U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
Evolution of the Contaminant Plume in an Aquifer Contaminated
with Crude Oil, Bemidji, Minnesota
by
Mary Jo Baedecker (U.S. Geological Survey, Reston, Va.), Isabelle
M. Cozzarelli (U.S. Geological Survey, Reston, Va.), Philip C. Bennett (University
of Texas, Department of Geological Sciences, Austin, Texas), Robert P. Eganhouse
(U.S. Geological Survey, Reston, Va.), and Marc F. Hult (Macalester College,
Department of Geology, St. Paul, Minn.)
Contents
Abstract
A long-term investigation of the geochemistry of a contaminant plume
in an aquifer where crude oil floats on the water table indicates that the
size of the plume of dissolved constituents has changed little over a 8-year
period. Even though the geochemical reactions have changed over time and
part of the plume has become anoxic, the biodegradation of dissolved hydrocarbons
under oxic and anoxic conditions has prevented the hydrocarbons, in concentrations
above Federal maximum contaminant levels, from moving more than 137 meters
downgradient from the oil body. Another factor that helps contain the plume
is the presence of silt layers of low hydraulic conductivity near the oil
and the top of the saturated zone. The findings of this study support the
conclusions that significant concentrations of petroleum-type hydrocarbons
can be attenuated or removed from aquifers by natural hydrologic and biogeochemical
processes.
INTRODUCTION AND OVERVIEW OF PROJECT
An investigation of the effects of a crude-oil spill on an
aquifer was undertaken near Bemidji, Minnesota, as part of
the Toxic Substances Hydrology
Program of the U.S. Geological Survey (USGS). An underground
pipeline carrying crude oil ruptured in 1979, and the oil
sprayed over the land surface (referred to as the spray zone
in fig. 1). After partial removal of the product, about 410 m3
of crude oil remained at the site (Hult, 1984). Some of the crude
oil reached the ground water and is floating on the water
table and a contaminant plume has developed in the aquifer downgradient
from the oil body. A preliminary investigation and site characterization
that began in 1983 was expanded in 1985 to examine (1) the
fate of the crude oil, (2) development of the contaminant plume,
(3) factors that affect the transport of chemical constituents,
and (4) the effect of biogeochemical processes on
aquifer solids. The scope of the work was expanded again in 1989
to investigate (5) the movement of the oil body, (6) partitioning
at the oil/water interface, and (7) solute transport.
A summary of the major topics that have been investigated at the site
is listed in table 1. The references are not inclusive and additional information
can be found in publications referred to in the table and in USGS reports
(see Mallard and Aronson, eds., 1991, for the most recent compilation of
papers on this work). Many of the studies mentioned above are continuing
and investigations are being expanded to include colloidal transport, sorption
characteristics of the sediment, and microbial activity in the aquifer.
This paper describes the evolution of the contaminant plume and discusses
the importance of assessing natural processes for removing petroleum-type
hydrocarbons from ground water. The geochemistry of the plume has been examined
in detail for a 8-year period to document the changes in the aquifer caused
by to the alterations and transport of hydrocarbons.
(50k)
Figure 1. Map of Crude-oil site near
Bemidji, Minnesota, showing the location of the pipeline that ruptured, approximate
location of the oil body, area over which oil was sprayed (spray zone), and
locations of wells.
Table 1.
Topics of investigation and selected references for research at the Bemidji,
Minnesota site from 1984-93
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DESCRIPTION OF SITE AND METHODS
The study site is located in the Bagley outwash plain Bemidji, Minnesota.
The surficial aquifer is about 20 m thick and consists of sand and gravel
outwash overlying till. Thin, discontinuous silt layers are interbedded
with sand near the water table (Franzi, 1988). The water table, in the part
of the aquifer studied in this report, is 6 to 10 m below land surface.
Only the upper 5 to 8 m of the aquifer has been affected by crude oil. Locally
ground water flows to the northeast and discharges to a small lake. The
average linear flow velocity of water in the aquifer is 0.1 to 0.5 m/d,
but velocities as low as 0.05 m/d were measured in the fine-grained silty
material (Bennett and others, 1993). The aquifer is mostly quartz with 30
percent feldspar, 5 percent carbonate, and less than 5 percent clay minerals
(Bennett and others, 1993).
Piezometers were installed by hollow-stem auger
and the wells were screened in depth intervals of
0.15, 0.61 or 1.52 m. The screens were stainless steel
and the casings were polyvinyl chloride. Water
samples were collected with submersible pumps for
analyses of inorganic constituents and dissolved
organic carbon and with a Teflon bailer for analyses
of organic constituents. Methods of chemical
analyses presented in this paper are found in
Eganhouse and others (1993) for hydrocarbons; in Baedecker
and Cozzarelli (1992) for dissolved oxygen (DO),
ferrous iron (Fe2+), volatile dissolved organic
carbon (VDOC), methane (CH4) and carbon isotopes;
and in Baedecker and others (1993) for
other analytical methods and field sampling methods.
DEVELOPMENT OF THE CONTAMINANT PLUME
Plume development depends primarily on aquifer transmissivity, the extent
of volatilization and solubilization of the contaminants, the amount of
sorption, and the degradability of the dissolved constituents. The main
components of crude oil at the Bemidji site are normal, alicyclic, and aromatic
hydrocarbons. The most soluble aromatic hydrocarbons, benzene and the alkylbenzenes,
are known to degrade by microbial processes (Atlas, 1984). When the plume
developed shortly after the spill, concentrations of oxygen present in the
aquifer were sufficient to oxidize the hydrocarbons by aerobic microbial
processes. After oxygen was depleted, the major processes that oxidized
hydrocarbons in the anoxic zone were iron and manganese reduction and methanogenesis
(Baedecker and others, 1989, 1993). It was demonstrated that these processes
are linked with hydrocarbon oxidation in laboratory experiments using pure
cultures for iron reduction (Lovely and others, 1989) and mixed cultures
for methanogenesis (Grbic-Galic and Vogel, 1987). Aerobic oxidation of hydrocarbons
continues to be a major process at the edges of the anoxic zone where hydrocarbons
come in contact with oxygenated ground water.
The plume at the Bemidji site can be defined by the distributions
of VDOC; specific hydrocarbons (Eganhouse and others, 1993);
geochemical indicators such as pH and DO (Baedecker and others,
1993); and inorganic solutes such as calcium (Bennett and others,
1993). The distributions of DO, VDOC, and Fe2+ in 1987
and 1992 are shown in fig. 2. Oxygenated ground water moved upward
in the middle of the anoxic zone in 1992. Concentrations of DO
ranged from 0.05 to 2.4 mg/L in a 4-m-thick zone below the
top of the saturated zone that extended 70 m downgradient from
the anoxic zone. These concentrations were less than background
DO concentrations of 8 mg/L. The distributions of VDOC were
similar in 1987 and 1992 as shown in fig. 2, but the zone
having VDOC concentrations greater than 10 mg/L has become smaller,
the plume has become less contaminated in the middle, and it
is sinking at the leading edge. A major change from 1987
to 1992 was observed for the leading edge of the 10 mg/L contour
of the Fe2+ plume which has moved downgradient at a rate of about
6 m/yr.
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Figure 2. Hydrologic sections (A-A' in
fig. 1) of the contaminant plume showing
the anoxic zone, and distribution of volatile dissolved organic carbon and ferrous
iron in 1987 and 1992. The water table is 6 to 10 meters below land surface.
The change over 8 years (1984-1992) in the chemical
composition of the plume near the oil body is
shown in fig. 3 for Mn2+, Fe2+, CH4,
and the 13C of the total inorganic carbon.
The data are for a sampling point in the plume at
the downgradient edge of the oil (location
533 on fig. 1), and the screened interval is about
1 m thick at the water table. These results indicate
that Mn(IV) reduction preceded Fe(III) reduction
and methanogensis (Baedecker and others, 1993). The
change in measurements of the total inorganic carbon
toward heavier numbers over time is due to the formation
of methane. Concentrations of Mn2+ increased to 0.12
mM and then decreased to 0.01 mM, whereas
Fe concentrations increased by a factor of 30 to 0.92
mM, and concentrations of CH4 increased from
the detection limit (0.006 mM) to 1 mM (fig. 3). Concentrations
of both Fe2+ and CH4 have decreased slightly
in recent years. The data indicate that Mn(IV) reduction
has become a less important reaction over time near
the oil body. The data suggest that both Fe(III)
reduction and methanogensis continue to be major
reactions in the anoxic plume.
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Figure 3. Concentrations of dissolved
ferrous iron, manganese, and methane, and 13C
measurements for total inorganic carbon for the years 1984-92 at
location 533 (see fig. 1).
Modified from Baedecker and others, 1933.
Concentrations of total benzene and alkylbenzenes in ground water were
10.4 to 18.4 mg/L, respectively, at three sampling locations within 1 meter
of the oil body (1990 and 1992 data). Concentrations of hydrocarbons in
the ground water varied over time and spatially near the oil body, probably
because small oil stringers were associated with the sediment near the source.
Oil stringers on a scale of millimeter to a few centimeters were observed
in sediment cores obtained downgradient from the oil body. Also, ground
water pumped near the oil body had an oily sheen even where a separate fluid
phase was not encountered.
At location 518 (fig. 1), 56 m downgradient from the middle
of the oil body, concentrations of benzene and alkylbenzenes
were more consistent over a 5-year period (fig. 4) than they
were at locations close to the oil body. Benzene was the
major hydrocarbon present, and its concentration decreased over
time, whereas concentrations of the other hydrocarbons were low
but increased over time. Concentrations of toluene were equal
to or less than 0.007 mg/L and concentrations of the
xylenes, the C3-benzenes and the C4-benzenes were
equal to or less than 0.35 mg/L. At locations beyond 137 m
the oil body, concentrations of hydrocarbons were less
than Federal drinking water standards.
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Figure 4. Concentrations of benzene and
alkylbenzenes at location 518 (see fig. 1)
for the years 1987, 1990, and 1992.
DISCUSSION AND CONCLUSIONS
Although the geochemical processes have changed over time, the plume
has not migrated as far as predicted considering the ground water flow velocities
and sorption constants for these compounds (Baedecker and others, 1993).
A factor that may affect migration of contaminanted ground water is the
presence of discontinuous silt layers (Franzi, 1988; Baehr and Hult, 1991)
that have low hydraulic conductivity. Thus, the flow velocities in these
zones may be low and retard the movement of fluids. However, the primary
reason that the anoxic plume has not farther migrated and that only trace
concentrations of hydrocarbons are found in ground water farther than 137
m downgradient from the plume is that the hydrocarbons have biodegraded
under oxic and anoxic conditions. The rate of removal of organic contaminants
by natural biodegradative processes and the factors that affect these rates
are important considerations in making decisions concerning cleanup of contaminated
ground water. Biodegradation of petroleum-derived hydrocarbons in aerobic
and sub-oxic environments is generally considered a more efficient attenuation
mechanism than is biodegradation in anoxic environments. However, biodegradation
in anaerobic environments also may remove significant amounts of hydrocarbons
from ground water. Estimated half-lives of benzene, toluene, and the xylenes
in methanogenic field sites were 0.5 to 3.8 years at five sites (Barker
and Wilson, 1992). Natural processes may account for the removal of significant
quantities of petroleum hydrocarbons in the subsurface. Additional work
needs to be done to determine ground-water flow velocities and rates of
intrinsic degradation on small scales in contaminated aquifers to evaluate
natural processes as part of long-term remedial action programs.
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