the variability and intrinsic remediation of a btex plume in anaerobic sulphate-rich groundwater

Ž .Journal of Contaminant Hydrology 36 1999 265–290

The variability and intrinsic remediation of a BTEXplume in anaerobic sulphate-rich groundwater

G.B. Davis a,), C. Barber a, T.R. Power a, J. Thierrin b,B.M. Patterson a, J.L. Rayner a, Qinglong Wu c

a Centre for Groundwater Studies, CSIRO Land and Water, PriÕate Bag, PO Wembley 6014, WesternAustralia, Australia

b UniÕersity of Neuchatel, Neuchatel, Switzerlandc Tsinghua UniÕersity, Beijing, China

Received 24 December 1997; revised 22 September 1998; accepted 22 September 1998

Abstract

Data from long-term groundwater sampling, limited coring, and associated studies are synthe-sised to assess the variability and intrinsic remediationrnatural attenuation of a dissolved

Žhydrocarbon plume in sulphate-rich anaerobic groundwater. Fine vertical scale 0.25- and 0.5-m. Ž .depth intervals and horizontal plume-scale )400 m characteristics of the plume were mapped

Žover a 5-year period from 1991 to 1996. The plume of dissolved BTEX benzene, toluene,.ethylbenzene, xylene and other organic compounds originated from leakage of gasoline from a

subsurface fuel storage tank. The plume was up to 420 m long, less than 50 m wide and 3 m thick.In the first few years of monitoring, BTEX concentrations near the point of leakage were in

Ž .approximate equilibrium with non-aqueous phase liquid NAPL gasoline. NAPL composition ofcore material and long-term trends in ratios of BTEX concentrations in groundwater indicated

Ž .significant depletion water washing, volatilisation and possibly biodegradation of benzene fromresidual NAPL after 1992. Large fluctuations in BTEX concentrations in individual boreholeswere shown to be largely attributable to seasonal groundwater flow variations. A combination oftemporal and spatial groundwater quality data was required to adequately assess the stationarity ofplumes, so as to allow inference of intrinsic remediation. Contoured concentration data for theperiod 1991 to 1996 indicated that plumes of toluene and o-xylene were, at best, only partially

Ž .steady state pseudo-steady state due to seasonal groundwater flow changes. From this analysis, itwas inferred that significant remediation by natural biodegradation was occurring for BTEXcomponent plumes such as toluene and o-xylene, but provided no conclusive evidence of benzenebiodegradation. Issues associated with field quantification of intrinsic remediation from groundwa-

) Corresponding author. Fax: q61-8-9387-8211; e-mail: [email protected]

0169-7722r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-7722 98 00148-X

( )G.B. DaÕis et al.rJournal of Contaminant Hydrology 36 1999 265–290266

ter sampling are highlighted. Preferential intrinsic biodegradation of selected organic compoundswithin the BTEX plume was shown to be occurring, in parallel with sulphate reduction andbicarbonate production. Ratios of average hydrocarbon concentrations to benzene for the period

Ž .1991 to 1992 were used to estimate degradation rates half-lives at various distances along theplume. The estimates varied with distance, the narrowest range being, for toluene, 110 to 260days. These estimates were comparable to rates determined previously from an in situ tracer testand from plume-scale modelling. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Natural attenuation; Biodegradation; Leaking underground storage tanks; Gasoline; Groundwater;Pollution

1. Introduction

Hydrocarbon compounds in gasoline, such as benzene, toluene, ethylbenzene andŽ . Žxylenes BTEX can be relatively mobile in groundwater Kennedy, 1992; Davis et al.,

. Ž .1993 . Non-aqueous phase liquid NAPL gasoline and other fuels provide a long-termŽ .source for these compounds e.g., Mackay et al., 1991; Hess et al., 1992 . Intrinsic

remediation, or what is also referred to as natural attenuation of the BTEX compoundsŽin groundwater has been reported by numerous authors e.g., papers in Hinchee et al.,

.1995; Alleman and Leeson, 1997 . However, there is considerable debate and uncer-tainty, especially in determining and quantifying the intrinsic remediation and fate ofbenzene in anaerobic groundwater environments. Since benzene exposure is often theprincipal risk at BTEX-contaminated sites, the processes governing the fate of benzeneand the assessment procedures for inference of intrinsic remediation of BTEX plumesneed further investigation.

Quantifying the attenuation mechanisms for BTEX compounds in anaerobic aquifersand determining their mobility and longevity in groundwater can be difficult. Protocolshave been developed to assess the applicability of intrinsic remediation mechanisms at

Ž .individual sites Wiedemeier et al., 1995 . Controlled release experiments in aerobicŽ . Ž .aquifers were carried out by Barker et al. 1987 and MacIntyre et al. 1993 to track the

Ž .fate of BTEX compounds. Yang et al. 1995 used BTEX concentration ratios to helpdefine plume dimensions and source characteristics. Also, degradation and attenuation of

ŽBTEX compounds has been well documented for oxic conditions papers in Hinchee and. ŽOlfenbuttel, 1991; Alleman and Leeson, 1997 and for some anaerobic conditions e.g.,

Haag et al., 1991; Acton and Barker, 1992; Barbaro et al., 1992; Beller et al., 1992;Edwards et al., 1992; Edwards and Grbic-Galic, 1992; Thierrin et al., 1992, 1993;

. Ž .Patterson et al., 1993b; Thierrin et al., 1995 . Rifai et al. 1995 catalogue many of thefield studies up to 1994 which look at intrinsic remediation of BTEX compounds in

Ž . Ž .different reducing groundwater environments. Wilson et al. 1990 , Borden et al. 1995 ,Ž . Ž . Ž .Rifai et al. 1995 , Wilson et al. 1995 and Chapelle et al. 1996 have reported BTEX

biodegradation within anaerobic sulphate-reducing zones of an aquifer, induced by thepresence of the contamination. Few studies, however, have reported data within anaturally occurring anaerobic sulphate-reducing groundwater environment, nor investi-gated year-to-year data variability over long time frames and its impact on assessmentand quantification of intrinsic remediation processes for BTEX compounds.

( )G.B. DaÕis et al.rJournal of Contaminant Hydrology 36 1999 265–290 267

In this paper, data are described from a gasoline leakage site, where individual BTEXand other organic compounds are variably attenuated within anaerobic sulphate-richgroundwater. The contamination affects groundwater in the superficial aquifer formationused for drinking water in Perth, Western Australia. The short- and long-term variabilityin the three-dimensional distribution of BTEX and other organic and inorganic com-pounds has been mapped over a 5-year period. The study aimed to determine three-di-mensional BTEX plume dynamics in an anaerobic aquifer and to determine intrinsicremediation and biodegradation rates for individual organic compounds at various scalesof the aquifer. The data have significant implications for sites where it may be wronglyconcluded, from limited spatial and temporal monitoring, that intrinsic remediation isoccurring. The long-term trend in the organic compound concentrations also provideindications of the longevity of residual NAPL in the near-water table zone as a sourcefor dissolved BTEX compounds in groundwater.

2. Hydrogeology of the site

The site is in metropolitan Perth, Western Australia, where gasoline has leaked togroundwater from a subsurface fuel storage tank at a service station. The duration orvolume of the leakage was not accurately known, but the tank had been inoperable since1990. The site is located on the eastern fringe of the Quaternary Bassendean Sands ofthe Perth Swan Coastal Plain. At the site, 7 to 12 m of medium to fine dune sandoverlies a thick clay aquitard of the Guildford Formation. A layer of dark-brown

Ž .iron-stained sands, with variable thickness typically 5 to 60 cm locally called coffeeŽ .rock is found in the zone of water table fluctuation Baddock, 1988 . The coffee rock is

a variably cemented layer composed principally of iron oxyhydroxide-coated sands andaccumulated organic matter leached through the soil profile. Seasonally, the water table

Ž .can fluctuate up to 1.8 m, with increases due to winter June to August rainfall. Annualrainfall is approximately 800 mmryr. The saturated thickness of the aquifer is approxi-mately 6 m.

Local groundwater flow is toward the southeast with a gradient of 0.003 to 0.005.From laboratory testing on aquifer sand samples from the site and a small-scale tracer

Ž .test Thierrin et al., 1995 , the groundwater velocity in the immediate vicinity of the sitewas estimated to be between 100 and 170 mryr. The hydraulic conductivity was

y4 y4 Ž .determined to be in the range 10 to 3.3=10 mrs 8.6 to 29 mrday , and theeffective porosity estimate ranged between 0.26 and 0.3 m3rm3.

3. Investigative methods

After initial investigation of groundwater quality at the site with short-screenedŽ . Ž .sighting boreholes Barber et al., 1991 , permanent multiport MP boreholes MP1 to

MP12 were installed, 11 within or on the perimeter of the BTEX plume and MP9up-gradient to serve as a background borehole. The MP boreholes were installed by firstauguring to the required depth with a hollow stem auger, insertion of the MP borehole


bundle through the centre of the auger, and then removing the augers to allow theunconsolidated sand to collapse around the installation. The locations of the sightingbores and the initial MP boreholes are shown in Fig. 1. Each MP borehole consisted of abundle of 12 stainless steel tubes of internal diameter 2.5 mm slotted over the bottom0.2 m. The slotted intervals were spaced 0.5 m apart in the vertical, except on theboreholes MP10, MP11 and MP12. MP10 and MP11 had vertical spacings of 0.25 m.MP12 had vertical spacings of 0.20 m, and was a replacement for MP10 which wasexcavated during removal of the underground storage tank in July 1993. An additional

Ž . Ž . Ž .Fig. 1. Contoured plumes for a benzene, b toluene and c sulphate in groundwater for April 1991, alsoshowing the locations of multiport and other sighting boreholes.


seven MP boreholes, denoted MP13 to MP19, were installed across two transects inAugust 1995, to provide a better definition of plume width. The locations of these MP

Ž .boreholes are noted on later contour plots for August 1996 e.g., Fig. 8 . Constructionwas similar to that of MP1 to MP9, except MP13 to MP19 were fabricated from 4.8-mminternal diameter nylon tubing, and had 10 ports with 0.2-m long slots placed at verticalspacings of 0.5 m.

Groundwater samples were recovered via suction directly from the access lines of themultiport boreholes using plastic syringes for inorganic compounds and glass syringes

Ž .for organic compounds Davis et al., 1992; Patterson et al., 1993a . An initial volume ofŽ .50 ml more than 3 times the access line volume was purged from the MP access lines

prior to recovery of each groundwater sample. This strategy was adopted to minimisedisturbance of the groundwater and sorptive andror volatile losses of the volatile BTEXcompounds, and also ensured minimal purging times for shallow aquifer systems with adepth of unsaturated zone less than approximately 8 m.

Groundwater was sampled from available MP boreholes every 1 to 2 months in 1991and less frequently in subsequent years to determine seasonal trends. There were 16complete samplings of all available MP boreholes. To assess short-term trends, weeklysampling was also carried out from selected MP boreholes over the June–July 1992winter period. The organic compounds were concentrated from groundwater samplesusing purge and trap or micro-solvent extraction techniques, and analysed by Gas

Ž . Ž .ChromatographyrMass Selective Detection GCrMSD Patterson et al., 1993a .Groundwater samples recovered from the MP boreholes were analysed for a range of

Ž .organic compounds including the BTEX compounds, 1,3,5-trimethylbenzene TMB ,Ž .and naphthalene, typical degradation products e.g., methane, phenol, cresols and

Ž .inorganic compounds e.g., nitrate, sulphate, iron, etc. .ŽIn general, data were hand-contoured computer-generated contours were not at-

.tempted due to the limited data in the plane . Also, plan contour plots of the plumeextent were drawn taking the maximum concentration in the vertical profile at eachmultiport borehole, and for sulphate, the minimum was taken. This strategy assumes thatthe maxima and minima were approximately located along the same flow path.

ŽTwo intact cores were recovered from close to MP11 30 m down-gradient of the.leaking tank location to establish the vertical distribution and composition of any

residual NAPL gasoline in the soil profile. The core was segmented into 5 to 10 cmsections, extracted with a diethyl ether and acetone mixture and the extracts wereanalysed by GCrMSD.

4. Results and discussion

4.1. General plume shape and the presence of NAPL

In general, the organic compounds dissolved in groundwater mapped out long, thinŽ .plumes see Figs. 1–3 for plan and cross-sectional views . The benzene plume for April

Ž . Ž .1991 had a minimum length of 420 m Fig. 1 , width 20 to 50 m Figs. 1 and 3 andŽ .thickness 0.5 to 3 m Fig. 2 . The toluene plume for April 1991 was less than 250 m


Ž . Ž . Ž .Fig. 2. Depth cross-sections along the plume showing a benzene, b toluene and c sulphate concentrationsin groundwater for April 1991. Dots are locations of monitoring ports.

long and 0.5 to 2 m thick. o-Xylene data were similar to toluene, while ethylbenzeneŽ .and m- and p-xylene data were similar to benzene data not shown .

Very steep concentration gradients were maintained vertically and laterally across theplume in the aquifer, probably due to very small dispersion coefficients in these

Ž .directions Thierrin et al., 1993 or due to enhanced microbial activity on the fringes ofthe plume, leading to degradation of the organic compounds. Longitudinal concentrationgradients were shallower, although still significant in the highly contaminated portion ofthe plume between MP11 and MP2. The longitudinal cross-sections also show apronounced dip towards the base of the aquifer at about 80 m down-gradient of the

Ž .source Fig. 2 , where up to 1 m of non-contaminated groundwater overlayed the plume.This is apparently due to enhanced rainfall recharge over this portion of the plume,which at this distance flows beneath a vacant lot, compared to sealed surfaces at other


Ž . Ž . Ž .Fig. 3. Depth cross-sections across the plume showing a benzene, b toluene and c sulphate concentrationsin groundwater for April 1991, March 1992 and August 1994. Dots are locations of monitoring ports.

locations above the plume. Even so, organic compounds were never detected at the baseof the aquifer, which is only 6 m below the water table.

In Table 1, general longitudinal trends for a range of organic compounds areindicated by the average maximum concentration of organic compounds sampled fromboreholes along the approximate centre line of the plume between April 1991 and March1992. Benzene, ethylbenzene and m- and p-xylene concentrations were significantlyabove background at 420 m down-gradient, however, toluene, o-xylene, TMB andnaphthalene concentrations decreased to below detection limits prior to 420 m. Ratios ofthe concentrations of the organic compounds to benzene are tabulated in Table 2. All

Table 1Ž .Average of highest hydrocarbon concentrations mgrl from multiport boreholes along the approximate centre

line of the plume, between April 1991 and March 1992

Ž .Compound Multiport borehole number and distance from the source m

MP10 MP11 MP3 MP6 MP7 MP80 30 80 150 260 420

Benzene 36,000 38,200 19,800 10,600 8800 8000Toluene 75,000 67,000 18,500 2600 1000 0Ethylbenzene 8200 3900 1800 900 490 260m- and p-Xylene 38,700 19,200 4900 2800 1300 460o-Xylene 15,600 8000 2400 980 375 01,3,5-trimethylbenzene 3200 510 210 130 100 0Naphthalene 1100 380 140 65 40 0


Table 2ŽRatios of organic concentrations to benzene concentrations, relative to ratios at the source calculated from

.Table 1

Ž .Compound Multiport borehole number and distance from the source m

MP10r12 MP11 MP3 MP6 MP7 MP80 30 80 150 260 420

Toluene 1.0 0.84 0.45 0.12 0.05 0Ethylbenzene 1.0 0.45 0.40 0.25 0.24 0.14m- and p-Xylene 1.0 0.47 0.23 0.22 0.14 0.05o-Xylene 1.0 0.48 0.28 0.25 0.10 01,3,5-trimethylbenzene 1.0 0.15 0.12 0.14 0.13 0Naphthalene 1.0 0.33 0.23 0.20 0.15 0

Ž .ratios, except for TMB, decreased with distance from the source assumed at MP10 ,especially between 80 and 260 m. Relative decreases along the length of the plume werenot uniform. For example, toluene ratios were the highest of all ratios within 30 m of the

Ž .source and lowest at 150 m Table 2 . Possible mechanisms for this are discussed below.Ž .In 1991, concentrations of benzene and toluene at MP10 and often MP11 near the

source of the leak were comparable to concentrations in equilibrium with gasolineŽ . Ž .NAPL see Table 1 , thus indicating the likely presence of NAPL. Cline et al. 1991

found for a typical gasoline that aqueous concentrations in equilibrium with gasolineNAPL would be approximately 43 mgrl for benzene and 70 mgrl for toluene, althoughthe percentage of aromatics in gasoline can vary considerably.

Core material from the water table zone was recovered for analysis of residual NAPLfrom approximately 5 m to the north of MP11 in February 1995 and in August 1996.The two cores were taken approximately 3 m apart across the direction of groundwater

Ž .flow. The 1995 core indicated significant BTEX concentrations Fig. 4 over anapproximate 0.4-m depth interval, with peak concentrations at about 3.375 m belowground level. The water table was approximately 3.25 m below ground level when thecore was recovered. Peak concentrations were 1200 mgrkg for m- and p-xylene, 670mgrkg for toluene, 450 mgrkg for o-xylene, 310 mgrkg for ethylbenzene, 140 mgrkgfor 1,3,5-TMB, and 33 mgrkg for naphthalene. Benzene concentrations were signifi-cantly depleted relative to the other organic compounds with peak benzene concentra-tions at the same depth only 4.3 mgrkg. Further discussion of benzene depletion is in

Ž .Section 4.3. The core recovered in August 1996 data not shown showed a similarBTEX composition to the core recovered in February 1995, however, in the latter core,significant NAPL concentrations extended over a 1.5-m depth interval, which is moretypical of the annual range of water table movement. Benzene concentrations were againdepleted relative to the other organic compounds. When potential adsorbed and waterphase fractions are accounted for, the concentrations noted here and in Fig. 4 are wellabove those necessary to maintain an equilibrium concentration in groundwater, i.e., theconcentrations measured imply that residual NAPL was present in the recovered core,and therefore that NAPL had moved down-gradient from the point of leakage by up toapproximately 30 m.


Fig. 4. Depth profile of hydrocarbon concentrations in a soil core recovered near MP11 in February 1995.

4.2. Inorganic chemistry and indicators of biodegradation

A summary of inorganic analyses is given in Table 3 for groundwater samples fromcontaminated and uncontaminated regions of the site. Mean dissolved O and NOy

2 3

concentrations were less than 0.6 mgrl and 0.2 mgrl, respectively, inside or outside theplume. This may be due to oxygen consumption and denitrification induced by high

Table 3Mean inorganic analyses in contaminated and uncontaminated groundwater at the sitea

Parameter Uncontaminated Contaminatedb cgroundwater groundwater

w xO mgrl 0.56 0.162

pH 5.1 5.7w xEh mv 103 y30y w xNO mg-Nrl 0.18 0.053q w xNH mg-Nrl 0.25 0.34

y w xHCO mgrl 14.3 7432y w xSO mg-Srl 27.5 10.84

w xH S mg-Srl 0.1 1.122q w xFe mgrl 1.6 1.4

a Ž .After Linge 1996 .b Mean values for inorganic and physical parameters from 170 uncontaminated groundwater samples close tothe contaminant plume from eight sampling occasions from April 1991 to February 1993, except for NOy and3

q Ž . ŽNH 80 samples in three sampling periods from April to July 1991 , and for H S 54 samples from February4 2.1993 .

c Ž .Mean values from 102 samples of contaminated groundwater with )1000 mgrl total BTEX for the samey q Ž . Žperiods, except for NO and NH 30 samples from April to July 1991 , and for H S 20 samples from3 4 2

.February 1993 .


Ž . Žlevels of background organic carbon OC within Perth groundwater Gerritse et al.,.1990 , although no direct measurements of dissolved OC were made at the site. Table 3

shows a significant decrease of Eh and sulphate concentrations, and increased HCOy3

and H S concentrations within, compared to outside the contaminant plume. Depth2Ž . Ž .profiles Fig. 5 for MP11, 30 m down-gradient of the source see Fig. 1 for location ,

also show these trends. No systematic trends were apparent for Fe2q, and methane wasŽ .not detected within the contaminant plume. Prommer et al. 1998a also observed small

amounts of pyrite in some core samples recovered within the plume, and none in abackground core.

Since mass loss of BTEX compounds was observed simultaneously with a drop in theEh value, production of CO and H S, as well as the consumption of SO2y, it is2 2 4

apparent that sulphate was acting as an electron acceptor for microbially-enhanceddegradation of the organic compounds. In Figs. 1–3, sulphate concentrations aresubstantially depleted at the centre of the plume of organic compounds, in some cases tobelow 1 mgrl from background concentrations of 20 to 50 mgrl. Sulphate concentra-tions were always less than 15 mgrl, and mostly less than 5 mgrl in regions of theplume with benzene concentrations greater than 100 mgrl. Reduction of sulphateconcentrations within the contaminated portions of the plume was also often accompa-nied by elevated H S concentrations up to 5.5 mgrl, compared with below 0.1 mgrl2

outside the plume.Assuming stoichiometrically that biodegradation of the BTEX compounds by sul-

Ž .phate reduction can be written as Borden et al., 1995 :

C H q4.5 SO2y q3 H O™2.25 H Sq2.25 HSyq7 HCOy q0.25 Hq7 8 4 2 2 3

then it is possible, from the depth profiles in Fig. 5 or data in Table 3, to carry out asimple mass balance calculation to assess whether sulphate is the major electron

Ž .acceptor for hydrocarbon mineralisation Linge, 1996 . For example, using the differ-ences in mean concentrations for S–SO2y and HCOy within and outside the plume in4 3

y Ž y4 .Table 3, the theoretical increase in HCO concentration 8.8=10 M calculated32y Ž y4 .from the observed decrease in mean S–SO concentrations 5.2=10 M were4

y Ž .comparable to observed HCO concentration increases within vs. outside the plume of3

Fig. 5. Depth profiles of groundwater quality parameters in MP11.


9.8=10y4 M. The ratio of that observed to that estimated from S–SO2y consumption4

was therefore 1.11. This adds weight to the premise that S–SO2y is the predominant4

electron acceptor at the site, and indicates that other electron acceptors such as ironŽ 3q.oxyhydroxides Fe on the aquifer matrix have either been significantly depleted

within the plume or have played a minor role in the degradation process. This is alsoŽ .supported by recent work by Prommer et al. 1998a , who, although not dismissing the

role of Fe3q in the biodegradation process at the site, found through wet extractionanalysis of soil cores and modelling that only low concentrations of Fe3q contributed tothe oxidation capacity of the aquifer.

Potential intermediate degradation products of m-, p- and o-xylene and toluene, suchŽ .as m-, p- and o-cresols and phenol Grbic-Galic and Vogel, 1987 , were also detected

in groundwater samples from the multiport boreholes. These were found in the highlyŽcontaminated portion of the plume generally where BTEX concentrations were greater

.than 10 mgrl , less than 80 m down-gradient of the source between May 1991 and JuneŽ .1992 data not shown . For example, in December 1991 phenol concentrations up to 1

mgrl and p-cresol concentrations up to 1.6 mgrl were determined in samples fromMP11, where toluene and xylene concentrations ranged up to 54 mgrl and 15 mgrl,respectively.

4.3. Temporal changes

4.3.1. IndiÕidual boreholesTemporal trends of BTEX concentrations in individual boreholes were highly vari-

able and seemed to be strongly associated with seasonal movement of the water table.For example, Fig. 6 shows the depth-integrated mass of benzene and toluene for MP11

Ž .and MP3 see locations on Fig. 1 over an approximate 60 month period of monitoring.Water table fluctuations from a piezometer near MP3 are superimposed. The correspon-dence between water table highs and hydrocarbon mass lows and vice versa seems

Ž .strong for MP11 although much less so at 30 months , with a time shift in the trend forborehole MP3, a further 50 m down-gradient. Note however, that only 3–4 cycles ofdata are available—often a larger number of cycles are required to determine frequencycorrelations. In any case, the delayed response at MP3 compared to MP11 may be due to

Žthe velocity of groundwater flow, or sideways migration of the BTEX plume see.below . If the delay is solely due to groundwater flow then from the migration of the

integrated-mass peak, a groundwater velocity of 100–200 mryr can be calculated—thisŽ .is in agreement with earlier estimates by Thierrin et al. 1993, 1995 .

Ž . Ž . Ž .Davis et al. 1993 also showed short-term weekly and medium-term 1 yeardepth-time trends for selected MP boreholes at the site. They showed that over the 1992

Žwinter period as water tables rose, shallower ports of MP10 i.e., higher in the soil.profile were able to be sampled and analyses of these samples continued to show high

benzene concentrations. The apparent plume thickness increased near the source over theperiod of groundwater rise. This was also observed for toluene and the other organiccompounds. Medium-term trends in the benzene profiles at MP10 show more markedvariations in the plume thickness, especially early in 1991. These data seemed to reflectseasonal dissolution of organic compounds from entrapped NAPL within the near-water


Fig. 6. Temporal changes in the groundwater level and the depth-integrated mass of benzene and toluenebetween April 1991 and August 1996 at MP11 and MP3.

table zone, or highly-contaminated capillary-zone water that was seasonally inundatedŽ .by the water table. Based on data from MP10, Davis et al. 1993 also reported that the

dissolved benzene mass in the near-water table soil profile over the 1992 winter periodand over a full year increased. These data indicated that significant depletion of theBTEX and other organic compounds had not occurred near the source up to March1992. Also, monitoring at a fine scale and in the capillary fringe was critical toassessment of loading rates to groundwater near what was a residual NAPL source inthis zone.

Despite the temporal variability of BTEX concentrations in individual boreholes,Ž .ratios of the TEX toluene, ethylbenzene, xylenes and naphthalene concentrations to

Ž .benzene showed more consistent trends Fig. 7 . These ratios indicate that the largeconcentration changes observed over time at individual boreholes may have been due tohydraulic variations in water table heights and groundwater flow paths, and perhaps alsovariability in the contact zone of NAPL dissolution, or possibly variability of microbialdegradation rates or sorption rates over time. The latter two processes seem less likely to


Fig. 7. Temporal trends in the ratios of the peak concentrations of selected organic compounds to benzenebetween April 1991 and August 1996 at MP10r12 and MP11.

induce such large fluctuations. It is apparent from the trends in BTEX concentrationratios near the source, i.e., in MP10r12 and MP11, that the plume source compositionwas approximately constant in the first year of monitoring, from April 1991 to March1992. This observation will be used later to qualify the use of benzene as a conservativetracer of the plume movement, and hence to determine biodegradation rates of the othercompounds relative to benzene.

4.3.2. NAPL concentrationsŽFor multiport MP10r12, beyond March 1992 an elapsed time of 11 months since

.April 1991 , there was a relatively slow increase in the ratio of the TEX and otherorganic compounds to benzene. This indicates preferential depletion of benzene from theNAPL source region relative to the other NAPL constituents. Despite ongoing depletionof benzene from NAPL in the vicinity of MP10r12, high concentrations were main-

Žtained in groundwater samples from MP11 over several samplings approximately 2.years . This indicates that concentrations in groundwater moving down-gradient of

MP10r12 re-equilibrated with residual NAPL andror any adsorbed phase between thetwo boreholes before being sampled at MP11.


No initial soil NAPL compositions are available near the source. However, the coreŽ .recovered in February 1995 Fig. 4 showed significant depletion of benzene from the

residual NAPL relative to the other components. This observed depletion is in agreementwith groundwater data trends over the same time, from samples recovered in August

Ž . Ž .1994 elapsed time of 40 months and September 1995 elapsed time of 53 months ,whereby benzene concentrations at MP10r12 were very low and concentrations weredecreasing sharply at MP11.

By simple modelling, we can also examine the time frames for dissolution of benzeneŽ .from the residual NAPL. Following Geller and Hunt 1993 , the time for the rate of

removal is proportional to the length of contact and inversely proportional to thegroundwater velocity, i.e., essentially the number of pore volumes that pass through theNAPL zone. Another way to express this is to consider the velocity of the zone of mass

Ž .transfer V , which is the ratio of the dissolved compound i leaving the initial mass ofmtz

compound i within the NAPL:

V sq C r d n SŽ .mtz i i i

where: qsDarcy velocity, C saqueous concentration of ith component, d sdensityi i

of ith component, nsporosity, S ssaturation of ith component. If we consideriŽparameters relevant to the field site see for example, Thierrin et al., 1993; Johnston and

. Ž y5 3. 3Rayner, 1996 then qs50 mryr, C s20 mgrl 2=10 grcm , d s0.86 grcm ,i iŽ .ns0.28 and S s0.05 5% residual NAPL saturation with benzene being 1% of this .i

This would give a V of 0.083 mryr. With a 30-m zone of NAPL contaminationmtz

between boreholes MP12 and MP11 it would take 360 years to remove the benzene bydissolution processes. A similar estimate can be obtained by following the formulation

Ž .of Mackay et al. 1991 which gives a timeframe for dissolution of 90–99% of thebenzene from the NAPL of 100 to 400 years.

Both of these methods give timeframes that appear to be one or two orders ofŽmagnitude greater than that observed at the field site which is probably less than 10

.years , and if non-equilibrium dissolution were considered, then the timeframes wouldbe even greater. It may be that the geometry of the NAPL, size of the source andperhaps other loss mechanisms for benzene are critical to this assessment. Clearly,dissolution alone cannot account for the short timeframe for significant depletion ofbenzene seen at the site. It is likely that significant volatilisation has occurred since the

Žtime of the spill at the site and that aerobic biodegradation of benzene and other.components in the NAPL would also occur in the zone of water table fluctuation and in

the zone of NAPLratmosphere contact near the source area. Little further aerobicbiodegradation is expected outside the source zone, since the BTEX plume is well belowthe water table beyond 80 m from the source down-gradient.

4.3.3. Depth cross-sectionsThe impact of variable groundwater flow directions is depicted more definitively in

Fig. 3, which shows transverse cross-sections across the plume through MP1, MP2 andŽ . ŽMP3 for April 1991, March 1992 elapsed time of 11 months and August 1994 elapsed

.time of 40 months . It could be inferred incorrectly from Fig. 3 for some boreholes, suchas MP1 and MP2, that considerable intrinsic remediation of benzene and toluene had


occurred between April 1991 and March 1992. Note also that the 1991 and 1992 dataare from similar times of the year, and therefore, seasonal influences could also beinferred to be minimal. However, from the August 1994 data in Fig. 3, where benzeneand toluene concentrations increase again at MP1 and MP2, it is apparent that seasonaland longer term influences had altered groundwater flow directions and redirected thebenzene plume around these borehole locations in March 1992. Note from Fig. 6 thatwater table maxima and minima in the vicinity of the plume were quite dissimilar fromyear to year over the period of monitoring, especially the minima. Together, theseobservations indicate that even year-to-year concentrations may be difficult to compare,and especially at individual boreholes, because of climatic variability between years.

These observations are supported by the results of two small-scale tracer tests carriedŽ .out in the vicinity of MP2 Thierrin et al., 1995 , which showed a reorientation of the

Žgroundwater flow direction of approximately 208 between March–April 1991 low water. Ž .table and July–October 1991 high water table . The flow direction variations are most

evident at MP1, as indicated above, and MP4, which periodically showed concentrationsof BTEX below 5 mgrl. When viewed over all the 16 sampling occasions, the trends atMP1 and MP4 seemed to be strongly negatively correlated, which may be expectedsince these MP boreholes are on opposite edges of the plume. These trends are

Ž .consistent with plume contouring see below , and approximate seasonal groundwaterflow direction changes. These transient features have significant implications for reliablemonitoring of groundwater plumes, and in assessment of plume stability, and thereforefor assessment of the effectiveness of intrinsic remediation in minimising plumemigration.

4.3.4. Plume contoursFigs. 8 and 9 depict benzene and toluene plumes for August 1994 and August 1996.

The apparent increase in width of the plumes for August 1994 compared to April 1991Ž .Fig. 1 , was largely due to uncertainties in locating the plume boundaries. Note that, for

Fig. 8. Contoured plumes for benzene concentrations in groundwater for August 1994 and August 1996.


Fig. 9. Contoured plumes for toluene concentrations in groundwater for August 1994 and August 1996.

August 1996 there was a greater surety in the location of the plume boundaries, due tothe additional MP boreholes along the transects through MP13 to MP15 and MP16 toMP19, which were installed in August 1995.

In overview, comparison of contoured plume data for August 1994, August 1996 andApril 1991 suggest that some of the individual organic plumes display approximate

Ž .steady-state static behaviour. The benzene plume contours for August 1994 and August1996 were similar in extent to the plume contours depicted for April 1991, although noextra boreholes were installed down-gradient of MP8 to observe further movement ofthe plume to the southeast. Also, as noted earlier, concentrations near the sourcedecreased significantly since April 1991. The toluene plumes for August 1994 andAugust 1996 were less than 250 m long. These plumes were truncated with respect tothe benzene plume, and showed a similar shape to that of the April 1991 plume,although toluene was detected in one sampling port of MP8 in August 1996. Note,

Ž .however, over all 16 sampling occasions, that toluene or o-xylene was rarely measuredabove detection levels at MP7 or MP8. As for benzene, peak toluene concentrations alsoreduced with time. However, the reduction was not as significant as for benzene. The

Ž .plumes for o-xylene data not shown here , although at lower concentrations, largelymimicked the shape of the plumes for toluene. In contrast, the m- and p-xylene plumesand other component plumes were typically more extensive than toluene and o-xylene.Therefore, for toluene and o-xylene especially, the plumes may be static and signifi-cantly intrinsically attenuated, whereas for benzene and some of the other hydrocarbons,steady state was difficult to confirm, even though the highest concentration contoursdecreased in extent. From time trends discussed earlier, it is apparent that the plumeswere at best pseudo-steady state, since water table fluctuations and groundwater flowdirections varied seasonally and over longer time frames.

The narrowness of the plumes is notable. In particular, if seasonal changes in flowdirection and water table fluctuations were significant as has been shown, then disper-


sion should be enhanced creating much broader plumes and shallower concentrationgradients. The narrowness could be due to enhanced biodegradation of the BTEXcompounds on the plume periphery, although little biodegradation of benzene would beexpected, except perhaps in the zone of oxygenated rainwater recharge to the water tableand perhaps in the source zone of residual NAPL exposed to the atmosphere. Thierrin et

Ž .al. 1995 also found that dispersion was small within the aquifer. Prommer et al.Ž .1998b recently modelled the effects of seasonal groundwater flow changes and foundenhanced natural attenuation of BTEX under sequential ironrsulphate reducing condi-tions. Under the scenario modelled, they found that groundwater flow direction varia-tions increased the contact zone between BTEX contaminants on the plume peripherywith non-exhausted ferric iron minerals and sulphate-rich groundwater. They also foundthat reduced precipitates could be locally re-oxidised by oxygen in recharge water.These processes may occur on the periphery of the BTEX plume leading to a reducedplume width, but have not been quantified in this study.

5. Degradation rate estimates

The above data provide evidence for active and selective degradation of individualhydrocarbon compounds under sulphate reducing conditions. In particular, systematicdecreases of S–SO2y concentrations, increases in HCOy concentrations and the4 3

presence of degradation products in regions of the plume where BTEX concentrationsŽare high, are indicative of degradation processes Grbic-Galic and Vogel, 1987; Wiede-

.meier et al., 1995 . Also, the relative extent of the benzene plume to the plumes for theother organic compounds suggested preferential degradation of some organic com-pounds relative to benzene. Preferential dissolution or sorption may also have played arole in differentiating the different organic plumes, although the sorption coefficients for

Ža number of the BTEX compounds are similar e.g., Barker et al., 1987; Mackay et al.,.1991 . Sorption may have had a more significant role for TMB and naphthalene

Ž .Whincup, 1994 , although groundwater data for these compounds from down-gradientŽ .boreholes e.g., MP7 and MP8 show no consistent increases that would indicate a

delayed breakthrough due to sorption effects. For example, at MP8, TMB was typicallyat non-detectable concentrations but reached a peak concentration of 30 mgrl in June1992.

Degradation rates for the BTEX compounds, TMB and naphthalene within the plumeŽ . Ž .were reported by Thierrin et al. 1992, 1993, 1995 . Thierrin et al. 1992, 1995

estimated degradation rates and retardation coefficients from an in situ natural gradienttracer test using deuterium-labelled benzene, toluene, p-xylene and naphthalene. Thetracer test was carried out from July to October 1991 over a 18-m travel distance south

Ž .east of the transect MP1, MP2 and MP3. Thierrin et al. 1993 compared thesesmall-scale rates with plume-scale average degradation rates estimated from three-di-mensional analytical modelling of the plumes obtained in 1991. They assumed that thedegradation rates were constant throughout the domain of interest. The degradation ratesŽ . Ž .half-lives estimated from the two methods Table 4 were comparable for benzeneŽ . Ž . Ž .greater than 800 days , toluene 100 to 120 days , and p-xylene 170 to 225 days , and


Table 4Ž .Half-life days estimates for hydrocarbon compounds from a small-scale tracer test and plume-scaleŽ .modelling after Thierrin et al., 1993

Compound Tracer test Model Howard et al.aŽ .estimates estimates 1991

Benzene )800 )800 112–720Toluene 100"40 120"25 56–210Ethylbenzene – 230"30 176–228p-Xylene 225"75 – –m- and p-Xylene – 170"10 28–112o-Xylene – 125"10 180–3601,3,5-trimethylbenzene – 180 –Naphthalene 33"6 160"20 25–258

aA compilation of literature values.

were different for naphthalene, perhaps due to strong sorption of naphthalene to aquifersediments. Also, apart from benzene and m- and p-xylene, the degradation rateestimates all lay in ranges found by other researchers and compiled by Howard et al.Ž .1991 —see Table 4. Ranking the hydrocarbon compounds by the plume-scale degrada-tion rate estimate, from highest to lowest rate gave the order: toluene, o-xylene,naphthalene, m- and p-xylene, TMB, ethylbenzene and benzene.

Ž .Below, degradation rates half-lives of the hydrocarbon compounds were approxi-mated at a larger scale than the tracer test and a smaller scale than average plume scale,to provide some information on the variability of degradation rates within the plume.Degradation rates were estimated relative to benzene for zones of the plume betweeneach successive pair of MP boreholes down-gradient of the site, i.e., between-borehole-scale degradation rate estimates. Data from each MP borehole from April 1991 to March1992 were used, since it was assumed that the hydrocarbon distributions within theplume were at an approximate steady state over that period. The apparent impact on

Ž .benzene concentrations within the plume of depletion dissolution of benzene in thesource NAPL relative to the TEX and other organic compounds made post-March 1992data unsuitable for such an analysis.

5.1. Theory

Assuming steady-state hydrocarbon distributions and assuming lateral dispersion issmall, then the advection–dispersion equation and boundary conditions governingtransport of the hydrocarbon compounds moving in groundwater in a semi-infinitedomain can be written as:

E 2 C E Ci iD yÕ yl C s0, 1Ž .i i i2 E xE x

C 0 sC 0 , 2Ž . Ž .i i

C is finite as x™`, 3Ž .i


Ž .where C x is the concentration of the ith hydrocarbon compound in groundwateriŽ . 0mgrl , C is the concentration of the ith hydrocarbon compound at the up-gradient MPi

Ž .borehole of successive pairs i.e., at xs0 , x is the distance down-gradient of a MPŽ . Ž 2 .borehole m , D is the dispersion coefficient for the ith organic compound m rs , Õ isi

Ž .groundwater velocity mrs and l is the first order degradation rate of the ithiŽ y1 .hydrocarbon compound of interest s . Note that if equilibrium sorption is assumed,

retardation does not play a role in the equations governing the steady-state distributionof the hydrocarbon concentrations in groundwater.

Ž . Ž . ŽSolution of Eqs. 1 – 3 gives the expression see, for example, Purcell, 1972, p..882 :

C x sC 0exp m x , 4Ž . Ž . Ž .i i i

where:1r22m s Õy Õ q4l D r2 D . 5Ž .Ž .i i i i

Ž .So from Eq. 5 , it is possible to calculate the first order degradation rate l as:i

w xl sD m m yÕrD , 6Ž .i i i i i

Ž .and from Eq. 4 , m can be written as:i

m s ln C L rC 0 rL, 7Ž . Ž .Ž .i i i

Žwhere L is the distance between pairs of MP boreholes e.g., Ls50 m for MP11 and.MP3 .

Note that the ratio ÕrD , estimated as 13 my1 for the study site from data in ThierriniŽ . y1et al. 1993 , is very large compared to m , which is typically -0.1 m . For this case,i

Ž .Eq. 6 reduces to:

l sym Õ , 8Ž .i i

which is equivalent to the solution for the longitudinal dispersion coefficient set to zero.If the dispersivity were larger, say 10 m in the longitudinal direction which may be

Ž .appropriate for a heterogeneous aquifer, then Eq. 6 would need to be used instead ofŽ .Eq. 8 , since m and ÕrD would be comparable.i i

Ž .The half-life for the hydrocarbon compounds, t days , can be calculated from the1r2

first-order degradation rate by the equation:

t s ln 2 rl syln 2 rm Õ , 9Ž . Ž . Ž .1r2 i i

Ž .with m given by Eq. 7 . The above discussion is valid if conditions are approximatelyi

steady state, if lateral dispersion is negligible, and if successive pairs of MP boreholesare aligned directly along the same groundwater flow line. The last of these conditions isdifficult to show at field scale without a conservative tracer of groundwater movement,although detailed modelling and contouring of water table levels may allow generationof closely aligned synthetic data.

To account for misalignment of the MP boreholes and lateral dispersion effects, theratio of the hydrocarbon component concentrations to benzene was considered. Benzeneis assumed to be conservative in groundwater and to not degrade readily under anaerobic


Ž .conditions as discussed above see also Patterson et al., 1993b . This is akin to theŽ .strategy of Yang et al. 1995 , who considered ratios of the BTEX compounds, and

Ž .Wilson et al. 1995 who used ratios to TMB, since TMB seemed persistent in theirŽ .study. Also, Wiedemeier et al. 1995 formalised aspects of this approach.

Taking ratios to benzene concentrations also minimises differences due to dispersioneffects, since the BTEX and other organic compounds of concern within the plume werelikely to have similar dispersion coefficients. After ratioing the hydrocarbon concentra-tions to benzene concentrations, and following the same mathematical derivations as

Ž . Ž .above, the following equations are obtained in place of Eqs. 7 – 9 :

t s ln 2 r l yl , 10Ž . Ž . Ž .1r2 i b

l yl sym Õ , 11Ž .i b r

m s ln h rL, 12Ž . Ž .r r

where:

h s C L rC 0 r C L rC 0 , 13Ž . Ž . Ž .Ž . Ž .r i i b b

and the subscripts i and b correspond to the ith hydrocarbon compound and benzene,respectively.

5.2. Results and comparison at different scales

Ž .The ratios h can be calculated using Eq. 13 and data in Table 1—the averager

maximum concentration of the hydrocarbon compounds for successive pairs of multiportŽboreholes along the approximate centre line of the plume i.e., MP11 and MP3, MP3

.and MP6, MP6 and MP7, MP7 and MP8 . Note that because NAPL is present overmuch of the distance between MP10 and MP11 and would continue to releasehydrocarbons into groundwater, degradation rates were not estimated for this intervaleven though concentrations relative to benzene decreased. The half-lives of the hydro-carbon compounds for each of the zones between the paired MP boreholes calculated

Ž . Ž .from Eqs. 10 – 12 are tabulated in Table 5. Note again, that since steady state isassumed, then sorption does not play a role in the estimate of the degradation rates, andÕ is given only by the groundwater velocity. The average groundwater velocity at the

Ž .site was taken to be 150 mryr Thierrin et al., 1993 .

Table 5Ž .Half-life days estimates for hydrocarbons within regions of the plume between multiport boreholes

Ž . Ž .Compound MP borehole interval m interval distance—m

30–80 80–150 150–260 260–420Ž . Ž . Ž . Ž .50 70 110 160

Toluene 140 110 260 200Ethylbenzene 780 )800 550 650m- and p-Xylene 130 – 340 490o-Xylene 160 400 280 2201,3,5-trimethylbenzene 420 – )800 270Naphthalene 280 730 730 260


The half-life estimates in Table 5 show some variability for each of the organiccompounds in different zones of the plume—although given the plume dynamicsŽ .discussed earlier , the estimates are reasonably consistent along the plume. Toluene,o-xylene, and m- and p-xylene had similar half-lives 30 to 80 m down-gradient, withhalf-lives for m- and p-xylene increasing with distance. Over all the zones, toluene had

Ž .a relatively narrow range of half-lives 110 to 260 days , whereas the range for o-xyleneŽ .was broader 160 to 400 days , and the other compounds much broader still, from 130

Ž .days to greater than 800 days. Toluene had the shortest half-life 110 days at a distance80 to 150 m down-gradient of the source area and the other organic compounds had

Ž .significantly longer half-lives greater than 400 days , while m- and p-xylene had theshortest half-life 30 to 80 m down-gradient. Despite these variations, Franzmann et al.Ž .1996 found no apparent variations in biomass or phospholipid signatures in coresrecovered at various locations within or outside the plume in 1995. For an aerobic,

Ž .nitrate-rich BTEX contaminated aquifer, Daniel and Borden 1997 found highestdegradation rates near the source of their plume, with decreasing degradation rates withdistance down the plume.

The estimates in Table 5 are indicative of degradation rates in different zones of theaquifer relative to benzene. Errors in the half-life estimates would be greatest whenestimated from MP boreholes at large spatial distances, and where benzene may bedegrading. However, the ratio of concentrations between successive boreholes providesa more accurate measure of the degradation rate than estimates based on the ratio to thesource concentrations. Also, since ratios of the hydrocarbon concentrations are relativeto benzene, then the degradation rates are underestimates if benzene is degrading.

All of the half-life estimates presented here, along with those from Thierrin et al.Ž . Ž .1993, 1995 , are typically larger than those summarised by Rifai et al. 1995 , or

Ž . Ž .observed by Acton and Barker 1992 , Beller et al. 1992 , Edwards and Grbic-GalicŽ . Ž .1992 , or Edwards et al. 1992 . Half-life estimates up to 100 days were obtained from

Ž .laboratory experiments by Edwards et al. 1992 for o-xylene and p-xylene. Daniel andŽ .Borden 1997 quote half-lives that are comparable to those presented here, and found

Ž .benzene to be most persistent, but found that o-xylene for example was highlypersistent compared to ethylbenzene, which is not the case in this study. For other

Ž .studies which considered sulphate-reducing conditions e.g., Wilson et al., 1995 ,o-xylene was found to degrade at a similar rate to toluene.

ŽThe average half-lives calculated for this site from the tracer test Thierrin et al.,. Ž .1995 and plume-scale modelling Thierrin et al., 1993 were generally lower than the

Ž .between-borehole estimates compare Tables 4 and 5 . In some portions of the plume,the degradation rates estimated by the three methods for toluene and the xylenes arecomparable. Each of the methods used to determine the degradation rates has limita-tions. The tracer test was a relatively small-scale estimate but provided a reliable in situmeasure. The plume-scale modelling assumed uniform velocities and aquifer properties,and required approximation of average dissolution rates for the organic compounds andthe source dimensions. The between-borehole estimates also relied on a uniformvelocity, and on benzene being reasonably conservative. Despite these limitations, thecomparison between methods is good, and especially since the hydrogeochemicalparameters that drive biodegradation change at a range of scales.


6. Conclusions and implications for assessment of intrinsic remediation

Fine-scale monitoring using multiport boreholes has defined long, thin hydrocarbonplumes in sulphate-rich anaerobic groundwater, with extremely high concentration

Žgradients. The thin plumes would be difficult to monitor with long-screened say 1 m.screened piezometers. The high concentration gradients were apparently maintained by

very low dispersion coefficients for the uniform sand aquifer, but also may be sustainedby enhanced degradation of hydrocarbon compounds on the plume periphery.

BTEX concentrations at individual boreholes were highly variable over time. Thesetemporal variations appeared to arise from hydraulic variations and seasonal water tablerises, but also preferential dissolution and biodegradation, and require careful interpreta-tion. The data showed a periodic influx of dissolved hydrocarbons to groundwater nearthe source due to water table rise and inundation of NAPL resident within thenear-capillary zone of the aquifer. Periodic or sporadic trends in groundwater flowdirection changes also significantly changed BTEX concentrations at individual borehole

Ž .locations Fig. 3 . These transient features have significant implications for reliablemonitoring and modelling of groundwater plumes, in assessing the effectiveness ofintrinsic attenuationrbiodegradation in minimising plume migration, and in establishingplume stability.

Ratios of hydrocarbon concentrations to benzene concentrations provided less vari-Ž . Ž .able temporal variations, as found by Wilson et al. 1995 and Yang et al. 1995 for

ratios to other compounds, and allowed estimation of between-borehole scale degrada-tion rates. Without a relatively conservative tracer of groundwater movement, such asbenzene in this case, then intrinsic bioremediation rate estimates based on between-borehole concentration reductions could lead to significant degradation rate errors. Bothover- and under-estimates of intrinsic remediation potential are possible. This is because

Žboreholes are often assumed to be along the same flow paths Wilson et al., 1990;.Chapelle et al., 1996 . Where groundwater flow directions vary seasonally as observed

in this study or due to aquifer heterogeneities, then the probability of boreholes along aplume intersecting the same flow-path is small, unless boreholes are very closely spaced,or if significant pumping occurs during sampling to capture passing flow-paths.

The ratio of hydrocarbon concentrations to benzene also indicated that the residualNAPL composition was changing over time. Significant depletion of benzene fromresidual NAPL near MP10r12 has taken longer than 5 years. Concentrations atMP10r12 of 30,000 to 40,000 mgrl in April 1991 reduced by two orders of magnitudeby August 1996. Along with other data presented here, these data indicated that thedepletion of residual NAPL has a potentially significant role in influencing relativeplume dimensions and groundwater composition, and therefore, in assessment ofintrinsic remediation. These data also emphasise the need to define and target theresidual NAPL in any remediation because of its longevity.

There was strong evidence for the preferential natural biodegradation of TEX andother organic compounds within the plume, with sulphate being the principal electronacceptor. There were expected parallel changes in inorganic chemical parameters,organic compound losses, and increases of degradation by-products in groundwater.However, there was little evidence in this study of substantial benzene degradation—


either from the observed extent of the plume or estimated degradation rates. Degradationrate estimates from the between-borehole calculations showed some variation withdistance down-gradient from the leakage point, except for the half-life estimates fortoluene, which had a range of 110 to 260 days. The plume-scale modelling gave muchmore uniform half-life estimates for the hydrocarbon compounds other than benzene,lying in the range 120 to 230 days. Tracer test data in general followed the same trends.Ranking the compounds according to their half-lives, from lowest to highest, gavetoluene, o-xylene, naphthalene, m- and p-xylene, TMB, ethylbenzene and then benzene.This ranking tends to be consistent with results from previous studies, although ofteno-xylene has been observed to degrade more slowly than the other xylene isomers.

From plume data, it can be concluded that the toluene and o-xylene plumes werepseudo-steady state, and were substantially intrinsically bioremediated. Despite this, theplume had migrated significantly off-site. However, it is unclear from the data whetherthe benzene plume or some of the other gasoline component plumes were stable. From

Ž .preliminary modelling Thierrin et al., 1993 , the benzene plume was not expected to beat steady state until it had extended to a distance beyond 4 km. This conclusion waslargely based on the very small longitudinal, transverse and vertical dispersion coeffi-

Žcients observed in the sand aquifer, and an assumption based on tracer test data,.Thierrin et al., 1992 of no biodegradation of the benzene in the anaerobic aquifer. At

this site, high concentrations of the dissolved hydrocarbons may therefore persistŽ .especially for benzene in groundwater for large distances down-gradient of the source.This has serious implications for long-term management or remediation of such a site,especially if located within groundwater abstraction regions supplying potable water.

Temporal data from frequent monitoring of groundwater chemistry at carefully placedŽ .boreholes combined with spatial mapping and perhaps modelling of plumes seems

essential to adequately assess the extent of intrinsic remediation. In this study, such dataenabled us to establish that the toluene and o-xylene plumes were pseudo-steady state,so were intrinsically remediating, despite highly variable changes in groundwater flowdirections and water table elevations. Evidence from a small number of short-screenedpiezometers or single boreholes, whether close to the source of leakage or furtherdown-gradient, were unreliable as the sole evidence for assessment of intrinsic remedia-tion of constituent organic compounds. This was because of the significant plumevariability in time and space, and possibly because of artefacts introduced throughpumping and sampling. Assessment of data trends based on ratios of componentcompounds offers a less data-intensive means of monitoring. However, to carry thisthrough effectively, the BTEX components need to be ratioed to a conservative markerin groundwater to provide reliable assessment of intrinsic remediation. In this study,benzene was used over a prescribed period. Additionally, the longevity and compositionchanges of residual NAPL need to be taken into account in using this technique.

Acknowledgements

This work was partly funded by the Water Authority of Western Australia and theAustralian Institute of Petroleum. David Briegel, Michael Lambert and Tracy Milligan


installed field infrastructure and assisted with sampling, and Alison Wells, TobyWhincup, Neil Fox and Kathryn Linge furthered components of the research. Thevaluable comments of Colin Johnston, Andrew Barry, Henning Prommer, Robert Bordenand Eugene Madsen on an earlier draft are much appreciated.

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