6. Organic Compounds:

Hydrocarbons

General Comments

Borden, Canada, tracer test

Laurens, SC, gasoline spill

Bemidji, MN, crude oil spill

In the subsurface, biogeochemical processes that

control contaminant fate vary in space and time, and

alter the aquifer aqueous and solid phase chemistry

Sequential Depletion of TEAs

(e.g. Acetic Acid)

Examples of Experimental and

Case Studies

of Organic Contaminant

Fate and Transport:

Hydrocarbons

Benzene,Toluene, and Xylene: Aerobic Degradation

Relative to Chloride in a Field-Scale Test at Base Borden

Barker, 1987

1800 L injected with 2.36 mg/L benzene, 1.75 mg/L toluene,

1.08 mg/L p-xylene, 1.09 mg/L m-xylene, 1.29 mg/L o-xylene, and

1280 mg/L chloride; samples taken up to 434 days

DO profile before

Injection, note low

DO zone at ~219 m

DO profile 32 days

after injection,

Some DO loss due to

biodegradation of B+X

Barker, 1987

Benzene and Xylene profile 32 days after injection,

note movement of B+X greatest in the low DO zone Barker, 1987

Mass of

Benzene,

Toluene,

and Xylene

Degradation

Relative to

Chloride in

a Field-

Scale Test

Chapelle

Significant mass loss

of all hydrocarbons

Only benzene

detected after

day 200, gone

at day 434

Barker, 1987

Approximate first-order rates in d-1 for BTX compounds:

Benzene 0.01 - 0.03

Toulene 0.02 - 0.05

Xylene 0.03

estimated using mass vs time data in fig. 7; Barker, 1987

Chapelle, 2000

Natural Attenuation

Of Petroleum

Hydrocarbons:

Laurens, SC,

Gasoline spill

- Piedmont Region

- Saprolite soil

(micaeous clay-sand)

- Fractured bedrock

80 feet

source

location

Chapelle, 2000

- Ground water ~6-17 ft BLS

- Hydraulic Conductivity ~ 3 ft/d

- Hydraulic Gradient ~ 0.01-0.03

- Porosity ~ 0.3

- GW velocity ~0.15 ft/d to SE

Groundwater levels,

contours, and

general direction

of flow at the

Laurens, SC site

June, 1998

Chapelle, 2000

Cross-section showing the total gasoline organic concentrations

in sediments beneath the Laurens, SC site

>100K 50-100K

10-50K

5-10K 1-5K

0.5-1K

Sediment concentration values in mg/Kg

No measured

free product

in any of the

monitoring

wells

Water table

Chapelle, 2000

Total BTEX

concentrations (in mg/L)

at the Laurens, SC site

Max C = 106,000 mg/L

at well MW-6

Benzene = 28,300 mg/L

Toluene = 66,700 mg/L

Ethylbenzene = 3260 mg/L

Xylene = <7500 mg/L

80 feet

Chapelle, 2000

DO concentrations at

the Laurens, SC site

(in mg/L)

Note reduced DO at

the location of the

highest petroleum

hydrocarbon

concentrations,

indicative of aerobic

biodegradation of

hydrocarbons

Using the biodegradation rates in the table above

(from Chapelle, 2000), calculate the time and distance

needed to reduce contaminants to below the appropriate MCL

Chapelle, 2000

Benzene Concentrations

(in mg/L)

First-order equation:

C/Co = exp(-kt)

From table 4, k for

Benzene is 0.005 - 0.01 d-1

MCL for Benzene is 5 mg/L

C/Co = 5/28,300 = 1.77E-4

time is 865 - 1,730 d

at v = 0.15 ft/d, distance

would be 130 - 260 ft 80 feet

~ 200 feet from

MW-6

Chapelle, 2000

MTBE Concentrations

(in mg/L)

First-order equation:

C/Co = exp(-kt)

From table 4, k for

MTBE is 0.001 - 0.006 d-1

at 200 feet and v = 0.15 ft/d

travel time =1330 d

and C/Co = 0.00034 - 0.26

or 4.4 to 3,350 mg/L

a 103 difference!

USGS Research Site for over 25 years

Ground Water Contamination by Crude Oil near Bemidji, Minnesota

Isabelle Cozzarelli, Barbara Bekins, and others

After the oil flowed to lower lying areas and

infiltrated, it formed 3 pools in the subsurface

~80% Oil

recovered

~300,000 L

remained

The

North Pool

of oil

has been

the

primary

focus of

research

Note GW flow is to Northeast

GW velocity ~ 0.05 – 0.08 m/d

Schematic Cross Section of North Pool

1

Unsaturated

Zone

Cross section -

Variable saturation, and over time, physical oil loss has

occurred due to dissolution and volatilization

(a)contours of oil saturation

in north pool cross section

(b) degree of oil degradation

compared with wt percent

oil

Upgradient>Downgradient

Indicates the degree of oil

degradation seems to vary

primarily with location

Effect of

oil saturation

vs location

on degradation

Upgradient Downgradient

(Spatial and Temporal Variations)

Initial crude oil composition dominated by saturated hydrocarbons (~60%) and

aromatic compounds (~35%); dominant hydrocarbons were alkanes (C6-C32)

Crude oil composition has changed over time (1983-99)

due to physical, chemical, and biological processes

Higher-number

Alkanes first

Lower-number

Alkanes next

Only recalcitrant

Compounds remain

1

2

3

4

DO in

1992-5

GW concentrations of BTEX down gradient

from the north oil pool, demonstrating

selective biodegradation of these compounds

Degradation of

T,o-X>B,E

Note depletion

of Fe3+ and

increased Fe2+

under the

oil body in the

same

location and

down gradient

Iron-reduction

dominates

where Fe3+

is at or near

background

concentration

Fe3+

Fe3+ Fe2+

Although the biogeochemical zones are relatively

stable, there is some change over time

Dissolved

Fe2+ in

1992-95

Fe2+, BTEX, and CH4 in the anoxic zone, 50 m down gradient

from oil pool center in 1993 (9322) and 1998 (9829/33)

1998

1993

Dissolved NVDOC, BTEX, CH4, Iron (II, III) at the edge of the

anoxic plume (70 m f/oil pool center) in 1993 (9320), 1997 (9701/2)

1997

1993

Fe3+ Fe2+

Benzene,

Ethylbenzene,

o-Xylene, and

1,3,5-TMB

at 50m (532b)

and 90m (530b),

between 1986

and 1996

(~ 50 m)

(~ 90 m)

Anoxic

2

Distribution of microbial types in the anaerobic plume beneath the

north oil pool; iron reducers dominate in the anoxic zone, note

reduced aerobes numbers, particularly in the methanogenic zone

Benzene

First-order degradation rates for Bemidji BTEX microcosms (black diamonds),

well data (black squares), modeling (black triangles); other studies are noted by

black circles, and grey squares, diamonds, and triangles.

Model-

GW monitoring

and simulation

of the plume

chemistry

indicated

significant

aerobic and

anaerobic

degradation

of the oil

V = Volatile; BTEX,

methane

NV = NonVolatile;

e.g. organic acids

DOC = Dissolved

Organic Carbon

(Mn2+)

(Fe2+)

Anoxic

Conditions

Saturated Zones

1 - Uncontaminated (native) GW 2 - Low DO; high DOC, DIC

3 – Anoxic; high HC, Mn2+, Fe2+, CH4 4 - Low HC, aerobic degradation

5 - Oxygenated w/ trace BTEX

Unsaturated Zones

6 - Atmospheric O2 7 - Lower O2; trace HC, CO2, CH4

8 - Anoxic, large HC, CO2, CH4 (note: HC = Hydrocarbons) 1

Generalized

Summary - Hydrocarbons are common contaminants in US aquifers

- Of these compounds, specific alkanes degrade more

quickly than others, and among aromatic compounds some

degrade much more quickly than others.

- Degradation rates vary depending on the redox conditions.

- In general, aerobic degradation rates are larger than

anaerobic rates.

- There were distinct differences in the degradation pattern

and rates of natural attenuation of hydrocarbons at the three

sites; Borden tracer test (selected hydrocarbons), Bemidji,

MN (crude oil spill), and Laurens, SC (gasoline spill).

Next

Organics: Solvents