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Geochemical 'Indicators of Intrinsic
<br /> Bioremediation
<br /> by Robert C.Borden", Carlos A. Gomezb,and Mark T.Becker`
<br /> I ,
<br /> Abstract
<br /> A detailed field investigation has been completed at a gasoline-contaminated aquifer near Rocky Point,NC,to examine
<br /> possible indicators of intrinsic bioremediation and identify factors that may significantly influence the rate and extent of
<br /> bioremediation.The dissolved,plume of benzene,toluene,ethylbenzene,and xylene(BTEX)in ground water is naturally
<br /> degrading.Toluene and o-xylene are most rapidly degraded followed by m-,p-xylene,and benzene.Ethylbenzene appears to
<br /> degrade very slowly under anaerobic conditions present in the center of the plume.The rate and extent of biodegradation
<br /> appears to be strongly influenced by the type and quantity of electron acceptors present in the aquifer.At the upgradient edge
<br /> of the plume,nitrate,ferric iron,and oxygen are used as terminal electron acceptors during hydrocarbon biodegradation.
<br /> The equivalent of 40 to 50 mg/1 of hydrocarbon is degraded based on the increase in dissolved CO2 relative to background
<br /> ground water.Immediately downgradient of the source area,sulfate and iron are the dominant electron acceptors.Toluene
<br /> and o-xylene are rapidly removed in this region.Once the available oxygen,nitrate,and sulfate arb consumed,biodegrada-
<br /> tion is"ted and appears to be controlled by mixing and aerobic biodegradation at the plume-fringes.
<br /> Introduction and geochemistry. At present, there are few well-_
<br /> Accidental releases of gasoline and other petroleum documented, full-scale demonstrations of intrinsic bio-
<br /> products from underground storage tanks(USTs)are one of remediation, although there has been significant research
<br /> the most common causes of ground-water contamination. into the processes that control the biodegradation of dis-
<br /> There are currently about 2 million underground tanks solved hydrocarbon plumes(Borden et al., 1986; Barker et
<br /> storing gasoline in the U.S. with 90,000 confirmed releases al., 1987;Hult, 1991;Kemblowski et al., 1987;Chiang eta].,
<br /> reported in 1989 and 1990(Office of Underground Storage 1989; Wilson et al., 1993).
<br /> Tanks,1990).However,field evidence suggests that many of In this work,a single site contaminated with gasoline
<br /> these spills naturally biodegrade before contaminants reach was examined for a twofold purpose-(1)to improve under
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<br /> a drinking water receptor(Hadley and Armstrong,1991).If standing of those factors that control the biodegradation of
<br /> i this degradation does occur,these spills can be thought to dissolved BTEX in ground water, and (2) to identify
<br /> j undergo intrinsic bioremediation. parameters that may be used as indicators of intrinsic
<br /> The basic concept behind intrinsic bioremediation is to bioremediation.
<br /> use the capacity of naturally occurring microorganisms to
<br /> degrade contaminants that have been released into the sub- Hydrocarbon Biodegradation
<br /> surface and at the same time minimize risks to public health The ability of microorganisms to degrade a wide vari-
<br /> and the environment. This approach requires an under- ety of hydrocarbons is well-known. In an early review,
<br /> standing of those factors that influence the biodegradation Zobell(1946)identified over 100 microbial species from 30
<br /> i capacity of an aquifer and the potential human and envi- genera that could degrade some type of hydrocarbon.These
<br /> Jronmental risks.Ongoing research has shown that an aqui- organisms are widespread in the environment and occur in
<br /> l fer's assimilative capacity depends on the metabolic capabii- fresh and salt water,soil,and ground water.Litchfield and
<br /> ities of the native microorganisms,the aquifer hydrogeology, Clark(1973)analyzed ground-water samples from 12 differ-
<br /> ent aquifers throughout the United States that were contam-
<br /> inated with hydrocarbons.These workers found hydrocarbon-
<br /> utilizing bacteria in all samples at levels up to 1.0 X 106 cells
<br /> "Corresponding Author,North Carolina State University, per milliliter of ground water.After a gasoline spill in south-
<br /> Box
<br /> 7908,Raleigh,North Carolina 27695-7908. ern California,Mckee et al.(1972)found 50,000 hydrocarbon-
<br /> e -�
<br /> American Agriculture Services, P.O. Box 1293, Cary, degrading bacteria per milliliter or higher in samples from
<br /> North Carolina 27512. �� gm�Be_ ��
<br /> wells cont4 tr rrunated
<br /> Dames&Moore,l2 Commerce Dr.,Cranford,New Jersey
<br /> 0,016. ,hell had only 200 organisms per milliliter. Ehrlich et al.
<br /> . .
<br /> Received September 1993, revised'March 1994, accepted 41985) observed elevated numbers of sulfate-reducing and
<br /> April 1994. methanogenic bacteria in a jet-fuel-contaminated aquifer.
<br /> LI180 Vol.33,No.2—GROUND WATER—March-April 1995
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