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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