Fate and Transport of Petroleum Hydrocarbons 

in the Subsurface near Cass Lake, Minnesota

Prepared in cooperation with the Leech Lake Band of Ojibwe, Department of Resource Management

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010-5085

© 2009 Europa Technologies

Image USDA Farm Service Agency

© 2009 Google
This page left intentionally blank.
Fate and Transport of Petroleum Hydrocarbons in the 

Subsurface near Cass Lake, Minnesota

By Dina M. Drennan, Barbara A. Bekins, Ean Warren, Isabelle M. Cozzarelli, Mary Jo Baedecker, William N. 

Herkelrath, Geoffrey N. Delin, Robert J. Rosenbauer, and Pamela L. Campbell

Scientific Investigations Report 2010-5085

Prepared in cooperation with the Leech Lake Band of Ojibwe, Department of Resource Management

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

FRONT COVER

Satellite image of the South Cass Lake Pumping Station and surrounding area.

U.S. Geological Survey, Reston, Virginia: 2010

This report and any updates to it are available online at:

http://pubs.usgs.gov/sir/2010/5085

For product and ordering information:

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

Telephone: 1-888-ASK-USGS (1-888-275-8747)

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

endorsement of the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report. 

Suggested citation:

Drennan, D.M., Bekins, B.A., Warren, E., Cozzarelli, I.M., Baedecker, M.J., Herkelrath, W.N., Delin, G.N., 

Rosenbauer, R.J., and Campbell, P.L., 2010, Fate and transport of petroleum hydrocarbons in the subsurface near Cass 

Lake, Minnesota: U.S. Geological Survey Scientific Investigations Report 2010-5085, 33 p. 
 iii

Contents

Abstract...........................................................................................................................................................1

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

Purpose and Scope ..............................................................................................................................2

Site Description.....................................................................................................................................2

Geography.....................................................................................................................................2

Hydrogeology................................................................................................................................2

Site History.............................................................................................................................................3

Biodegradation of Petroleum Hydrocarbons ...................................................................................5

Evidence Used to Demonstrate Biodegradation ....................................................................5

Description of the Bemidji Research Site................................................................................5

Methods...........................................................................................................................................................6

Sample Collection by Push-Probe .....................................................................................................6

Installation and Sampling of Well Network......................................................................................6

Groundwater Analyses ........................................................................................................................6

September 2007 and May 2008..................................................................................................6

June 2008.......................................................................................................................................7

Sediment Sampling and Analysis.......................................................................................................8

Hydraulic Conductivities.............................................................................................................8

Oil....................................................................................................................................................8

Analyses of Nonvolatile Fraction .....................................................................................8

Analyses of Volatile Fraction ............................................................................................9

Iron....... ........................................................................................................................................11

Microbial Populations...............................................................................................................11

Results...........................................................................................................................................................12

Characterization of Hydrogeology ..................................................................................................12

Head Distribution and Flow Directions .................................................................................12

Hydraulic Conductivities, Discharge Rate, and Flow Velocity ...........................................12

Distribution of Petroleum Hydrocarbons ........................................................................................13

Separate Phase Oil....................................................................................................................13

Nonvolatile Hydrocarbons ..............................................................................................14

Volatile Hydrocarbons......................................................................................................15

Groundwater Plume ..................................................................................................................17

Differences Between Datasets ......................................................................................21

Distribution of Redox Species ..........................................................................................................21

Groundwater...............................................................................................................................21

Sediments....................................................................................................................................23

Microbial Populations........................................................................................................................24

Analysis of Biodegradation Processes ...................................................................................................25

Comparison of the Bemidji and Cass Lake Sites ...........................................................................25

Biodegradation of Oil Body ...............................................................................................................27

Rate...... ........................................................................................................................................27

Long-Term Fate of Oil ................................................................................................................28

Biodegradation of Groundwater Plume ..........................................................................................28

Biodegradation Rate .................................................................................................................28

Long-Term Fate of Plume..........................................................................................................28

Conclusions...................................................................................................................................................29

References....................................................................................................................................................31
Figures

 1. Location of the Cass Lake site ..............................................................................................................2

 2. Locations of previous monitoring wells and borings at the Cass Lake site ..................................4

 3. Water table elevations (m) beneath the Cass Lake site..................................................................12

 4. Saturated thickness (m) of the surficial sand-and-gravel aquifer at the Cass Lake site .........13

 5. Frequency distribution of log hydraulic conductivity (m/d) determined from grain size 

analyses of 16 samples collected at the Cass Lake site .........................................................14

 6. Comparison of n-alkanes in oil and sediment samples as determined by gas 

chromatograph/mass spectrometry ..........................................................................................14

 7. Comparison of concentrations of benzene, toluene, ethylbenzene, and xylenes (BTEX); 

cycloalkanes; methylbenzenes; and methylnaphthalenes (mg/g) in Cass Lake 

spilled oil, Cass Lake pipeline oil, and oil extracted from sediment .....................................15

 8. Maps of the benzene plume at the Cass Lake site ......................................................................... 18

 9. Benzene concentrations (μg/L) in the Cass Lake plume ................................................................20 

10. Maps of reduced iron Fe(II) (mg/L) in the Cass Lake plume ..........................................................22

11. Concentrations of redox derivatives (mg/L) along line of section D-D’ .......................................23

12. Average oxidized iron Fe(III) measured in sediment cores ...........................................................23

13. Most probable number (MPN) of four major physiological types of microorganisms along 

line of section E-E’ ........................................................................................................................ 24

14. Geochemical zones in the unsaturated and saturated hydrologic zones at the Bemidji 

crude-oil spill site ..........................................................................................................................26

15. Conceptual model of the oil spill and plume at the Cass Lake site, showing major 

geochemical zones .......................................................................................................................27 

16. Average concentrations (mg/L) of selected constituents at the Cass Lake site versus 

distance ...........................................................................................................................................29

17. Position of oil body and 10-μg/L benzene contour line in the Cass Lake plume ........................30

List of Tables

1. Biodegradation reactions of benzene under aerobic and anaerobic conditions ..........................5

2. Site identification numbers for wells installed in May 2008 by U.S. Geological Survey ...............7

3. Locations of samples collected for sediment iron and microbial analyses at the 

Cass Lake site, 2008 ........................................................................................................................9 

4. Results of grain-size analyses and calculated hydraulic conductivities for core samples 

collected at the Cass Lake site ...................................................................................................10

5. Selected results of oil analyses: d13C values for oil extracted from sediments at the South 

Cass Lake Pumping Station .........................................................................................................13

6. Results of analyses of oil collected from a South Cass Lake Pumping Station pipeline ............16

7. Values of selected parameters in water from the background well (MW-1) and 

selected contaminated wells ......................................................................................................19

8. Results of iron analyses from core samples collected from the Cass Lake site, 2008 ................25 
By Dina M. Drennan, Barbara A. Bekins, Ean Warren, Isabelle M. Cozzarelli, Mary Jo Baedecker, William N. 

Herkelrath, Geoffrey N. Delin, Robert J. Rosenbauer, and Pamela L. Campbell

Fate and Transport of Petroleum Hydrocarbons in the Subsurface 

near Cass Lake, Minnesota

Abstract

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

natural attenuation of subsurface petroleum hydrocarbons 

leaked over an unknown number of years from an oil pipeline 

under the Enbridge Energy Limited Partnership South Cass 

Lake Pumping Station, in Cass Lake, Minnesota. Three 

weeks of field work conducted between May 2007 and July 

2008 delineated a dissolved plume of aromatic hydrocarbons 

and characterized the biodegradation processes of the 

petroleum. Field activities included installing monitoring 

wells, collecting sediment cores, sampling water from 

wells, and measuring water-table elevations. Geochemical 

measurements included concentrations of constituents in 

both spilled and pipeline oil, dissolved alkylbenzenes and 

redox constituents, sediment bioavailable iron, and aquifer 

microbial populations. Groundwater in this area flows eastsoutheast at approximately 26 meters per year. Results from 

the oil analyses indicate a high degree of biodegradation, 

characterized by nearly complete absence of n-alkanes. 

Cass Lake oil samples were more degraded than two oil 

samples collected in 2008 from the similarly contaminated 

USGS Bemidji, Minnesota, research site 40 kilometers away. 

Based on 19 ratios developed for comparing oil sources, the 

conclusion is that the oils at the two sites appear to be from 

the same hydrocarbon source.

In the Cass Lake groundwater plume, benzene 

concentrations decrease by three orders of magnitude within 

150 meters (m) downgradient from the oil body floating on 

the water table (between well MW-10 and USGS-4 well nest). 

The depths of the highest benzene concentrations increase 

with distance downgradient from the oil, a condition typical 

of plumes in shallow, unconfined aquifers. Background 

groundwater, which is nearly saturated with oxygen, becomes 

almost entirely anaerobic in the plume. As at the Bemidji site, 

the most important biodegradation processes are anaerobic 

and dominated by iron reduction. The similarity between 

the Cass Lake and Bemidji benzene degradation rates, redox 

conditions, and aquifer material all support a hypothesis that 

the Cass Lake plume, like the Bemidji plume, is decades old. 

As concentrations of alkylbenzenes in the oil decrease 

over time, the benzene concentrations in the groundwater 

plume will also decrease and the plume is expected to shrink. 

The Fox Creek wetland, about 250 m south of the Cass Lake 

site, is the nearest receptor to the south. 

Introduction

Crude-oil contamination was detected in the subsurface 

beneath an oil pipeline pumping station (the Enbridge Energy 

Limited Partnership South Cass Lake Pumping Station) in 

Cass Lake, Minnesota (fig. 1) in May 2001 (Natural Resources 

Engineering Company, 2003). The Leech Lake Band of Ojibwe 

Department of Resource Management (DRM) is concerned that 

crude-oil contaminants may threaten water supplies, human 

health, or the environment by migrating off site to nearby 

domestic water wells and surface water bodies. To address this 

concern, USGS did a study in cooperation with Leech Lake 

Band of Ojibwe, Department of Resource Management. Previous studies on the fate of crude oil in the subsurface in a similar 

sand-and-gravel aquifer at a long-term research site 20 kilometers (km) northwest of Bemidji, Minnesota, and 40 km northwest of the Cass Lake site, provide a comprehensive scientific 

framework within which to evaluate the Cass Lake spill. 

Studies at the oil-contaminated Bemidji research site 

(Baedecker and others, 1993; Eganhouse and others, 1993; 

Bennett and others, 1993) and other hydrocarbon-contaminated 

sites nationwide (for example, Rice and others, 1995) show that 

groundwater contaminants from crude oil and gasoline biodegrade naturally in the subsurface. When the degradation rate 

is rapid compared to the groundwater velocity, the distance of 

migration of contaminants away from the relatively stationary 

oil is limited to less than a few hundred meters. The long-term 

outlook for contaminant migration depends on the continued 

ability of subsurface microbial communities to degrade the contaminants. How long a hydrocarbon groundwater contaminant 

plume will persist depends on the amount of oil in the subsurface and the rate that contaminants are leached from the oil. 

Degradation rates must be examined individually at each site 
2 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

where natural biodegradation may be used to manage groundwater contamination (National Research Council, 2000). 

Purpose and Scope

The purpose of this study is to examine whether natural 

attenuation can control the spread of the crude-oil contamination 

from the South Cass Lake Pumping Station (the Cass Lake site). 

This report describes the important natural attenuation processes 

of petroleum hydrocarbons in the shallow surficial aquifer below 

and downgradient of the Cass Lake site. The results extend and 

supplement data provided by Enbridge Energy and its consultants 

to assist DRM and Enbridge in their management of the Cass Lake 

site. The work consisted of three phases: (1) delineation of the 

plume of dissolved petroleum hydrocarbons in three dimensions, 

(2) characterization of the redox zones and physiologic types of 

subsurface microorganisms in the plume, and (3) evaluation of the 

natural attenuation capacity and sustainability of the biodegradation reactions. The methods, approach, and hypotheses for this 

study are derived from the knowledge gained from more than 25 

years of research at Bemidji and other petroleum-contaminated 

sites. An important goal of this study is to determine whether degradation rates differ at the two sites (Cass Lake and Bemidji) and, 

if so, to understand the most important contributing factors.

Site Description

Geography

The Cass Lake site (fig. 1A) is located in north-central 

Minnesota, 2.25 km west of downtown Cass Lake and within 

the reservation environment of the Leech Lake Band of 

Ojibwe. The nearest surface water bodies are Spike Lake, 

1 km south by southwest, and a wetland at the head of Fox 

Creek, about 250 m to the south (fig. 1B). There are about 

a dozen homes or small businesses within 1 km of the site, 

primarily to the northwest and northeast, with no homes or 

businesses within 1 km to the southwest or southeast. Annual 

precipitation is approximately 61 centimeters (cm) a year.

Hydrogeology

The glacial outwash aquifer beneath the Cass Lake 

site constitutes part of the Bemidji-Bagley outwash plain 

(Kanivetsky, 1979; Stark and others, 1991). The aquifer 

consists primarily of moderately well sorted fine to mediumgrained sand and silt with some interbedded gravel. Grey 

calcareous clay-rich till underlies the aquifer 8 to 14 m 

Figure 1. Location of 

the Cass Lake site. A, 

Surface geology and 

hydrologic features 

(modified from 

Kanivetsky, 1979). 

B, Satellite image of 

the South Cass Lake 

Pumping Station and 

surrounding area.

94°40'W 94°30'W

47°20'N

Pike

Bay

Cass

Lake

South Cass

Lake Pumping

Station Cass Lake

Fox Creek

and wetlands

Mississippi River Mississippi River

EXPLANATION

Major road

Town

Stream, river

Wetlands

Outwash

Lake

Till, loess, peat

2

Lake

371

Spike

0

0

5 miles

Oil pipelines

Oil pipelines

5 10 kilometers

A

Minnesota

Minneapolis

St. Paul 

Mississippi

River 

2

2
Introduction 3

below the land surface and crops out at the land surface 

to the northeast and southwest of the site (fig. 1A). The 

glacial outwash has a high horizontal hydraulic conductivity 

compared to the till (Lohman, 1972). Depth to the water table 

generally varies with topography and is roughly 7 to 9 m 

below land surface (Wenck Associates, 2006). 

Site History

Enbridge Energy Limited Partnership (Enbridge), an 

energy transporter and distributor with extensive crudeoil pipelines in the United States and Canada (http://www.

enbridge.com/utility/site-map.php, last accessed April 9, 

2010), runs the South Cass Lake Pumping Station. The station was completed in 1971, and the final testing occurred on 

August 4, 1971 (Karl Beaster, Enbridge Energy, email commun., July 7, 2009). Pumping of oil through the pipeline began 

shortly thereafter.

In 2001, Enbridge consultants evaluated the shallow 

groundwater quality at the Cass Lake site to evaluate the geology 

and aquifer properties, determine if any groundwater contamination occurred, and establish a release-detection warning system. 

Four monitoring wells were installed in May 2001 (MW-1 to 

MW-4, fig. 2). The first indication of impacted soil and groundwater was on May 23, 2001, when MW-3 was installed. The 

Minnesota State Duty Officer was notified, and the Cass Lake 

site was assigned spill #54827. On the basis of the monitoring 

Figure 1.—Continued.

South Cass Lake 

Pumping Station

Fox Creek and wetlands

Railroad tracks

0 250 500 750 1,000 feet

0 100 200 300 meters

© 2009 Europa Technologies

Image USDA Farm Service Agency

© 2009 Google

B
4 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

well data, borings GP-1 through GP-16 were collected using a 

Geoprobe in September 2001 to define the magnitude and extent 

of the dissolved and free-phase hydrocarbons. 

In February 2002, a weeping flange connecting two 

pipes was found 23 m upgradient from well MW-3 (Natural 

Resources Engineering Company, 2003) and was identified 

as the location of the original spill site. It was repaired soon 

after its discovery, but the date that the leak first started is 

unknown. About 278 cubic meters (m3) of contaminated soil 

were removed in 2002; some impacted soil was left in place to 

maintain the integrity of the pipeline. A second Geoprobe survey (GP-17 through GP-20, fig. 2) was conducted on August 

13-14, 2002, to define the area of offsite contamination. 

The extent of the separate-phase oil body shown in figure 2 

was established by the two Geoprobe surveys conducted by Natural Resources Engineering Company (2003). The maximum 

thickness of oil contamination observed in the drill cuttings 

was 0.61 m. About 182,000 liters (L) of crude oil were estimated to be floating on the water table in 2005 (Wenck Associates, 2005). Kinematic viscosity of the crude oil measured at 

10°C varied from a low of 23×10-6 meters squared per second 

(m2/s) at MW-13 to a high of 421×10-6 m2/s at MW-5 (Natural 

Resources Engineering Company, 2004). Similarly, the density of the crude oil varied from a low of 0.85 grams per cubic 

centimeter (g/cm3) at MW-13 to a high of 0.938 g/cm3 at MW-5. 

Crude oil thickness was measured in the wells 13 times from 

June 2001 to January 2006, with thicknesses ranging from 

2.7 cm to 49.7 cm (Wenck Associates, 2006). Crude oil 

thicknesses were greater than 30 cm in wells MW-3, MW-5, 

MW-11, and MW-13 (fig. 2) in January 2006. Current remediation at the Cass Lake site consists of natural attenuation 

and periodic bailing of oil from the monitoring wells. In 2008 

about 1.4 L were removed from the wells (Natural Resources 

Engineering Company, 2009). 

Initial groundwater quality samples were collected 

from monitoring wells in June 2001, and the wells have been 

sampled regularly since 2004 (Natural Resources Engineering Company, 2009). The dissolved plume of benzene was 

previously estimated to extend to the southeast from the Cass 

Lake site to beneath the adjacent railroad tracks. The greatest 

concentrations of dissolved petroleum hydrocarbon constituents (benzene, 6,500 micrograms per liter, μg/L) were detected 

in well MW-5 (closest to the leaking flange) on January 6, 2004 

(Wenck Associates, 2006). The greatest concentration of disExtent of crude oil

Centerline of Railroad

BUILDING

Line of trees

Water

Well

0 50 100

200 40 60 meters

150 200 feet

Fence

around

gravel

area

Buried 

pipelines

EXPLANATION

PP-8 Approximate push-probe location

GP-18 Approximate Geoprobe location

MW-7 Monitoring well

Section A - A’ (figure 15)

Weeping flange

Approximate core location

Buried 

pipelines

PP-12

PP-9 PP-15

PP-10

PP-16, 17

PP-18

PP-3

PP-7

PP-8

PP-5

PP-4

PP-13

PP-14

PP-1

PP-2

PP-6

MW-4 MW-11

MW-3

MW-10

MW-2

(sealed)

MW-14

(sealed)

MW-15

MW-1

MW-6 USGS-4A, B, C

USGS-3A, B, C

USGS-2A,B,C

MW-5

MW-13

MW-7

MW-9

MW-8

USGS-1A,B

GP-9

GP-12

GP-10

GP-11

GP-13

GP-8

GP-1 GP-2

GP-5

GP-6

GP-7

GP-20

GP-14 GP-15

GP-17

GP-18

GP-19

GP-4

GP-16 GP-3

A

A’

PP-11

C0809

C0801

C0808 C0810

C0803

C0807

C0805

C0804

C0802

C0806

C0811

C0801

C GRAVEL ROAD

Figure 2. Locations of previous 

monitoring wells and borings at the Cass 

Lake site from Wenck Associates (2006) 

and new U.S. Geological Survey (USGS) 

push-probe, core, and well sites. Also 

shown is extent of subsurface crude oil 

from Wenck Associates (2005).
Introduction 5

solved benzene offsite, detected on September 25, 2001, in 

GP-4, was 1,300 μg/L. Periodic detections of benzene were 

measured in MW-9, downgradient from the oil, with a high 

concentration of 18 μg/L in April 2005.

Previous work indicated that groundwater movement in 

the area is generally from northwest to southeast. Slug tests 

conducted on the monitoring wells to determine the hydraulic conductivity of the aquifer indicated a mean horizontal 

hydraulic conductivity of approximately 6 meters per day 

(m/d) (Natural Resources Engineering Company, 2004). The 

measured water-table gradient was about 0.03 to 0.1 percent. 

On the basis of these data, the groundwater flow velocity was 

estimated to be approximately 9 meters per year (m/yr) to the 

east-southeast, assuming an effective porosity of 0.25 (Natural 

Resources Engineering Company, 2004). The dissolved benzene 

transport velocity was estimated to be about 6.3 m/yr, based on 

a benzene retardation factor of 1.59 (Natural Resources Engineering Company, 2004).

Biodegradation of Petroleum Hydrocarbons

Evidence Used to Demonstrate Biodegradation

Natural biodegradation transforms petroleum contaminants initially into organic acids and ultimately into final 

reaction products consisting of carbon dioxide and water. In 

order to document biodegradation, chemical and microbiological properties of soil and groundwater samples from a 

contaminated site must be analyzed for contaminants, microorganisms, metabolic reactants and products, and growth 

nutrients (National Research Council, 1993). As microorganisms consume contaminants, they grow and divide. An 

increase in microbial population is an indication that natural 

bioremediation is occurring. When microorganisms metabolize contaminants, they consume oxygen, nitrate, sulfate, and 

sediment-bound oxidized iron (iron oxyhydroxides) and create 

characteristic products such as carbon dioxide, ammonia, 

reduced iron, sulfide, and methane (table 1). Thus, decreased 

reactant concentrations and the presence of products are evidence of biodegradation. Some contaminant mixtures, including crude oil, contain both relatively recalcitrant (degradation 

resistant) compounds and labile (easily degradable) compounds. An increased ratio of recalcitrant to labile compounds 

can be another indication of biodegradation. Because microorganisms preferentially consume lighter carbon isotopes, 

contamination undergoing bioremediation also exhibits a 

depletion of lighter carbon isotopes (Clark and Fritz, 1997).

Description of the Bemidji Research Site

The investigation strategy at the Cass Lake crude-oil spill 

site was based largely on results from research at a site located 

about 20 km northwest of Bemidji, Minnesota, and 40 km from 

the Cass Lake site. The Bemidji site became contaminated when 

a pipeline burst on August 20, 1979, dispersing approximately 

1,700,000 L (10,700 barrels) of crude oil onto the land surface. 

Cleanup efforts immediately after the spill removed about 75 

percent of the spilled oil. The remaining 25 percent infiltrated 

the sand-and-gravel outwash aquifer, forming three subsurface, 

separate-phase oil bodies. At the largest of these, known as the 

“north pool,” separate-phase oil is present within the sediment 

pore space from the land surface down through the 6 to 8 m 

deep unsaturated zone and to 1 to 2 m below the water table. A 

plume of dissolved hydrocarbons in the groundwater extends at 

least 150 m downgradient from the source (Cozzarelli and others, 2001). Oil saturations in the sediments (the volume fraction 

of pore space filled with oil) range from 10 to 20 percent in the 

unsaturated zone to 30 to 70 percent near the water table (Dillard and others, 1997).

Data from outside the contaminated area indicate 

that before the Bemidji oil spill the microbial population 

consisted primarily of aerobic and fermentative bacteria with 

concentrations of about 100 per gram (g) of sediment. In the 

contaminated aquifer aerobes, fermenters, iron reducers, and 

methanogens have increased in concentration up to 1,000,000 

per g of sediment (Essaid and others, 1995). Three microbial 

physiologic types composed of methanogenic, iron-reducing, 

and aerobic microorganisms delineate the primary redox 

reaction zones in the contaminated aquifer. A methanogenic 

Type of reaction Balanced chemical equation

Aerobic1 C6

H6 + 7.5O2 → 6CO2 + 3H2O

Nitrate ammonification2 C6

H6 + 3.75NO3

-

 + 37.5H+

 + 0.75H2O → 6CO2 + 3.75NH4

+ + 30H+

Sulfate reduction1 C6

H6 + 3.75SO4

2- + 7.5H+ → 6CO2

 + 3.75H2S + 3H2O

Iron reduction1 C6

H6 + 30Fe(OH)3 + 60H+ → 6CO2 + 30Fe2+ + 78H2O

Methanogenesis1 C6

H6 + 4.5H2O → 2.25CO2 + 3.75CH4

1

Wiedemeier and others, 1996

2

Cozzarelli and Baehr, 2003

Table 1. Biodegradation reactions of benzene under aerobic and anaerobic 

conditions.
6 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

zone exists near the crude-oil source, where sediment iron has 

been depleted. Farther from the source, where significant iron 

is still available, an iron-reducing zone is well developed, with 

elevated concentrations of iron-reducers relative to the source. 

Beyond the iron-reducing zone, an aerobic zone is present. 

Degradation of the oil by reduction of sulfate and nitrate is not 

significant at the Bemidji site because these chemical species 

have low concentrations in the groundwater.

Methods

To delineate the groundwater contaminant plume, a 

low-cost push-probe was devised to obtain groundwater 

samples that were then analyzed in the field. The results of the 

push-probe survey provided data to determine the placement 

of monitoring-well installations along the centerline of the 

plume. The push-probe results also guided locations for soil 

cores used to determine the microbial populations and sediment iron oxyhydroxide concentrations in the aquifer.

Sample Collection by Push-Probe

In September 2007, groundwater samples were collected 

at the Cass Lake site using a push-probe tool that was advanced 

into the ground at 14 locations (fig. 2). The first three locations 

(PP-1 to PP-3) were sampled using a small-diameter downhole 

bladder pump (Amos and Blowes, 2008). To improve sample 

recovery a modified push-probe sampling procedure, termed 

“peristaltic bailing” (Herkelrath and others, 2008), was developed and successfully employed at the next 11 locations (PP-4 

to PP-14). The peristaltic bailing equipment consisted of a 

Solinst Canada drive-point piezometer attached to the bottom 

of a 2-cm-diameter stainless-steel pipe with a 15-cm screen. 

The pipe was manually pounded into the ground using a length 

of pilot drill rod and a jack hammer. At each water sampling 

depth, 0.6-cm Teflon tubing was lowered to the bottom of the 

screen and a peristaltic pump was used to draw water into the 

tubing. Because all of the sampling depths were below the peristaltic lift depth of about 7.6 m, the Teflon tubing needed to be 

raised about 2 to 3 m in order to bring the water to the surface. 

Sampling at each location continued at target spacings of 20 to 

50 cm until the till was encountered. The depth to till was noted 

at each site. A total of 49 water samples were obtained at varying depths below the water table. A second round of push-probe 

sampling was carried out in May 2008. In order to allow drill rig 

access to find the leading edge of the plume, a path was cut into 

the forest along a line extending downgradient from the location 

where the highest benzene, toluene, ethylbenzene, and xylenes 

(BTEX) concentrations were detected. The push-probe was 

advanced at four locations (PP-15 to PP-18), and a total of 19 

groundwater samples were obtained using the peristaltic bailing 

method. 

Saturated thickness of the sand-and-gravel aquifer was 

determined by creating grids by triangulation with linear 

interpolation in Surfer (Golden Software, Golden, Colo., version 

9.7.543) of water-table and till elevations and subtracting.

Installation and Sampling of Well Network

In May 2008, nests of groundwater monitoring wells 

were installed in the forested area beyond (south of) the 

railroad right of way (fig. 2). On the basis of the push-probe 

work, four well locations were chosen along the main transect 

of the plume with two to three different screen depths at each 

location. The wells are constructed of 5-cm PVC with 0.3-m 

screens. Altogether 11 monitoring wells labeled “USGS” 

were installed (fig. 2): one well nest of two wells (USGS-1A 

and -1B) and three well nests of three wells each (USGS-2A, 

-2B, and -2C; -3A, -3B, and -3C; and -4A, -4B, and -4C). The 

depths of the screens and other location data are given in 

table 2. All 11 USGS wells and 4 of the existing monitoring 

wells (MW-6, MW-8, MW-9, and MW-10) were sampled in 

June 2008.

A mini-typhoon pump was used for collecting waterquality parameters from the wells, including pH, temperature, 

and specific conductivity. The pump screen for the minityphoon pump was approximately 0.3 m from the bottom of 

the well. Once these parameters were taken, the mini-typhoon 

pump was replaced by a Grundfos pump with Teflon tubing 

to sample for volatile organic carbon, dissolved oxygen, 

ammonium, major cations, major anions, dissolved methane, 

dissolved ferrous iron, alkalinity, and nonvolatile organic 

carbon. At least three well-casing volumes were pumped 

through the Grundfos pump before water samples were 

collected. The Grundfos pump screen was approximately 0.46 

m from the bottom of the well.

Groundwater Analyses

September 2007 and May 2008

During the screening phase of this study, water samples 

were analyzed for BTEX and reduced iron in the field. 

Samples for BTEX analysis were collected without headspace 

in 40- or 125-milliliter (mL) brown glass bottles, depending 

on the expected concentrations. Estimates of BTEX concentrations were obtained in the field using a SRI 8610C gas 

chromatograph (GC) with the flame ionization detector (FID), 

a 1.0-mL fixed-loop injection, internal air compressor, and a 

Restek MXT-1 capillary column. A 5-cubic-centimeter gas 

bubble (ultra pure helium) was created in each sample bottle 

using a syringe. The sample was equilibrated by shaking for at 

least 45 minutes (min) at ambient temperature. The headspace 

gas was injected in the GC. Gas concentrations were converted to water concentration using a calibration curve based 

on headspace measurements on BTEX standard solutions 

prepared in the field. 
Methods 7

Dissolved, reduced iron concentrations in water samples 

were measured in the field by filtering the sample, adding 

ferrozine solution (following the methods of Gibbs, 1979), 

and then reading the absorbance with a GENESYS 6 UV-Vis 

spectrophotometer at 562 nanometers (nm). Concentrations 

were calculated from a calibration curve based on ferrous 

ammonium sulfate standards.

June 2008

Samples collected in June 2008 were analyzed for volatile organic carbon compounds by the U.S. Geological Survey 

(USGS) National Water Quality Laboratory (NWQL) in 

Denver, Colo. (http://nwql.usgs.gov/nwql.shtml, last accessed 

April 9, 2010) using heated purge and trap, gas chromatography, and mass spectrometry. Analyses for other geochemical 

indicators of BTEX degradation processes were performed in 

the field and at the USGS laboratories in Reston, Va. 

Specific conductance, pH, and downhole temperature of 

groundwater were determined in the field using a YSI multimeter (model 682-C-M) after parameters had stabilized. 

Dissolved oxygen (DO) was measured in the field 

using CHEMets® Kits (CHEMetrics Inc., Calverton, Va.). 

Dissolved oxygen is typically determined in the field using 

spectrophotometric techniques or dissolved oxygen sensors. 

Low concentrations of dissolved oxygen cannot be accurately 

measured with a dissolved oxygen sensor because of 

difficulties calibrating at zero dissolved oxygen (Baedecker 

and Cozzarelli, 1992; Radtke and others, 1998). In lowoxygen waters, spectrophotometric techniques are preferred, 

and in those wells with lower dissolved oxygen levels a 

spectrophotometric technique was employed. Samples for 

DO measurements must be collected in a manner to eliminate 

aeration during pumping. For this reason positive displacement 

pumps were used and gas-impermeable high-density plastic 

tubing. Wells were purged and samples were collected in a 

manner to eliminate aeration of the water in the well or the 

sample. The measurements were collected in triplicate, and the 

lowest value measured was reported.

Ammonium concentrations were determined in the field 

on samples using an Ammonia Vacu-vials® Kit and CHEMetrics V2000 photometers. Ferrous iron concentrations were 

determined in the field on samples that were filtered in line 

Table 2. Site identification numbers, latitude (lat.), longitude (long.), measuring point elevations, surface elevations, screen 

elevations, and water table elevations for wells installed in May 2008 by U.S. Geological Survey. 

[Elevations are in meters. The water table elevations were taken in November 2009.]

Well name Site identification number Lat. Long.

Measuring

point 

elevation

Surface 

elevation

Elevation, 

top of 

screen

Elevation, 

bottom of 

screen

Water table 

elevation

USGS-1A 47223709

4381201

47.3772 -94.6367 408.92 407.99 399.76 399.51 400.16

USGS-1B 47223709

4381202

47.3772 -94.6367 409.17 407.95 399.06 398.75

USGS-2A 47223709

4381001

47.3772 -94.6362 409.15 408.20 399.70 399.39 400.05

USGS-2B 47223709

4381002

47.3772 -94.6362 409.15 408.25 399.14 398.84

USGS-2C 47223709

4381003

47.3772 -94.6362 409.16 408.11 398.38 398.14

USGS-3A 47223709

4380901

47.3771 -94.6358 408.82 407.90 399.71 399.41 400.12

USGS-3B 47223709

4380902

47.3771 -94.6358 408.83 407.83 399.08 398.78

USGS-3C 47223709

4380903

47.3771 -94.6358 408.71 407.82 397.21 396.90

USGS-4A 47223609

4380701

47.3770 -94.6355 408.32 407.68 399.71 399.42 400.11

USGS-4B 47223609

4380702

47.3770 -94.6355 408.54 407.65 399.04 398.74

USGS-4C 47223609

4380703

47.3770 -94.6355 408.64 407.71 397.29 396.99
8 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

through a 0.2-micrometer (μm) NucleporeTM filter. Samples were 

immediately treated in the field with reagents specified by the 

colorimetric ferrozine technique and analyzed on site using a 

GENESYS 6 UV-Vis spectrophotometer as described previously 

for the push-probe sample analyses. Anion samples (including 

nitrate and sulfate) were filtered in line through a 0.20-μm NucleporeTM filter into plastic bottles and analyzed by ion chromatography using a Dionex 120. Nitrate and sulfate detection limits 

were both 0.5 milligrams per liter (mg/L). Cation samples were 

filtered through 0.10-μm NucleporeTM filters into acid-washed 

plastic bottles, preserved to pH 2 with nitric acid, and analyzed 

by inductively coupled plasma mass spectrometry. Alkalinity 

samples were filtered through a 0.20-μm NucleporeTM filter into 

glass bottles and analyzed in the laboratory with a Radiometer 

(Copenhagen, Denmark) TIM 900 automatic titrator.

Samples for the analysis of the nonvolatile component 

of dissolved organic carbon (DOC) were filtered through 

0.20-μm Supor® filters into baked glass bottles, preserved 

with hydrochloric acid to a pH of <2, and analyzed using 

a Shimadzu TOC Vcsn analyzer (Shimadzu Corporation). 

Methane samples were collected in Glaspak syringes that were 

connected directly to the sample-pump outlet following the 

method of Baedecker and Cozzarelli (1992). Methane samples 

were analyzed using a 5890 Series II HP Gas Chromatograph 

split/splitless inlet FID with a fused silica capillary column.

Sediment Sampling and Analysis

Eleven soil cores (C0801-C0811) were collected for 

grain-size determinations, sediment iron, and microbial analyses. The core locations are shown in figure 2, and table 3 lists 

the samples collected from each core. Ten cores were collected in June 2008 from outside the fenced area enclosing the 

pumping station. The eleventh core was collected in July 2008 

from inside the fenced area, where separate-phase oil is floating on the water table. Cores were collected by drilling with a 

hollow-stem auger to the top of the desired coring interval, followed by driving of a piston core barrel beneath the augers to 

the desired depth. The bottom 10 cm of the core was frozen in 

place to prevent loss of fluids during core extraction following 

the method of Murphy and Herkelrath (1996). 

Hydraulic Conductivities

Grain-size analyses were done on 16 sediment samples 

(table 4). Each sample was dried, weighed, and separated by 

grain sizes of >2 millimeters (mm), 500 μm to 2 mm, 106 μm 

to 500 μm, 63 μm to 106 μm, and <63 μm. Horizontal hydraulic conductivities were estimated from the grain-size data 

using the method of Hazen (1892):

where K is hydraulic conductivity (m/d), T is temperature 

(°C), and D10 is the grain diameter (mm) for which 10 percent 

of the soil is finer. For these samples, the temperature was 

assumed to be 10°C. 

Past research on a primarily sand-and-gravel glacial 

outwash aquifer in Cape Cod, Mass., which is very similar to 

that of the Cass Lake site, confirmed that hydraulic conductivities determined by the Hazen method closely matched those 

measured with a permeameter on aquifer core samples. In the 

range of measured values from 0.01 to 0.1 centimeters per 

second, the hydraulic conductivities estimated from grain-size 

analysis showed no significant difference from the permeameter results. Furthermore, hydraulic conductivities determined 

in the aquifer by borehole flowmeter were similar to those 

measured with the permeameter (Wolf and others, 1991).

Oil

For this study, one core from the oil body was collected 

in July 2008. The core was located 4.6 m from MW-13 and 

7.6 m from MW-5 on a line between the two (C0811, fig. 2). 

The top of the core was at 6.95 m below land surface, and 

the core extended to 8.47 m. From visual inspection, oil was 

present in this core over an interval of 1.03 m. Two sediment 

samples were preserved for oil analyses: one from above the 

water table and the second below the water table (sample 

intervals were 70 to 82 cm and 108 to 111 cm below the top of 

the core, respectively). In addition, two other samples of Cass 

Lake oil were analyzed: one bailed from MW-3 in September 

2007 and one collected from the pipeline in July 2008. Results 

from the analyses of the Cass Lake samples were compared to 

analyses of oil extracted from two sediment samples collected 

in July 2008 from the Bemidji research site (Bemidji Core 

B0801 interval 22 to 36 cm, located above the water table, and 

Bemidji Core B0802 interval 10 to 13 cm, located below the 

water table) and to an archived oil sample collected from the 

Bemidji pipeline in 1984.

Analyses of Nonvolatile Fraction

Sediment samples for oil analysis were extracted and 

analyzed using the method described in Hostettler and others 

(2007). Ten g of sediment were added to a 50-mL glass beaker. Extractions with dichloromethane (DCM) were performed 

using successive solvent rinse volumes of 20, 15, and 10 mL. 

For each extraction, several g of sodium sulfate were added to 

the beaker containing the sediment/DCM slurry. The extracts 

were filtered through a champagne funnel containing a small 

plug of precombusted glass wool. The extracts were collected 

into a 50-mL pear-shaped flask and blown down to near-dryness with nitrogen. 

The extract was dissolved into 5 mL of hexane for column fractionation. The two whole oil samples were weighed 

(25 to 50 milligram, mg) directly into a 5-mL centrifuge tube, 

and DCM was added to the sample, making the final volume 

5 mL. The sediment oil and whole oil were added directly to 

columns containing 2.5 g activated alumina (40/60 mesh), 2.5 g 

silica gel 62 (6-200 mesh, 150 angstroms), and 5.0 g silica 

K = 1000 (0.7+0.03T) (D10)2, (1)
Methods 9

Table 3. Locations of samples collected for sediment iron and microbial 

analyses at the Cass Lake site on June 20–23, 2008, and July 18, 2008. 

[Locations of cores C0801 through C0811 shown in figure 2. Depths are in meters (m) and 

centimeters (cm).] 

Sampling depth in core, cm

Core General 

location

Depth 

to top of 

core, m

All MPN1

except iron 

reducers

Iron 

reducer 

MPN1

Sediment for 

iron extraction

C0801 MW-6 7.79 35 35 35-70

70 70

C0802 PP-15 8.32 50 50-60

100 60-70

100-114

C0803 PP-13 8.18 30 30 11-25

C0804 USGS-3 7.85 20

70

 30-70

70

C0805 USGS-3 9.98 60 30-60

C0806 USGS-4 8.99 50 0-40

90 80-90

C0807 USGS-3 8.85 50 50 50-80

C0808 USGS-1 7.78 30 30 20-30

C0809 USGS-2 9.29 40 40 50-80

C0810 USGS-2 8.41 30 30 30-40

90 70-90

C0811 Between 

MW-5 and 

MW-13

6.95 54 54 91-101

108 108

1

Most Probable Number analyses

gel 923 (100-200 mesh), which were used to fractionate the 

extract into the F1 fraction eluted with hexane and the F2 

fraction eluted with 30-percent DCM in hexane (volume for 

volume). The F1 fraction contained the aliphatic and branched 

hydrocarbons and cyclohexanes, whereas the F2 fraction contained the parent and alkylated polycyclic aromatic hydrocarbons. Analyses of the fractions were carried out on a HP 6890 

GC equipped with a mass-selective detector. The GC was fitted with a 30-m DB-5 capillary column. The oven temperature 

was programmed from 60°C to 310°C.

Analyses of Volatile Fraction

Two oil samples were analyzed for volatile hydrocarbons 

(VHC). One sample was collected from oil residing in well 

MW-3 collected in 2007, and the other sample was provided by 

Lakewood Pipeline Company in 2008 as representative of the 

oil carried in the pipeline. The samples were stored in a freezer, 

brought to room temperature and gently shaken before removing 

a 1-mL sample. A known volume of an internal standard solution 

consisting of a mixture of perdeuterated compounds was added to 

the 1-mL sample, which was then mixed on a Vortex mixer before 

analysis by solid phase microextraction-gas chromatography/

mass spectrometry (SPME-GC/MS). The following perdeuterated 

compounds were added as internal standards: d-benzene, d-toluene, d-ethylbenzene, d-m-xylene, d-o-xylene, d-1,3,5-trimethylbenzene, d-naphthalene, d-hexane, d-nonane, and d-dodecane. 

In addition to the two oil samples, oil-contaminated sediment from core C0811 (table 3) at 91 cm below the top of the 

core was extracted and analyzed for comparison to the oil at 

well MW-3. The soil core was obtained in 2008 in the most 

contaminated part of the aquifer about 17 m north of MW-3 

(fig. 2). A 1-g sediment sample was extracted with 1.7 g dichloromethane (DCM) in a 4-mL vial by sonication for 30 min. An 

aliquot of the deuterate standards was added before sonication, 

and a subsample of DCM was analyzed by solid phase microextraction using the method as described for the oils. 
Table 4. Results of grain-size analyses and calculated hydraulic conductivities for core samples collected at the Cass Lake site. [Dry weights in sediment and total weights are in grams (g). All grain sizes (in micrometers, μm, and millimeters, mm) are displayed as percent of dry weight. D10 (mill

the diameter of grain size for which 10% of the soil is finer. Hydraulic conductivities (K) are in meters per day (m/d).] Core/ Distance,1 cm Dry weight sediment, g Coarse sand to gravel >2 mm Medium sand 500 μm-2 mm Fine sand 106-500 μm Silt 63-106 μm Silt and clay <63 μm Total weight, g % Loss of sediment D

C0801/35-70 23.93 48.02 40.87 9.03 0.48 0.87 23.75 0.74 

C0801/60-70 23.27 12.85 30.51 40.35 7.86 8.04 23.18 0.39 

C0802/60-70 25.02 38.65 38.29 19.82 1.06 1.57 24.87 0.60 

C0802/100-114 19.22 0.90 54.21 43.96 0.21 0.19 19.12 0.52 

C0803/9-25 24.25 32.23 56.25 8.87 9.69 1.49 26.32 0.--2 

C0804/30-70 26.29 61.39 27.39 8.37 1.05 1.67 26.26 0.13 

C0804/60-70 22.04 35.95 49.55 11.68 1.27 1.50 22.03 0.05 

C0806/0-40 25.35 36.52 51.14 11.18 2.92 0.52 25.93 0.--2 

C0806/80-90 23.00 3.94 49.39 42.35 1.51 2.17 22.85 0.64 

C0807/50-80 25.84 45.39 47.52 6.48 0.26 0.21 25.81 0.13 

C0808/20-30 32.70 42.60 43.79 11.59 0.80 1.02 32.64 0.20 

C0809/50-80 26.23 13.84 68.28 15.80 0.88 1.12 26.21 0.08 

C0810/30-40 26.15 30.57 35.76 19.40 6.47 7.58 26.09 0.22 

C0810/70-90 22.13 22.46 59.38 15.45 0.82 1.56 22.06 0.33 

C0811/60-70 23.29 0.38 46.25 49.74 0.36 0.19 22.57 3.08 

C0811/91-101 26.78 3.63 71.66 21.22 0.46 0.41 26.08 2.62 

1Distance from top of core (centimeter, cm, intervals). See table 3 for depth to top of cores2Total weight is greater than dry weight.
Methods 11

Using a syringe, a 10-microliter (μL) sample of oil or 

20-μL sample of DCM was placed in a 4-mL vial with 500 mg 

of vacuum pump oil and mixed on a Vortex mixer for 1 min. 

A 100-μm polydimethylsiloxane-coated fiber was exposed 

to the headspace of the vial for 10 min. The fiber was then 

inserted into the heated inlet (250°C) of an Agilent 6890 GC 

coupled with a 5973 mass spectrometer. The GC was equipped 

with a 30-m DB-5ms column (0.25 mm ID and 0.25 μm thick 

coating) and programmed as follows: hold at -50°C for 2 min, 

20°C per min to 20°C, hold 5 min, 5°C per min to 100°C, 

25°C per min to 225°C, and hold for 1 min. 

The concentrations of compounds reported were calculated using the internal perdeuterated standards with the 

following exceptions. The normal hydrocarbons C7 and C8

were calculated assuming a linear response of the perdeuterated internal standards C6

 and C9, respectively; C10 and C11 

were based on C9 and C12, respectively. Cyclohexane, methylcyclohexane, ethylcyclohexane, the tetramethylbenzenes, 

and the methylnaphthalenes were quantified using the nearest 

perdeuterated internal standard based on retention time and 

structure and a relative response factor. The relative response 

factor was calculated from an external standard that contained 

these compounds and the perdeuterated standards listed 

above. This corrected for differences in response of the ion 

current for different compounds. These concentrations should 

be considered less accurate than the concentrations calculated 

using internal standards.

Iron

Analyses of sediment-iron content were performed on a 

total of 17 sediment samples from the Cass Lake site. Cores 

collected for sediment-iron analyses were left in the polycarbonate core liners, cut into sections, frozen in the field, and 

shipped frozen to the lab. In the lab, the cores were thawed and, 

while cold, placed in an anoxic glove bag. About 50 grams (g) 

of sediment from each core was placed in a centrifuge tube 

and mixed with a spatula, in order to homogenize the sample. 

Pebbles greater than 1 cm in diameter were removed. Sediment 

samples of about 2.5 g were placed in serum bottles and 20 mL 

of 0.5-molar (M) hydrochloric acid was added to each bottle; 

all samples were prepared in triplicate. The serum bottles were 

sealed with rubber stoppers, crimped, and wrapped with aluminum foil while in the glove bag.

Outside the glove bag, the bottles were gently shaken for 

24 hours. About 5 mL of acid was removed with a syringe and 

filtered through a 0.2-μm Supor® syringe tip filter. The sample 

was immediately analyzed using the ferrozine colorimetric 

method (modified from the method of Gibbs, 1979). In a 

cuvette, a 100-μL sample was added to 3 mL of ferrozine solution (2.0-millimolar (mM) ferrozine solution made in 0.05-M 

HEPES buffer and adjusted to pH 6.5). The color complex 

was read at 562 nanometer (nm) wavelength for reduced iron 

and compared to standards that were prepared using the same 

volumes as for the samples. A 10-mM standard iron solution 

was made from ferrous ammonium sulfate (Sigma-Ultra), and 

dilutions were made from 0.005 mM to 2 mM. Total iron was 

measured on the filtered samples on a Perkin-Elmer 4300DV 

optical emission spectrometer; oxidized iron was determined 

by difference.

A separate sediment sample of about 25 g was placed in an 

aluminum pan and weighed. This sample was then dried at 60°C 

for 24 hours, and the sediment was again weighed to determine 

the percent water in the sediment for calculation of the iron concentrations in millimoles per kilogram (mmol/kg) dry weight 

sediment. The dried sediment was then sieved into 5 fractions 

through sieves of the same sizes as for grain-size analysis.

Microbial Populations

Sediment samples for microbial analyses were collected using sterile techniques at one or two locations within 

each core (table 3). Concentrations of aerobic heterotrophic, 

anaerobic heterotrophic, iron-reducing, sulfate-reducing, and 

methanogenic microorganisms were measured in sediment 

samples by a Most Probable Number (MPN) analysis (Warren and others, 2006). MPN analysis is a statistical method in 

which samples are diluted by factors of 10 and then used to 

inoculate selective media that encourage the growth of specific 

physiologic types of microorganisms. The concentrations in 

the original sample are determined from the growth patterns of 

the inoculated media. Five replicate analyses provide an estimate of the 95-percent confidence intervals. The upper end of 

the 95-percent confidence interval corresponds to about three 

times the MPN and the lower end to one-third of the MPN 

(Meyall and Meyall, 1970). On a log10 scale this is approximately plus or minus one-half log unit.

Microorganisms attached to sediments must be suspended 

in a solution before dilution. The method for the sample preparation and the preparation and analysis of different media can 

be found in Warren and others (2006). Briefly, microorganisms 

were extracted from sediments using a surfactant solution and 

sonication. Extracted cell suspensions were diluted in series, 

by orders of magnitude into prereduced anaerobic mineralsalts solutions. Aliquots of the dilutions were used to inoculate 

six different media designed to promote growth of the target 

physiological groups of microorganisms.

Microorganisms capable of aerobic, heterotrophic growth 

were enumerated by counting test tubes containing Standard 

Methods Broth with visible growth after incubation. Microorganisms capable of anaerobic, heterotrophic growth or fermentation 

were enumerated using anaerobically prepared Schaedler’s Broth 

with turbid growth or clumps of particulates after incubation. 

Iron-reducing microorganisms were enumerated using the method 

found in Warren and others (2006) with goethite (Lovley and 

Phillips, 1986) as the oxidized iron source. Media for sulfatereducing and methanogenic microorganisms consisted of anaerobic mineral-salts media with acetate as well as sulfate or hydrogen, 

respectively, as electron acceptors. The activity of sulfate-reducing 

bacteria was determined from the presence of sulfide using lead 

acetate paper. Similarly, the presence of methane was used to indicate methanogen activity (Warren and others, 2006). 
12 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

Results

Characterization of Hydrogeology 

Head Distribution and Flow Directions 

The water table is relatively flat at the Cass Lake site, 

with groundwater flow primarily to the east or east-southeast 

(fig. 3). Starting at MW-13, in the oil body source zone, the 

direction of flow is east-southeast toward well nest USGS-4. 

The head gradient at the water table along the flow line from 

MW-13 to USGS-4 is 0.00057 meter per meter (m/m), which 

falls in the previously estimated range of 0.0003 to 0.001 m/m 

(Natural Resources Engineering Company, 2004).

Geologic information gathered during the 2007-2008 

push-probe work indicates that the saturated thickness of the 

aquifer and depth to the glacial till decrease to the south and 

increase to the east and northeast from the Cass Lake site. The 

contour plot in figure 4 shows that the saturated thickness of 

the sand and gravel exceeds 6 m at MW-1 but thins southward 

to only about 1 m at MW-9 and USGS-1. The thinning of the 

saturated thickness causes the groundwater flow in the sandand-gravel aquifer beneath the Cass Lake site to be diverted 

eastward from the southeast regional flow direction.

Hydraulic Conductivities, Discharge Rate, and 

Flow Velocity

The grain-size data from 16 samples indicate that most of 

the sediments are poorly sorted (table 4), with grain sizes ranging from gravel to sand and silt, with a few samples containing 

cobble-size particles. The dominant grain-size fraction in the 

samples is coarse sand. The horizontal hydraulic conductivities 

estimated with the Hazen method ranged from 5 to 216 meters 

per day (m/d), with a geometric mean value of 47 m/d (fig. 5). 

Combining the head gradient and mean hydraulic conductivity values gives an estimated specific discharge rate of 

9.8 m/yr. Converting this to a pore-water velocity using a 

porosity of 0.38 (Essaid and others, 1995) results in a velocity of 26 m/yr. The previous estimate of 9 m/yr was based on 

an assumed effective porosity of 0.25, a measured gradient of 

BUILDING

400.20

400.21

400.19

400.18

400.17

400.16

400.15

400.14

400.13

400.12

400.11

EXPLANATION

Centerline

of Railroad

Line of trees

Water

Well

0 50 100

200 40 60 meters

150 200 feet

Fence

around

gravel

area

Buried 

pipelines Buried 

pipelines

Extent of crude oil

Monitoring well with water table elevation, m

Weeping flange

Water table contour elevation above sea level, m

MW-4

MW-11

MW-3

MW-10

MW-2

(sealed)

MW-14

(sealed)

MW-15

MW-1

MW-6 USGS-4A, B, C

USGS-3A, B, C

MW-5

MW-13

MW-7

MW-9

MW-8

USGS-1A,B

USGS-2A,B,C

400.205

400.218

400.190

400.148

400.141

400.175

400.190

400.196

400.163 400.123

400.108

C GRAVEL ROAD

Figure 3. Water table elevations 

(meters, m) beneath the Cass Lake 

site measured in November 2009 

(table 2).
Results 13

Table 5. Selected results of oil analyses: d13C values for oil 

extracted from sediments at the South Cass Lake Pumping 

Station and collected from the pipeline, weight percent of oil 

extracted from sediment samples, and n-heptadecane/pristane 

(n-C17/Pr) ratio for each of the four sediment samples and the two 

oil samples.

Sample d13C, per 

mil

Percent 

oil

n-C17 /Pr

Bemidji B0801/22-36 cm1 1.6 0.66

Bemidji B0802/10-13 cm1 2.6 0.79

Cass Lake C0811/70-82 cm1 -29.45 2.7 0.04

Cass Lake C0811/108-111 cm1 -29.24 0.1 0.04

Cass Lake pipeline -30.13 2.15

Cass Lake MW-3 bailed 0.05

1

Core/Distance from top of core (centimeter, cm, intervals). See table 3 for 

depth to top of cores

Centerline of Railroad

BUILDING

Line of trees

Water

Well

Fence

around

gravel

area

PP-12

PP-9 PP-15

PP-10

PP-16, 17

PP-18

PP-3

PP-7

PP-8

PP-5

PP-4

PP-13

PP-14

PP-1

PP-2

PP-6

MW-10

MW-1

PP-11

Buried 

pipelines

C GRAVEL ROAD

6

5

4

3

2

0 50 100

200 40 60 meters

150 200 feet Approximate push-probe location

Monitoring well

EXPLANATION

Saturated thickness, m

Weeping flange

Buried 

pipelines

Extent of crude oil

Figure 4. Saturated thickness 

(meters, m) of the surficial sand-andgravel aquifer at the Cass Lake site. 

0.03 to 0.01 percent, and a hydraulic conductivity of 6 m/d 

(Natural Resources Engineering Company, 2004; see also 

section on Site History). While the velocity estimate from this 

study is higher than the previous estimate for the Cass Lake 

site, it is similar to the estimated mean value for the Bemidji 

research site (22 m/yr) (Essaid and others, 2003).

Distribution of Petroleum Hydrocarbons

Separate Phase Oil

The weight percent oil, determined in triplicate for each 

core sample, is shown in table 5. The lowest average percent 

of oil in the sediments was in the C0811 (108 to 111 cm) 

sample with an average of 0.1 percent oil. The highest oil 

percent is found in C0811 (70 to 82 cm) with an average of 

2.65 percent oil. Similar values were obtained from the two 

Bemidji samples (table 5).
14 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

Nonvolatile Hydrocarbons

Table 5 also lists the n-heptadecane/pristane (n-C17/Pr) 

ratios and the d13C values, which are indicators of biodegradation. The n-C17/Pr ratio decreases as degradation progresses 

because n-C17 degrades while Pr remains unchanged (Peters 

and others, 2005). Very low values for this ratio were obtained 

for all of the Cass Lake oil-body samples, indicating that the 

n-alkane fraction is highly degraded. As oil biodegrades, the 

d13C value increases (Clark and Fritz, 1997). The change in the 

d13C values from -30.13 per mil for the pipeline oil to -29.24 

per mil in the oil body is consistent with biodegradation.

Figure 6A shows the n-alkanes in the range of n-C11

to n-C31 for all the sediment and oil samples (pipeline and 

MW-3). In order to assess the degree of degradation, the 

n-alkanes were normalized to C30 hopane, a conservative 

(relatively nondegradable) component of this oil. The highest 

values for the n-alkanes are from the crude oil taken directly 

from the pipeline. The normalized concentrations exhibit peak 

values in the n-C13 to n-C17 range, decreasing linearly in the 

range of n-C19 to n-C31. The most highly degraded oils are 

Figure 6. Comparison of n-alkanes 

in oil and sediment samples from the 

Cass Lake and Bemidji sites as determined by gas chromatograph/mass 

spectrometry. A, Ratio of n-alkanes to C30

hopane in oil samples from Cass Lake and 

in Cass Lake and Bemidji sediment cores. 

Distances from top of core in legend are 

in centimeters (cm). B, Comparison of 

n-alkane distributions in Bemidji pipeline 

oil from 1984 and Cass Lake pipeline oil 

from 2008 showing the similarity of the two 

samples. The difference in peak heights is 

caused by instrument differences between 

2002 and 2008. Elution time of n-alkanes 

increases with number of carbons. 

Elution time

Bemidji crude oil

Cass Lake crude oil

n

A

B

-alkanes

Detector response

Carbon number of n-Alkane

Ratio of n-alkane to C30 hopane

18

16

14

12

10

8

6

4

2

0

11

1213141516171819202122232425262728293031

Bemidji Core B0801 22-36 cm

Bemidji Core B0802 10-13 cm

Cass Lake Core C0811 70-82 cm

Cass Lake Core C0811 108-111 cm

Cass Lake pipeline

Cass Lake MW-3 (bailed)

Figure 5. Frequency distribution of log hydraulic 

conductivity in meters per day (m/d) determined from 

grain size analyses of 16 samples collected at the Cass 

Lake site.

12

10

8

6

4

2

0

0.0-0.5 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3.0 0.5-1.0

Frequency

Log hydraulic conductivity range, m/d
Results 15

from the Cass Lake sediment samples C0811 (70 to 82 cm) 

and C0811 (108 to 111 cm) and the sample from MW-3. The 

concentrations for all n-alkanes and the geochemical ratios 

(n-C17/Pr) for samples C0811 (70 to 82 cm) and C0811 (108 

to 111 cm) were similar to the oil sample bailed from MW-3 

(table 5). The similarity of floating oil from one location and 

oil extracted from sediments in another location suggests the 

degradation state is uniform over a sizeable area of the oil 

body. All of the Cass Lake samples are highly degraded compared to the two Bemidji samples (fig. 6A, table 5).

To determine if the Cass Lake oil came from the same 

original source as the Bemidji oil, the Cass Lake pipeline 

sample was compared to Bemidji source oil collected from the 

pipeline in 1984 and analyzed in 2002 (Hostettler and others, 2004). The analysis was based on 19 ratios developed for 

comparing oil sources (Hostettler and others, 2004; Peters and 

others, 2008) that were computed from the data for each oil 

sample. The results showed that all ratios were within 20 percent and 13 ratios were within 10 percent of each other. The 

three highest differences in ratios were attributed to instrument 

differences between when the Bemidji samples were analyzed

in 2002 and when the Cass Lake samples were analyzed in 

2008. Figure 6B shows that the n-alkane fractions in the two 

oils are identical. The difference in peak heights is caused by 

instrument differences between 2002 and 2008. The conclusion is that the two oils are from the same source.

Volatile Hydrocarbons

The composition of volatile hydrocarbons in crude oil 

from well MW-3 was compared to that in the pipeline oil 

and oil-contaminated sediment. The concentrations of compounds analyzed are in table 6 and figure 7. Two samples were 

analyzed from MW-3, and the average of the two results is 

reported. The concentrations of hydrocarbons are reported in 

milligram per gram (mg/g) oil for the oils and in mg/g dryweight sediment for the sediment. 

Assuming the oil at MW-3 and in sediment at C0811-91 

had the same composition initially as the pipeline oil, comparisons can be made on changes in composition over time. Evidence that the pipeline oil might not have varied significantly 

over time was obtained from comparison of the VHC from 

the Bemidji pipeline oil (sample obtained in 1984) to the Cass 

Figure 7. Comparison of concentrations of benzene, toluene, ethylbenzene, and xylenes (BTEX); cycloalkanes; methylbenzenes; and 

methylnaphthalenes (milligrams per gram, mg/g) in Cass Lake spilled oil, Cass Lake pipeline oil, and oil extracted from sediment. Distance 

from top of core C0811 is 91 centimeters (cm). benzene toluene ethylbenzene

m-xylene

p-xyleneo-xylene

cyclohexane

methylcyclohexane

ethylcyclohexane

1,3,5-trimethylbenzene1,2,4-trimethylbenzene1,2,3-trimethylbenzene

1,2,4,5-tetramethylbenzene1,2,3,5-tetramethylbenzene1,2,3,4-tetramethylbenzene

naphthalene

2-methylnaphthalene1-methylnaphthalene

0

1

2

3

4

5

6

Pipeline Oil

MW-3 Well Oil

C0811 91 cm Sediment

0

4

8

12

16

20

24

Concentration in oil in

pipelines and wells, mg/g oil

Concentration in oil on

sediment, mg/g dry weight sediment

BTEX and cyclohexanes C3

- and C4-alkylbenzenes

and naphthalenes
16 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

Lake pipeline oil (sample obtained in 2008), which indicated 

that the compositions of the oils are very similar. Some caution 

should be applied because there are no data indicating that the 

composition of the leaking oil was constant over time. 

The VHC in the well oil and oil-contaminated 

sediment, compared to the VHC in pipeline oil, fall into 

three groups based on structure and degradation from 

biological and physical processes. In the first group, BTEX 

and the cyclohexanes are attenuated within the oil body 

(fig. 7). The most abundant compound in this group is 

methylcyclohexane, and about 60 percent of it is depleted 

in the MW-3 well oil compared to the pipeline oil. About 

90 percent of the benzene is depleted in the well oil. Some 

components are below detection limit, such as toluene and 

o-xylene, whereas others, such as m- and p-xylenes, were 

relatively unchanged. In the second group, the C3

- and 

C4-alkylbenzenes, naphthalene, and the methylnaphthalenes 

show similar patterns between the pipeline, well oil, and 

sediment. There is little or no loss of these compounds in 

the well oil compared to the pipeline oil, and some of the 

trimethylbenzenes and tetramethylbenzenes are higher in 

concentration in the well oil compared to the pipeline oil. 

The sediment sample has a similar distribution of VHC as 

in the well oil. In the third group, the normal hydrocarbons 

in the C6 to C12 range are present in the pipeline oil and not 

detected in the well oil or in the sediment sample (table 6).

The loss of BTEX in the well oil and oil-contaminated 

sediment compared to the pipeline oil is likely due primarily 

Table 6. Results of analyses of oil collected from a South Cass Lake Pumping Station pipeline (2008), 

bailed from MW-3 (2007), and extracted from C0811-91 centimeters (cm) (2008).

[ND, not detected. Results in milligrams per gram (mg/g)]

Hydrocarbons identified Pipeline oil, 

mg/g oil

Well MW-3 oil, mg/g 

oil1

C0811-91 cm, mg/g 

dry weight sediment

benzene 1.186 0.132 0.494

toluene 4.025 ND ND

ethylbenzene 2.182 0.514 3.312

o-xylene 1.231 0.022 ND

m-xylene 2.089 1.711 8.411

p-xylene 0.737 0.486 0.702

cyclohexane2 3.808 0.584 3.783

methylcyclohexane2 5.326 2.010 10.543

ethylcyclohexane2 1.905 0.980 4.888

1,3,5-trimethylbenzene 0.602 0.853 2.727

1,2,4-trimethylbenzene 2.037 2.319 9.238

1,2,3-trimethylbenzene 0.970 0.871 3.105

1,2,3,4-tetramethylbenzene2 0.439 0.742 2.455

1,2,3,5-tetramethylbenzene2 0.305 0.417 1.345

1,2,4,5-tetramethylbenzene2 0.138 0.175 0.567

naphthalene 0.360 0.342 1.106

1-methylnaphthalene2 0.522 0.446 1.088

2-methylnaphthalene2 0.674 0.650 1.635

hexane 10.400 ND ND

heptane 15.391 ND ND

octane 6.952 ND ND

nonane 7.951 ND ND

decane 7.873 ND ND

undecane 7.687 ND ND

dodecane 6.037 ND ND 1

Average of two samples analyzed

2

External standard used to quantify
Results 17

to biodegradation. The observed pattern of greatest to least 

amount of loss is toluene = o-xylene > benzene > ethylbenzene 

> p- and m-xylene. The well oil and oil-contaminated sediment are in anoxic environments, and their similar loss patterns 

suggest that the same processes are occurring in this part of the 

aquifer. Although the two samples were collected in a contaminated part of the aquifer, the oil is floating on the groundwater 

surface at MW-3 at an elevation of approximately 400.04 m, 

whereas the oily sediment sample was collected about 17 m to 

the north at an elevation of 399.90 m. 

The observed pattern of loss is similar to the loss of 

BTEX in groundwater at the Bemidji site (Eganhouse and others, 1996). The complete loss of o-xylene and not the m- and 

p-xylenes is an indication of the importance of microbial degradation, because these compounds have similar physicochemical properties. The biodegradation of BTEX under anaerobic 

conditions is well documented in the literature (Alvarez and 

Vogel, 1991). The considerable depletion of cyclohexane, 

methylcyclohexane, and ethylcyclohexane in the Cass Lake 

well oil relative to the pipeline oil (fig. 7) is somewhat surprising, because these compounds are relatively insoluble in water 

and more difficult to biodegrade under anoxic conditions than 

BTEX and normal hydrocarbons (Perry, 1984).

The C3

- and C4-alkylbenzenes, naphthalene, and the 

dimethylnaphthalenes are only slightly soluble in water and 

are known to degrade under some conditions. Although some 

trimethyl- and tetramethylbenzenes have been shown to 

degrade under anoxic conditions in groundwater (Cozzarelli 

and others, 1990), it is possible that the oil body is too toxic 

for their biodegradation. Likewise, naphthalene and the methylnaphthalenes are more likely to be degraded under oxic conditions (Mihelcic and Luthy, 1988) but were shown to degrade 

under anaerobic conditions by sulfate-reducers (Meckenstock 

and others, 2000). In this environment these compounds are 

stable compared to BTEX, cyclohexanes, and normal hydrocarbons. The finding that several alkylated benzenes were in 

higher concentrations in the well oil compared to the pipeline 

oil suggests that they are resistant to degradation and becoming enriched as other compounds are being attenuated.

The normal hydrocarbons in the C6

 to C12 range appear 

to be labile under the conditions at the site. This is consistent 

with a study at the Bemijdi site (Bekins and others, 2005a). 

Although normal hydrocarbons are not detected in the Cass 

Lake well oil or in the oil-contaminated sediment, alkylated 

hydrocarbons are still abundant, but they were not analyzed 

quantitatively in this study. Alkylated or branched-chained 

hydrocarbons are known to be more difficult to degrade than 

normal alkanes (Solano-Serena and others, 1999).

Although most of the VHC in the Cass Lake oil body are 

attenuated over time in a pattern that is consistent with that 

observed at the Bemidji site (Eganhouse and others, 1993; and 

M.J. Baedecker, USGS, unpublished data, 2009), some differences are notable. For example, normal hydrocarbons from C6

to C12 are absent in the Cass Lake well oil and in the sediment 

sample. In contrast, at the Bemidji site normal hydrocarbons 

from C6 to C11 were found in all the oil samples analyzed. 

Also, at the Bemidji site the concentrations of the cyclohexanes identified in the well oils and pipeline oil were similar, 

whereas in the Cass Lake well oil and sediment sample these 

compounds were depleted compared to the pipeline oil. These 

results indicate that the Cass Lake oils floating on the groundwater and in the sediment are more degraded than are the oils 

at the Bemidji site.

Groundwater Plume

Figures 8 and 9 show plan-view and cross-section plots, 

respectively, of benzene concentrations measured in the pushprobe surveys conducted in September 2007 and May 2008. 

These data provide the most complete three-dimensional 

picture of the plume, with 68 different sample locations. In 

figure 8A the contours are based on the highest benzene concentration measured at any depth. Concentrations used for the 

contours are written next to the sample location. The data show 

that the leading edge of the 10-μg/L benzene plume lies in the 

wooded area south of the railroad tracks, about 150 m east of 

well MW-10. The dissolved plume of BTEX extends eastsoutheast from MW-10. This direction of transport represents 

an eastward deviation from the generally southeast direction of 

regional groundwater flow. The thinning of the sand and gravel 

aquifer to the south of the site (fig. 4) appears to cause the eastward diversion of the groundwater flow (fig. 3) and plume.

Figure 9A shows a cross section of the benzene concentrations along the centerline of the plume marked in figure 8B. 

The contamination occurs mainly in the sand-and-gravel layer 

overlying the till. The benzene plume spans the vertical interval 

from the water table to the base of the sand-and-gravel aquifer. 

There is some indication that the plume splits vertically at PP-9, 

located 30 m downgradient from PP-7, where a low permeability layer impeded sample collection in the center of the aquifer.

The well nests USGS-1, USGS-2, USGS-3, and USGS-4 

(fig. 2) were installed to monitor temporal changes along the 

axis of the plume. In some cases their locations were several 

meters south of the closest push-probe site in order to provide 

access for the drill rig through the cleared area in the woods. 

Figure 9B is a comparison between the push-probe data and 

the USGS wells, showing that benzene concentrations at the 

USGS-1 and USGS-2 well nests are considerably lower than 

at PP-7 and PP-9, respectively. 

Several lines of evidence indicate that these differences 

are not caused by sampling methodology or differences in the 

analytical methods but are due to the location of the plume. 

For example, in May 2007, a sample was collected from 

MW-10 and analyzed in the field, resulting in a value of 2,751 

μg/L. Then in June 2008 another sample was collected and 

analyzed at the NWQL, resulting in a value of 3,146 μg/L. 

These results are in the same order of magnitude and are 

within the yearly variation expected within the plume, thus 

indicating that the sampling and analytical methods gave comparable results during May 2007 and June 2008. 

Results from the analyses for redox constituents (table 7) 

indicate that well nests USGS-1 and USGS-2 are located on 
18 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

Centerline of Railroad

BUILDING

Line of trees

Water

Well

Fence

around

gravel

area

Buried 

pipelines Buried 

pipelines

PP-11

PP-12

PP-9 PP-15 PP-10

PP-16, 17

PP-18

PP-19

PP-3

PP-8

PP-5

PP-4

PP-14

PP-1

PP-2

PP-6

MW-4 MW-11 MW-10

MW-2

(sealed)

MW-14

(sealed)

MW-15

MW-1

MW-6 USGS-4A, B, C

USGS-2A,B,C USGS-3A, B, C

MW-5

MW-13

MW-7

MW-9

MW-8

USGS-1A,B

PP-7

PP-13

4

0 4 1

182 37

2

464

1

60

0

12

4,346 1,417 133 2

41

1

0

3

3,146

1

0

3

0

0

0

2

PP-8

MW-7

EXPLANATION

Weeping flange

Benzene concentration, μg/L

Monitoring well

Approximate push-probe location

Benzene concentration contour

(dashed where inferred), μg/L 10

182

MW-3

10 100

1,000

Section B-B’ (figs. 9A and 9B )

Section C-C’ (fig. 9B “Well data”)

EXPLANATION

Weeping flange

Benzene concentration contour

(dashed where inferred), μg/L 10

PP-8 Approximate push-probe location

MW-7 Monitoring well

10

B’

B

C

C’

Centerline of Railroad

BUILDING

Line of trees

Water

Well

0 50 100

200 40 60 meters

150 200 feet

Fence

around

gravel

fence

Buried 

pipelines

Buried 

PP-11 pipelines PP-12

PP-9 PP-15 PP-10

PP-16, 17

PP-18

PP-19

PP-3

PP-8

PP-5

PP-4

PP-14

PP-1

PP-2

PP-6

MW-10

MW-4 MW-11

MW-2

(sealed)

MW-14

(sealed)

MW-15

MW-1

MW-6 USGS-4A, B, C

USGS-2A,B,C USGS-3A, B, C

MW-5

MW-13

MW-7

MW-9

MW-8

USGS-1A,B

PP-7

PP-13

MW-3

A

B

extent of

crude oil

extent of

crude oil

Figure 8. Maps of the benzene plume at the 

Cass Lake site. A, Maximum dissolved benzene 

concentrations (micrograms per liter, μg/L) 

found at any sample depth based on push-probe 

sampling and field gas chromatograph analyses, 

September 2007 and May 2008. B, Outline of the 

10-μg/L benzene contour of the plume, with lines

of sections B–B’ and C–C’ (figs. 9A and 9B).
Table 7. Values of selected parameters in water from the background well (MW-1) and selected contaminated wells at the Cass Lake site.[Spec. cond., specific conductivity in microsiemens (μS); concentrations in milligrams per liter (mg/L) and micrograms per liter (μg/L); TDOC, total dissolved organic carbon

Parameter MW- 1 MW- 10 USGS- 1A USGS- 1B USGS- 2A USGS- 2B USGS-2C USGS- 3A USGS- 3B USGS- 3C USGS- 4A 

pH 7.33 6.57 7.13 7.07 6.91 6.99 7.07 7.12 7.30 7.04 7.10 

Spec. cond. (μS) 433 1,012 526 560 610 617 677 634 693 749 658 

Alkalinity (mg/L) 232.5 675.1 366.8 378.0 372.8 395.8 470.8 470.2 488.1 561.3 453.4 

TDOC(mg/L) 1.31 20.97 0.77 0.64 1.72 2.58 4.26 4.96 8.68 3.47 3.36 

Benzene (μg/L) na 3,146 0.27 0.50 1.31 2.07 2.12 39.52 143.10 2.94 <0.50 

O2 (mg/L) 9.57 0.00 0.01 0.00 0.00 0.00 0.00 0.30 0.00 0.00 <0.01 

NO3- (mg/L) 0.45 1.20 <0.52 <0.55 <0.59 <0.58 <0.51 <0.62 <0.66 1.12 <0.58 

Fe2+ (mg/L) na 25.60 1.80 7.10 19.67 16.46 15.31 8.10 17.76 1.24 1.56 

Mn2+ (mg/L) na 0.58 2.50 2.09 1.46 1.02 1.02 1.96 1.23 1.43 1.60 

NH4+ (mg/L) 0.23 8.99 0.26 1.10 3.83 6.12 3.41 2.10 na 0.75 0.26 SO42- (mg/L) 1.30 0.82 4.95 5.40 5.14 3.78 2.00 0.95 0.95 5.42 2.30 

CH4 (mg/L) 0.04 19.26 0.45 0.69 0.55 0.55 0.61 2.73 3.12 0.07 0.08 
20 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

10

10

1

40 60 80 100 120 140

396

397

398

399

400

401

1

10

100

1,000

4,000

17

2,691

2,955

3,802

4,346

1,417

1,275

349

28

547

593

441

237

107

130

133

92

47

5

1

0

30

41

12

2

4

4

1

12

1

1

Distance from MW-10, m

Elevation above sea level, m 

PP-7 PP-9 PP-15 PP-16/17 PP-18

B

Northwest

B’

Southeast

Benzene,

μg/L

515

1,000

1,000

10

100

100

100

100

0

1

10

100

1,000

10,000

0 20 40 60 80 100 120 140 160

PP data

Well data

Benzene concentration, μg/L

Distance from MW-10, m

Section C

B’

Section B

C’

MW-10

USGS-1

USGS-2

USGS-3

USGS-4

PP-7

PP-9

PP-15

PP-16, 17

A

B

Figure 9. Benzene concentrations (micrograms per liter, μg/L) in 

the Cass Lake plume. Elevations and 

distances are in meters (m). A, Benzene concentrations along a cross 

section through the plume based on 

push-probe sampling and field gas 

chromatograph analyses, September 2007 and May 2008 (section line 

shown in figure 8B, section B–B’). 

B, Comparison between benzene 

concentrations measured in the 

push-probe profiles and in the USGS 

wells. Error bars are maximum and 

minimum values measured in each 

vertical profile (section line shown 

in figure 8B, section C–C’).
Results 21

the lateral edge of the plume. In particular, dissolved methane 

concentrations are lower and sulfate concentrations higher 

than at USGS-3, indicating less reduced conditions typical of 

the periphery of the plume. Since the USGS-1 nest is located 

only about 5 m from PP-7, these results show that the areal 

extent of the plume is quite limited along the southern edge, 

probably a result of the shallow depth of the glacial till. In summary, the combined dataset from all of the groundwater sampling indicates that the Cass Lake plume is narrow (40 to 60 m 

in the middle) compared to the Bemidji plume (60 to 80 m). 

Benzene concentrations from PP-15 through PP-18 are 

comparable to values from well nests USGS-3 and USGS-4, 

indicating that these wells lie in or near the center of the 

mapped plume. Benzene concentrations in the USGS-4 nest 

were all below 5 μg/L; thus this well is suitably located as a 

monitoring well for detecting future plume expansion.

Differences Between Datasets

The three-dimensional picture of the plume in figures 8 

and 9 shows some differences with other data from the Cass 

Lake site. Typically, the highest concentrations in petroleum 

plumes become deeper with distance from the source as clean 

recharge water enters the aquifer above the plume (National 

Research Council, 2000). This increasing depth with distance 

is apparent in the benzene data cross section in figure 9A. 

Previous groundwater data from the Cass Lake site are based 

on samples collected near the water table from either the MW 

wells or temporary Geoprobe boreholes.

To understand the three-dimensional nature of the Cass 

Lake plume, direct comparison of data collected from nearby 

locations is instructive. Boreholes PP-7 and GP-16 are separated by about 10 m (fig. 2). At PP-7, the concentration of 

benzene detected in 2007 at the deepest level was 4,346 μg/L 

(fig. 9A). In contrast, the benzene concentration at GP-16 was 

only 19 μg/L in 2003 (Natural Resources Engineering Company, 2003). Although the samples were collected in different 

years, using different equipment, and analyzed with different 

instruments, the primary source of this difference may be the 

depth from which the samples were collected. The borehole 

log for GP-16 indicates that the water sample was collected 

at a depth of about 0.3 m below the water table (Natural 

Resources Engineering Company, 2003). For PP-7, samples 

were collected at 0.2-m intervals from 0.2 m to 1.0 m below 

the water table. In the 0.2-m sample benzene was 17 μg/L (fig. 

6A), which is comparable to the GP-16 value of 19 μg/L at 

0.3 m below the water table. Concentrations in PP-7 increased 

with depth, reaching the maximum value of 4,346 μg/L in the 

deepest sample collected at 1.0 m below the water table. No 

deeper samples were collected at PP-7 because the base of the 

sand-and-gravel aquifer was encountered just below this depth.

Longer well screens may also lead to lower concentrations than observed in USGS wells and push-probes, if water 

from near the edge of the Cass Lake plume mixes with water 

from the center. This effect is apparent when the push-probe 

data are compared to well data. The push-probe screen consisted of ten 1.0-cm-diameter ports distributed over a 10-cm 

vertical interval. By comparison, the USGS well screens are 

slightly longer at 30 cm, but the existing monitoring-well 

screens are 3.05 m long. In May 2007, the concentration of 

benzene in MW-10 was 2,751 μg/L, whereas a peak value of 

4,346 μg/L was found at PP-7 using the smaller push-probe 

screen. If all samples from 0.20 m to 1.0 m at PP-7 are averaged, the resulting value of 2,762 μg/L is closely comparable 

to the MW-10 value. 

Distribution of Redox Species

Groundwater

Analysis of the water chemistry in wells located 

upgradient and downgradient from the oil body indicates 

that biodegradation at the Cass Lake site is occurring by the 

five primary reactions in table 1: oxygen reduction, nitrate 

ammonification, iron reduction, sulfate reduction, and methanogenesis. Manganese is not a major electron acceptor at 

the Cass Lake site and will not be discussed. Well MW-1 is 

representative of the background aquifer unaffected by the 

oil spill, and MW-10 is the most contaminated well located 

outside the oil (table 7).

Figure 10A is a plan-view contour map of groundwater 

concentrations of dissolved reduced iron (Fe(II)), a product 

of degradation by iron reduction. The contours are based on 

the highest value measured at any depth during the pushprobe survey and the concentration of reduced iron in the 

most contaminated monitoring well outside of the oil (MW10). Reduced iron concentrations are highest near the source 

of the contamination at MW-10 (23.52 mg/L), decreasing to 

below 1 mg/L downgradient and on the sides of the plume. 

Concentrations of reduced iron decrease as water containing oxygen mixes with the plume along the edges, oxidizing 

Fe(II) to Fe(III). The Fe(III) is insoluble and forms coatings on 

the sediments, resulting in the observed decrease in dissolved 

Fe(II). Because dissolved Fe(II) is only present where oxygen 

is absent, the 1-mg/L contour of dissolved Fe(II) provides an 

indication of the extent of the anaerobic portion of the plume 

(fig. 10B). Whereas the background groundwater at MW-1 is 

almost saturated with DO, groundwater at MW-10 and all the 

wells downgradient from the oil are anoxic, with DO concentrations below detection. 

Figures 11A-D are cross-section contour maps of redox 

species along the line of section D-D’ shown in figure 10B. 

Ammonia, a product of nitrate ammonification, is low in 

the background well but increases in the plume (table 7, fig. 

11A). The concentrations of reduced iron increase from the 

background well to well MW-10 and then decrease from the 

source of contamination to the downgradient wells (table 7, 

fig. 11B). The concentration of sulfate, an electron acceptor,

is higher at the background well than in well MW-10, indicating that sulfate is consumed by degradation coupled to sulfate 
22 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

Centerline

of Railroad

PP-11

PP-12

PP-9 PP-15

PP-10

PP-16,17 PP-18

PP-3

PP-8

PP-5 PP-4

PP-14

PP-1

PP-6

MW-10

0 50 100

40200 60 meters

150 200 feet

PP-7

PP-13

0.00

0.00

0.00 1.12 4.84 26.45

0.00

12.4

20.00

4.00

0.00

10.00

0.00

3.67

0.17 0.42

extent of 

crude oil

extent of 

crude oil

23.52 10 1 20

EXPLANATION

Weeping flange

PP-3 Approximate push-probe location

MW-10 Monitoring well

Line of trees

Fence around

gravel area

Buried 

pipelines

Centerline

of Railroad

Line of trees

Fence around

gravel area

Buried 

pipelines

Buried 

pipelines

D

D’

E’

E

1

USGS2A,B,C

PP-11 PP-12

PP-9 PP-15

PP-10

PP-16, 17

PP-18

PP-3

PP-8

PP-5 PP-4

PP-14

PP-1

PP-6

MW-10

PP-7

PP-13

USGS3A, B, C USGS-4

A, B, C

10 Reduced iron concentration contour

(dashed where inferred), mg/L

Section D-D’ (figures 11A-D and 16)

Section E-E’ (figures 12A

A

B

, 12B, and 13A-D)

Approximate core location 

Buried 

pipelines

C0808

C0811

C0808

C0810

C0806

C0804

Figure 10. Maps of reduced iron (Fe(II), in milligrams per liter, mg/L) in the Cass Lake plume. A, Contour plot of concentrations of Fe(II) in groundwater based on maximum value measured in push-probe profiles. B, Anaerobic outline of the 

plume, which is defined by the 1-mg/L Fe(II) contour line. Also shown are lines of section for figures 11, 12, 13, and 16.
Results 23

reduction (table 7, fig. 11C). Methane, which is a product of 

methanogenic degradation, increases from the background well 

to well MW-10 and then decreases from MW-10 to the wells 

downgradient from the oil (table 7, fig. 11D).

Sediments

Biodegradation of hydrocarbons coupled to iron reduction 

depends on the presence of sediment iron oxyhydroxides in the 

aquifer (Fe(OH)3 in table 1). The reaction produces reduced 

iron (Fe(II) or Fe+2), which is mainly retained in the sediments 

(Baedecker and others, 1993) but also is dissolved in the groundwater (figs. 10A and 11B). Degradation by iron reduction can be 

assessed by measuring the depletion of sediment iron oxyhydroxide and the increase in reduced iron in the plume compared 

to the uncontaminated aquifer. 

Figure 11. Concentrations of redox derivatives (milligrams per 

liter, mg/L) along line of section D-D’ in figure 10B based on data 

from USGS wells and MW-10 sampled in June 2008. Constant 

concentrations across the well screen were used for MW-10. 

Elevations and distances are in meters (m). A, Ammonium. B, Iron. 

C, Sulfate. D, Methane.

8

6 4

2

2

2

4

4

6

15

10

5

25.60

19.67

16.46

15.31

8.10

17.76

1.24

1.56

1.59

Approximate base of aquifer 1.21

8.99

3.82

6.12

3.41

2.10

5.56

0.75

0.26

0.75

Approximate base of aquifer 0.19

Approximate base of aquifer

0.82

5.14

3.78

2.00

0.95

0.95

5.42

2.30

0.98

8.29

Distance from MW-10, m

Elevation above sea level, m 

Approximate base of aquifer

19.26

0.55

0.55

0.61

2.73

3.12

0.07

0.08

0.15

0.01

D ’D MW-10

USGS-2 USGS-3 USGS-4

8

10

20

25

15

5

Ammonium, mg/L

Iron(II), mg/L

Sulfate, mg/L

Methane, mg/L

A

B

C

D

Water table

MW-10 MW-10 MW-10

Water table

Water table

Water table

396

397

398

399

400

396

397

398

399

400

396

397

398

399

400

396

397

398

399

400

0 20 40 60 80 100 120 140

0 50 100 150

A

B

-50

Distance from MW-10, m

Background range

Background range

Fe(III), mmol/kg dry weight Percent Fe(III) relative to total Fe

E E’

70

60

50

40

30

20

10

0

100

80

60

40

20

0

Figure 12. Average oxidized iron (Fe(III)) measured in sediment 

cores plotted against distance from well MW-10 along line of section 

E-E’ in figure 10B. Data points, from left to right, are from sediment 

cores C0811, C0808, C0810, C0804, and C0806. Shaded areas show 

range of values measured on background sediments. Error bars on 

points represent minimum and maximum values measured in each 

core. Distances are in meters (m). A, Oxidized iron on sediments 

(millimoles per kilogram, mmol/kg). B, Percent of oxidized iron relative 

to total iron on sediments.
24 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

Results from the sediment iron analyses are shown in 

table 8. Figure 12A shows the concentration of iron oxyhydroxides in the sediment cores along the line of section E-E’ 

in figure 10B, and figure 12B shows the percentage of the total 

extracted iron that is in the oxidized form.

Outside the plume, the iron oxyhydroxide content of the 

sediments ranges from 14 to 50 mmol/kg (fig. 12A), with 94 

percent of the total iron in the oxidized form (fig. 12B). The 

greatest depletion of iron oxyhydroxide occurs in the oil body, 

where the concentration is 8 mmol/kg and iron oxyhydroxide 

makes up 16 percent of the total iron. Farther from the oil, less 

of the iron oxyhydroxide has been used, and average concentrations range from 9 to 30 mmol/kg, making up 34 to 63 

percent of the total iron, respectively.

At 150 m downgradient from the oil body, the iron oxyhydroxide concentration is 14 mmol/kg, making up 82 percent 

of the total iron, which is close to the background value. The 

same pattern, characterized by depletion of iron oxyhydroxide near the plume source, occurs at the Bemidji site. As time 

progresses, iron oxyhydroxide decreases and reduced iron 

increases in the anaerobic portion of the plume (Cozzarelli and 

others, 2001). Near the fringes of the plume, where groundwater 

containing dissolved reduced iron mixes with water containing 

dissolved oxygen, significant precipitation of iron oxyhydroxides sometimes occurs. Evidence that this process occurred in 

the past is apparent at 91 m downgradient from MW-10 (130 m 

from the center of the oil body), where the maximum iron oxyhydroxide concentration is significantly greater than the highest 

background level (fig. 12A). At the Bemidji site a similar 

buildup of iron oxyhydroxides was found 105 m downgradient 

from the center of the oil body (Cozzarelli and others, 2001). Microbial concentration, MPN per gram

1

1

-50 0 50 100 150 -50 0 50 100 150

Aerobes

Distance from MW-10, m

Iron reducers

Sulfate reducers Methanogens

100,000

100,000

10,000

10,000

1,000

1,000

100

100

10

10

A B

C D

E E’ E E’

Figure 13. Most probable number 

(MPN) of four major physiological 

types of microorganisms along line 

of section E-E’ in figure 10B. Values 

given are average for profile, 

and error bars are maximum and 

minimum values in each profile. 

Data points, from left to right, are 

from sediment cores C0811, C0808, 

C0810, C0804, and C0806. Distances 

are in meters (m). Dashed lines 

indicate numbers for a relatively 

uncontaminated site outside 

the plume (Core C0801, fig. 2) for 

aerobes and iron reducers and 

indicate detection limits for sulfate 

reducers and methanogens, which 

were not found outside the plume. 

A, Aerobes. B, Iron reducers. 

C, Sulfate reducers. D, 

Methanogens.

Microbial Populations

The microbial population data, shown in figures 13A–D, 

support the redox constituent data, indicating that anaerobic 

degradation occurs in the core of the plume and aerobic degradation occurs at the edges. Populations of microorganisms detected 

with aerobic growth media are higher than background levels 

throughout the plume (fig. 13A). These numbers include facultative bacteria that are capable of growing under either anaerobic or 

aerobic conditions. Their increased numbers in anaerobic portions 

of the plume reflect the increased numbers of fermenting bacteria 

compared to outside the plume. Aerobic populations peak at 116 

m downgradient from well MW-10, where dissolved oxygen was 

measured near the water table in well USGS-3A. Peak aerobe concentrations are expected where there is mixing of reduced carbon 

from the plume and dissolved oxygen from outside the plume.

The anaerobe population data indicate that biodegradation 

coupled to iron reduction, sulfate reduction, and methanogenesis 

are all important, but the relative contribution of each process varies with position in the plume. Peak iron-reducer concentrations 

of 2,000 times background occur at 91 m downgradient from well 

MW-10, corresponding to the location where the highest sediment 

iron oxyhydroxide concentrations were found (core C0809 adjacent to well cluster USGS-2 in fig. 13B). Iron-reducers within the 

oil are lower in concentration, but still at least 200 times average 

background levels. These lower iron-reducer concentrations in the 

oil body reflect the decreased concentrations of iron oxyhydroxides remaining on the sediments. Concentrations of iron reducers 

drop to background levels by 146 m downgradient from well 

MW-10, reflecting mixing of the plume with oxygenated water 

and decreasing importance of anaerobic processes.
Analysis of Biodegradation Processes 25

Table 8. Results of the sediment iron analyses collected from 

the Cass Lake site on June 20–23, 2008, and July 18, 2008.

[Core depth intervals are in centimeters (cm) from top of core; see 

table 3 for depth to top of cores. Locations of cores C0801 through 

C0811 are shown in figure 2. Iron concentrations are in millimoles per 

kilogram (mmol/kg).]

Core Interval, cm Fe(III), mmol/kg Fe(II), mmol/kg

C0801 38-41 13.76 0.86

 64-67 50.72 5.22

C0802 53-56 13.51 17.36

 64-67 2.91 19.96

103-106 12.32 1.12

C0803 19-22 4.39 12.91

C0804 33-36 4.37 15.02

 64-67 14.91 17.88

C0806 34-37 14.58 4.70

 84-87 15.26 2.10

C0807 53-56 7.99 12.23

C0808 23-27 19.73 21.69

C0809 53-56 5.29 24.70

C0810 33-36 67.96 107.95

 84-87 16.95 10.47

C0811 63-66 9.66 38.55

 95-98 6.82 50.47

Sulfate-reducing bacteria were below detection limits (0.42 

Most Probable Number per gram dry weight sediment, MPN/g, 

fig. 13C) in background samples (core C0801). Their concentrations peak at more than 2,000 MPN/g within the oil (core C0811), 

presumably reflecting the continued supply of sulfate in water 

infiltrating from the surface. Their concentrations are at intermediate levels in the center of the plume (for example, cores C0809 and 

C0810). Like the iron reducer concentrations, they drop to nearbackground levels by 146 m downgradient from well MW-10, 

consistent with a switch from anaerobic to aerobic conditions. 

Concentrations of methanogenic microorganisms were also 

below the detection limit in the background samples (fig. 13D). 

However, in the oily sediments their concentrations were more 

than 70 MPN/g, comparable to mixed methanogenic and ironreducing areas of the Bemidji plume (Bekins and others, 1999). 

The location of the peak concentration of methanogens in the oil 

is similar to that at the Bemidji site (Bekins and others, 2001). 

As sediment iron oxyhydroxides become depleted, a microbial 

consortium performing degradation by fermentation coupled to 

methanogenesis becomes competitive. Methanogen concentrations 

remain higher than detection limits throughout the contaminant 

plume, indicating that reduced conditions have existed for sufficient time to deplete the aquifer sediment iron oxyhydroxides and 

allow the methanogen population to become established.

Analysis of Biodegradation Processes 

Comparison of the Bemidji and Cass Lake Sites

Many aspects of the Cass Lake site are similar to the 

Bemidji site, suggesting that the biodegradation processes are 

comparable. At both sites the contaminant is crude oil from the 

same pipeline and oil field, with an estimated 182,000 L in the 

aquifer at Cass Lake and 147,000 L at the Bemidji north oil 

pool (Herkelrath, 1999). Both aquifers are composed of glacial 

outwash material with silt, sand, and gravel. The 8 to 9 m depth to 

the water table at Cass Lake is comparable to the 6 to 8 m depth 

at the Bemidji north oil pool (figs. 14 and 15). The groundwater 

velocity estimate for Cass Lake of 26 m/yr is very close to the 

average velocity for Bemidji of 22 m/yr. Oxidized iron is the 

dominant electron acceptor at the Bemidji site, and iron reduction 

is occurring near the oil body in the Cass Lake plume. Concentrations of methane indicate that methanogenic degradation is occurring near the oil at both sites. Both sites have low concentrations 

of nitrate in their background groundwater (Baedecker and others, 

1993). The background nitrate values are 0.45 mg/L at Cass Lake 

and 0.19 mg/L at Bemidji. Similarly, sulfate background concentrations are also low at both sites, with 1.3 mg/L at Cass Lake and 

2.3 mg/L at Bemidji (Baedecker and others, 1993).

The geochemistry around the subsurface contamination at 

the Bemidji north pool site has been categorized into five different 

zones in the groundwater below the water table (Zones 1 to 5 in 

fig. 14; Bennett and others, 1993) and three additional chemical 

zones in the unsaturated sediments above the water table (Zones 

6, 7, and 8 in fig. 14; Delin and others, 1998). Zone 1 constitutes the uncontaminated background groundwater with natural 

background organic matter. Zone 2 underlies the area where the 

land surface was contaminated by sprayed oil. The groundwater is 

chemically characterized by decreased oxygen concentrations and 

contamination with DOC compounds. Zone 3 is the area containing the crude oil floating on the water table and the adjacent 

portion of the groundwater contaminant plume. The groundwater 

in Zone 3 is very reduced and anaerobic, with high concentrations 

of dissolved contaminants and characteristic degradation products 

including methane and reduced iron. The primary hydrocarbon 

degradation reactions in Zone 3 occur by reduction of sedimentbound iron oxyhydroxides and by methanogenesis. Zone 4 transitions back to aerobic conditions, with low concentrations of DOC 

in the aquifer. Zone 5 constitutes the reoxygenated groundwater 

downgradient from the plume, with slightly elevated concentrations of contaminants. 

Zones 6, 7, and 8 are in the unsaturated zone. Zone 6 is 

uncontaminated and exhibits near atmospheric concentrations 

of oxygen. Zone 7, a transition zone, is defined by lower 

concentrations of oxygen compared to background, 

hydrocarbon concentrations less than 1 μg/L, and higher 
26 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

440

435

430

425

420

415

Southwest Northeast

-250

Elevation above sea level, m

Oil

Pipeline

break

Distance from approximate center of oil, m

Water table

Zone 1 Zone 5

Zone 7 Zone 6

Zone 8

Groundwater flow

Zone 3

Zone 2

Datapoint

Zone 4

-200 -150 -100 -50 0 50 100 150 200

Spray zone Unsaturated

zone

containing

liquid crude

oil

Land surface

unaffected by spill

Enhanced recharge

concentrations of carbon dioxide and methane compared to 

background. Zone 8, immediately adjacent to and within the 

unsaturated zone oil body, is anaerobic and contains maximum 

concentrations of more than 10 percent mole fraction carbon 

dioxide and 20 percent methane (Amos and others, 2005), with 

hydrocarbons >1 μg/L (Chaplin and others, 2002). 

Figure 15 provides a graphical summary of the 

geochemical zones at the Cass Lake site for comparison with 

those at the Bemidji site (fig. 14). The descriptions for Zones 

1, 3, 4, and 5 for the Bemidji site also apply for the Cass Lake 

section. Zone 2 is not present at the Cass Lake site because oil 

was not sprayed onto the land surface there.

A number of important differences also exist between 

the two sites, providing a potential opportunity to increase 

understanding of processes that control crude-oil degradation 

rates in the subsurface. The Bemidji spill occurred in a single 

incident more than 30 years ago, whereas the Cass Lake spill 

occurred as an ongoing leak over an unknown period of time 

ending in early 2002. Because of the large area over which the 

oil infiltrated the Bemidji aquifer, about 40 percent of the oil is 

trapped in the unsaturated zone, whereas the small area of the 

Cass Lake site leak presumably resulted in oil mainly floating 

at the water table with a minor amount in the unsaturated zone 

(figs. 14 and 15). Although the climate is identical, the recharge 

may be much higher through the oil body at the Cass Lake 

site because the fenced yard of the pumping station is devoid 

of vegetation and covered with gravel (fig. 15). The kinematic 

viscosity range of 23-421×10-6 m2

/s (Wenck Associates, 2005) 

of the Cass Lake site oil is higher than the range measured 

Figure 14. Geochemical zones in the unsaturated and saturated hydrologic zones at the Bemidji crudeoil spill site (modified from Delin and others, 1998). Elevations and distances are in meters (m). Dots 

represent locations of gas sample ports and monitoring well screens.

for the Bemidji oil in 1988-1989 of 10-25×10-6 m2/s (Landon, 

1993). Enhanced recharge through the gravel is expected to 

slightly increase the oil degradation rates at the Cass Lake site 

and could result in measurably faster degradation rates 

compared to the Bemidji site.

The similarity between the Cass Lake and Bemidji 

benzene degradation rates, redox conditions, and aquifer 

material all support a hypothesis that the Cass Lake plume 

is decades old. The presence of culturable methanogens 

at a location requires that anaerobic conditions be well 

established and that some iron-oxide depletion has occurred 

(Bekins and others, 2001). The Cass Lake methanogen 

data (fig. 13D) and the sediment iron data (fig. 12B) both 

indicate that anaerobic conditions are well established at the 

location of USGS-4. In the early years at the Bemidji site, 

there was an abrupt transition from anaerobic to aerobic 

conditions downgradient from the oil body, and much of 

the benzene degradation occurred during this transition 

(Baedecker and others, 1993; Eganhouse and others, 1993). 

More recent evidence suggests that benzene at the Bemidji 

site is degrading under anaerobic conditions in the zone of 

the plume that has mixed methanogenic and iron-reducing 

microbial populations (Cozzarelli and others, 2010). The 

Bemidji plume is now just over 30 years old, and the present 

pattern of a slow, uniform benzene degradation rate under 

anaerobic conditions took decades to establish. The Cass 

Lake plume has a very similar slow benzene degradation rate 

under anaerobic conditions and thus also can be assumed to 

be decades old.
Analysis of Biodegradation Processes 27

Figure 15. Conceptual model of the oil spill and plume at the Cass Lake site, showing major geochemical zones. 

Elevations and distances along section A-A’ (fig. 2) are in meters (m).

395

400

405

410

0

Distance from MW-10, m

Elevation above sea level, m

Gravel in fenced area

Hypothesized enhanced recharge

Weeping flange

Oil

Land surface

Water table

Unsaturated zone

Groundwater flow

Zone 3

Zone 5

Zone 1 Zone 4

Till

West-northwest

A

East-southeast

A’

USGS-2USGS-3

MW-10

USGS-4

A

B

C B

C

B

C

EXPLANATION

Unsaturated zone

Zone 3

Zone 4

Zone 5

Zone 1

Glacial till

Well screen

-60 -40 -20 20 40 60 80 100 120 140

A A

395

400

405

410

Biodegradation of Oil Body

Rate

The highly degraded state of the Cass Lake oil represents one of the significant findings of this study. Although 

the start date of the Cass Lake leak is unknown, the Bemidji 

spill is known to have occurred on August 19, 1979. Even 

though more than 30 years have passed since the Bemidji 

spill, the spilled oil in some locations is less degraded than the 

Cass Lake samples. Forensic geochemistry researchers have 

proposed that unknown oil-spill dates can be inferred from the 

degradation state of the oil (Christensen and Larsen, 1993). 

However, work at the Bemidji site has shown that dating of 

spills based on degradation state is invalid when site conditions differ from those required by the dating method. The 

method requires the overlying surface to have been paved 

since the time of the spill, which is rarely the case. 

In spite of its uniform age, the Bemidji oil exhibits 

degradation states ranging from highly degraded with all 

n-alkanes absent to minimally degraded states such as shown 

in figure 6 and 7. Bekins and others (2005a) showed that the 

higher degradation rates occur in locations where recharge 

to groundwater is enhanced by lack of trees and topographic 

focusing of surface runoff. On the basis of these results, three 

possible aspects of the Cass Lake spill may contribute to the 

highly degraded state of the Cass Lake oil: (1) the leak has 

been active for many years, (2) high recharge rates stimulate 

degradation, and (3) the leak occurred slowly. These factors 

are discussed in the following paragraphs.

Although the date that the flange began leaking oil into 

the subsurface is unknown, the highly degraded nature of the 

Cass Lake oil suggests the leak was active for decades. A 

time scale of decades since the onset of the spill is consistent 

with the highly reduced nature of the plume as indicated by 

depletion of oxidized iron in the aquifer sediment and by the 

presence of methanogenic bacteria. In addition, the most-
28 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

degradable aromatic hydrocarbons (toluene and o-xylene) are 

now completely absent from the oil. However, even if the leak 

began when the pumping station opened in 1971, the degree 

of oil degradation is high compared to the Bemidji spill which 

occurred just eight years later in 1979.

As noted above, recharge can accelerate degradation of 

oil (Bekins and others, 2005a). Recharge to groundwater in 

northern Minnesota occurs primarily in the spring and, to a 

lesser extent, fall months. In general, the amount of precipitation that enters the groundwater as recharge is decreased 

by runoff, evaporation, and transpiration by plants. The flat, 

gravel-covered yard of the South Cass Lake Pumping Station 

is devoid of vegetation, which likely contributes to rapid infiltration of precipitation and a higher groundwater recharge rate 

compared to surrounding areas. A high rate of recharge stimulates biodegradation by providing necessary nutrients and 

moisture to the degrading microbial population and carrying 

away degradation products. That the Cass Lake oil is so highly 

degraded compared to the Bemidji oil suggests that enhanced 

recharge at the site may be an important factor accelerating 

degradation. Additional information and data analysis on 

recharge rates at the Cass Lake site are needed in order to 

verify this hypothesized link between a higher recharge rate 

and enhanced biodegradation within the gravel-covered yard.

A third hypothesis to explain the highly degraded oil is 

that the slow nature of the leak allowed aerobic degradation 

and weathering to take place in the unsaturated zone before 

the oil reached the water table. Using the estimate of 181,700 L 

of oil in the aquifer (Wenck Associates, 2005), it is possible 

to calculate a minimum leakage rate by assuming the leak 

began when the oil first entered the pipeline in August 1971. 

The resulting value is 14.4 liters a day. At this rate, the oil 

would rapidly migrate to the water table and join the accumulating floating oil body at the water table. Thus, the oil is not 

expected to be significantly retained in the unsaturated zone 

before reaching the water table.

Long-Term Fate of Oil

The progression of biodegradation of oil has been 

summarized by Peters and others (2005). According to their 

summary, biodegradation of oil proceeds in a quasi-sequential 

manner such that easily degradable compounds are lost first, 

but more resistant classes of compounds begin degrading 

before complete loss of less-resistant compounds occurs. Data 

for the Cass Lake site oil samples show near complete depletion of the n-alkane fraction, placing the oil in the category of 

“heavily degraded.” The alkylcyclohexanes are only partially 

degraded, indicting that the oil has not yet reached the category of “severely degraded.”

As oil degradation proceeds through higher levels of 

severity, the soluble fraction decreases to the point where very 

little dissolution of compounds into the groundwater occurs. 

Once the oil reaches this degradation state, at an unknown time 

in the future, the groundwater plume is not expected to pose a 

risk for further expansion. In the long term, a residual fraction 

of heavily degraded oil consisting mainly of an unresolvable 

complex mixture of compounds will remain in the aquifer.

Biodegradation of Groundwater Plume

Biodegradation Rate

The attenuation rate of benzene along the main flow path 

was determined using the maximum concentrations measured 

at PP-7 and PP-9 (fig. 9A). In this portion of the plume the 

observed benzene concentrations exceed 1,000 μg/L, so the 

degradation rate was assumed to be independent of benzene 

concentration. (This is known as a zero-order rate model.) 

Using an estimated flow velocity of 26 m/yr, the resulting benzene attenuation rate is 8 micrograms per liter per 

day (μg/L/d). The concentration data for electron acceptors 

indicate that the main degradation process in the area between 

PP-7 and PP-9 is iron reduction.

At the Bemidji site, a benzene degradation rate of 8 

μg/L/d was measured under iron-reducing conditions in an 

in-place microcosm study conducted during 1998-2001 (Cozzarelli and others, 2010). In 2007, data from the anoxic portion 

of the Bemidji north pool plume yielded a rate of 4 μg/L/d. 

The similarity of rates between the two sites suggests that the 

anaerobic degradation processes are quite similar. However, it 

is important to note that the estimated rate depends directly on 

the value assumed for the groundwater flow velocity, which is 

difficult to measure.

Long-Term Fate of Plume

The soluble fraction of the oil is the source of dissolved 

compounds forming the groundwater plume. In 2007, the 

groundwater plume contained alkylbenzenes and naphthalenes. 

The oil had very little toluene or o-xylene but still contained 

significant benzene, ethylbenzene, and naphthalenes. The time 

for complete loss of alkylbenzene and naphthalene compounds 

from the oil depends on the oil saturation and rate of groundwater flow through the oil body (Essaid and others, 2003). The 

dissolution rate becomes slower as the concentrations in the oil 

decrease, resulting in an exponential decay in concentration with 

time. As concentrations of alkylbenzenes in the oil decrease, the 

concentration in the groundwater plume will also decrease, and 

eventually the plume is expected to shrink. The length of time 

required for the dissolution rate to slow to the point where the 

plume begins to shrink is a topic of active research at the Bemidji 

site and cannot be predicted with certainty.

Theoretical calculations predict that the Bemidji plume 

will grow slowly as iron oxyhydroxides become depleted in 

the aquifer (Bekins and others, 2005b). However, recent results 

from the Bemidji site show that calculating the growth rate of 

the benzene plume requires additional research to better understand two issues. The first issue is the bioavailability of different 
Conclusions 29

Figure 16. Average concentrations (milligrams per liter, mg/L) of 

selected constituents at the Case Lake site versus distance (meters, m)

from well MW-10 along line of section D-D’ in figure 10B.

0

5

10

15

20

25

0 50 100 150

Distance from MW-10, m

Concentration, mg/L

Ammonium

Methane

Dissolved organic carbon

MW-10

USGS-2 USGS-3

USGS-4

D D’

types of iron oxyhydroxides for benzene degradation. Previous 

results from the Bemidji site indicated that rapid degradation 

of benzene under iron-reducing conditions required iron oxide 

to exceed 60 percent of the poorly crystalline iron coatings on 

the sediments (Bekins and others, 2005a). However, 2007 data 

from the Bemidji site show that slow degradation of benzene 

is occurring where iron oxides make up 20 to 30 percent of the 

poorly crystalline iron. The Cass Lake data show that benzene 

degradation occurs with 30 to 40 percent iron oxide (figs. 9A

and 12B). It is very encouraging that benzene degradation 

coupled to iron reduction may occur with lower iron concentrations than previously recognized.

The second issue is that the plume contains many components that can be oxidized by iron-reducing bacteria. For 

example, methane can be oxidized by iron reduction (Beal and 

others, 2009), and this process appears to be important at the 

Bemidji site (Bekins and others, 2008). At the Cass Lake site 

the maximum concentration of methane occurs in the oil body, 

and concentrations decrease with distance downgradient from 

the oil, suggesting that methane is consumed faster than it is 

produced in the anaerobic plume (fig. 16). A similar pattern 

of decreasing concentrations with distance is apparent in the 

ammonium data (fig. 16). Moreover, the high concentration of 

nitrate at USGS-4C (table 7) may be due to oxidation of ammonium to nitrate. Finally, dissolved organic carbon is transported 

from the oil body and decreases in concentration, indicating 

degradation with distance by microorganisms (fig. 16). Altogether, the oxidation of methane, ammonium, and dissolved 

organic carbon could consume much of the dissolved oxygen 

and iron oxyhydroxide. A full accounting of all these reactants 

requires multispecies reactive transport modeling.

Although predicting the timeframe and migration rate of 

the plume requires more research, natural attenuation is currently protecting the environment and water supplies in the area 

near the Cass Lake site. For sites contaminated with petroleum 

hydrocarbons, the biodegradation processes are considered reliable under aerobic and anaerobic conditions (National Research 

Council, 2000). Therefore, requirements for demonstrating that 

natural biodegradation is an adequate management strategy for 

petroleum hydrocarbon plumes are somewhat simpler than for 

other contaminants. The most important issue is monitoring the 

plume to show that biodegradation is controlling the growth. The 

data from USGS-4 show that benzene is present at low concentrations (<1 μg/L), indicating that the location of this well nest 

can be used to monitor growth of the plume. Figure 17 illustrates 

the position of the oil body and the dissolved plume with respect 

to the wetland in the upper reaches of Fox Creek. The Fox Creek 

wetland, located about 250 m south of the Cass Lake site, is the 

nearest receptor (building, surface water, or wells) south of the 

Cass Lake site. Regarding the threat of contamination of Fox 

Creek and the wetlands that feed into it: (1) The direction of 

groundwater flow and therefore plume migration is parallel to 

Fox Creek rather than toward it. (2) The geochemical and microbial data show the redox conditions are similar to the Bemidji 

plume, which is decades old. Thus, the Cass Lake Pumping 

Station oil leak may have been present when the pumping station 

first began operations in 1971. The evidence that the plume is old 

implies that its advance rate to date must have been very slow 

(less than 4 m per year). Even if the plume doubled in size over 

the next 40 years (which is unlikely) it would still not impact 

Fox Creek or the wetlands. However, the well cluster at USGS-4 

should be monitored in order to ensure that the current biodegradation processes continue to limit expansion of the plume. In 

addition, installation of a nest of observation wells in the vicinity 

of PP-10 would help in this regard. 

Conclusions

A 2-year study by the U. S. Geological Survey of the 

crude oil spill at the Enbridge South Cass Lake Pumping 

Station included three-dimensional characterization of 

the contaminant plume, assessment of the important 

biodegradation processes, and installation of monitoring 

wells to assess future changes in the plume. The delineated 

contaminant plume of greater than 10 mg/L extends 

approximately 150 m east of the pumping station fence (fig. 

17). The direction of migration is diverted eastward from 

the generally southeast regional groundwater flow direction, 

probably because of thinning of the sand-and-gravel aquifer 

to the south of the Cass Lake site. The dissolved plume is 

approximately 40 m across and extends 2 to 3 m vertically, 

and the location of maximum concentration becomes deeper 

with distance. Biodegradation in the oil body and in most of 

the plume occurs under anaerobic conditions. The important 

processes are nitrate ammonification, iron reduction, sulfate 
30 Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota

Figure 17. Position of oil body and 10-micrograms-per-liter (μg/L) benzene contour line in the Cass Lake plume with respect to the 

nearest surface water in the wetland at the headwaters of Fox Creek. 

reduction, and methanogenesis. Two samples of oil extracted 

from sediments and one bailed from a well were all heavily 

degraded. Sediment iron oxyhydroxide concentrations from 

sediment samples in the oil body and plume show that iron 

oxyhydroxides are being utilized by iron-reducing bacteria. 

The remaining iron oxyhydroxide percentage on the sediments 

ranges from 15 percent in the oil body, to 34 to 40 percent 

in the core of the plume, and to 81 percent at the farthest 

downgradient extent of the plume. The benzene degradation 

rate is estimated to be 8 μg/L/d, which is comparable to 

iron-reducing rates at the Bemidji site. The well-established 

anaerobic microbial populations, degree of iron depletion, and 

highly degraded state of the oil suggest that the Cass Lake 

plume has been present for decades. 

South Cass Lake 

Pumping Station

Fox Creek and wetlands

10 μg/L benzene contour

Railroad tracks

Groundwater flow direction

0 250 500 750 1,000 feet

0 100 200 300 meters

Extent

of oil

© 2009 Europa Technologies

Image USDA Farm Service Agency

© 2009 Google
References 31

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Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, April 14, 2010

Text edited by Peter H. Stauffer

Layout and design by Jeanne S. DiLeo
Drennan and others—Fate and Transport of Petroleum Hydrocarbons in the Subsurface near Cass Lake, Minnesota—U.S. Geological Survey Scientific Investigations Report 2010-5085