04-290 or 04-317 to monitor 

contaminant concentrations near the source over time.

Existing sentinel well 04-601 also could be sampled to 

monitor intermediate concentrations in the DRO contaminant 

plume, or it could be sampled in the future to document 

shrinking of the dissolved-phase plume when concentrations 

become non-detectable at TS-01. As of 2003, data from TS-01 

was optimal for delimiting the downgradient extent where 

concentrations were less than the 1,500 μg/L cleanup level.

Borehole or 

well No.

Date 

sampled

Date of 

historical 

petroleum 

data

Sampling 

depth 

(ft)

Water-level 

altitude 

(ft)

Diesel range 

organics (μg/L) pH 

(units)

Specific 

conductance 

(μS/cm)

Carbon 

dioxide 

(mg/L)

Dissolved 

oxygen 

(mg/L)

Ferrous 

iron

(mg/L) Field Laboratory

Boreholes

TSB-1A 05-21-03 – 10 2.24 <100 50e 5.8 192 35 0.5 15

-1B 05-21-03 – 18 2.31 – – 5.6 206 19 6 .8

-2 05-21-03 – 10 2.46 – – 5.6 93 10 7 .4

-3 05-21-03 – 10 2.67 – – 5.9 143 23 1.5 7.0

Monitoring wells

TS-01 05-21-03 10-08-01 7 2.31 – 240 6.1 492 30 0.8 0.14

-03 05-21-03 10-08-01 11 4.53 – 153 5.9 175 35 7 .09

04-175 05-21-03 10-07-01 9 4.99 – 12,400 6.0 369 135 <.025 1.5

-290 05-21-03 10-07-01 9 4.24 – 9,220 6.0 453 180 <.025 2.0

-317 05-21-03 10-08-01 9 4.53 – 9,220 6.0 380 180 <.025 2.1

-601 05-21-03 10-14-02 11 3.36 – 2,600 5.5 265 35 3 .96

Table 12. Selected diesel range organics, geochemical, and water-level data from boreholes and wells at the Tanker Shed site, Operable Unit A, 

Adak Island, Alaska, May and June 2003.

[Borehole or well No. are shown in figure 17. Sampling depths are approximate feet below land surface of the mid-point of the sampling device or well 

screen. Water-level altitudes are in feet above or below (-) NAD27. Diesel range organics: Historical petroleum data collected by the Navy are in italics. 

Abbreviations: ft, foot; μg/L, microgram per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligram per liter. Symbols: e, compound 

was detected but concentration was estimated; <, actual value is less than value shown; –, not analyzed]

Monitoring for Natural Attenuation at Specific Petroleum Sites 45
Monitoring Strategy for Natural 

Attenuation of Petroleum Throughout 

the Downtown Area

A summary of the proposed monitoring locations for 

the investigated petroleum sites is presented in table 13. The 

proposals were based in large part on the overall monitoring 

strategy for natural attenuation throughout the Downtown 

area.

The 1999 ROD specified three objectives that groundwater monitoring in affected areas must satisfy:

• To verify whether natural attenuation is occurring.

• To monitor locations where chemical concentrations 

exceed specified cleanup levels.

• To estimate the rate of natural attenuation to 

demonstrate achievement of cleanup levels within 

75 years.

Site Well No. 2002 sampling status 2004 and proposed future monitoring status

Former Power Plant 01-118 Sampled for MNA Sample to monitor source strength over time for MNA.

01-150 Not available Sample to monitor plume stability for MNA.

01-151 Not available Sample to monitor discharge to East Canal.

E-701 Sampled for background/sentinel Discontinue sampling.

GCI 04-100 Not available Sample to monitor plume stability (towards southwest) for MNA.

04-202 Not sampled Sample to monitor source strength over time for MNA.

04-701 Sampled-sentinel Sample to monitor plume stability (towards west) for MNA.

Arctic Acres 03-420 Sampled for MNA Sample infrequently to confirm no westward contaminant 

migration.

03-421 Sampled for MNA Discontinue sampling.

03-422 Sampled for MNA Discontinue sampling.

03-890 Sampled for MNA Sample to monitor source-strength and NAPL over time for MNA.

AA-01 Sampled for MNA Sample to monitor plume stability (towards east) for MNA.

AA-02 Sampled for MNA Sample infrequently to confirm no westward contaminant 

migration.

AA-05 Sampled for MNA Discontinue sampling.

AA-06 Sampled for MNA Sample infrequently to confirm no westward contaminant 

migration.

ROICC 08-153 Sampled for MNA Discontinue sampling.

08-160 Sampled for MNA Discontinue sampling.

08-175 Not available Sample to monitor plume stability for MNA.

08-200 Sampled for limited monitoring Sample to monitor source strength over time for MNA.

08-201 Sampled for limited monitoring Discontinue sampling.

08-202 Sampled for limited monitoring Discontinue sampling.

Avgas Valve Pit 14-100 Sampled for MNA Sample to monitor source strength over time for MNA.

14-110 Not available Sample to monitor plume stability for MNA.

SA 80 04-103 Not available Sample to monitor plume stability for MNA.

04-801 Sampled for sentinel monitoring Discontinue sampling.

SP4-3 Not sampled Sample to monitor source strength over time for MNA.

SWMU 14 01-153 Not available Sample to monitor plume stability for MNA.

MW14-5 Sampled for MNA Sample to monitor source strength over time for MNA.

MW14-423 Sampled for sentinel monitoring Discontinue sampling.

Tank Farm B 14-113 Not available Sample to monitor plume stability near Sweeper Creek.

14-210 Sampled for sentinel monitoring Discontinue sampling.

TFB-MW-4A Sampled for MNA Discontinue sampling.

TFB-MW-4B Sampled for MNA Sample to monitor source strength over time for MNA.

Sandy Cove Housing 03-155 Not available Sample to monitor plume stability for MNA.

03-619 Sampled for sentinel monitoring Discontinue sampling.

MW134-11 Not sampled Sample to monitor source strength over time for MNA.

Tanker Shed 04-290 or 04-317 Not sampled Sample to monitor source strength over time for MNA.

04-601 Sampled for sentinel monitoring Sample in future years to monitor shrinking plume.

TS-01 Not sampled Sample to monitor plume stability for MNA.

Table 13. Wells proposed for monitoring natural attenuation at investigated petroleum sites, Operable Unit A, Adak Island, Alaska.

[Locations of petroleum sites are shown in figure 2. Site is abbreviated site name; see table 1 for full site name. Well No. in bold indicates proposed future 

monitoring well. Abbreviations: MNA, monitored natural attenuation; NAPL, non-aqueous phase liquid.]

46 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
The U.S. Navy developed a comprehensive monitoring 

plan to achieve these objectives (URS Greiner Inc., 2001a), 

and they update that plan annually based on annual findings. 

Their overall monitoring strategy for MNA sites includes 

three types of sampling—natural attenuation, sentinel, 

and background. The goal of natural attenuation sampling 

is to verify that natural attenuation is occurring. Natural 

attenuation sampling is done at wells within the dissolvedphase petroleum plumes, and usually includes one well 

per MNA site. Samples from those wells are analyzed for 

petroleum chemicals of concern to monitor concentrations 

within the plumes and to assess the rate of degradation. 

Samples also are analyzed for NAPs to assess biodegradation. 

Degradation rates are estimated (when possible) based on a 

statistical determination of significant concentration trends 

over time at individual wells. The goal of sentinel sampling is 

to monitor the protectiveness of the remedy. Sentinel sampling 

is done at wells outside of dissolved-phase petroleum plumes 

and upgradient from the nearest receptor (generally surface 

water), and samples are analyzed for petroleum chemicals of 

concern. The goal of background sampling is to collect NAP 

data from uncontaminated ground water for demonstration 

of contaminant degradation by comparison to NAP data 

from within dissolved-phase petroleum plumes. The 2001 

monitoring plan (URS Greiner, Inc., 2001a) proposed 

evaluating the MNA data using a peer-reviewed computer 

model (approved by EPA and the Alaska Department 

of Environmental Conservation) that simulates natural 

attenuation of hydrocarbons in ground water. The specific 

objective of such modeling is unclear, although it may be used 

to demonstrate achievement of cleanup levels within 75 years.

Overall, the rationale behind that existing strategy is 

reasonable, but three years of monitoring data (1999-2002) 

suggest that the strategy needs some refinement because 

it does not appear that the monitoring objectives will be 

achieved in the specified 5-year evaluation period. This 

conclusion was reached in a USGS evaluation of the data 

(R.S. Dinicola, U.S. Geological Survey, written commun., 

December 23, 2002). A primary objective of this investigation 

was to install new wells at selected sites to better meet 

monitoring objectives. Because of the new sampling locations 

and the additional data collected while installing the new 

wells, specific refinements in the monitoring strategy are 

proposed. The sampling plan based on the proposed revisions 

should be considered as the minimum sampling effort 

needed to meet the first two monitoring objectives at the 

sites (a discussion concerning the third objective follows). 

A number of published regulatory agency and Department 

of Defense guidance documents recommend much more 

extensive monitoring at MNA sites, particularly with regard 

to the lateral and vertical extents of all contaminant plumes. 

However, the methods used to position the new monitoring 

wells during this investigation have addressed many of the 

concerns discussed in the guidance documents (at least in the 

near term). Ground-water flow directions over time may shift 

and contaminant plumes may migrate in different directions. 

A minimal monitoring program may not detect such changes.

The most direct approach for verifying that natural 

attenuation is effective is to monitor petroleum concentrations 

at the downgradient margin of contaminant plumes to 

demonstrate that the plumes are not expanding or that they are 

getting smaller. Regardless of the efficiencies of the individual 

attenuation mechanisms at work (biodegradation, dispersion, 

dilution, volatilization, and sorption), a stable or shrinking 

contaminant plume demonstrates that natural attenuation 

is effectively containing the migration of contamination. 

Thus, the monitoring plans for each MNA site should 

include sampling at least one strategically placed well at the 

downgradient margin of the contaminant plume, preferably 

where contaminant concentrations are detectable, but less than 

the cleanup level. Monitoring wells placed in such locations 

are now available for sampling at the 10 sites investigated.

An ancillary benefit of properly located downgradient 

wells is that they also are optimal for improved sentinel 

sampling. Although the theory of locating sentinel wells 

in uncontaminated downgradient locations is sound, the 

real-world difficulties of locating such wells precisely on 

contaminant flow paths are substantial, as was illustrated by 

the field work completed for this investigation. It was often 

challenging to find the contaminant plume over distances of 

a few hundred feet, and that challenge is much more difficult 

when trying to locate sentinel wells many hundreds of feet 

downgradient. Whether or not an uncontaminated and aerobic 

sentinel well in the Downtown area of Adak Island is actually 

downgradient from a contaminant source will always be 

uncertain. The area is too large to install the required density 

of sentinel wells near all potential surface-water receptors 

to substantially reduce the uncertainty that a plume will be 

missed. Similarly, if a far downgradient sentinel well does 

become contaminated, the source of the contamination will 

be uncertain. As detected at the Former Power Plant and GCI 

sites, there may be petroleum contamination in the Downtown 

area very near to but not associated with a specific MNA site. 

Installing sentinel wells at the distal parts of known plumes 

where contaminant concentrations are low and geochemical 

indicators of petroleum contamination and biodegradation are 

definitive is a more certain approach for sentinel monitoring. 

Thus, it would be beneficial to use the downgradient natural 

attenuation wells as sentinel monitoring wells for the MNA 

sites, and not include less accurately located sentinel wells in 

the MNA monitoring program. Four non-useful sentinel wells 

were identified at the sites investigated: 04-801 at the SA 80 

site, MW14-423 at the SWMU 14 site, 03-619 at the Sandy 

Cove housing area, and 04-601 at the Tanker Shed site. Well 

04-701 at the GCI site was proposed as one possibly useful 

sentinel well for future sampling.

Monitoring Strategy for Natural Attenuation of Petroleum Throughout the Downtown Area 47
In addition to the downgradient sentinel wells, 

continued monitoring is appropriate for at least one natural 

attenuation well within or immediately downgradient from the 

contaminant source area at each site. Continued monitoring 

at most existing natural attenuation wells is suggested 

because they are already suitably located. Contaminant 

concentration data from those near-source-area wells are 

useful for clearly demonstrating natural attenuation (or in a 

few cases, the lack of attenuation) over distance when their 

contaminant concentrations are compared to downgradient 

wells. Demonstration of attenuation over distance is more 

important than demonstrating natural attenuation over time 

at a single well when protection of downgradient receptors 

is a primary remediation goal. Data from upgradient and 

downgradient wells also would be beneficial for any fate 

and transport modeling at MNA sites, as was suggested 

in the comprehensive monitoring plan. Although “off-theshelf” contaminant degradation values often have been 

used for similar modeling exercises, the use of observed 

field attenuation rates calculated from observed data adds 

credibility to those modeling efforts.

Benefits from continued “background” sampling for 

MNA seem to have become marginal. This conclusion 

is based on the preponderance of existing NAP data 

demonstrating on-going petroleum biodegradation in the 

Downtown area of Adak Island, and the sound scientific 

basis for when to expect (and not to expect) biodegradation 

to occur. At most sites sampled for this investigation, the 

aerobic and uncontaminated background water characteristic 

of the shallow aquifer became anaerobic in contaminant 

plumes, along with increased ferrous iron and carbon dioxide 

concentrations. Even at the few sites that have naturally 

anaerobic and iron-rich ground-water (such as the ROICC and 

Avgas Valve Pit sites), elevated carbon dioxide concentrations 

within contaminant plumes clearly indicated petroleum 

biodegradation. A consistent relation between carbon dioxide 

concentrations and petroleum concentrations was observed 

at all sites except Tank Farm B (figs. 18 and 19). That 

relation indicates that, in general, the more dissolved-phase 

petroleum available, the more biodegradation occurs. At the 

lone site where biodegradation appeared to be inconsequential 

(the flood plain area at Tank Farm B), the contaminant 

concentration data alone clearly demonstrate the ineffective 

attenuation processes. Even there, the NAP data were 

somewhat superfluous. And despite some earlier data quality 

issues, the 1999-2002 NAP data from Downtown area sites 

lead to the same conclusions; petroleum biodegradation has 

been demonstrated to be nearly ubiquitous.

Continued collection of NAP data is not suggested 

to achieve the ROD monitoring objective of verifying the 

occurrence of natural attenuation. The overall effectiveness 

of MNA is better demonstrated through contaminant 

concentration data from downgradient monitoring wells. 

In addition, the NAP data collected to date are adequate to 

demonstrate that biodegradation plays a significant role in that 

attenuation (figs. 18 and 19).

A benefit of NAP data collection at intensively monitored 

MNA sites is the potential for detecting small-scale expansion 

of methanogenic layers within a contaminant plume that can 

lead to subtle (a few meters) plume expansion over a few 

decades (Bekins and others, 2001; Cozzarelli and others, 

2001). As available electron-acceptors (iron in particular) 

are consumed in narrow preferred-flow pathways within a 

plume, the switch to slower petroleum degradation linked to 

methanogenesis can result in petroleum plume expansion. 

However, this phenomena has only been detected through 

analysis of porewater samples from aquifer cores, and it 

probably cannot be detected using plume-scale monitoring 

well data. Thus, realization of those benefits for continued 

NAP data collection at Adak Island MNA sites would require 

a substantially enhanced monitoring program, and the risk of 

not being able to immediately identify relatively subtle plume 

expansion at most Adak Island sites is minimal. A second 

potential benefit for continued NAP data collection at the 

MNA sites is to facilitate predictive modeling of petroleum 

attenuation over time; however the existing data are probably 

sufficient to complete the type of predictive modeling proposed 

in the comprehensive monitoring plan.

Predictive modeling of contaminant plume behavior 

was proposed to achieve the third ROD specified monitoring 

objective “to estimate the rate of natural attenuation to 

demonstrate achievement of cleanup levels within 75 years.” 

The comprehensive monitoring plan (URS Greiner, Inc., 2001a) 

describes another strategy for meeting that objective that relies 

primarily on a statistical trend analysis on 5 years of petroleum 

concentration data from individual within-plume wells. 

Demonstrating (predicting) achievement of cleanup levels will 

be difficult to achieve within any timeframe in a technically 

defensible manner using any type of short-term monitoring and 

evaluation, and will be particularly difficult to achieve through 

monitoring and evaluation of dissolved-phase petroleum only. 

The majority of petroleum contamination at MNA sites 

resulted from past spills or leaks of pure-phase petroleum into 

the ground. Thus, the continuing source of dissolved petroleum 

in ground water probably is residual NAPL petroleum held 

by capillary forces in the unsaturated zone or near the top of 

the saturated zone. Volumes of residual NAPL at Adak Island 

MNA sites are not well known, but the presence of NAPL is 

clearly indicated by the many within-plume wells that show 

no clear decreasing trends in petroleum concentrations. If 

that residual NAPL is the predominant continuing source 

of dissolved-phase contaminants, time required to achieve 

cleanup levels is primarily controlled by the volume of residual 

NAPL remaining, and by the rate at which the NAPL dissolves 

into ground water. NAPL itself is essentially not susceptible 

to biodegradation (U.S. Environmental Protection Agency, 

1995). Many of the NAPL-related compounds are amenable 

to biodegradation, but they must first dissolve into water 

before biodegradation can occur. Thus, the dissolution rate of 

residual NAPL (rather than the biodegradation rate of dissolved 

petroleum compounds) primarily controls the time it takes to 

achieve cleanup levels throughout an MNA site.

48 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
Figure 19. Relation between gasoline range organics (GRO) and carbon dioxide concentrations from all sites investigated where GRO 

was a contaminant of concern, Operable Unit A, Adak Island, Alaska.

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a contaminant of concern, Operable Unit A, Adak Island, Alaska.

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Monitoring Strategy for Natural Attenuation of Petroleum Throughout the Downtown Area 49
The trend-analysis strategy for predicting cleanup times 

relies on the assumptions that the change in dissolved-phase 

concentrations over time at a monitoring location is directly 

proportional to the NAPL dissolution rate, and that the 

monitoring location is located close enough to the NAPL 

source that the observed dissolved-phase concentration is the 

maximum concentration that is dissolving from the NAPL 

source. Those assumptions are not likely true. Laboratory and 

modeling studies conducted by many researchers (Borden 

and Kao, 1992; Geller and Hunt, 1993; Huntley and Beckett, 

2002; Powers and others, 1991) have shown that when NAPL 

is present, nearby dissolved-phase contaminant concentrations 

are initially high, followed by a rapid decline primarily 

due to volatilization, followed by a slow asymptotic decline 

over a long period. In that case, a trend calculated from 

concentration data collected during the rapid decline would 

not be representative of the following slower decline. Also, 

given that most MNA sites have only one monitoring well 

within the dissolved-phase contaminant plume, the probability 

is low that those lone wells at each site are directly adjacent to 

the residual NAPL source. And finally, as the NAPL weathers 

over time, the more soluble petroleum compounds (such as 

benzene) will be depleted in the remaining NAPL and the 

residual less soluble compounds will dissolve at a slower rate.

Unfortunately, no simple method exists to demonstrate 

(predict) achievement of cleanup levels in all ground water 

under an MNA site within 75 years. BIOSCREEN modeling 

results presented in Appendix C of the Final Focused 

Feasibility Study for Petroleum Sites (URS Greiner, Inc., 

1999) present time-to-cleanup estimates for MNA sites that 

probably are more realistic than those that can be derived 

from dissolved petroleum data alone. The BIOSCREEN 

model considers both attenuation of dissolved-phase 

petroleum through biodegradation, and attenuation of the 

source through dissolution of the remaining NAPL. However, 

the estimates of how much NAPL remained are highly 

uncertain. BIOSCREEN modeling applications were not 

evaluated in detail for this report, but in general, the results 

appear realistic. 

BIOSCREEN modeling results indicate that within 80 

years: 

1. Contaminant concentrations at nearly all MNA 

source areas are predicted to remain above specified 

cleanup levels, but 

2. Dissolved-phase contaminant concentrations are 

predicted to be below cleanup levels except at 

distances of only a few feet to a few tens of feet from 

the sources.

An alternative approach for more precise time-to-cleanup 

estimates would be to more rigorously simulate NAPL 

dissolution. The BIOSCREEN model uses a simplified and 

unverified theory (a caveat listed in the model documentation) 

to simulate NAPL dissolution—it assumes that dissolution 

can be approximated as a first-order decay process. A 

modeling toolkit that is similar to BIOSCREEN with regard 

to dissolved-phase processes, but more rigorous with regard 

to NAPL dissolution would be one option (Huntley and 

Beckett, 2002). However, NAPL dissolution is a complex 

process, particularly in heterogeneous environments with 

fluctuating water tables. As a result, NAPL simulation models 

have extensive data requirements and the accuracy of their 

results often are uncertain. Published research from the wellstudied crude-oil spill site at Bemidji, Minnesota, illustrates 

the intensity of monitoring required and the sophistication of 

modeling needed to realistically attempt to predict petroleum 

fate and transport far into the future (Bekins and others, 2001; 

Cozzarelli and others, 2001; Dillard and others, 1997).

Overall, demonstrating (predicting) achievement 

of cleanup levels within any timeframe in a technically 

defensible manner will be difficult to meet using any type of 

short-term ground-water monitoring data. Natural attenuation 

processes appear to have greatly limited the extent of groundwater contamination at most sites investigated, and therefore 

have limited the risk that petroleum contaminants pose. 

However, relatively small areas where contaminants remain at 

concentrations exceeding cleanup levels will likely persist for 

many decades, but how many decades is a difficult question 

to answer. Clarification or refinement of the monitoring 

objective to demonstrate cleanup within 75 years would be 

a reasonable prelude to developing a monitoring and data 

evaluation strategy to meet the objective.

Conclusions

The expedited site-assessment strategy used for this 

investigation allowed us to quickly and effectively characterize 

ground-water conditions at the sites of interest. The extent of 

petroleum contamination and natural attenuation parameter 

evidence for petroleum biodegradation in ground water were 

determined on-site using field analyses of selected petroleum 

compounds, specific conductance, dissolved oxygen, ferrous 

iron, and carbon dioxide, and 10 new monitoring wells were 

optimally positioned and installed. Specific refinements in 

the natural attenuation monitoring strategy were proposed 

with regard to the newly available sampling locations and the 

additional data collected during this investigation.

The monitoring objective of verifying that natural 

attenuation is occurring can be achieved at the 10 investigated 

sites by including in the sampling plans the new wells 

installed at the downgradient margin of the contaminant 

plumes. An ancillary benefit of sampling the new wells is 

that they also can serve as sentinel monitoring wells for the 

monitored natural attenuation sites. Continued collection of 

natural attenuation parameter data and sampling background 

wells is no longer essential to achieve the monitoring objective 

of demonstrating the occurrence of natural attenuation. 

50 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003
Collecting and analyzing contaminant concentration data 

from downgradient monitoring wells better demonstrates 

the overall effectiveness of monitored natural attenuation. In 

addition, the natural attenuation parameter data collected to 

date are adequate to demonstrate that biodegradation plays 

a significant role in natural attenuation. A preponderance of 

existing data demonstrates on-going petroleum biodegradation 

in the Downtown area of Adak Island, and a sound scientific 

basis exists for when to expect (and not expect) biodegradation 

to occur.

The objective of monitoring locations where chemical 

concentrations exceed specified cleanup levels can be 

achieved by monitoring at least one natural attenuation well 

within or immediately downgradient from the contaminant 

source area at each site, and most existing natural attenuation 

wells are in appropriate locations. Contaminant concentration 

data from the near source-area wells are useful to clearly 

demonstrate natural attenuation over a distance, and that 

demonstration is more important than demonstrating natural 

attenuation over time at a single well when protection of 

downgradient receptors is a primary remediation goal. 

Achieving the monitoring objective of estimating the rate of 

natural attenuation to demonstrate achievement of cleanup 

levels within 75 years will be problematic. Demonstrating 

(predicting) achievement of cleanup levels within any 

timeframe in a technically defensible manner will be 

difficult to achieve using any type of short-term monitoring 

and evaluation, and will be particularly difficult to achieve 

through monitoring and evaluation of dissolved-phase 

petroleum only. Despite the difficulties in achieving that 

final objective in a technically defensible manner, natural 

attenuation processes appear to have greatly limited the extent 

of ground-water contamination at most sites investigated and 

have limited the risk that petroleum contaminants pose to 

downgradient receptors.

References Cited

Alaska Department of Environmental Conservation, 2000, 

Guidance on the selection of natural attenuation as a 

cleanup alternative for the restoration of soil and ground 

water at contaminates sites: ADEC Division of Spill 

Prevention and Response Guidance No. SPAR 2000-1, 22 p.

Alaska Department of Environmental Conservation, 2002, 

Underground storage tanks procedures manual: November 

7, 2002, Appendix D. Alaska Series Laboratory Methods 

for the Analysis of Gasoline Range Organics (AK101), 

Diesel Range Organics (AK102), and Residual Range 

Organics (AK103), 70 p.

References Cited 51

Bekins, B.A., Cozzarelli, I.M., Godsy, M., Warren, E., Essaid, 

H.I., and Tuccillo, M.E., 2001, Progression of natural 

attenuation processes at a crude-oil soil site—II. Controls 

on spatial distribution of microbial populations: Journal of 

Contaminant Hydrology, v. 53, p. 387-406.

Borden, R.C., and Kao, C.M., 1992, Evaluation of groundwater extraction for remediation of petroleum-contaminated 

aquifers: Water Environment Research, v. 64, no. 1, 

p. 26-28.

Cozzarelli, I.M., Bekins, B.A., Baedecker, M.J., Aitken, G.R., 

Eganhouse, R.P., and Tuccillo, M.E., 2001, Progression 

of natural attenuation processes at a crude-oil soil 

site—I. Geochemical evolution of the plume: Journal of 

Contaminant Hydrology, v. 53, p. 369-385.

Dillard, L.A., Essaid, H.I., and Herkelrath, W.N., 1997, 

Multiphase flow modeling of a crude-oil spill site with 

a bimodal permeability distribution: Water Resources 

Research, v. 33, no. 7, p. 1617-1632.

Geller, J.T., and Hunt, J.R., 1993, Mass transfer from 

nonaqueous phase organic liquids in water-saturated porous 

media: Water Resources Research, v. 29, no. 4. p. 833-845.

Huntley, D., and Beckett, G.D., 2002, Persistence of LNAPL 

sources—Relationship between risk reduction and LNAPL 

recovery: Journal of Contaminant Hydrology, v. 59, p. 3-26.

ICRC, 2003, Annual groundwater monitoring report, October 

2002: Integrated Concepts and Research Corporation, Eagle 

River, Alaska, for Engineering Field Activity Northwest, 

Naval Facilities Engineering Command, Poulsbo, 

Washington, under contract N44255-00-D-2538, delivery 

order 008.

National Research Council, 2000, Natural attenuation for 

groundwater remediation: National Academy Press, 

Washington, D.C., 274 p.

Powers, S.E., Loureiro, C.O., Abriola, L.M., and Weber, 

W.J., 1991, Theoretical study of the significance of 

nonequilibrium dissolution in nonaqueous phase liquids in 

subsurface systems: Water Resources Research, v. 27 no. 4, 

p. 463-477.

Sigma-Aldrich Co., 1998, Monitor BTEX compounds and 

fuels in water using solid phase microextraction/capillary 

GC: Application Note 81, Supelco, Bellefonte, Penn., 2 p.

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nonaqueous phase liquids: Technology Innovation Office, 

Office of Solid Waste and Emergency Response, Ground 

Water Issue, EPA/504/S-95/500, Washington, D.C., 28 p.

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for evaluating solid waste, physical/chemical methods: U.S. 

Environmental Protection Agency, SW-846, accessed March 

20, 2003, at http://www.epa.gov/epaoswer/hazwaste/test/

pdfs/8021b.pdf
U.S. Geological Survey, variously dated, National field manual 

for the collection of water-quality data: U.S. Geological 

Survey Techniques of Water-Resources Investigations, book 

9, chaps. A1-A9, available online at http://pubs.water.usgs.

gov/twri9A. [Chapter updates and revisions are ongoing and 

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U.S. Navy, Alaska Department of Environmental 

Conservation, and U.S. Environmental Protection Agency, 

2000, Draft final record of decision, Operable Unit A, 

Former Adak Naval Complex, Adak Island, Alaska: 

Prepared by URS Greiner, Inc., for Engineering Field 

Activity Northwest, Naval Facilities Engineering Command, 

Poulsbo, Washington, under CLEAN Contract No. N62474-

89-D-9295. Accepted as final April 2000.

52 Monitoring the Natural Attenuation of Petroleum in Ground Water, Adak Island, Alaska, May and June 2003

URS Greiner, Inc., 1999, Final focused feasibility study for 

petroleum sites, Adak Naval Complex, Adak Island, Alaska, 

Final Addendum: Prepared for Engineering Field Activity, 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, Washington, CLEAN Contract Task Order 0232.

URS Greiner, Inc., 2001, Final comprehensive monitoring 

plan, Operable Unit A, former Adak Naval Complex, Adak 

Island, Alaska: Prepared for Engineering Field Activity 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, Washington, CLEAN Contract Task Order 0238.

URS, Inc., 2003, Revised comprehensive monitoring plan, 

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Island Alaska: Prepared by URS, Inc., Seattle, Washington, 

for Engineering Field Activity Northwest, Naval Facilities 

Engineering Command, Poulsbo, Washington, Delivery 

Order 0008.
Manuscript approved for publication, January 10, 2005

Prepared by the Publishing Group, U.S. Geological Survey, 

Washington Water Science Center, Tacoma, Washington

USGS Publishing staff

Linda S. Rogers

 Debra Grillo

Bill Gibbs

Bobbie Jo Richey

Ginger Renslow

For more information concerning the research in this report, contact 

the Washington Water Science Center Director, 

U.S. Geological Survey, 1201 Pacific Avenue – Suite 600 

Tacoma, Washington 98402

http://wa.water.usgs.gov
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