6. organic compounds: hydrocarbons - epoc...initial crude oil composition dominated by saturated...
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6. Organic Compounds:
Hydrocarbons
General Comments
Borden, Canada, tracer test
Laurens, SC, gasoline spill
Bemidji, MN, crude oil spill
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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
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(e.g. Acetic Acid)
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Examples of Experimental and
Case Studies
of Organic Contaminant
Fate and Transport:
Hydrocarbons
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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
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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
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Benzene and Xylene profile 32 days after injection,
note movement of B+X greatest in the low DO zone Barker, 1987
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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
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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
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Chapelle, 2000
Natural Attenuation
Of Petroleum
Hydrocarbons:
Laurens, SC,
Gasoline spill
- Piedmont Region
- Saprolite soil
(micaeous clay-sand)
- Fractured bedrock
80 feet
source
location
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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
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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
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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
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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
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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
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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
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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!
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USGS Research Site for over 25 years
Ground Water Contamination by Crude Oil near Bemidji, Minnesota
Isabelle Cozzarelli, Barbara Bekins, and others
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After the oil flowed to lower lying areas and
infiltrated, it formed 3 pools in the subsurface
~80% Oil
recovered
~300,000 L
remained
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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
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Schematic Cross Section of North Pool
1
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Unsaturated
Zone
Cross section -
Variable saturation, and over time, physical oil loss has
occurred due to dissolution and volatilization
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(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
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Upgradient Downgradient
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(Spatial and Temporal Variations)
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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
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Higher-number
Alkanes first
Lower-number
Alkanes next
Only recalcitrant
Compounds remain
1
2
3
4
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DO in
1992-5
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GW concentrations of BTEX down gradient
from the north oil pool, demonstrating
selective biodegradation of these compounds
Degradation of
T,o-X>B,E
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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+
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Although the biogeochemical zones are relatively
stable, there is some change over time
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Dissolved
Fe2+ in
1992-95
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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
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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+
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Benzene,
Ethylbenzene,
o-Xylene, and
1,3,5-TMB
at 50m (532b)
and 90m (530b),
between 1986
and 1996
(~ 50 m)
(~ 90 m)
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Anoxic
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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
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Benzene
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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.
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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
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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
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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).
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Next
Organics: Solvents