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Potential for bioremediation of petroleum hydrocarbons ingroundwater under cold climate conditions: A review

Dale Van Stempvoorta,, Kevin Biggarb

aNational Water Research Institute, PO Box 5050, Burlington, Ontario, Canada L7R 4A6b BGC Engineering Inc. Suite 207-5104, 82 Avenue, Edmonton, AB, Canada T6B 0E6

Received 10 November 2006; accepted 22 June 2007

Abstract

Globally, bioremediation is a common choice for remediation of petroleum hydrocarbon-contaminated sites. For application atcold climate sites, bioremediation approaches are appealing because they have potential to be more efficient and cost-effective thanalternative, more energy intensive approaches. Several bioremediation approaches have been reported to be successful for

petroleum hydrocarbon-contaminated soils at cold climate sites. In contrast, there are relatively few publications on applications ofbioremediation for petroleum-contaminated groundwater at cold climate sites. Most of the existing relevant groundwater studieswere conducted at sites with either no permafrost, or with sporadic to discontinuous permafrost. To date, the majority of coldclimate groundwater investigations were at fuel spill sites; few studies on bioremediation of dissolved hydrocarbon plumes derivedfrom crude oil or gas condensate have been published. Some studies reported that extents of hydrocarbon plumes in groundwater

were limited by natural attenuation, including intrinsic bioremediation. At other sites, oxygenation of groundwater or amendmentswith nitrate were reported to be successful techniques for enhancing biodegradation of petroleum hydrocarbons. Both aerobic andanaerobic processes appear to be important at these sites. Based on three case studies, bioremediation (in situ or ex situ) may befeasible for sites with extensive permafrost. Further research and field demonstrations are required to establish or confirm theapplicability of bioremediation technologies to clean up hydrocarbons in groundwater in various hydrogeological settings at coldclimate sites. 2007 Elsevier B.V. All rights reserved.

Keywords: Bioremediation; Petroleum; Hydrocarbons; Groundwater; Cold climate, Natural attenuation

1. Introduction

Petroleum hydrocarbon pollution is a pervasive,global problem. The main sources of petroleumcontamination at cold climate terrestrial sites can beattributed to spills and leaks of crude oil and natural gascondensate as consequences of oil and gas production(e.g., Alaska USA; Canada, Russia), the transport of

petroleum products by pipelines and other means, andthe above-ground or underground storage of fuel(Margesin and Schinner, 1999). The widespread use ofpetroleum fuel and other products in cold climateregions has led to contamination of soil and groundwa-ter at many sites. In these regions, the cold temperaturesand remote locations pose additional challenges forremediation.

This paper addresses a science gap by reviewinginformation on the feasibility to apply bioremediationfor petroleum-contaminated groundwater at cold

Available online at www.sciencedirect.com

Cold Regions Science and Technology 53 (2008) 1641www.elsevier.com/locate/coldregions

Corresponding author.E-mail address: [email protected] (D. Van Stempvoort).

0165-232X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.coldregions.2007.06.009
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climate sites. Previous reviews of information on thebiodegradation of petroleum hydrocarbons in ground-water (e.g., Suarez and Rifai, 1999) have generally notaddressed cold climate sites specifically. Reviews ofbioremediation techniques for petroleum hydrocarbon

contamination at cold climate sites have generallyfocused on aerobic biodegradation processes by cold-adapted microorganisms in soils (e.g., Atlas andCerniglia, 1995; Margesin and Schinner, 2001; Filleret al., 2006).

In contrast to soil, groundwater lacks an air phase andthus the availability of oxygen as an electron acceptor isgreatly diminished, limited by the low aqueoussolubility of oxygen (approximately 12mg/L at 5C).This typically results in anoxic conditions withinpetroleum-contaminated plumes in groundwater. How-

ever, some anaerobic microorganisms can degradehydrocarbons using other electron acceptors, such assulfate, nitrate or ferric iron (see Section 2). Hence theintrinsic (i.e. unassisted) biodegradation of petroleumhydrocarbons in groundwater may be dominated byanaerobic processes (Lovley, 1997; Weidemeier et al.,1999).

The objective of this review is to address the abovenoted science gap by summarizing available informationregarding bioremediation of petroleum-contaminatedgroundwater at cold sites, including:

(1) information regarding aerobic and anaerobicbiodegradation processes at such sites;

(2) evaluation of the potential to utilize such process-es to remediate the groundwater;

(3) identification of key research needs for advancingsuch bioremediation applications for groundwaterat cold climate sites.

1.1. Recent expansion of bioremediation technologies

Bioremediation has been defined as a managed or

spontaneous process in which biological, especiallymicrobial, catalysis acts on pollutant compounds,thereby remedying or eliminating environmentalcontamination (Madsen, 1991). This paper reviewsinformation relevant to the applicability of bioremedi-ation technologies for petroleum-contaminated ground-water under cold climate conditions.

Bioremediation includes a range of environmentalremediation techniques/technologies that were intro-duced largely in the 1980s and 1990s. Since thenapplications have grown rapidly world-wide. In recentdecades, bioremediation has become one of the mostcommon approaches used to remediate petroleum

hydrocarbon-contaminated sites. Bioremediation tech-nologies are appealing because they offer the potentialfor significant cost savings compared to conventionalremediation technologies, such as excavation anddisposal in landfills, excavation and thermal treatment,

and pump and treat. Proponents have argued thatbioremediation technologies are safer and less disrup-tive (e.g., in situ techniques) than some of theconventional technologies; the latter are often notfeasible or practical at remote sites in the north.

Table 1 provides a list of some bioremediation tech-niques to treat groundwater, as well as some alternativephysical/chemical/thermal approaches, including a briefdefinition of each. Explanations of various bioremedi-ation approaches and technologies are provided by Atlas(1995a,b,c, 1997); Sikdar and Irvine (1998), Downey et

al. (1999); Weidemeier et al. (1999); Suthersan (2001);Hughes et al. (2002) and others. Active bioremediationtechnologies accelerate the reduction or elimination ofcontaminants through environmental modification(Atlas, 1995c; Margesin and Schinner, 1999). Passivebioremediation technologies make use of naturalbiodegradation processes in soil and groundwater. Exsitu bioremediation technologies involve excavation ofcontaminated soil or removal of contaminated ground-water for treatment on- or off-site, whereas in situtechnologies provide remediation of contaminated soilor groundwater in place (Riser-Roberts, 1998). Biore-

mediation approaches may also incorporate physical andchemical processes along with biological processes.

For groundwater applications, in situ bioremediationapproaches include biosparging with air or oxygen,introduction of specially designed oxygen releasingcompounds, liquid delivery of nutrients and/or electronacceptors such as H2O2 or nitrate, and intrinsic(naturally-occurring) bioremediation (sometimes re-ferred to as monitored natural attenuation). Someremediation systems combine existing proven techni-ques (physical and/or chemical and/or biological). For

example, bioslurping (multiphase extraction), devel-oped in the 1990s, combines vacuum removal of freeproduct near the water table in combination with in situbioventing (Riser-Roberts, 1998 p. 105).

Biostimulation and bioaugmentation are activebioremediation approaches that can be applied underboth ex situ and in situ conditions (cf. Table 1). Theseinvolve the addition of essential nutrients for enhancingthe existing natural bacterial population (biostimulation)or the addition of cultured bacteria with specifichydrocarbon-degrading potential (bioaugmentation).

In addition to climate/temperature factors, which arethe focus of this review, there are many other site-

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specific factors that may affect biodegradation pro-cesses, such as: presence of a hydrocarbon-degradingmicrobial population, the chemical composition of theoil/petroleum source, availability of oxygen or otherelectron acceptors, nutrient supply, presence of toxicor inhibiting chemicals, and characteristics of the soil/geological conditions. This report summarizes theresults from various studies that have indicated theroles that these various factors play at cold climatesites.

1.2. Definitions

In this review:

The term hydrocarbons refers specifically to petro-leum hydrocarbons, and the term treatmentshould beconsidered as being synonymous with remediation.

A cold climate site is one at which the average annualair temperature is 8C, and where measuredgroundwater temperatures are typically 10C or

Table 1Ex situ and in situ physical/chemical and biological remediation approaches for groundwater

Ex situ groundwater treatment In situ groundwater treatment

Technique Definition Technique Definition

Physical/chemical processes Physical/chemical processes

Air stripping Volatile organics are partitioned from extractedground water by increasing the surface area ofthe contaminated water exposed to air. Aerationmethods include packed towers, diffusedaeration, tray aeration, and spray aeration. a

Treatment walls/passive reactivebarriers

Barriers allow the passage of water while causing thedegradation or removal of contaminants. a

Carbonadsorption

Removal of hydrophobic organic contaminantsfrom aqueous phase to carbon by physicaland chemical forces. b

Air sparging The injection of air below the water table in order toinduce volatilization of contaminants into the unsaturatedzone, which can be removed by soil vapor extraction. b

Phasefiltration/separation

Use of filter membranes and/or conventionaloil-water separator technology to removenonaqueous phase/emulsions of hydrocarbonsfrom water.

Steam sparging/flushing

Steam is forced into an aquifer through injection wells tovaporize volatile and semivolatile contaminants, whichare removed by vacuum extraction in the unsaturatedzone and then treated. c

Chemical oxidation Bringing chemical oxidants (various materials) into

contact with subsurface contaminants to remediate thecontamination.d

Hydrofracturingenhancement

Injection of pressurized water through wells to crack lowpermeability and over-consolidated sediments; cracks arefilled with porous media that serve as substrates for

bioremediation or to improve pumping efficiency. a

Biological processess e

Constructedwetlands

Use of natural geochemical and biologicalprocesses inherent in an artificial wetlandecosystem to accumulate and removecontaminants from influent waters. a

Intrinsicbioremediation

Unmanipulated, unstimulated, non-enhanced biologicalremediation of an environment; i.e. natural attenuation. f

Bioreactors A contained vessel in which biologicaltreatment takes place. f

Biosparging The injection of air or specific gases below the watertable to enhance bacterial activity for remediation. c

Phytoremediation The use of natural plants to remove contaminants throughbioaccumulation or through enhancing biodegradation.b

Bioslurping Combines vaccum removal of petroleum hydrocabonfree product with in situ bioventing. Designed forremoval of free-floating LNAPL on the water table aswell as residual product in the vadose zone. b

Biofilter (groundwater) Refers to treatment of groundwater via passage through abiologically active area in the subsurface. f

a Van Deuren et al., 2002.b Riser-Roberts, 1998.c USEPA, 2004b.d USEPA, 2004a.e Most of the biological techniques except intrinsic bioremediation include biostimulation, and some bioaugmentation, as defined in the text.f Hazen, 1997.

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lower these values, which are slightly higher thanthose used in a recent review by Van Stempvoort andGrande (2006), were selected to include severalrelevant groundwater studies in the Great Lakesregion of North America and the Baltic region of

Scandinavia/Europe, where annual air temperaturesare typically in the range 48C.

Permafrost is defined as subsurface soil, sediment orrock having a temperature persistently below 0C for aperiod of at least two years (Williams and Smith, 1989);it generally occurs in areas with mean annual air tem-peratures (MAAT) below 1.5C (Vidstrand, 2003).

Frozen groundincludes soil or rock in which part or allof the pore water has turned into ice, whereas unfrozengrounddoes not contain any ice (National Snow andIceData Center, 2006). In thesubsurface, theboundary

between liquid and frozen water is not sharp. Soil-water-ice systems contain liquid water as thin films onsoil/mineral surfaces at temperatures well below 0C(van Everdingen, 1990; Vidstrand, 2003).

The active layer is a near-surface zone of groundabove permafrost that changes seasonally fromfrozen ground to unfrozen ground (Fig. 1).

A talik is a localized layer or body of ground thatremains unfrozen in a permafrost area as a result of athermal, hydrological, hydrogeological, or hydro-chemical anomaly.

A thaw bulb is a zone of thawed ground below or

surrounding a man-made structure placed on or inpermafrost that is maintained at temperatures above0C (National Snow and Ice Data Center, 2006)(Fig. 1).

Groundwater refers to the saturated or phreaticzone, including the capillary fringe above the watertable, where pores are saturated with water.

Suprapermafrost water occurs in unfrozen groundabove permafrost; it may be present in active layers,taliks and/or thaw bulbs. At sites underlain by zonesof continuous permafrost, the suprapermafrost water

that is particularly vulnerable to hydrocarbon con-tamination may include groundwater in taliks or thawbulbs, or at the base of the active layer (i.e., seasonal).

Intrapermafrost water occurs in unfrozen groundwithin permafrost; it may be present in taliks and/orthaw bulbs.

1.3. Applications of hydrocarbon bioremediation

technologies at cold sites

There are many cold climate terrestrial sites where soiland groundwater are contaminated with hydrocarbons,largely the result of spills and leaks at petroleum

production, transmission and storage facilities. Whileapplications of bioremediation at cold climate sites haveexpanded in recent years, there is limited agreement as towhich bioremediation technologies are effective undervarious cold climate conditions. Some have questioned

whether bioremediation approaches are suitable optionsfor cold climate sites because of their low temperatureregime (Mohn et al., 2001). Studies have generally shownthat the microbial degradation of hydrocarbons tends to beslower under cold temperature conditions, and that it istypically negligible under frozen conditions (e.g., Atlas,1985). However, a recent study by Rike et al. (2005) hasindicated some hydrocarbon degradation can occur attemperatures ranging from 2 to 6C within seasonallyfrozen ground. At sites which contain permafrost, thegroundwater temperatures are typically lower than 5C,

either seasonally or on a year-round basis. Although theannual fluctuations in groundwater temperatures aretypically less drastic than those in the overlying soil/vadose zone, at some sites thin zones saturated withsuprapermafrost water occur only in the warm seasoneach year, at the base of the active layer. Such extremeconditions may jeopardize the successful application ofbioremediation technologies. Even at relatively warmsites, the intrinsic bioremediation of hydrocarbon-con-taminated groundwater may take considerable time. Slowrecoveries may be acceptable at some sites, depending onthe inferred risks to receptors (adjacent water bodies, etc.),

and the land use. However, in other cases, engineeredbioremediation approaches may be more appropriate tospeed up the process through the supply of electronacceptors, nutrients, and heat.

2. Microbial degradation of hydrocarbons at cold

climate sites

Many microorganisms have metabolic capabilities todegrade petroleum hydrocarbons (e.g., Atlas, 1995b;Spormann and Widdel, 2000; Chakraborty and Coates,

2004). Hydrocarbon-degrading microorganisms havebeen reported to increase from less than 0.1% of totalbacterial population in pristine environments to up to100% of the total microbial population if the environ-ment is exposed to hydrocarbon contamination (Atlas,1981). Hydrocarbons ranging from C10 to C26 andaromatics of low molecular weight are considered themost readily degraded (Atlas, 1995b), whereas morecomplex molecular structures are generally moreresistant to biodegradation.

Early reviews of the bioremediation of hydrocarbons(e.g., Atlas, 1981, 1985) focused mainly on marineenvironments and almost exclusively on aerobic

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oxidation pathways. Later reviews reported thatanaerobicprocesses contribute to the biodegradation of petroleumhydrocarbons (e.g., Spormann and Widdel, 2000; Phelpsand Young, 2001; Chakraborty and Coates, 2004).Zwolinski et al. (2001) concluded that our understandingof the diversity of the microbial organisms and commu-

nities involved in hydrocarbon degradation is still verylimited. With respect to groundwater, reviewers haveprovided information on biodegradation of petroleumhydrocarbons under both aerobic and anaerobic condi-tions (e.g., Suarez and Rifai, 1999; Weidemeier et al.,1999; Lovley, 2000; Chapelle, 2001). But these reviewshave generally provided little if any information on howthese processes are affected by temperature, or whetherbioremediation of hydrocarbons in groundwater is afeasible approach at cold climate sites.

2.1. Cold adapted microorganisms

Microorganisms adapted to environments subjectedto low temperatures are generally classified as psychro-philes (cold preference) or psychrotrophs (cold tolerant).Morita (1975) defined psychrophiles as organisms thathave an optimal temperature for growth of 15C orlower, a maximum growth temperature of about 20C,and a minimum growth temperature of 0C or lower. Bycontrast, Morita used the term psychrotrophs to referto other organisms that grow at these low temperatures(015C), though this is not the optimal temperaturerange for them. Subsequent studies have sometimes

referred to psychrotrophs as psychrotolerant micro-organisms (e.g., Nedwell and Rutter, 1994).

Russell (1990) reviewed information on how somemicroorganisms have adapted to cold temperatureconditions. These microorganisms tend to have cellmembranes that are enriched in unsaturated lipids and

depleted in branched chain lipids, which may be amechanism to maintain fluid properties at low tempera-tures. In a more recent review, Nedwell (1999)suggested that stiffening of lipids in the membranes atlower temperatures might account for the decreasedaffinity of the microorganisms for substrates.

Recent studies have indicated that some microorgan-isms are adapted to temperatures below 0C, includingpermafrost (Steven et al., 2006). For example, Rivkinaet al. (2000) conducted a laboratory study of themetabolism and growth of bacteria from Siberian

permafrost at temperatures between 5 and 20C. Insediment samples, metabolism (uptake of 14C-labelledacetate) declined with temperature, and was very slowbut still measurable at temperatures of15 to 20C.During growth, the minimum doubling times of thebacterial populations ranged from 1day (5C) to 20days(10C) to 160days (20C). At the end of the growthphase, the stable populations that were reached (i.e.,stationary phase) declined with temperature, following acurve similar to that of the predicted thickness ofremaining unfrozen water for the same temperaturerange. This suggests that the stationary phase, which isgenerally considered to be reached when the availability

Fig. 1. Conceptual model of groundwater-permafrost relationships at cold climate sites. Modified aftervan Everdingen (1990) and Vidstrand (2003).Dashed arrows indicate flow of water.


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of nutrients becomes limiting, was controlled by thediffusion through the layers of unfrozen water, whichdecreased in thickness with decreasing temperature.Films of unfrozen water in permafrost are apparentlyessential for the mass transfer of nutrients and metabo-

lites, and thus the survival and growth of microorganisms(Steven et al., 2006).

Viable microorganisms isolated from permafrost tendto be psychrotrophs rather than psychrophiles, andinclude both aerobic and anaerobic bacteria, such assulfate-reducers and methanogens (Steven et al., 2006).Analyses of gases extracted from the pore spaces ofpermafrost and redox potentials measured in permafrost(Steven et al., 2006) suggest that both aerobic andanaerobic conditions are common.

2.2. Microbial degradation of hydrocarbons at coldtemperatures

Many of the laboratory experiments that havedemonstrated the biodegradation of hydrocarbons bymicroorganisms have been conducted at 20 to 35C(Margesin and Schinner, 1997). Similarly, many of thefield tests and demonstrations of hydrocarbon bioreme-diation have been conducted under either warm ortemperate climate conditions. Typically, for such fielddemonstrations and applications, temperature data havenot been collected and/or reported.

In one of the earliest studies to indicate the potentialfor microbial degradation of hydrocarbons at lowtemperatures, Zobell (1973) reported on laboratorymicrocosm experiments in which samples of crude oil-contaminated soil or water from the tundra region ofnorthern Alaska were used to inoculate mixtures ofmineral salts media, while being incubated at tempera-tures of1.1 (depressed freezing point), 4 or 8C. At4C and 8C, most microcosms had strong bacterialactivity based on the visible emulsification anddisappearance of the oil phase over two to four weeks.

In tests at 1.1C, growth of microorganisms wasdetected after a few days, and emulsification of oiloccurred within two or three weeks, associated withconsumption of O2. Under the conditions of Zobell'sexperiments, growth rates of microorganisms at1.1Cand 4C were indistinguishable, whereas the rate at 8Cwas approximately double that of the lower tempera-tures. Over a ten week period, the average amounts ofvarious crude oils degraded in the microcosm tests were:55% at 8C, 44% at 4C and 38% at 1.1C. Zobellconcluded that psychrophilic bacteria were active intheir tests, and reported that many more varieties ofbacteria were present in the cultures they grew at 4 and

8C than in the 1.1C tests. Based on a stoichiometriccomparison of O2 uptake rates and disappearance ofhydrocarbons, Zobell suggested that some of thehydrocarbons were only partially degraded in theirtests. Based on the detection of psychrophilic

hydrocarbon-degrading bacteria in samples from naturaloil seeps by Zobell (1973), Agosti and Agosti (1973)inferred that it might be useful to provide cultures fromthese seeps as inoculants to clean-up pipeline oil spills.

Atlas (1981, 1985) summarized the earliest labora-tory studies which collectively indicated that hydrocar-bon biodegradation can occur at significant rates at lowtemperatures. For experiments with either soil or beachsand, significant biodegradation rates were reported fortemperatures as low as 36C, but no degradation wasobserved below 0C. Q10 values were sometimes

calculated to indicate changes in biodegradation rateswith temperature: For a given temperature T, Q10 is theratio of the rate of biodegradation at T plus 10C to therate at T. Q10 values were reported for experiments withbeach sand or seawater, with temperature ranges of 6 to26C or 5 to 20C. The results suggested that the rate ofhydrocarbon biodegradation decreases by a factor of 2to 4 for every 10C decline in temperature. However, asAtlas (1981) pointed out, the influence of temperatureon the biodegradation rate is more complex than theseapparent Q10 values. The influence of temperature onthe biodegradation rate is interactive with other factors,

such as the composition of the hydrocarbon mixture, themicrobial community, the physical state of the hydro-carbons, the amount of water and nutrients present, andthe availability of oxygen (or other electron acceptors).

In general, the pioneering laboratory studies reviewedby Atlas indicate that psychophilic or psychrotrophicmicroorganisms growing at low temperature conditionswere not as capable at degrading some of the hydrocarbonfractions (e.g., isoprenoids, branched or aromatic groups)as mesophilic microorganisms growing at highertemperatures. Atlas (1991) and Atlas and Bartha (1992)

summarized the main limiting factors that influencebiodegradation of petroleum hydrocarbons in coldclimates. Atlas (1991) identified two main categoriesinto which the rate-limiting factors can be divided; 1) thephysiological capabilities of the indigenous hydrocarbon-degrading microbial population, and 2) the abioticvariables that affect the activity (i.e., growth andmetabolism) of that microbial population.

A review by Margesin and Schinner (2001) alsosummarized information on the biodegradation of hydro-carbons by cold-adapted microorganisms. The reviewersobserved that cold habitats generally possess a sufficientpopulation of indigenous hydrocarbon-degrading


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microorganisms which are able to respond to contamina-tion, and that these tend to be psychrotrophic. Similar toearlier reviews, they noted that bioaugmentation hasgenerally not been successful, though it may reduce thelag time for the growth of a hydrocarbon-degrading

population, whereas biostimulation by fertilization withnutrients has been an effective strategy. They also observedthat the physical environment is a factor that effectshydrocarbon degradation, and provided a few examples(e.g., sand content). Margesin and Schinner (2001)summarized information that indicated a considerablerange of hydrocarbon compounds can be degraded bybacteria at cold climate sites, including alkanes, aromatichydrocarbons, and polycyclic aromatic hydrocarbons(PAHs). Short-chain alkanes were more readily degradedthan longer chain alkanes. In terms of biodegradation of

aromatics, much of the research has focused on BTEX:benzene, toluene, ethylbenzene and xylenes. Some studieshave indicated rates of biodegradation of toluene orbenzene under cold climate conditions that were similarto rates observed atwarm sites, whereas others indicatedslower degradation of BTEX at lower temperatures.

To date, the reviews of biodegradation of hydro-carbons at cold climate sites have generally focused onaerobic processes in soils. However, over the past decade,a growing number of studies of the intrinsic biodegrada-tion of hydrocarbons in groundwater have indicated thatanaerobic biodegradation processes are important (e.g.,

Lovley, 1997; Weidemeier et al., 1999), including somestudies that have considered cold climate sites specifically(e.g., Herrington et al., 1997; Armstrong et al., 2002;Ulrich et al., 2006). The anaerobic processes typicallyinvolve one or more of the following electron acceptingprocesses: reduction of dissolved sulfate or nitrate,reduction of mineral phase ferric iron or manganese,and methanogenesis (also referred to as methanefermentation: Stumm and Morgan, 1996).

3. Bioremediation of hydrocarbons at cold sites:

previous reviews pertinent to groundwater

Information on the kineticsof microbial degradation ofhydrocarbons in groundwater has typically been providedas rates orrate constants with reference to processesdescribed as attenuation, decay, degradation orbiodegradation. For consistency, this section uses theterm biodegradation rate constant. Reviews by Aronsonet al. (1997) and Suarez and Rifai (1999) havesummarized biodegradation rate constants for dissolvedhydrocarbons in groundwater, mainly BTEX, each reviewbased on data from approximately 150 field andlaboratory studies. The estimated biodegradation rate

constants based on field investigations have typicallytaken into account concentration gradients and inferredgroundwater velocities, and may accountfor other factors,such as sorption and dispersion, as applicable. Some fieldbiodegradation rate constant estimates are based on tests

that involved injection of hydrocarbons together withconservative tracers. The majority of the reportedbiodegradation rate constants refer to the first ordermodel of biodegradation:

C C0ekt 1

where C0 is the initial dissolved hydrocarbon concentra-tion, Cis the concentration after time interval t, and kis afirst order rate constant (units of time1). Other rateequations used to simulate biodegradation, such as the

zero-order model have been discussed by Aronson et al.(1997), Suarez and Rifai (1999) and others.In the reviews by Aronson et al. (1997) and Suarez and

Rifai (1999), there was little discussion of the role oftemperature. Aronson and Howard noted that temper-ature and redox environment did not appear to be correlatedto the anaerobic biodegradation of benzene in aquiferenvironments. Suarez and Rifai (1999) made a generalstatement that the optimum temperature range for mostorganisms is 1035C, while noting that below thisoptimum range growth rates of microorganisms tend todouble with every 10C increase in temperature (i.e., the

Q10 approach). The lack of further discussion about the roleof temperature is not surprising, given that there is verylittle published information on the effect of temperature onhydrocarbon degradation rates in groundwater.

Salanitro (1993) summarized information on thebiodegradation of aromatic hydrocarbons (BTEX) ingroundwater for several case studies in which both fieldinvestigations and microcosm experiments were avail-able. Several of these case studies involved relativelycold sites in Michigan and Ontario, and correspondingmicrocosm experiments at 1012C. The results of

these studies indicated that monoaromatic compoundswere readily biodegraded in aquifers at low tempera-tures (circa 10C). The inferred biodegradation rateconstants for these low temperature sites were similar tothose at warmer sites that were also reviewed bySalanitro (e.g., Florida and Texas, USA).

Herrington et al. (1997) summarized the results ofinvestigations of the natural attenuation of aviation/jetfuel plumes in groundwater at five US Air Force Bases,including one in northern Michigan, and four in Alaska.At the Michigan site, the groundwater temperaturesranged from 9.8 to 14.9C. At the Alaska sites, thegroundwater temperatures ranged between 3.4 and


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11.7C. Herrington et al. found evidence for receding(shrinking) hydrocarbon plumes at all sites, linked toaerobic processes, as well as anaerobic processes ofnitrate, iron, manganese and sulfate reduction andmethanogenesis. They estimated that the overall first

order cumulative BTEX degradation rate constants atthese cold climate sites ranged from 0.19 to 2.99%day1, including aerobic and anaerobic processes. Theauthors pointed out that these biodegradation rates weresimilar to those reported for warmer sites.

Armstrong et al. (2002) analyzed a database ofgroundwater chemistry results for monitoring programsat 124 contaminated sites in western Canada. The siteswere mainly upstream oil and gas sites in Alberta, wheretypically the hydrocarbon contaminants in groundwater arederived from releases of crude oilor natural gas condensate.

In this region groundwater temperatures typically arewithin the range of 510C. Where sufficient data wereavailable, more than 90% of the monitored hydrocarbonplumes were either stable or shrinking, rather than expand-ing. This evidence supported the interpretation that naturalattenuation of hydrocarbons was important at the majorityof these sites. Based on geochemical indicators, sulfatereduction appeared to be the most important terminalelectron accepting processes linked to hydrocarbonoxidation, followed by iron reduction. Oxygen, nitrateand manganese also appeared to be significant terminalelectron acceptors. The relative importance of methano-

genesis could not be assessed due to a lack of data.Armstrong et al. reported that theoretical biodegradationcapacity for these plumes, based on dissolved concentra-tions of electron acceptors, generally exceeded hydrocar-bon concentrations (expressed as BTEX), by 77% inshrinking plumes, and by 56% for stable plumes.

The estimated first order biodegradation rate constantsfor BTEX at the sites in western Canada reviewed byArmstrong et al. (2002)ranged from 0.0002to 0.017day1,with the majority of plumes having rates ranging from0.001 to 0.005day1. These results indicated plume half-

lives for the individual BTEX components of approxi-mately 1 to 2years. These ranges are of the same order inmagnitude and overlap with ranges of BTEX biodegrada-tion rate constants observed in plumes at warmer sites, asdocumented for example in surveys conducted in theUnited States (Suarez and Rifai, 1999). The authors sug-gested that possible causes for a tendency for marginallylower biodegradation rates for the western Canadian siteswere their cooler temperatures, and possible influences ofco-contaminants, which are often present in hydrocarbonplumes at upstream oil and gas facilitiesin western Canada.

Recently Ulrich et al. (2006) provided a summary ofpublished information on biodegradation rate constants

for BTEX components under anaerobic conditionswhere temperatures have been reported. This includeda graphical summary of inferred first-order rateconstants either reported, or calculated using theBuscheck and Alcantar (1995) solution (Fig. 2). Ulrich

et al. (2006) reported 95% confidence intervals for therate constants (a) as reported, and (b) normalized to 5C,based on an assumed Q10= 2. Approach (b) was chosento create a data set that could be used by regulators andpractitioners for estimating biodegradation rate con-stants for natural attenuation in cold-climate fieldsettings. Given that the ranges in temperature normal-ized rates provided by Ulrich et al. (2006) (Fig. 2) aredrawn from 60 studies (including 49 case histories atfield sites), using 95% confidence intervals, these rangesof values appear to provide reasonable conservative/

proxy estimates of rate constants for biodegradation ofBTEX in fuel plumes in groundwater at cold sites. Theentire database of information is available at http://environment.gov.ab.ca/info/library/6684.pdf.

4. Bioremediation of hydrocarbons in groundwater

at cold sites: review of individual studies

As recently reviewed by Van Stempvoort and Grande(2006), a significant and growing number of studieshave indicated successful applications of bioremediationto clean up hydrocarbon-contaminated soils at cold

sites, but in contrast, information on the feasibility and/or successful application of bioremediation to clean-upof hydrocarbon-contaminated groundwater at cold sitesis much more limited. This section provides a summaryof results from some of the pertinent groundwaterstudies.

Typically the pioneering groundwater studies thatwere conducted in the 1980s at relatively cold sites inNorth America did not emphasize the role of temper-ature in the bioremediation, and often temperature datawere not reported. Particularly rare are studies of the

biodegradation of hydrocarbons in groundwater at sitesthat have permafrost.

4.1. Geographic range and geologic settings of field

studies

The focus of this review is on field investigations thathave been conducted in North America (24 sites: Tables 2and 3); studies from Scandinavia and the Baltic region ofEurope have also been included (11 sites: Table 4). Someof the earliest relevant field studies were conducted in theGreat Lakes area of North America, including the USA(Wilson et al., 1986; Rifai et al., 1988; Chiang et al., 1989)

http://environment.gov.ab.ca/info/library/6684.pdfhttp://environment.gov.ab.ca/info/library/6684.pdfhttp://environment.gov.ab.ca/info/library/6684.pdfhttp://environment.gov.ab.ca/info/library/6684.pdf

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and Canada (Barker et al., 1987; Acton and Barker, 1992).By the mid-1990s, colder sites north of the 60th parallel in

both the USA (Braddock and McCarthy, 1996; Westerveltet al., 1997) and Canada (Carss et al., 1994) had beenstudied. Over the past decade, the geographic range of theinvestigations in North America has expanded to includevarious other locations in Canada, including Alberta(Lai et al., 2001; Van Stempvoort et al., 2002, 2005, 2007,in press), Labrador (Curtis and Lammey, 1998), northernManitoba (Shields et al., 1997), northern Ontario(Bickerton et al., 2005), Northwest Territories (VanStempvoort et al., 2006; Van Stempvoort and Talbot,2006), and Yukon Territory (Whyte et al., 1998; Billowits

et al., 1999; Soloway et al., 2001).The majority of the studies (Tables 2, 3, and 4; Fig. 3)

were investigations of hydrocarbon plumes in sand orsand/gravel aquifers. Other geologic media includedfractured rock (Carss et al., 1994; Van Stempvoort et al.,2006; Eriksson et al., 2006), gravel fill over peat (Mitchelland Friedrich, 2001) and silt/clay deposits (Van Stemp-voort et al., 2002, 2007, in press). Based on the limitedavailable information, geology does not appear to be amajor constraint on the in situ biodegradation ofhydrocarbons in groundwater. Authors reported evidencefor significant intrinsic or enhanced in situ bioremediationof hydrocarbons in all of the different types of geologic

media that were investigated, including all of the fieldstudies of groundwater in fractured rock (3 studies in

Canada, 1 study of 4 sites in Sweden), for example.Though studies of hydrocarbon bioremediation in cold,fractured rock environments are rare, this topic is veryrelevant to large cold regions, including the CanadianShield, for example, which includes an area of approx-imately 4.8million square km in Canada.

4.2. Thermal regime, permafrost setting

For the sites considered in this review (Tables 2, 3and 4), the average annual air temperatures vary

between 12 and 8C, whereas groundwater tempera-tures reported in studies at these sites ranged from 0.3 to15.8C. Typically the groundwater temperatures weremeasured in summer. Almost all of the sites have eitherno permafrost, or sporadic to discontinuous permafrost.Three exceptions are the three case studies included inthis review, two of which provide evidence forsignificant biodegradation of hydrocarbons in supraper-mafrost and/or intrapermafrost groundwater.

When groundwater is present in the upper few metersbelow ground surface, its temperature fluctuates sea-sonally, which may include freezing during the winter(e.g., suprapermafrost water in zones of continuous

Fig. 2. Ranges of estimated first order rate constants for biodegradation of petroleum hydrocarbons under anaerobic conditions, based on field data,modified afterUlrich et al. (2006). The data compiled for this figure was obtained from 49 case histories at field sites: 8 in western Canada; 1 inOntario, Canada; 1 in Australia, 3 in Alaska and 36 at other USA locations.


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Table 2Summary of information on cold climate sites from the USA included in this review

Geographiclocation(reference(s))

Plume type (geologic medium) Temperature annual averageair (reportedgroundwater)

Biodegradation evidence Type of microorganismsindicated/suspected (type of fieldapproach/treatment)

Traverse City,Michigan, USA(Wilson et al.,1986; Rifaiet al., 1988)

Benzene, toluene and xylenesfrom aviation fuel spill (sandand gravel aquifer)

7 C (9.3 to15.8 C)

Field data indicated rapidbiodegradation of the aromatichydrocarbons under both aerobic andmethanogenic, anaerobic conditions;modeling of 1985 plume data: aromatichydrocarbon mass loss: 1% day1;microcosm tests (Table 5)

Aerobic and methanogenic(evidence for intrinsic

bioremediation)

NorthernMichigan, USA(Chiang et al.,1989)

Gas condensate at a gas plant(sand aquifer)

Est. 7 C Natural attenuation of benzene, tolueneand xylenes was found to beapproximately 1% day1, but slower inthe O2 depleted, central portion of

plume, microcosm tests (Table 5)

Aerobic, anaerobic processesalso suggested (evidence forintrinsic bioremediation)

Bemidji,

Minnesota,USA(Baedeckeret al., 1993;Bennett et al.,1993;Eganhouseet al., 1993;Rooney-Vargaet al., 1999)

Crude oil from pipeline (sand

and gravel aquifer)

3.2 C Plume became anoxic, with dissolved

Fe, Mn and methane released togroundwater, appearance of organicacids, shift in isotopic composition ofdissolved inorganic C: plume migration(aromatics and alkanes) attenuated by

biodegradation; enrichment ofGeobacter sp. in plume

Aerobic and anaerobic Fe, Mn

reducers and methanogens(evidence for intrinsic

bioremediation)

Adak, Alaska,USA (Bradleyand Chapelle,1995)

Jet fuel (shallow sand aquifer) 4.7 C (4 to6 C)

Toluene degradation in aerobicmicrocosms (Table 5)

Aerobic (concluded that intrinsicbioremediation might be a viableapproach)

Near Barrow,Alaska, USA(Braddock andMcCarthy,1996) (Casestudy #1)

Gasoline, jet fuel spill in 197678 (sand and gravel deposits,suprapermafrost groundwater)

12 C (1.2 to7.4 C)

Depleted oxygen and nitrate in plume,with increases in ferrous iron andsulfide, and higher microbial

populations; aerobic microcosm testsindicated significant rates of benzenemineralization, stimulated by nutrientaddition (Table 5)

Aerobic and anaerobic nitrate,iron, sulfate reducers (concludedintrinsic remediation could be

part of management strategy,planned to combine with novelartificial high layer in permafrostto contain contaminant plume)

Fairbanks, Alaska,USA(Westerveltet al., 1997)

Arctic diesel from undergroundstorage tank (unconfinedalluvial sand and gravelaquifer)

2.8 C Geochemical evidence for anaerobichydrocarbon degradation linked toreduction of sulfate, iron andmanganese, methanogenesis; calculatedfirst-order BTEX biodegradation rates:8 to 21% day1

Anaerobic iron, manganese,sulfate reducers, methanogens(natural attenuation of BTEX tonon-detectable levels within45 m of source area, 50 m up-gradient of municipal water

supply wells)Near Fairbanks

Alaska, USA(Richmondet al., 2001)

Benzene with trichloroethene,trichloroethane; toluenedetected until 1994,contaminant source(s)unknown (alluvial sand andgravel aquifer)

2.8 C Concentrations of ferrous iron andsulfide, hydrocarbons

Iron and sulfate reducers(concluded intrinsic

bioremediation was notsignificant, that natural dilutionreduced hydrocarbonconcentrations)

Fairbanks Alaska,USA (Braddocket al., 2001)

Petroleum-distillate (alluvialsand and gravel aquifer)

2.8 C (3 to4 C)

Microbial activities and mineralizationof selected aromatics in laboratory tests(Table 5); aquifer tends to be anaerobic,observed 0.2 to 3.6 mg/Ldissolved oxygen in groundwater

Aerobic (evidence for intrinsicbioremediation)


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Table 3Summary of information on cold climate sites from Canada included in this review


Plume type (geologic medium) Annual averageair temperature(measuredgroundwater

temperature)


Alliston, Ontario,Canada (Barkeret al., 1987;Acton andBarker, 1992)

Injected aromatichydrocarbons, associated withlandfill leachate (surficial sandaquifer)

7.4 C Loss of aromatics injected withchloride/bromide tracer; concluded thatthe inferred field biotransformationrates in agreement with lab results;

benzene recalcitrance

Aerobic and anaerobic (evidencefor intrinsic bioremediation)

North Bay,Ontario, Canada(Acton andBarker, 1992)

Landfill leachate (sand aquifer) 3.8 C Extensive biodegradation of aromaticsinjected with bromide tracer, mostdisappeared after 2040 days

Anaerobic, fermentative andmethanogenic bacteria (evidencefor intrinsic bioremediation)

Great Slave Lake,NorthwestTerritories,

Canada (Carsset al., 1994)

Not identified (fracturedbedrock)

5 C (0.3 to8 C)

Oxygen consumption, decrease in TPHconcentrations by 30%; decline inBTEX to mostly non-detectable levels;

increase in hydrocarbon-degradingmicroorganisms by N3 orders inmagnitude; estimated 1200 L of

petroleum mineralized

Aerobic (pump and treat, oxygenand nitrate addition, reinjection)

NorthernManitoba,Canada (Shieldset al., 1997)

Diesel fuel (not reported) 0.5 C After 2 years contaminated zone(undefined) was reduced by 50%

Aerobic, (circulation of nutrientand oxygen-enrichedgroundwater)

Goose Bay,Labrador,Canada (Curtisand Lammey,1998)

Fuel including arctic diesel(sand and silt aquifer)

0.3 C Electron acceptors apparently involvedin biodegradation of BTEX: oxygen,nitrate, ferric iron and sulfate; increasein alkalinity

Aerobic and anaerobic nitrate,iron and sulfate reducers(intrinsic bioremediation;suggested adding nitrate to thegroundwater)

Whitehorse,Yukon Territory,Canada (Whyteet al., 1998;Billowits et al.,1999; Solowayet al., 2001)

Diesel fuel/heating oil (shallowsand and silt)

0.7 C Microbial enumeration and molecularanalyses indicated cold-adapted aerobicand anaerobic bacteria present inaquifer, aerobic and anaerobic diesel-degraders; DNA testing indicated genesinvolved in alkane degradation labtests indicated mineralization ofhexadecane (Table 5)

Aerobic and anaerobic diesel-degraders (evidence for intrinsic

bioremediation; groundwaterwas treated by biosparging andnutrient biostimulation)

Haines Junction,Yukon Territory,Canada(Billowits et al.,1999)

Pit disposal of oil refineryproducts (shallow sand-gravelaquifer)

Est. 0.5 C Plate growth of bacteria in aquifersamples greater at 5 C than 37 C,DNA testing indicated genes involvedin alkane degradation; lab testsindicated degradation of aromatics andhexadecane

Suggested nitrate-reducers mayhave been involved inhydrocarbon degradation(evidence for intrinsic

bioremediation)

Central Alberta,Canada (Laiet al., 2001)

Aromatic hydrocarbons(BTEX) and othercontaminants (chorinatedhydrocarbons) in landfillleachate (sand aquifer)

Est. 4 C Mixed contaminant plume includingBTEX is depleted in oxygen andsulfate, and enriched in methane anddissolved iron; microbial assaysindicated the presence of iron andsulfate reducing bacteria andhydrocarbon degraders

Aerobic and anaerobichydrocarbon degraders (evidencefor intrinsic bioremediation)

Komakuk Beach,Yukon Territory,Canada(Mitchell andFriedrich, 2001)(Casestudy #2)

Fuel oil (gravel pad over peatand silt, permafrost present)

11.4 C BTEX were removed during treatment Aerobic (multiphase extraction,groundwater treated ex situ in

bioreactors: aeration, nutrients,capacity for intrinsic

bioremediation reported to below)


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permafrost). At greater depths, groundwater has arelatively stable temperature. For example, at Wrigley,Northwest Territories, Canada, the water table in anunconfined sand and gravel aquifer occurs at 25m belowground, where the temperature remains at2C year-round (Van Stempvoort and Talbot, 2006). Suchdifferences in thermal regimes, including the extent oftemperature fluctuations and freezing events, may haveimportant effects on the type of hydrocarbon-degrading-microorganisms/consortia that are present in groundwa-

ter, and specifically those that degrade hydrocarbons.Aside from a few laboratory studies on the effect ofcyclic freezing on bioremediation of hydrocarbons insoils (Eriksson et al., 2001; Brresen et al., 2006), thereappears to be a research gap regarding this topic.

All (21) of the field studies listed in Tables 2, 3 and 4that had mean annual air temperatures (MAAT) at orabove 0C (10 in North America, 11 in Baltic region)reported evidence for in situ bioremediation of hydro-carbons in groundwater. For the remaining studies in

Table 3 (continued)


Plume type (geologic medium) Annual averageair temperature(measuredgroundwatertemperature)


Alberta Canada(Cross et al.,2003)

Diesel (fractured bedrock) Est. 4 C Laboratory tests (see Table 5) Aerobic and anaerobic(concluded intrinsic

bioremediation of groundwatercould potentially be enhancedwith nutrient addition)

Alberta, Canada(VanStempvoortet al., 2002,2007)

Natural gas condensate (silt/clay/sand aquitard)

Est. 4 to 6 C (5to 8 C)

Field injection of sulfate with a bromidetracer; observed loss of sulfate overseveral months of45 mg/L per dayelevated iron, methane in plume

Sulfate and iron reducingbacteria dominant; aerobes,methanogens; (inferred intrinsic

bioremediation, suggestedenhancement possible)

Alberta, Canada(VanStempvoort

et al., 2005)

Natural gas leaking along wellbore (confined sand and graveaquifer)

1.2 C(approximately5 C)

Lower sulfate concentrations, stableisotope shift in sulfate and dissolvedinorganic carbon

Sulfate reducing bacteria,possibly in consortia

Yellowknife,NorthwestTerritories,Canada(Barnette et al.,2005)

Diesel (sand and gravel, silt) 5 C Reduction of BTEX concentrations insome monitoring wells observed overan 8 month period

No details; (focus was bioventingfocus of the hydrocarbon-contaminated vadose zone)

Moose Factory,Ontario, Canada(Bickertonet al., 2005)

Diesel fuel(surficial sand aquifer)

1.1 C (0.5 to9 C)

Monitoring suggested plume (BTEX,others) was stable, depleted oxygen,sulfate, elevated dissolved iron andmanganese in contaminant plume;molecular analyses indicated different

bacterial strains outside, inside theplume; some same/similar to those inother hydrocarbon plumes


Colomac mine,NorthwestTerritories,Canada (Casestudy #3)

Diesel fuel/fractured bedrock,permafrost

Est. 7.8 C (2to 6 C)

Plume has low O2, relatively highconcentration of dissolved iron, mostdissolved N is ammonia, sulfateapparently decreases downgradient,with isotope shift suggesting sulfatereduction, short-chain fatty acids in

plume


Wrigley,NorthwestTerritories,Canada (VanStempvoort and

Talbot, 2006)

Diesel fuel, jet fuel, possiblyother (unconfined sand andgravel aquifer)

Est. 5 C (2 to2.4 C)

Higher dissolved iron and lower sulfateassociated with hydrocarbons present ingroundwater

Anaerobic (evidence for intrinsicbioremediation)


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Table 4Summary of information on cold climate sites from the Baltic region (northern Europe and Scandinavia)included in this review

Geographiclocation/study

Plume type/geologicmedium

Annual averageair temperature

Biodegradation evidence Type of microorganisms indicated/suspected (type of field approach/treatment)

Vejen, Denmark(LyngkildeandChristensen,1992a,b)

Landfill leachate(unconfined sandyaquifer)

Est. 7.5 C Disappearance of aromatic hydrocarbonsand other organic contaminantsdowngradient, associated with loss of O2,nitrate and sulfate, appearance of methane,ammonia, Fe2+, Mn2+ and sulfide,appearance of some organic intermediatemetabolites in the plume

Aerobic, nitrate-, sulfate-, Fe- and Mn-reducers, methanogens in distinct zones(evidence for intrinsic bioremediation)

Riga, Latvia(Banks et al.,1998)

Fuel, largely diesel(unconfined sandaquifer)

7.3 C Increase in dissolved iron and alkalinity inhydrocarbon plume; depletion of sulfateand nitrate within the plume

Iron-, nitrate- and sulfate-reducers(evidence for intrinsic bioremediation)

Radsted,Denmark(Mossinget al., 2001)

Petrol fuel/gasoline(sand aquifer)

Est. 7.5 C BTEX in plume are being degraded underaerobic, iron- and nitrate-reducingconditions; BTEX degradation rates areestimated to be 0.0003 to 0.016 day1

Aerobic, nitrate- and iron-reducers(evidence for intrinsic bioremediation)

Near Riga,Latvia(Spalvinset al., 2001)

Jet fuel (sand aquifer) 7.3 C Numerical modeling result: estimated halflife of fuel plume in groundwater of1800 days; concluded that as a result ofintrinsic bioremediation only a small partof fuel plume reaches river

Details not included (evidence forintrinsic bioremediation)

Szprotawa,Poland(Paweczyket al., 2003)

Aircraft fuel (shallowsand, gravel, silt,clay)

8 C Reduced hydrocarbon levels ingroundwater from 22.36 mg/L to 0.023.61 mg/L

Aerobic, based on treatment approach(inoculation of groundwater with

bacteria, in situ aeration and addition ofbiogenic substances)

Hanko, Finland(Salminenet al., 2004;Purkamoet al., 2004)

Lightweight fuel,lubrication oil atindustrial dumpsite(sand, gravel, silt,clay)

6 C Groundwater is anoxic, laboratory studiesindicated biodegradation of mineral oil,including n-alkanes in subsurface samplesunder aerobic and anaerobic,methanogenic conditions; highest aerobicand methanogenic activities were in mostcontaminated samples; microbial diversitystudied by DNA/molecular analysesindicated strain with similarity to knownanaerobic alkane degraders

Aerobic and anaerobic includingmethanogens (evidence for intrinsic

bioremediation)

Oslo, Norway(Konowskiet al., 2005)

Jet fuel (sand andgravel)

5.7 C Injected toluene, xylene and otheraromatics along with bromide tracer;observed losses; in main plume observeddepletion of O2, nitrate and sulfate, highFe, Mn, alkalinity; metabolites ofhydrocarbon degradation in plume;laboratory tests: Zheng et al. (2002a,b,c)

Aerobic; nitrate-, iron- and sulfate-reducers (evidence for intrinsic

bioremediation; inferred that intrinsicbiodegradation of toluene could be anefficient approach)

Murjek, Sweden

(Erikssonet al., 2006)

Diesel (fractured

rock)

Est. 0 C Compared to pristine setting, diesel-

contaminated groundwater had highertotal number of microorganisms,somewhat higher populations of iron-,sulfate- and nitrate-reducers, elevatedalkalinity; low sulfate, trace of nitrite[evidence of nitrate-reduction]

Suggested aerobic bacteria dominant

(evidence for intrinsic bioremediation)

Ludvika,Sweden(Erikssonet al., 2006)

Diesel (fracturedrock)

Est. 5 C Compared to pristine setting, diesel-contaminated groundwater had highertotal number of microorganisms,higher populations of iron-, sulfate-and nitrate-reducers, detection ofmetabolites of hydrocarbon degradation;elevated alkalinity; trace of nitrite,indicating denitrification

Suggested anaerobic bacteria dominant(evidence for intrinsic bioremediation)


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North America, which had MAAT below 0C (12 to0.3C), the large majority (12 of 14) reportedsignificant in situ biodegradation of hydrocarbons ingroundwater (intrinsic or enhanced). This included thesite with the coldest MAAT of12C (see Section 4.7,Case study 1). Collectively, this information indicatesthat the potential to utilize in situ bioremediation as atechnology to clean-up petroleum-contaminated

groundwater in cold regions is very promising,including sites with sub-zero MAAT and presence ofpermafrost. At the second coldest site included in thissurvey, investigators reported thatcapacity for intrinsicbioremediation was found to be low. However, an exsitu bioremediation technology was applied to treat thegroundwater, which included aeration and nutrientaddition in bioreactors (see Section 4.7, Case study 2).

4.3. Types of hydrocarbon sources, contaminants

investigated

For the majority (23 of 35) of the field sites reviewed(Tables 2, 3 and 4; Fig. 4), the hydrocarbon plumes ingroundwater were derived from fuel, including diesel,gasoline, jet/aviation fuel, and other unidentified fueltypes. Other types of petroleum sources investigatedwere crude oil, refinery wastes, natural gas and/or gascondensates, landfill leachate, and sources unknown ornot identified. Most studies focused on the attenuation ofplumes of dissolved monoaromatic hydrocarbons, par-ticularly BTEX (benzene, toluene, ethylbenzene andxylenes), although some considered other aromaticspecies or focused on total extracted/detected hydro-

carbons (Fig. 5). In situ biodegradation was reported tobe significant in all types of plumes that wereinvestigated. However, given the variety of conditionsand methodologies for the studies included in thisreview, there remains a gap in definitive, quantitativeinformation regarding the relative biodegradability ofhydrocarbon plumes derived from various types ofpetroleum sources in various hydrogeological settings

under cold climate conditions.

4.4. Types of remediation investigated

4.4.1. Intrinsic bioremediation

The large majority (27 of 35) of the field studies ofpetroleum hydrocarbon plumes in groundwater at coldsites were investigations of intrinsic bioremediation

Table 4 (continued)

Geographiclocation/study

Plume type/geologicmedium

Annual averageair temperature

Biodegradation evidence Type of microorganisms indicated/suspected (type of field approach/treatment)

Sala, Sweden

(Erikssonet al., 2006)

Gasoline (fractured

rock)

Est. 5 C Compared to pristine setting, gasoline-

contaminated groundwater had higher totalnumber of microorganisms, higherpopulations of iron-, sulfate- and nitrate-reducers, low sulfate, elevated alkalinity,trace of nitrite

Suggested anaerobic bacteria dominant

(evidence for intrinsic bioremediation)

Bldinge,Sweden(Erikssonet al., 2006)

Diesel and gasoline(fractured rock)

Est. 7 C Fuel-contaminated groundwater had moreabundant microorganisms, includingnitrate-, iron- and sulfate reducers, presenceof ferrous iron and sulfide, elevatedalkalinity, depleted sulfate and ferric iron,detection of metabolites of hydrocarbondegradation; RNA-based analysesindicated presence of bacteria similar toother hydrocarbon-contaminated

environments

Anaerobic bacteria dominant (evidencefor intrinsic bioremediation)

Fig. 3. Geological setting of the hydrocarbon plumes at the 35 sitesreviewed in this study.


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(Tables 2, 3, and 4; Fig. 6). In 20 of these cases, theevidence for intrinsic bioremediation that was gatheredincluded an emphasis on hydrogeochemistry, specificallydata regarding the distribution of electron acceptors, suchas oxygen, sulfate, nitrate, or potential products of redoxreactions associated with hydrocarbon biodegradation,such as dissolved ferrous iron and manganese, sulfide or

methane.This review found that evidence for intrinsic

bioremediation included the detection of lower concen-trations of electron acceptors in the contaminant plume,and/or higher concentrations of reduced product species(e.g., Fig. 7). Particularly common were reports ofhigher concentrations of dissolved iron or depletednitrate or sulfate in the hydrocarbon plumes as evidencefor the role of anaerobic bacteria (iron-, nitrate- andsulfate-reducers) (Fig. 7). Reports of evidence foraerobic hydrocarbon degraders (relatively low or

negligible oxygen in the plumes) and methanogens(presence of methane) were also common.

In this review, some of the cold-climate sites lackedevidence for oxygen depletion (lower or negligibleconcentrations) in the hydrocarbon plumes, compared topristine conditions, because the groundwater was anaer-obic both inside and outside of the hydrocarbon plumes(e.g., Van Stempvoort et al., 2005). At other sites,background concentrations of sulfate and/or nitrate werevery low, precluding the importance of these electronacceptors in biodegradation of hydrocarbons. The role ofmethanogens is likely to have been under-reportedbecause methane was not analyzed in all studies.

In some studies, chemical evidence for intrinsicbioremediation included the detection of probablebiodegradation metabolites including various partiallyoxidized petroleum hydrocarbons (e.g., Eganhouse et al.,1993; Eriksson et al., 2006) and short-chain fatty acids(Van Stempvoort et al., 2006, in press). In some plumes,higher alkalinity (e.g., Konowski et al., 2005; Van

Stempvoort et al., in press) or alkalinity/hardness ratio(Eriksson et al., 2006) was apparently an indicator thathydrocarbons had been mineralized to dissolved CO2.Some authors have reported shifts in the isotopiccomposition of dissolved inorganic carbon that wereassociated with mineralization of hydrocarbons to CO2(Baedecker et al., 1993; Van Stempvoort et al., 2002,2005). Stable S and O isotope data have supported the

Fig. 4. Source types of the hydrocarbon plumes at the 35 sites reviewed in this study.

Fig. 5. Diagram illustrating the emphasis on analyses of aromaticcompounds, including BTEX, at the field sites.


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interpretation that dissolved sulfate had been an importantelectron acceptor during intrinsic bioremediation of anatural gas plume (Van Stempvoort et al., 2005)andafuel

plume (Van Stempvoort et al., 2006, see Case study #3).Some relevant studies of intrinsic bioremediationincluded microbial analyses, including various enumera-tion techniques for total bacteria, and some techniquesmore specifically for hydrocarbon-degraders, sulfatereducers, iron-reducers, nitrate-reducers and/or methano-gens (e.g., Carss et al., 1994; Lai et al., 2001; Cross et al.,2003). Typically the studies found that bacteria, either totalpopulations or more specific groups, were more abundantand/or active in plumes compared to un-contaminatedgroundwater, and more active/abundant in treated ground-water compared to untreated. For example, Carss et al.

(1994) found that the population of hydrocarbon-degradingmicroorganisms increased by more than three orders inmagnitude in response to aeration and biostimulation.

Over the past decade, some relevant intrinsic biore-mediation studies have focused specifically on microbi-

ology, employing both comprehensive microbialenumeration procedures and molecular analyses (Whyteet al., 1998; Billowits et al., 1999; Purkamo et al., 2004;Eriksson et al., 2006). For example, Eriksson et al. (2006)enumerated total bacteria, most probable numbers of

anaerobes, nitrate-, iron-, and sulfate-reducing bacteria,and also conducted DNA/RNA analyses to determinespecific types/strains. They noted that the bacteria foundin contaminated groundwater in fractured rock in Swedenwere similar to bacteria in other hydrocarbon-contami-nated environments.

Based on reported molecular analyses, it appears thatsome strains or types of bacteria that occur in hydrocarbonplumes may be widespread geographically. For example,Dojka et al. (1998), Purkamo et al. (2004) and Bickertonet al. (2005) reported Syntrophus sp. or closely related

strains in Michigan USA, Finland, and northern Ontario,Canada respectively. Purkamo et al. indicated that Syn-trophus sp. are known to participate in anaerobic alkanedegradation. At the northern Ontario fuel plume site,Bickerton et al. found several other strains of bacteria thatwere close matches to those previously detected atother hydrocarbon-contaminated sites, including a Spiro-chaeta sp., previously detected in a jet fuel-contaminatedaquifer in Michigan undergoing intrinsic bioremediation(Dojka et al., 1998). Rooney-Varga et al. (1999) andBickerton et al. (2005) found Geobacter sp. in plumes innorthern Minnesota USA and northern Ontario Canada,

respectively.Collectively, the evidence summarized above indicates

that a wide range of aerobic and anaerobic microorgan-isms are active in hydrocarbon plumes in groundwater atcold climate sites. It appears that further research is

Fig. 6. Bioremediation approaches investigated at the 35 groundwatersites included in this review.

Fig. 7. Evidence for anaerobic electron accepting processes in studies of plumes that emphasized the application of hydrogeochemical analyses.


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required to compare the diversity and structure ofmicrobial communities present in plumes at cold climatesites to those at warmer sites. These communities mayinclude consortia of anaerobic microorganisms in syn-trophic relationships, such as sulfate reducers, fermenta-

tiveacetogens, and/or archaea (methanogens), where eachgroup benefits from the metabolites of others, with thefinal result being partial or complete degradation ofhydrocarbons (Dojka et al., 1998; Bickerton et al., 2005;Van Stempvoort et al., in press).

4.4.2. Active groundwater bioremediation approaches

Six of the studies reported applications of activebioremediation techniques to clean up hydrocarbon-contaminated groundwater at cold climate sites. Theseinvolved either a combination of ex situ aeration and

biostimulation (Carss et al., 1994; Shields et al., 1997;Mitchell and Friedrich, 2001), in situ biosparging withbiostimulation (Soloway et al., 2001), in situ aeration withbacterial inoculation and addition of biogenic sub-stances (details not provided) (Paweczyk et al., 2003), orby bioventing (Barnette et al., 2005). Two of the three exsitu applications included groundwater recirculation(Carss et al., 1994; Shields et al., 1997), while the thirdutilized a bioreactor (Mitchell and Friedrich, 2001).Details regarding the success of the biosparging approachwere not available at the time of reporting by Solowayet al. (2001). Success was reported for each of the other

active bioremediation approaches, measured as disap-pearance or reduction of BTEX concentrations (Carsset al., 1994; Mitchell and Friedrich, 2001; Barnette et al.,2005), decline in TPH/oil concentrations (Carss et al.,1994; Mitchell and Friedrich, 2001; Paweczyk et al.,2003), oxygen loss (Carss et al., 1994), and/or shrinkageof the plume (Shields et al., 1997).

Other studies have suggested that in situ biostimulationwith nutrients might enhance the bioremediation of thehydrocarbon-contaminated groundwater, based either oninterpretation of field results (a study of intrinsic

bioremediation by Curtis and Lammey, 1998) or labora-tory test results (Billowits et al., 1999; Cross et al., 2003).Following field injection tests that had indicated conser-vative estimates of sulfate reduction rates of5mg L1

day1, Van Stempvoort et al. (2007, in press) suggestedthat it might be helpful to add sulfate as an electronacceptor to gas condensate plumes in Western Canada.

4.5. Biodegradation rates in cold groundwater

A small minority of the field studies examined in thisreview included estimates of the rates of biodegradationof the dissolved hydrocarbons in the contaminant plumes

(Tables 2, 3 and 4). For example, Rifai et al. (1988)reported field-based first order biodegradation rateconstants (or half life equivalents) for BTEX ingroundwater that fall within the normalized rangesderived by Ulrich et al. (2006) for anaerobic biodegrada-

tion, as shown on Fig. 2. In contrast, Chiang et al. (1989)and Westervelt et al. (1997) inferred higher biodegrada-tion rateconstants for BTEX in cold groundwater, rangingfrom 0.03 to 0.1day1 and 0.08 to 0.21day1 respectively,both ranges above the normalized ranges inferred byUlrichetal.(Fig.2).InthecaseofChiang et al. (1989),thebiodegradation was, at least in part, inferred to be anaerobic process, which may account for the faster inferredrates (see following section). Positive inferences abouthigh biodegradation rates have been encouraging forproponents of bioremediation for cold climate sites.

However, given the uncertainty in various parametersused in such calculations (e.g., estimated groundwatervelocities), we recommend that fast outliers (e.g.Westervelt et al., 1997), above the normalized rangesprovided by Ulrich et al., should be viewed withdiscretion.

Although most of the reported biodegradation rates forhydrocarbons in cold groundwater are for BTEXcompounds, some researchers have reported rates forother hydrocarbon components. For example, Spalvinset al. (2001) estimated that the first order biodegradationrate constant foroil in groundwater in Latvia (immis-

cible plus dissolved) was 0.0004day1 (half life of1800days).

Low or insignificant intrinsic rates of hydrocarbondegradation were reported in two field studies ofgroundwater at cold climate sites (Mitchell andFriedrich, 2001; Richmond et al., 2001). One of thesewas one of coldest sites (MAAT=11C) included inthis review, located at Komakuk Beach in YukonTerritory, Canada (Mitchell and Friedrich, 2001: seeCase study 2). The other report of negligible hydrocar-bon biodegradation was a study of a mixed contaminant

plume at Fairbanks Alaska (Richmond et al., 2001). Incontrast, other studies of fuel plumes in the samegeological setting in the Fairbanks area have indicatedsignificant biodegradation of hydrocarbons (Westerveltet al., 1997; Braddock et al., 2001). Furthermore, despitetheir negative conclusion about intrinsic bioremediation,Richmond et al. (2001) reported concentrations offerrous iron and sulfide in that plume, which suggestedmicrobial iron and sulfate reduction were dominantterminal electron accepting processes in the aquifer (i.e.that anaerobic bacteria were active in the groundwater atthis site). Together, this evidence suggests that thecomplex mixture of contaminants in the plume studied


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by Richmond et al. may have limited the hydrocarbonbiodegradation potential. Confirmation of this hypoth-esis would require further study.

4.6. Relationship of biodegradation rates to oxygen

availability, temperature, other factors: evidencefrom laboratory tests

Some of the studies listed as field investigations inTables 2, 3 and 4 included laboratory microcosm tests withaquifer and/or groundwater samples (Table 5). Themajority of tests with aquifer/groundwater samples listedin Table 5 were conducted either completely under aerobicconditions, or using combination of both anaerobic andaerobic tests. A few researchers applied only anaerobictests (Acton and Barker, 1992; Zheng et al., 2002a,b,c).

Where compared, mineralization/biodegradation ratestended to be slower under anaerobic conditions comparedto aerobic, for example by factors of440 in studiesreported by Wilson et al. (1986), Barkeret al. (1987), Crosset al. (2003), and Salminen et al. (2004). However,Billowits et al. (1999) reported that aeration did notenhance the hexadecane degradation rate in tests withaquifer samples saturated with groundwater from a fuelplume at Whitehorse, Yukon Territory, Canada.

The effect of temperature on microbial growth and/orthe biodegradation rate of hydrocarbons in groundwaterand/or aquifer sediments has been examined in

laboratory studies by Bradley and Chapelle (1995),Braddock et al. (2001), Cross et al. (2003) and VanStempvoort et al. (2004). Bradley and Chapelle (1995)challenged the conventional assumption that psychro-philic microorganisms have slower metabolism thanmesophilic microorganisms. The conventional Q10assumption had inferred that the rate of biodegradationof contaminants would be lower by half with each 10Cdecrease. Bradley and Chapelle studied microorganismsin microcosms containing samples from a jet fuel-contaminated shallow sand aquifer in Adak, Alaska,

which they reported to be psychrophiles. In experimentsunder aerobic conditions, these microorganisms werecapable of degrading toluene at 5C at a faster rate (byapproximate factor of two) than mesophiles growingaerobically in other microcosms containing sedimentfrom a petroleum-contaminated shallow aquifer inSouth Carolina at 20C. The groundwater temperatureat the Alaska site was reported to be 46C. Theyconcluded that intrinsic remediation (i.e., monitorednatural attenuation) might be a viable approach for coldsites.

Braddock et al. (2001) also conducted tests underaerobic conditions, using hydrocarbon-contaminated soil

and groundwater from Alaska, at 4, 10, 15 and 25C. Theyfound thatmicrobial growth (heterotrophs andhydrocarbondegraders)was enhanced as the temperature increased from4 to 10C, and was optimal at either 1015C or 25C.

The above tests (Bradley and Chapelle, 1995;

Braddock et al., 2001) were conducted under aerobicconditions, whereas it appears that intrinsic bioremedia-tion of hydrocarbons in groundwater at cold sites is oftendominated by anaerobic processes. In anaerobic labora-tory tests by Cross et al. (2003), an increasein temperaturefrom 10 to 20C resulted in an approximately two-foldincrease in the degradation rate of total extractablehydrocarbons (i.e. Q10= 2). In contrast, Cross et al.(2003) inferred a Q10 of 1.38 for biodegradation ofdodecane in aerobic, nutrient amended tests over thetemperature range 10 to 28C.

In aerated tests, Van Stempvoort et al. (2004) foundthat the overall rates of losses of total and C6C10fraction of hydrocarbons increased by 80 and 50%,respectively, when the temperature was increased from 4to 23C. However, in O2-limited batch tests, an increasein temperature from 5 to 23C had no observable effecton the hydrocarbon loss/degradation rate.

Several investigators have reported laboratory evi-dence that hydrocarbon biodegradation rates in ground-water increase with addition of various nutrient mixtures,including mixtures containing various K, PO4, NH4, NO3,SO4, and Cl salts (Braddock and McCarthy, 1996;

Braddock et al., 2001), NO3, NH4+, urea, and K3PO4(Whyte et al., 1998; Billowits et al., 1999; Soloway et al.,2001)andamixtureofK2HPO4, NH4ClandKNO3 (Crosset al., 2003). Van Stempvoort et al. (2004) found thataddition of (NH4)2HPO4 enhanced therate of hydrocarbondegradation in batches amended with ferrous sulfate, butnot in batches amended with ferric iron (FeOOH).

ThestudybyWhyteetal.((Whyte et al., 1998; Billowitset al., 1999; Soloway et al., 2001) with samples from a fuel-contaminated aquifer in Whitehorse, Yukon Territory inCanada had mixed results. The nutrient addition increased

hexadecane mineralization in microcosms containingaquifer sediment samples saturated with groundwater, butnot in tests with groundwater alone. Whyte et al. (1998)provided several hypotheses to explain these results,including the possibility that sessile microorganismsattached to aquifer sediment particles were more effectiveat degrading alkanes than planktonic microorganismssuspended in the groundwater.

Laboratory evidence that nitrate (Cross et al., 2003;Fan et al., 2006), sulfate (Cross et al., 2003; VanStempvoort et al., 2004; Fan et al., 2006) or ferric ironamendments (Van Stempvoort et al., 2004) may serve asan electron acceptor to enhance degradation has been


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Table 5Summary of laboratory tests included in this review

Geographiclocation ofsamples (study)

Plume type (geologicmedium)

Type of test Lab temperature (test period)

Measured/observed rate/amount ofdegradation

Traverse City,Michigan,USA (Wilsonet al., 1986;Hutchinset al., 1991)

Benzene, toluene andxylenes from aviation fuelspill (sand and gravelaquifer)

Microcosms with aquifersamples, some with nitrate/nutrient amendments

12 C (aerobic 2 weeks;anaerobic 8 weeks)

In study by Wilson et al., aromatichydrocarbons declined by one order inmagnitude under anaerobic conditions,

by two orders under aerobic conditions;in anaerobic microcosms reported byHutchins et al, benzene was recalcitrant,first order rate constants for otheraromatics under denitrifying conditionsranged from 0.022 to 0.067 day1

Alliston,Ontario,Canada(Barker et al.,1987)

Aromatic hydrocarbons(surficial sand aquifer)

Microcosms with aquifer coresand groundwater from site,aerobic and anaerobic

10 C (6080 d) Inferred zero order rates of0.03 mg/Lday1 for benzene, toluene and xylene

biodegradation under aerobic conditions,much slower rates under anaerobicconditions

Ottawa, Ontario,Canada(Berwangerand Barker,1988)

Landfill leachate (sandaquifer)

Aerobic microcosmscontaining aquifer sedimentsand groundwater; spiked withBTEX; addition of peroxide assource of oxygen to mediate

biodegradation

10 C (80 days) Added benzene, toluene andethylbenzene eliminated by 20 days,xylenes partially degraded

Michigan, USA(Chiang et al.,1989)

Gas condensate at a gasplant (sand aquifer)

Microcosms with groundwaterand aquifer material; dissolvedO2: 0.0, 0.1, 0.5, 1, 2, 4, 5 or8 mg L1

10 C (2835 days) 80 to 100% of added BTX weredegraded, with half lives of 520 days inmicrocosms with O22 mg/L; ratesslower with O2b2 mg/L; negligibledegradation at lowest or no O2

North Bay,Ontario,Canada(Acton andBarker, 1992)

Landfill leachate (sandaquifer)

Anaerobic microcosms withgroundwater and aquifersamples, nitrate and glucoseamendments

10 C (187 days) No degradation observed

Adak, Alaska,USA (Bradleyand Chapelle,1995)

Jet fuel (shallow sandaquifer)

Aerobic microcosms withaquifer sediments:mineralization of14C-labelledtoluene and acetate

5 C (3, 25 h) Psychrophiles were capable ofdegrading toluene at 5 C at a faster rate(factor of two) than mesophilesgrowing aerobically in sediment from a

petroleum-contaminated shallow aquiferin South Carolina at 20 C; overallmicrobial metabolic rates were similar attheir respective in situ temperatures

Near Barrow,Alaska, USA(Braddock and

McCarthy,1996)

Gasoline and jet fuel spills,(sand, gravel deposits, thinsaturated zone above

permafrost)

Aerobic microcosm tests withgroundwater samples, nutrientadditions: mineralization of14

C-labelled benzene

10 C (10 days) Groundwater in contaminant plume hadgreater benzene mineralization potentialthan groundwater sampled outside

plume; benzene mineralization ratesranged from 0.1 to 0.5 mg L1 day1 insamples from contaminated wells andincreased to 0.7 mg L1 day1 whennutrients were added

NorthernManitoba,Canada(Shields et al.,1997)

Diesel fuel (not reported) Bench scale batch-activatedsludge test

5 C (unspecified) Removal of up to 60% of thehydrocarbons

Whitehorse,Yukon,Canada(Whyte et al.,

1998;

Diesel fuel (shallow sandand silt)

Aerobic and anaerobicmicrocosms containing aquifersamples, saturated withgroundwater

5 C (60 days) Mineralization of14C-labelledhexadecane (C16) by indigenousmicroorganisms,aeration did notenhance; nutrient treatment had very

little effect on overall TPH


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reported. In microcosm tests by Cross et al. (2003) withgroundwater from a diesel-contaminated fractured rockaquifer in Alberta, Canada, amendments with nitrate ornutrients had a larger positive effect on increasing the

biodegradation rate than the raising of temperature from10 to 20C. Cross et al. (2003) also reported that sulfateamendments increased the hydrocarbon biodegradationrate in anaerobic microcosms.

Table 5 (continued)

Geographiclocation ofsamples (study)

Plume type (geologicmedium)

Type of test Lab temperature (test period)

Measured/observed rate/amount ofdegradation

Billowits

et al., 1999)

concentrations. Anaerobic denitrification

occurred in some testsHaines Junction,YukonTerritory,Canada(Billowitset al., 1999)

Pit disposal of oil refineryproducts (shallow sand-gravel aquifer)

Anaerobic and aerobicmicrocosm tests with aquifersamples - mineralization of14C-labelled hexadecane;aerobic soil column tests reduction of TPH and oxygen

Microcosms: 5 C(60 days) columns:7 C (58 days)

Nutrient addition enhanced thedegradation of TPH, whereas aerationdid not; degraded fraction ofhydrocarbons appeared to consist largelyof linear compounds; indigenous bacteriawere capable of degrading large fractionsof naphthalene (50%) and toluene (30%)over 60 days, but not14C-labelledhexadecane

FairbanksAlaska, USA(Braddocket al., 2001)

Petroleum-distillate (alluvialsand and gravel aquifer)

Aerobic tests with groundwatersamples, nutrient addition;microbial growth;mineralization of naphthalene,

benzene

Microbial growth: 4,10, 15 and 25 C;mineralization: 10 C(up to 43 days)

Microbial growth was enhanced astemperature increased from 4 to 10 C,optimal at either 1015 C or 25 C;nutrient addition generally enhanced the

mineralization of benzene andnaphthalene at 10 C

Olso, Norway(Zheng et al.,2002a,b,c)

Jet fuel (sand and gravel) Anaerobic column tests ofaquifer sediment withgroundwater

810 C (8 to17 days)

Intrinsic biodegradation of toluene(estimated average intrinsic rate of 0.160.27 mM day1) was coupled with themicrobial reduction of ferric iron (Fe(III)); intrinsic biodegradation of 1,2,4-trimethylbenzene was slower: 0.050.13 mM day1)

Alberta Canada(Cross et al.,2003)

Diesel (fractured bedrock) Anaerobic microcosms withgroundwater, some withnutrient/nitrate or sulfateamendments aerobicmicrocosms with groundwater,some with nutrientamendments

Anaerobic: 10 and20 C (790 days)aerobic: 10 and 28 C(187 days)

Anaerobic: sulfate, nitrate/nutrients andhigher temperature (20 C) resulted ingreater losses of total extractablehydrocarbons (38, 5170% and 46%respectively), compared to unamended,10 C (20%) aerobic tests: mineralizationof14C-labelled dodecane: withoutnutrients - 3%; with nutrients - up to 23%(10 C) to 36% (28 C)

Moose Factory,Ontario,Canada (VanStempvoortet al., 2004)

Fuel (shallow sand aquifer) Aerobic and oxygen-limitedbatch tests with aquifersamples; some of latter withsulfate, ferric iron (FeOOH),ammonium phosphate, and/orhumic substances added

45 and 23 Caerobic: (60 d) O2limited: (110 d)

Overall losses/degradation rates ofhydrocarbon degradation similar in alltests (aerobic or O2 limited, 45 or23 C). In O2 limited batches, thoseamended with ferric iron had largestlosses of hydrocarbons; nutrient additionenhanced the hydrocarbon losses in

batches amended with sulfate, suggestingrole of sulfate reducers; addition of

humic substances had no noticeableeffect in O2-limited batches

Hanko, Finland(Salminenet al., 2004;Purkamoet al., 2004)

Lightweight fuel,lubrication oil at industrialdumpsite (sand, gravel, silt,clay)

Aerobic and anaerobicmicrocosms with soil samples

8 C anaerobic: (1012 mo) aerobic: (34 mo)

Average of 1544% (up to 64%) removalof mineral oil in anaerobic tests, methane

production;, average of 2731% (up to75%) removal in aerobic tests;

preferential degradation of C10C15fraction


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The results summarized above indicate that a range offactors, including temperature, the availability of variouselectron acceptors, and nutrients, exert some control on therate of biodegradation of hydrocarbons in groundwater.However, given the limited scope of the studies conducted

to date, significant further research would be required tofirmly establish quantitative models of the interactive rolesof these various factors on the rate of biodegradation ofhydrocarbonsin groundwater. Based on current results, wecan conclude that temperature does not exert a consistent,predictable, over-riding control on the rate of biodegrada-tion of hydrocarbons in groundwater.

4.7. Extreme case studies: coldest sites, permafrost

settings

4.7.1. Case study 1: Barrow, Alaska, USABraddock and McCarthy (1996) investigated biodeg-radation in hydrocarbon-contaminated groundwater insand and gravel deposits at an Arctic site near Barrow,Alaska, adjacent to the Arctic Ocean. The soil andgroundwater at this site were contaminated by gasolineand jet fuel spills in 197678. Here the annual average airtemperature is 12C, with groundwater present as arelatively shallow, thin layer above the permafrost (i.e.suprapermafrost water),with short, localizedgroundwaterflow paths inferred. Air temperatures at this site rise abovefreezing for 90days per year (JuneAugust). The

authors found that although twenty years had elapsedsince the hydrocarbon spills, BTEX concentrations werestill elevated in the groundwater near the source of thecontamination. During their monitoring program, thegroundwater temperatures ranged between 1.2 and 7.4C.They found strong evidence for intrinsic bioremediationof the BTEX plume, including depleted oxygen andnitrate, increases in ferrous iron and sulfide, and highermicrobial populations. The groundwater in the BTEXp

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