Modeling of transient groundwater flow, pollutant transport,

and biodegradation in an aquifer with large hydraulic head variations

Sandro Rinaldi & Sihem Louati & Hocine Bendjoudi &Ghislain de Marsily

Abstract Industrially sourced dense non-aqueousphase liquids (DNAPLs) contaminated an alluvialaquifer in France decades ago. The location(s) andnature of the pollution source zone(s) were unknown,and the dissolved concentrations of volatile organiccompounds in the monitoring wells varied greatly withtime. The aquifer was in hydraulic equilibrium with anartificial canal whose water level was highly variable(up to 5m). These variations propagated into theaquifer, causing changes in the groundwater flowdirection; a transient numerical model of flow andsolute transport showed that they correlate with theconcentration variations because the changes in theflow direction resulted in the contaminant plumeshifting. The transient hydrogeological numericalmodel was built, taking into account solvent biodeg-radation with first-order chain, since biodegradationhas a significant influence on the pollutant concentra-tion evolution. The model parameterization confirmsthe position of the source zones among the potentialtroughs in the bedrock where DNAPLs could haveaccumulated. The groundwater model was successfullycalibrated to reproduce the observed concentrationvariations over several years and allowed a rapidvalidation of the hypotheses on the functioning of thepolluted system.

Keywords Transient groundwater modeling .Groundwater/surface-water relations . Chlorinatedhydrocarbons contamination . Geophysical surveys .France

Introduction

The characterization of pollution source zones and ofthe dynamics of biodegradation of chlorinated solventsin an alluvial aquifer is a common problem in manyindustrialized countries. Different techniques, measure-ments and studies provide information on theseproblems but often additional work is needed to betterconstrain the conceptual model of the functioning ofthe system, and thus the efficacy of remediationmeasures.

In groundwater flow modeling, it has been shownthat measurements of hydraulic head variations withtime greatly help to improve the calibration of anumerical model; see for instance the inverse problemsolution developed by Vázquez González et al. (1997),Giudici (1991, 2001, 2003) and Giudici et al. (2008).Presented here is a case-study where such a calibrationwas successfully completed, because of the existenceof a very strong temporal variation of the hydraulichead within the aquifer, which generated strongtemporal and spatial variations of the solvent concen-trations; by calibrating a groundwater flow andtransport model, the location, nature and strength ofthe solvent sources, as well as the dynamics of solventbiodegradation, were identified.

Dense non-aqueous phase liquids (DNAPLs) in anaquifer are located at the interface with the imperviouslayer on which the aquifer material lies, and move as afunction of its topography and of the groundwater flowdirection (Luciano et al. 2010), accumulating in thetroughs of the bedrock. Thus, the locations wherespilled DNAPLs have accumulated do not necessarilycorrespond to the superficial source zones where thepollutants were released (Huling and Weaver 1996).Geophysical seismic surveys of the studied siteprovided the shape of the impervious bottom layer ofthe aquifer and located potential troughs whereDNAPLs might have accumulated. This informationwas used to restrain potential locations of pollutionsource zones in the numerical groundwater model.

Several microorganisms capable of biodegradingchlorinated solvents have been recognized in theliterature (e.g. Wiedemeier et al. 1999). Differentbiodegradation mechanisms may be active, depending

Received: 6 May 2013 /Accepted: 16 December 2013

* Springer-Verlag Berlin Heidelberg 2014

S. Rinaldi ()) : S. Louati :H. Bendjoudi :G. de MarsilySorbonne Universités, UPMC Univ Paris 06, UMR 7619,Sisyphe, F-75005 Paris, Francee-mail: sandro.rinaldi@upmc.fr

Hydrogeology JournalDOI 10.1007/s10040-013-1096-7

on the environmental conditions and on the availabilityof reactants. In general, chlorinated solvents with threeor more chlorines are more easily biodegraded underanaerobic conditions than those with one or twochlorines, which are easily biodegraded in an oxidizingenvironment (ADEME 2006; US EPA 2000; AFCEE2007). At the studied site, microbiological studiesconfirmed the presence of a biological potential totransform the historical pollution (trichloroethane anddichloroethane, 1,1,2-TCA and 1,2-DCA) into vinylchloride and enabled the estimation of the biodegrada-tion transformation rate used in the numerical ground-water model. To the authors’ knowledge, this is thefirst time that a transport model has been calibratedunder transient flow conditions with sequentialbiodegradation.

Description of the site and the pollutionThe study site is in France but, due to legal constraints,no further information can be given on its exact location.All the data come from this actual site (Rinaldi 2012),which is located on an alluvial plain downstream of ahistorical industrial site where chlorinated solvents havebeen produced since the 1960s; a historical dumpsite,closed since 1975, was known to have received largedeposits of 1,1,2-trichloroethane (1,1,2-TCA) and 1,2-dichloroethane (1,2-DCA), respectively 60 and 30 % ofthe total disposal. 1,1,2-TCA and 1,2-DCA are chlori-nated solvent DNAPLs and are also known as volatileorganic compounds (VOCs). Until 2006, no aquifercontamination had ever been reported; however, in2006, a routine aquifer monitoring campaign revealedthe presence of very large concentrations of vinylchloride (VC) in a few observation wells, which wastotally unexpected. Since the site contains a large tank ofpressurized VC as well as several lines to distribute it tovarious reactors in the plant, a leak of VC was firstproposed as an explanation. This assumption was furtherreinforced when successive sampling campaigns in thesame few observation wells showed extremely largevariations of VC concentrations over a period of lessthan 3 months; however, despite many tests and checks,no VC leaks could be detected.

Additional observation wells were drilled andmonitored, all showing the same type of spatial andtemporal variations with a clear increase of VC withthe distance from the dumpsite and a decrease of theother more chlorinated solvents. This study wasconducted when 4 years of hydraulic head andchlorinated solvents solute concentration measurementswere available. Firstly, the report describes the site,then the new measurements that were made, andfinally it shows how a transient flow and transportnumerical model with realistic parameterization wasable to fit the observations and describe the ground-water system functioning, suggesting the position andnature of the pollution source zones.

Hydrogeological site characteristic and conceptualmodelThe studied zone is located on alluvial deposits ofhomogeneous coarse unconsolidated sediments overly-ing impervious marls, close to an artificial canal whosewater level is governed by a gate connecting it to alarge river (Fig. 1). The lateral recharge from themoraine plateau feeds the east side of the alluvium. Noprecise quantification of this lateral flux is availablebut, in the alluvium deposits, the water table is stableon this side, so it is supposed that the lateral rechargeis approximately constant. It will be represented laterin the model by a prescribed head condition on theeast boundary. The surface of the industrial site isalmost everywhere covered by an impervious liner(cement or bitumen) to eliminate the infiltration ofliquids; therefore, direct rainfall recharge is considerednegligible. Occasionally, a few meters of heteroge-neous structural fill material overlie the alluvium, butthey never reach the water table in the alluvium, whichremains unconfined. The alluvial deposit is in generalmore than 20 m thick (between 21 and 25 m) andhighly permeable with hydraulic conductivity (K)values between 10–2 and 10–3 m/s in the area; at thestudied site, a K value of 3.8×10–3 m/s was foundfrom an old pumping test. The site lies on Miocenemarls with some sand intercalations (called “Molasse”)with low hydraulic conductivity (10–4 to 10–6 m/s inthe area).

On the studied site, during the drilling of observa-tion wells, the Molasse was observed to be verycompact (cemented), so it was concluded that it wasmuch less permeable than in the region generally. Inthe study zone, the geological profile is quite homo-geneous; there are exceptional areas with smallvariations of the elevation of the top of the Molasse,possibly creating local troughs, and a thin impermeablelayer of clay that has occasionally been found on topof the Molasse during drilling of observation wells; theclay is between 0 and 2 m thick (Fig. 2). It wasassumed that the Molasse and/or the clay layer is theimpervious layer on which lies the alluvial aquifer andthis impervious layer does not need to be representedin the model.

Regionally, the hydraulic heads of the two forma-tions (alluvium and Molasse) are similar, but there areno wells in the Molasse in the studied zone. The meanhydraulic gradient in the alluvium is 0.3 ‰. Upstreamof the canal gate (Fig. 1), the water level in the riveris higher and more stable than in the canal downstreamof the gate, where the water levels vary strongly andrapidly (up to 5 m in 2 days). The water table isaround 10 m deep in the studied zone, but thegroundwater levels in the observation wells arestrongly influenced by the canal water level variationsand can vary rapidly, thus influencing the groundwaterflow direction and velocity. On the east and north sidesof the study area, the water table is stable and higherthan the water level in the west side of the study area

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

(bounded by the canal) and also higher that the watertable in the aquifer in the south of the study area(Fig. 3). The groundwater flow direction is east–westwhen the canal water level is low (Fig. 1), and it isnorth–south when the canal water level is high. Thegroundwater velocity also changes with time, propor-tionally to the hydraulic gradient, which is about 0.5‰ during low canal water level and 0.1 ‰ duringhigh canal water level.

Aquifer contaminationNine observation wells (Fig. 4) were considered in theconcentration measurements; however, they do not havethe same characteristics because they were drilled atdifferent times. Furthermore, the temporal frequency ofwater sampling was not the same for all observation wells,as shown in Table 1. Three main issues were raised at thestart of this study:

1. The observation wells with the highest solvent soluteconcentrations were not those closest to the zone ofhistorical chlorinated waste dumps (Fig. 4).

2. A strong temporal variation (more than one order ofmagnitude) of dissolved chlorinated solvents concen-tration was often observed between two successivesampling campaigns (Fig. 5).

3. The nature of the DNAPLs dumped at the site(1,1,2-TCA and 1,2-DCA) did not correspond toall the major dissolved constituents (in particular,high concentrations of VC were measured, al-though this constituent had never been releasedinto the dump; Table 2).

To understand the system functioning and to findexplanations for the three observations highlighted inthe preceding, three additional series of studies weremade:

– Groundwater analyses to characterize biogeochemicalconditions and to detect the presence of biodegradationmarkers.

– Microbiological studies, to evaluate the potential forsolvent biotransformation within the aquifer.

– Geophysical analyses, to locate potential DNAPLtroughs on top of the Molasse.

Fig. 1 Overview of the plant site and location of the studied zone, showing the hydraulic head in the aquifer (blue lines) on 8 May 2009(with a low water level in the canal). Hydraulic head is given in meters above mean sea level (mamsl) every 0.5 m; observation wells P3(blue), P4 (green) and P9 (orange) are used in Fig. 3 to show the hydrodynamic functioning of the aquifer; black arrows in river and canalshow water flow direction; Pnorth is an observation well used just in numerical model calibration

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

New data

Groundwater analyses for biogeochemical conditionsA special geochemical measurement campaign wasconducted on 20 June 2008 to determine if biodegradationoccurred in the study area (Table 3). Contrary to theconventional sampling policy, groundwater samples were

collected without observation well purging (bail samples).In the deeper observation wells, sampling was done atdifferent depths: 2 m (top of the aquifer) and 10 m(bottom); a more complete set of constituents than normalwas measured (ethylene, ethane, methane, chloride,dissolved organic carbon (DOC)) and the groundwaterbiogeochemical conditions were measured in situ (Eh, pH,dissolved oxygen and electrical conductivity). The waterlevel of the canal was near its average (around 152.35mamsl). The results provided a spatial distribution ofconcentrations notably different from that obtained in theregular measurement campaigns.

On 20 June 2008, the highest concentrations weremeasured at P5 and not at P1 as in regular chlorinatedsolvents campaigns (Table 3). A comparison was made onobservation well P5 between sampling methods withpurging (WP) and without purging (WOP). The resultsconfirmed the difference between the sampling methods:water taken with the WOP method provided considerablyhigher concentrations than that sampled with the WP one.However, for other observation wells, this difference inchlorinated solvent concentrations did not appear.

In Table 3, note the presence of ethylene and VC inobservation wells P1, P2 and P3 (located hydraulicallydownstream from the dumpsite), whereas these com-pounds were not found in observation wells P5 and P6(located upstream from P1, P2, P3 and closer to thedumpsite). The biogeochemical measurements in theaquifer at the bottom level provided several indices ofthe presence of anaerobic biodegradation (Table 4):

1. Anoxic conditions: less than 0.2 mg/l of dissolvedoxygen in the groundwater in P1, P2, P3, P5 and P6.

2. Sufficiently reducing conditions: redox potential Eh<200 mV (Wiedemeier et al. 1999).

3. The chloride ion concentrations, which are supposed toincrease when chlorinated solvents are biodegraded,actually increase from upstream to downstream: obser-vation well P4 (upstream from the dump zone, Cl– about40 mg/l), P5 (dump zone, about 70 mg/l) and P1(downstream but close to the dump zone, 130 mg/l). It

Fig. 2 Geological profile of the studied zone. The thick horizontalline is the water table in the alluvium (mean depth at the studiedzone is approximately 11 m below ground surface, whichcorresponds to 152 mamls)

Fig. 3 Water level variations upstream and downstream of the canal gate and in three observation wells (P3, P4 and P9, see Fig. 1 forlocations). Elevations in mamsl

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then decreases slightly downstream, but remains consis-tently at values close to 100 mg/l in P2, P3 and P10.

4. The total alkalinity is high (CaCO3>200 mg/l) and isable to partly buffer the potential pH decrease causedby the increase of chloride ion concentration.

5. The pH decreases slowly in the flow direction: P4(7.51); P5 (6.8); P1 (6.5); P2 (5.95); P3 (4.78).

6. Dissolved organic carbon (DOC) has minimum valuesin observation wells upstream of the historic dumpsitenear P4 (5.2 mg/l), and maximum values in theobservation wells most affected by pollution includedin the campaign, P5 (20 mg/l) and P1 (17 mg/l).

7. The temperatures of the most polluted observationwells (P1, P2, and P3) are 1 °C higher than those ofupstream observation wells (P4, P5 and P6).

Microbiological analyses and microcosmexperimentsFor anaerobic and reductive conditions, Dyer et al. (2000),Hunkeler et al. (2002, 2005), and others have proposed apossible dehydrochlorination of 1,2-DCA to VC; Grosternand Edwards (2006), Moe et al. (2009), Yan et al. (2009),and Maness et al. (2012) later identified the bacterial

strains (Dehalobacter and Lykan) capable of biodegradingchloroethanes (1,1,2-TCA) into chloroethenes (VC) bydihaloelimination.

Microbiological analyses of the quantitative polymerasechain reaction (qPCR) and the reverse transcriptase-qPCR(RT-qPCR) were made on the water of observation wells P1,P2 and P5; qPCR provides information on the presence ofspecificmicroorganism genes (deoxyribonucleic acid, DNA)in the water and RT-qPCR on the activity of specificmicroorganism genes (ribonucleic acid, RNA). Theseanalyses, for qPCR and RT-qPCR, were seeking to detectspecific genes known (from the literature cited in theprevious) to be involved in such chlorinated solventbiodegradation reactions. The analyses were done onsamples collected on 31 October 2009; the bacterial strainscapable of transforming 1,1,2-TCA and 1,2-DCA into VC(dcaA1, rdhA2, tceA and lykan) were found and showed tobe active in this water (Table 5).

A microcosm experiment, which showed a rapidbiodegradation of 1,1,2-TCA, was then conducted withP1 water. The water was collected on 8 December 2010,in 30 bottles of 1 l (Schott glass), with no addedchlorinated solvents or soil material, as this water alreadycontained chlorinated solvents; the bottles were storedisothermally at 20 °C in the dark and equipped with

Fig. 4 Spatial distribution of the time-averaged sum of major soluble chlorinated solvents (VOCs) measured in observation wells (valuesof Table 2 are used); location of two small troughs in the impervious bottom layer beneath the alluvium (grey zones); historical dumpsite(red zone); and the three principal groundwater flow directions on the studied zone

Table 1 Observation well characteristics, sampling frequency, and years of well drilling and the start of chlorinated solvents concentrationmeasurements

Observation well P1 P2 P3 P4 P5 P6 P7 P8 P9 P10

Well bottom (depth, m) 13.6 14 14 22 23 22.5 20 16 15 14Length of screen below water table (m) 2.6 3 3 11 12 11.5 10 15 8 13Well diameter (mm) 40 60 60 110 110 110 60 60 300 60Sampling frequency (no. per month) 1 1 1 3 3 3 1 1 1 3Year drilled 1985 1985 1985 1992 1992 1992 2008 2009 1970 1985Year that sampling started 2006 2006 2006 2006 2006 2006 2008 2009 2006 2006

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

hermetic Teflon caps. Three types of microcosms werebuilt; ten bottles were used for each series (30 bottles intotal). To ensure that anaerobic conditions were main-tained, the bottles were never opened during incubationand a complete bottle was used for each analysis.

The three series were:

1. A microcosm without modification of the conditions,called “control.” (Anaerobic conditions were maintainedin the system, as the bottle was completely filled.)

2. A microcosm in which a 100 ml headspace was left for thepurpose of allowing aeration of the medium. As thedegradation pathways leading to vinyl chloride are anaero-bic, they are inhibited by the presence of oxygen (10 bottles)

3. A microcosm in which acetate was added to reach afinal concentration of 1 g/l. The acetate was added tostimulate the metabolism of the bacterial population(anaerobic conditions were maintained in the system bycompletely filling the bottle).

In all the series, the 1,1,2-TCA concentration decreased(Fig. 6), but in the acetate series, the decrease was more

significant than in the aired and control series. Theestimated half-lives (t½) indicated by these microcosmexperiments for 1,1,2-TCA are between 17 days (acetateseries) and 155 days for the other series. This difference isexplained by the presence of acetate, which enhancesbiodegradation and better reproduces the field conditionswhere dissolved organic matter is present in the ground-water: large DOC values were indeed measured (DOC inTable 4). An increase of VC concentration in themicrocosms was also observed in the control and acetateseries, although the mass balance was not perfect in theacetate series. These biodegradation results are consistentwith other laboratory studies (Maness et al. 2012;Grostern and Edwards 2006) where a similar 1,1,2-TCAhalf-life was observed; the half-lives are not given in thisarticle, but from the decay curves they are on the order of20 days.

Geophysical survey resultsThe DNAPLs are most likely partly trapped in theunsaturated zone and alluvial fill, retained by capillarity,

Table 2 Average concentrations of the major constituents, calculated from analyses where the detection threshold was exceeded betweenJanuary 2007 and March 2011; no dissolved chlorinated solvents have been detected in P9

Observation well P1 P2 P3 P4 P5 P6 P7 P8 P10

Number of samples 40 40 45 26 16 16 31 31 19Vinyl chloride (μg/l) 5,720 1,500 1,250 <10 <10 <10 230 600 <101,2-DCA (μg/l) 4,340 240 20 20 200 40 <10 <10 <101,1,2-TCA (μg/l) 45,300 13,500 6,020 150 1,700 1,800 610 2,700 701,1-DCE (μg/l) 160 700 60 <10 30 10 20 130 <10TCE (μg/l) 1,400 650 160 <10 20 40 14 250 10PCE (μg/l) 3,400 1,410 340 140 1,500 840 80 250 50All chlorinated solvents (μg/l) 60,500 17,300 7,800 290 3,500 2,600 850 4,000 140

Fig. 5 Temporal variation of the sum of the major dissolved chlorinates solvents (VOCs) in the oldest and most polluted observation wells(P1, P2 and P3), together with the water level in the canal (downstream of the gate) and the groundwater level at observation well P3; fromMay 2009 to November 2009 a period of stable water level was used for parameter calibration of the numerical model

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

and have partly migrated downward into the saturatedalluvium. The heavy phase would have stoppedmigrating and accumulated when it encountered alayer of sufficiently low hydraulic conductivity, andthen moved in the direction of the slope of this layer,but it would also be influenced by the groundwaterflow (Luciano et al. 2010; Huling and Weaver 1996;Pankow and Cherry 1996 and Schwille 1988); it wouldmost likely be stored in troughs downstream of thehistorical DNAPL dumpsite. Therefore, the site whereDNAPLs had been dumped did not necessarilycorrespond to the source zone of the groundwatersolute pollution. A geophysical survey (seismic refrac-tion) was conducted in 1995, to measure the depth ofthe interface between the alluvium and the Molasse.On the studied zone, this interface was found betweena minimum of 21 m and a maximum of 25 m, with anaverage depth of 23 m; small irregularities were alsovisible at the top of the Molasse (variations of lessthan 2 m, i.e. less than 18 % of the saturated alluviumthickness). A more detailed geophysical study (2010)confirmed these results and was able to locate smalltroughs. It could therefore be assume that DNAPLswere stored in small depressions at the top of theMolasse (Fig. 4, grey zones in the west and south-westof the dumpsite). These troughs are thought to be largeenough to store an accumulation of historically spilledDNAPLs, even if their influence on groundwater flowis still negligible because they do not create asignificant relative variation of the saturated thicknessof the aquifer.

The hydrogeological model

Even if the pollution by DNAPLs might cause complexflow of different phases (oil-water), the release ofcontaminants into the aquifer was several decades ago(1960s) and therefore it is supposed that the residualDNAPL deposits in the ground are immobile and stable.

As the focus was only on the mechanisms occurring inthe saturated zone and on the transport of solutes, the codeMODFLOW was chosen for the hydraulic simulation(Harbaugh et al. 2000) and the code MT3DMS for thetransport (Zheng and Wang 1999) through the interfacePMWIN Pro (Chiang and Kinzelbach 2001). These codesare widely used in hydrogeology and are simple toimplement. They take into account the phenomena thatshould be represented (spatially and temporally variablehydraulic heads, sequential biodegradation transformation,etc). MODFLOW uses the finite difference method forcalculating the transient groundwater head and velocity ineach cell of a two-dimensional (2-D) domain. TheMT3DMS code solves the dispersion equation for solutetransport by an Eulerian-Lagrangian approach; a 2-Doption based on the transient 2-D velocity field given byMODFLOW was used.

This approach assumes that the change in theconcentration does not affect the hydraulic field, whichis an acceptable assumption in this case, where theconcentrations are on the order of tens of mg/l.MT3DMS simulates the transport of dissolved speciesand proposes many method options. A numericalmethod, among all those proposed, was chosen based

Table 3 Concentrations of the major constituents for the 20 June 2008 campaign

Obs. well Sampling depth (m) 1,1,2-TCA (μg/l) 1,2-DCA (μg/l) PCE (μg/l) TCE (μg/l) VC (μg/l) Ethylene (μg/l)

P4 3 220 1.9 140 4.6 <0.1 <2P5 3 74,000 19,000 1,800 210 <15 <2P5 9 52,000 14,000 2,400 200 28 <2P6 8 2,000 12 830 30 1,6 <2P6 3 2,900 13 840 32 1,7 <2P1 3 67,000 4,900 3,600 1,200 7,300 750P2 3 8,700 22 980 310 430 4.9P3 3 10,000 17 400 170 750 110P10 3 770 1.6 52 12 3.9 <2P10 9 700 1.7 51 12 4.3 <2

Table 4 Biogeochemical parameters and other concentrations measured in groundwater sampled on 20 June 2008

Obs. well Samplingdepth (m)

Cl–(mg/l) DOC(mg/l)

Temp (°C) Eh (mV) O2 (mg/l) pH CaCO3(mg/l)

Methane(μg/l)

P4 3 39 5.2 15.09 281 1.59 7.51 240 <0.2P5 3 77 21 15.62 102 <0.2 6.83 440 <0.2P5 9 66 19 15.68 137 <0.2 6.08 370 <0.2P6 8 62 6 15.58 155 <0.2 7.47 380 <0.2P6 3 61 8.3 15.55 121 <0.2 6.04 370 <0.2P1 3 130 17 16.36 85 <0.2 6.61 490 <0.2P2 3 97 6 16.32 132 <0.2 5.95 390 2.3P3 3 97 8.6 15.9 146 <0.2 4.78 470 6.8P10 3 110 2.5 17.03 74 1.77 6.86 360 <0.2P10 9 110 2.5 16.88 119 1.28 7.84 380 <0.2

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

on modeling experiments and transient tests; the mainselected characteristics were:

& The 3rd-order total variation diminishing (TVD)scheme numerical-resolution method

& No sorption (It was assumed that the total sorptioncapacity of the soil is depleted because all sorptionplaces are probably already occupied since the pollu-tion is very old.)

& First-order biodegradation rate for the dissolved phase& Constant concentration in the source-zone

The mesh, boundary conditions, and locations of thesources are represented in Fig. 7. The source area wassimulated as a prescribed concentration (mol/l) whosevalue was determined by calibration of the groundwatermodel. The source zones were positioned at the troughs onthe top of the Molasse identified by the geophysicalsurvey.

Prescribed groundwater heads were used as theboundary conditions on the four sides of the hydrody-namic model. The prescribed heads are not constant on allfour sides; the levels in the west (canal side) and southvary during the simulation, whereas the levels in the northand east remain constant. To reproduce the variations inthe water level of the canal (Fig. 3), a discretization step of0.25 m was used, generating 18 different levels between150.5 and 155 mamsl and a succession of 147 timeperiods between 2007 and 2010. Each one of theseperiods of varying durations is divided into 8 time steps.

During each period, one of the 18 levels between 150.5and 155 mamsl was prescribed at the upstream section ofthe canal. A different hydraulic gradient in the canal wasused for each of the 18 defined water levels upstream. Toestimate this gradient and its variations, the canal waterlevels measured immediately downstream of the gate, andat the confluence of the river and the canal (about 11 kmdownstream of the gate) were used. The mean differenceof water levels measured between these two pointsdivided by the length of the canal (11 km) gives theaverage hydraulic gradient, which varies between 0.02 and0.16 ‰, depending on the water level in the canal.

On the other boundaries of the model, the meanobserved groundwater levels in the existing observationwells were interpolated along the boundaries. They areconstant at the north and east boundaries, but vary withtime at the south boundary (Fig. 7).

Based on the geophysical measurements and boreholedata, the mean alluvium depth (23 m) was used on thewhole domain. It was estimated that the influence of smallchanges of the alluvium depth showed by geophysicalmeasurement (less than 2 m or 18 % of the saturatedthickness of the aquifer) can be neglected. The variationsof the saturated thickness in the aquifer due to the waterhead variations with time were taken into account byusing the “unconfined aquifer” option in MODFLOW.

As explained previously, there is no vertical aquiferrecharge by rainfall, as the surface of the industrial site hasbeen made impervious, and there are no withdrawal wells.All inputs and outputs of the aquifer occur through the

Table 5 Average concentrations of replicates of qPCR (DNA) and RT-qPCR (RNA) analyses

Bacterial straindcaA1 rdhA2 tceA lykan

Obs. well Analyzedacid

Mean replicate SD Mean replicate SD Mean replicate SD Mean replicate SD

P5 DNA <200 NC ND NC 4.2E+2 3.2E+2 1.6E+2 1.8E+1P2 DNA 1.6E+2 2.0E+1 1.9E+4 4.6E+3 7.0E+2 1.2E+2 2.6E+2 1.2E+0P1 DNA 9.6E+2 2.6E+2 8.7E+3 NC 1.8E+7 6.3E+6 <100 NCP5 RNA 5.6E+3 NC ND NC 3.7E+3 NC ND NCP2 RNA 3.2E+2 3.2E+2 ND NC <100 NC ND NCP1 RNA 2.3E+2 2.5E+1 ND NC 3.1E+2 6.0E+1 ND NC

ND not detected; NC not calculable; SD standard deviation

Fig. 6 Variation of a 1,1,2-TCA and b VC concentrations in the microcosm series

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

boundaries and will be estimated once the model iscalibrated, as they can hardly be measured.

The specific yield (S) was set at 25 % and the hydraulicconductivity was set at 3.8×10–3 m/s in the western partof the model (consistent with existing pumping testresults). The hydraulic conductivity of the eastern part ofthe mesh was fitted in the simulation to match the headobserved in the observation wells, with a uniform value of1.25×10–3 m/s. Initially, a steady-state model was used tofit the 8 May 2009 observed heads (the most completemeasurement day; Fig. 8 compares observed and calcu-lated heads); later on, a transient model was used withparticular attention to the P1, P2 and P3 head variations;the fitted model reproduced the measurements well duringthe whole period (October 2006–March 2010; Fig. 9).

The effective porosity for transport calculations was setat 30 %. This porosity represents the mobile water in the

saturated part of the aquifer, which can be invaded bysolutes. It is in general larger than the specific yield, whichrepresents the amount of water that can be drained fromthe aquifer matrix, the remaining water being retained bycapillary forces. The longitudinal dispersivity was initiallyset at 25 m, consistent with the literature (e.g. de Marsily1986), where it is in general on the order of 1/10th of themean distance between the source and the observationpoints, and the transverse dispersivity was set at 1/10th ofthe longitudinal one.

The water balance was checked through time. Theamount of water that enters the model is always very closeto the amount coming out when the variations of storageare taken into account (Table 6). In general, groundwaterflows from the moraine plateau (north-east) to the canal(south-west), although with strong directional variations(up to 90°), from north to south during high water levelsin the canal, and east to west during low water levels.Figure 10 shows the pollution plume of 1,1,2-TCA and itsmovement between the high and low water in the canal.

The transient model allows one to reproduce thehydrodynamic and pollution observations. The measureddissolved 1,1,2-TCA concentrations were compared withthe simulations. They reproduced the very rapid variationsin concentrations due to the lateral displacement of theplume, which explains the large temporal variationsobserved in observation wells P1, P2 and P3.

Results and discussion

The transport model of 1,1,2-TCAThe parameter calibration of the transport model wasbased on the 1,1,2-TCA concentrations measured inobservation wells P1, P2 and P3 during the stable period(May–November 2009, Fig. 5) by varying the followingparameters and using realistic values of:

Fig. 7 Mesh used to simulate the pollution-plume variationsbetween low and high hydraulic heads, with the pollution sourcesand the observation points (wells P1, P2 and P3 inside the blacksquare). The boundary conditions are prescribed fixed heads (blueline) and prescribed time-varying heads (green line)

Fig. 8 Scatter diagram of hydraulic head (simulated as a steady-statevs observed) on 8 May 2009 (low canal water level); the resultingstandard deviation is small (0.046 m); Pnorth is a observation wellused in model hydrodynamic calibration (see Fig. 1)

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

– The number, extensions and locations of source zonesof the contamination

– The initial prescribed concentration in the source zones– The half-life of the 1,1,2-TCA biodegradation– The dispersivities in the longitudinal and transverse

directions

The results are very encouraging: after parametercalibration, the simulated concentrations in observationwells P1, P2 and P3 are close to the measured ones duringthe stable period and their variations are of the same orderof magnitude for the rest of the simulation (Fig. 11),excepted for one measurement in P1 in the stable periodthat could be due to an analytic or sampling problem.

These results were obtained with:

– Two spatially separated source areas, as shown inFig. 10

– An initial concentration of 1,1,2-TCA at the source of2,000 μmol/l (tested values between 750 and7,500 μmol/l)

– A half-life of 1,1,2-TCA of 16 days– Dispersivity values (longitudinal and transversal) of 20

and 1 m, respectively, still on the order of magnitudesuggested by e.g. de Marsily (1986)

The initial concentrations of 1,1,2-TCA at thesources are consistent with the literature and do notexceed its solubility (~33,000 μmol/l at 20 °C). Thehalf-life is much shorter than some literature values(years: US EPA 2001), but consistent with more recentlaboratory experiments (Maness et al. 2012; Grosternand Edwards 2006), and similar to that which wasestimated with the microcosm experiments as part ofthis study.

In conclusion, the transient modeling supports thehypothesis of the presence of at least two source areas for1,1,2-TCA, close to the troughs in the bedrock of the alluviumfound by geophysical exploration. It can be said that:

1. Part of the organic phase (DNAPLs) is trapped in thealluvium.

2. The major portion that had plunged into the saturatedzone was stopped by an impermeable layer (betweendepths of 20 and 25 m).

3. The organic phase was divided into at least two sourceareas.

This modeling also confirms that changes in thedirection of the groundwater flow are sufficient to explainthe observed variations in 1,1,2-TCA concentration andthat its biodegradation is active across the whole studiedarea, as confirmed in the following.

The transport model of VCFor this study, it was assumed that VC was createdonly from the 1,1,2-TCA biodegradation. Thereforethe simulation of 1,1,2-TCA was used as a startingpoint for the modeling of VC concentrations, becauseit is by far the dominant compound. A simplesequential biodegradation of 1,1,2-TCA into VC wasused in the simulation. Several factors were taken intoaccount for the parameter calibration of the VCtransport model:

1. The prescribed initial VC concentration at the source; itwas assumed that, in the source area, a fraction of1,1,2-TCA may already have been degraded andformed VC.

2. The stoichiometry (amount of VC created for eachmole of degraded 1,1,2-TCA).

3. The half-life of VC biodegradation.

Fig. 9 Scatter diagram of hydraulic heads (simulated in transientconditions vs observed) of wells P1, P2 and P3, from October 2006to March 2010. The three ovals correspond to low (A), middle (B)and high (C) water levels in the canal (see in Fig. 10)

Table 6 Water balance of the fitted model through time, as givenby the groundwater flux through the limits of the studied zone (Fig.10a), m3/day; positive values are inputs; negative values are outp-uts; positive storage is depletion and negative is replenishment ofthe storage

Quasi-steady state:low water level incanal during thestable period

Transientstate: middlewater level incanal

Transientstate: highwater level incanal

North 1,182 1,165 1,675East 2,246 746 –496South –841 –763 –1,200West –2,583 –2,096 698Storage –0.06 948 –677Balance 4 0 0

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

Fig. 10 Simulation of the pollution-plume variations (1,1,2-TCA mmol/l) during transient simulations at a low canal water level (scenarioA), b middle water level (scenario B) and c high water level (scenario C). Circles are wells and black lines are calculated hydraulic heads inmamsl. The limits of the studied zone for the water balance are shown in a

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

4. The geometry and the extension of the area in theaquifer where VC is biodegraded; it was assumedthat in some parts of the studied zone, the VCconcentration is not high enough compared withthat of 1,1,2-TCA or 1,2-DCA near the sourceszones to permit VC biodegradation; this assumptionproved necessary to calibrate the numerical modeland is consistent with redox observations, see thefollowing.

After calibration, the simulatedVC concentrations and theirvariability are of the same order of magnitude as those shownby the measurements (Fig. 12). The obtained results were:

1. Initial low concentration of VC at the source: 50 μmol/l.2. Stoichiometry of 1.3. Very short calibrated VC half-life (10 days),

shorter than that of 1,1,2-TCA. Thus, this modelindicates a very substantial degradation of VC,especially during periods when the water level inthe canal remains unchanged. However, the esti-mated VC half-life is unusually short but compa-rable to recent laboratory experiments (Maness etal. 2012; Grostern and Edwards 2006); the half-lives are not given in these articles, but can be

estimated from the decay curves, and are on theorder of 20 days.

4. Area of active biodegradation of VC as shown in Fig. 10.

The effective porosity for transport calculations for VCwas set at 30 %, the longitudinal and transversaldispersivities at 20 and 1 m, as for the 1,1,2-TCAmodel. The redox potential was measured at observa-tion well P1 at different periods in time and signifi-cantly different values were found. In June 2008 (lowwater level in the canal, Fig. 5), the measured redoxpotential was lower than 100 mV, and in March 2010(high water level in the canal), the value was near300 mV. Indeed, fast changes in groundwater flow canchange the biogeochemical conditions (oxido-reduc-tion), thus reducing the effectiveness of 1,1,2-TCAbiodegradation. When no or little change occurs in thegroundwater flow over a long period of time (constantcanal water level), the effectiveness of 1,1,2-TCAbiodegradation intensifies, increasing the amount ofVC produced. This could explain at least partially theP1 and P2 difference between observed and modeledVC concentrations, but analytical or sampling problemscould also exist.

Fig. 11 Comparison between simulated (line) and observed (small dot) concentrations of 1,1,2 TCA in observation wells a P1 (blue), b P2(green) and c P3 (red) for the model with two spatially separated pollution sources. Time 0 (days) is 16 October 2006

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

Conclusions

This work demonstrates how the transient nature ofgroundwater flow in a polluted aquifer, with changesin the flow direction reaching 90°, made it possible tocalibrate a flow and contaminant transport model of anaquifer at an industrial site with unknown buriedsources of chlorinated solvents stored as DNAPLs inunderground troughs at the aquifer bottom. Theuncertainty on the choice of the aquifer parametersand on the location of the source zones was greatlyreduced by the transient nature of the flow, as the“degree of freedom” in the parameter calibration wasmuch more constrained than for a steady-state plume.

The numerical groundwater model made it possible,over a short period, to show that:

1. The concentration variations in the observationwells from 1 month to another are mainly due tothe water level variations in the near-by canal,which make the direction of the groundwater flowvary significantly in the studied zone.

2. There is significant biodegradation in the alluvialaquifer. This biodegradation explains the presence

of dissolved VC in the aquifer water as adegradation product of the major compound respon-sible for the pollution (1,1,2-TCA); VC is alsobiodegraded.

3. The organic phase (DNAPL oil phase) is presentmostly in the alluvium, in troughs of the aquiferunderlying bedrock, as detected by a geophysicalsurvey.

4. There are at least two distinct contaminant sourceareas.

After the parameter calibration of the model, tennew drillings and surveys made in April 2011 foundDNAPLs as close as 10 m from the assumed centersof the troughs in the aquifer bedrock, which con-firmed the assumption made in the transient model.With the model and the microbiological studies, theefficiency of the biodegradation was estimated. Thekinetics of this degradation of 1,1,2-TCA and VC(half-lives of 16 and 10 days, respectively) seems tobe rapid but comparable to values found in theexisting literature and consistent with the acetateseries of the microcosm experiment conducted(17 days for 1,1,2-TCA).

Fig. 12 Comparison between simulated (line) and observed (small dot) vinyl chloride (VC) concentrations in observation wells: a P1(blue), b P2 (green) and c P3 (red) for the model based on 1,1,2-TCA transformation into VC and with two spatially separated pollutionsources. Time 0 (days) is 16 October 2006

Hydrogeology Journal DOI 10.1007/s10040-013-1096-7

Acknowledgements The authors wish to express their gratitude forthe comments and suggestions received from two anonymousreviewers of an earlier draft of this article, and for those of theassociate editor and technical editorial advisor.

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