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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

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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.

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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 delta13C 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 delta13C 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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