Aquifer washing by micellar solutions:: 3 Field test at the Thouin Sand Pit (L'Assomption, Québec, Canada)

Ž .Journal of Contaminant Hydrology 30 1998 33–48

Aquifer washing by micellar solutions:ž3 Field test at the Thouin Sand Pit L’Assomption,

/Quebec, Canada´

Richard Martel a,), Pierre J. Gelinas b, Laurent Saumure bá INRS-Georessources, 2535 boul. Laurier, C.P. 7500, Sainte-Foy, Qc., Canada G1V 4C7´

b GREGI, Department of Geological Engineering, LaÕal UniÕersity, Quebec, Qc., Canada G1K 7P4

Received 6 June 1996; revised 17 February 1997; accepted 19 February 1997

Abstract

Ž .A field test was performed to recover DNAPL Dense Non Aqueous Phase Liquid in ashallow aquifer at the Thouin Sand Pit near Montreal, to evaluate a new technique of aquiferrestoration involving surfactant solutions. Laboratory tests have shown that washing solutionscontaining alcohols, surfactants, and solvents are very efficient in recovering DNAPL as a

Ž . Ž .miscible phase. The Thouin field test was designed to: 1 study in situ recovery of DNAPL; 2Ž .evaluate an injection–pumping strategy; 3 test the use of polymer solutions to control the

mobility of a washing solution slug and to improve the vertical sweep efficiency throughout thesand unit. The test was performed in a shallow medium sand aquifer containing both contaminatedsaturated and unsaturated zones. The washing experiment was done on 17 m3 of the saturated zonewith an average DNAPL initial concentration of 55 000 mg kgy1 dry soil. Solutions were injectedthrough a central well and pumped into four wells arranged in a five-point square pattern. In thezone swept by the washing solution, 86% of residual DNAPL was recovered using only 0.9 porevolume of a surfactant solution. These results confirm laboratory sand column experimentsalthough the washing solution used in the field experiment had not yet been optimized to meetsite-specific criteria. The use of a polymer solution before and after injection of the washingsolution proved beneficial in insuring that the washing solution effectively swept all the sandlayers in spite of soil heterogeneities. As the rinsing cycles were not completed because of weather

Ž .problems freezing conditions , small but significant amounts of the washing solution ingredientswere still present in the aquifer at the end of the test and their fate is being monitored. For thisfield test, DNAPL recovery could have been better if an optimal washing solution had been

) Corresponding author.

0169-7722r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0169-7722 97 00031-4

( )R. Martel et al.rJournal of Contaminant Hydrology 30 1998 33–4834

injected, a larger volume of the selected solution had been circulated, or a different injection–pumping strategy had been chosen. q1998 Elsevier Science B.V.

Keywords: Field test; DNAPL recovery; Soil washing; Surfactant; Polymer; Alcohol; Solvent

1. Introduction

This field test is part of a 4 year research project designed to develop surfactantŽ .solutions able to dissolve deep-located DNAPL Dense Non Aqueous Phase Liquid

Ž .lenses such as those observed at the contaminated site of Mercier Quebec, Canada .Ínitially, laboratory procedures were developed to optimize the composition of washing

Žsolutions for in situ NAPL recovery in contaminated aquifers. Previous studies De-.snoyers et al., 1983; Martel et al., 1993; Martel and Gelinas, 1996 show that solutions´

composed of both alcohols and surfactants can be used for the efficient miscibleŽ Ždisplacement of mixtures of petroleum and organic compounds TCE 1,1,2,2-tetrachlo-

. Ž . .roethane , PCE 1,1,2,2,-tetrachloroethylene , diesel, gasoline at residual saturation incontaminated soils. Dissolution of the organic contaminants results from the formation

Ž .of oil-in-water microemulsions in a Winsor Type I system Martel and Gelinas, 1996 .´However, for efficient displacement of heavy and viscous oils, such as those at the

ŽMercier site, an organic solvent must be added to the alcohol–surfactant system Martel.et al., 1998a .

The experimental program was designed for different scales. In the laboratory, phasediagrams were established to select active agents and test surfactant systems for NAPLdissolution. Oil recovery tests in sand columns were performed to study interactionswith mineral soil components and to optimize the systems. Other laboratory tests haveevaluated the compatibility of the optimized solutions with aquifer material at fieldgroundwater temperature. At a larger scale, sand boxes were used to evaluate the sweep

Žefficiency of the washing solution in porous media by polymer solutions Martel, K.E.,.et al., 1997 . DNAPL recovery mechanisms with this type of washing solution were

Ž .examined in detail in a large sand column experiment Martel, K.E., et al., 1997 . Beforeperforming deep DNAPL recovery tests at the Mercier site, a field experiment wasperformed at shallow depth at the Thouin Sand Pit, where DNAPLs similar to those inMercier are present in a sand layer underlain by impervious silty clays. The objective ofthe Thouin test was to assess the scale effects between laboratory and field experimentsand to decide on the feasibility of the washing technique in field conditions beforeattempting a demonstration test at the Mercier site, where the contaminated zone is 30 mdeep and geological conditions are more complex.

Ž .The objectives of this experiment were to: 1 evaluate the efficiency of the washingŽ .solution; 2 evaluate the injection–pumping strategy including field testing of polymer

Ž .solution to control mobility; 3 study the effects of the washing solution density and theŽ . Ž .polymer viscosity; 4 better identify the oil-recovery mechanisms involved; 5 explore

bio-treatment of sediments, groundwater, and recovered fluids after chemical washing.Air, soil, and fluid samples were taken in the test plot, before, during, and after thewashing experiment. The first three objectives are discussed in this paper. Another paper

( )R. Martel et al.rJournal of Contaminant Hydrology 30 1998 33–48 35

in preparation looks at the DNAPL recovery mechanisms and washing solution behaviorobserved during the field test and compares them with those obtained in the large sand

Ž .column test in Martel et al. 1998b .

2. Methods

2.1. Washing solution optimization

Phase diagrams were drawn to select the most efficient alcohols, surfactants, andsolvents which could be combined in a washing solution. These diagrams were used toselect the best alcohol–surfactant–solvent ratios to create a solution able to solubilizeMercier DNAPL. All diagrams were done at 88C, the field groundwater temperature.Sand column experiments using Mercier sand and DNAPL were used to evaluatemineralogical effects on DNAPL recovery and to optimize the systems. These solutions

Ž .were then adapted for the conditions at the Thouin site. Sand columns Fig. 1 containŽ .150 g of sand. All sand column tests were done using the following procedure: 1 after

filling with Thouin sand and compaction, CO is injected and deaerated water is2Ž .introduced; 2 about 5 g of Thouin DNAPL is injected downwards at high velocity

Ž y1 .354"33 cm h , pushed by 0.5 l of deaerated water to reach residual DNAPLŽ . Ž . Ž .saturation of 12.3"2.5% volume ; 3 3 PV pore volumes of washing solution are

y1 Žinjected downwards with a peristaltic pump at a velocity of 5.15"0.73 cm h about.six times the groundwater velocity . DNAPL in excess after saturation, transported with

the effluent and remaining in the soil sample after washing, was determined byŽ .spectrophotometry HACK DRr200 at 550 nm. Dichloromethane was used as a solvent

for mass determination of DNAPL. With this technique DNAPL mass balance in thesand column is determined with a precision of "6%. Downward washing was selectedbecause the displacement front is stable when a lighter and more viscous washing

Fig. 1. Setup for sand column washing experiments.


Ž .solution is displacing water Lake, 1989 . Viscosity and density of washing solutions arephysical properties that can influence DNAPL recovery. Solution density was measuredwith a pycnometer and the viscosity with a Cannon–Fenske viscometer.

2.2. Field test

Ž .The Thouin Sand Pit is located 20 km north east of Montreal Fig. 2 . This sitereceived several million liters of oil wastes from Montreal refineries during the 1960s.Waste oils were dumped by truck in ponds and sometimes burnt in the open. By 1968,the provincial government closed the site and local sand was used to backfill the oil

Ž .ponds. This site is characterized by a thin silty sand layer 2 m underlain by a 30 mŽ .thick deposit of silty clay 30 m which acts as an impervious base. Because of the waste

oil disposal method, NAPL saturation in the sand is locally higher than residual. Sincetheir disposal, waste oils and organic compounds dissolved in water have been flowingin ditches and nearby creeks. Characterization studies of the site show that chlorinated

Ž .solvents are locally present in the soil. The test plot 4.3 m=4.3 m covers less thanŽ 2 .0.075% of the contaminated area 25 000 m . It is located where chlorinated solvents

associated with waste oils are present. The test plot was equipped with one centralinjection well and four recovery wells in a five-point square pattern, and 12 multilevelobservation wells. The recovery wells are located 3 m from the injection well. The

3 Žwashing experiment cell contains 17 m of contaminated sediments the lower half of the. 32 m thick layer of silty sand which represent a PV of about 6 m . The washing

Ž .proceeded in several steps: 1 water flooding with 1.34 PV and polymer flooding with0.54 PV to reach steady-state flow in the cell and to decrease non-residual oil saturation;Ž . Ž .2 injection of 0.9 PV of surfactant solution to wash the sediments; 3 polymerflooding with 1.6 PV and water flooding with 1.4 PV to push the washing solution out

Fig. 2. Location of the Thouin Sand Pit.


Ž .and rinse the sediments; 4 injection of acclimated autochthonous bacteria and nutrientsto increase biodegradation of the remaining DNAPL and ingredients of the washingsolution.

ŽDNAPL recovery is a function of DNAPL displacement efficiency miscible or. Žimmiscible and volumetric sweep efficiency in the aquifer ratio of DNAPL volume

.contacted by the washing solution to the volume of DNAPL in place before washing .DNAPL displacement efficiency depends mainly on the washing solution formulation.Volumetric sweep efficiency considers both areal and vertical sweep efficiencies.Mathematical modeling is used to optimize the areal sweep efficiency. The injection–pumping wells pattern and flow rates were decided after numerical simulations of

Ž .groundwater flow using the MODFLOW model Martel and Gelinas, 1994 . Conse-´quently, to prevent any leak of the washing solution outside the test plot, the pumpingrate was fixed at twice the injection rate. Vertical sweep efficiency depends on fluidviscosity and density, and on porous media heterogeneity. To prevent preferential flow

Žof washing solutions in the most permeable zones, a polymer solution xanthan gum in.water was injected. This shear-thinning polymer has a viscosity which decreases as the

shear rate on the fluid increases in the fine-grained layers. Conversely, the polymerŽ . Ž .solution velocity is slowed down in the coarse layers Fig. 3 Martel, K.E., et al., 1997 .

Viscosity differences in a stratified porous medium promote piston-like displacement offluids. Because the washing solution is not shear-thinning, a polymer solution wasinitially injected to condition the porous medium. A second polymer solution is alsoinjected following the surfactant solution to create a stable front at the back edge of thewashing solution slug. For horizontal flow, the condition for stability is simply that theviscosity of the displacing fluid should be larger than that of the displaced fluid. Densitycontrast between the polymer and the washing solution should be minimal, to avoid

Ž Ž ..Fig. 3. Viscosity of xanthan gum in water as a function of shear rate adapted from Martel, K.E., et al. 1997 .


gravity effects which can induce uneven fluid circulation in the upper and lower parts ofa saturated zone.

2.3. Soil sampling and analysis

During well installation, 42 soil samples were taken in the cell from the surface downŽ .to the silty clay layer using a split-spoon 60 cm length=3.5 cm at three levels

Ž .0–60 cm, 60–120 cm and 120–180 cm in 14 boreholes. The spoon was brushed, rinsedwith acetone, and steamed between each sampling. The soil samples were described,

Ž .placed in 1 l Mason glass jars, and cooled. Other samples 25 were obtained fromplastic liners taken with the split-spoon in three stratigraphic boreholes located outside

Ž .the cell Fig. 4 . After the washing experiment, the top 60 cm of the unsaturated zonewere dug with a hand auger and, because of difficult access between wells, samples of

Ž .sediment were taken by driving ABS casings 3 m length=5 cm diameter down with a

Fig. 4. Location of soil samples taken before and after the test.


Table 1DNAPL concentrations in soil before and after the test

Ž .Depth cm DNAPL in soil before test DNAPL in soil after test DNAPL recoverya b c a b cConcentration Mass S Concentration Mass S Mass Masso or

Ž . Ž . Ž . Ž . Ž . Ž .% kg % kg % kg

0–60 19.1 3580 0.96 n.a. n.a. n.a. n.a. n.a.d60–90 9.9 929 0.50 9.8 918 0.49 1.3 12d90–120 9.9 929 0.50 5.1 482 0.26 48.1 447d120–150 3.3 309 0.16 0.7 64 0.04 79.4 246d150–180 3.3 309 0.16 0.2 22 0.01 92.9 287

90–180 5.5 1547 0.27 2.0 568 0.10 63.3 980e120–180 3.3 618 0.16 0.5 86 0.03 86.1 532

a Weighted average concentration of DNAPL in dry sand is calculated for each layer with Surfer w by the sumŽof the product of concentration on each element of the grid divided by the total area of the grid wt.

.DNAPLrwt. solids .b Mass of DNAPL in a layer is calculated by multiplying the weighted average concentration of DNAPL in the

Ž y3 .layer by the bulk density 1.7 g cm of the soil and by the volume of soil in the layer.cS or S is the proportion of the pore volume occupied by DNAPL and is calculated by the weighted averageo or

concentration of DNAPL in the layer multiplied by the bulk density of the soil, and divided by the DNAPLdensity and the soil porosity.d Because the split-spoon sample was 60cm long the concentration is considered the same for both 30cmsubsamples.e Water-saturated zone.

sledge-hammer. With this technique, the maximum sampling depth was 2.4 m and theŽ .sample recovery varied from 32% to 100% mean of 74% . For soil recovery less than

100%, samples are assumed to come from the upper part of the plastic tube. ABS casingand plastic liners were sealed in the field and cut in sections of 10, 15, or 30 cm lengthin the laboratory. From the 18 ABS tubes driven, 77 soil samples were obtained. Themineralogy of sediment was described by observation of a clean sample by binocular

Ž .microscopy. DNAPL in soil was determined by spectrophotometry HACK DRr200 at550 nm after DNAPL extraction on 50 g soil samples with dichloromethane. AfterSoxhlet extraction of surfactants in sediment samples with methanol, surfactant concen-

Ž .trations were determined by titration Zhen Cao, 1978 . Volatile organic compounds insediment were determined by gas chromatography–mass spectrometry according to theUS EPA 624 method. Data on DNAPL concentrations before and after the test wereprocessed using a kriging interpolation technique using Surfer w v. 3.1 of Golden

Ž .Software Colorado, USA . Results are presented in Table 1.

3. Results

3.1. Washing solution optimization

Various proportions of alcohol–surfactant–solvent–water were combined in phaseŽ .diagrams to identify solubilization zones of Mercier DNAPL Martel et al., 1998a .


Fig. 5. DNAPL recovery in sand columns with various aqueous solutions of n-butanol, SAS, D-limonene andtoluene.

Twenty sand column experiments were performed to optimize the washing solutionsselected by phase diagrams. An optimal washing solution for DNAPL in Mercier sandcan recover more than 70% of residual DNAPL after the injection of only 1 PV ofwashing solution. Laboratory tests using the same solution for the Thouin sand recov-

Ž .ered less than 25% of the residual DNAPL saturation Fig. 5 . The difference in DNAPLrecovery is related to changes in interfacial and surface properties. In fact, DNAPLs

Žfrom Thouin and Mercier show little difference in physical properties Thouin: densityy3 y3 .1020 kg m , viscosity 18 mPa s; Mercier: density 1048 kg m , viscosity 25 mPa s but

differences in chemical compositions may be more significant. Sand mineralogy atŽ .Thouin 82% quartz, 11% feldspar, 6% igneous rock fragments, and 1% mica is

Ž .different from that at Mercier 62% carbonates, 34% quartz, and 4% feldspar . Mineral-Žogy affects wetting properties of DNAPL and influences oil recovery Stegemeier,

.1977 . The optimal washing solution for the Mercier site was adapted for the Thouinsite. The proportions of ingredients in the Mercier washing solution were adapted forThouin to find DNAPL-in-water microemulsion zones in phase diagrams. Sand columntests were performed with these solutions, and the washing solution showing the bestrecovery was selected. For this test, the alcohol is n-butanol, the surfactant is Hostapur

R Ž .SAS 60 Hoechst GmbH, Frankfurt , and D-limonene and toluene were used assolvents. In laboratory columns, 1 PV of the selected washing solution recovered 45% ofThouin DNAPL in downward flow experiments.

3.2. Field test

The experimental plot is flat, covers a surface area of 18.5 m2 and intersects amedium sand layer resting on low-permeability silty clay at a depth varying between 1.8and 2.1 m. The average water-saturated thickness is 0.8 m. The sand has a porosity of

Ž .0.30–0.35, a particle size range of 0.05–2 mm with a median particle diameter d of50

0.4–0.8 mm, a specific surface area of 0.006 m2 gy1, and contains 0.72"0.35% organic


Ž .matter Walky Black Method, Agriculture Canada, 1977 . The sand has a meanŽ . y4 y1 Ž .hydraulic conductivity of 8"2 =10 m s estimated from the Hazen formula .

The field measured hydraulic conductivity from slug tests in observation wells is10y4 m sy1. Differences in hydraulic conductivity can be explained by the interferencecreated by partial DNAPL saturation in sand. In the immiscible flow regime the relative

Žpermeability is always smaller when two fluids are present. A thin silt layer 10–15 cm.in thickness was observed between 1.0 and 1.2 m depth, close to the groundwater table.

This less permeable layer is probably not present over all the Thouin site because ofexcavations made in the sand pit. Groundwater flows east with a natural gradient of0.009 and at a mean velocity of 80 m yeary1. A packer was installed in the injection welljust below the silt layer to prevent injection of washing solution into the unsaturated

Žzone and to prevent its circulation only in the upper part of the saturated zone becauseof the buoyancy forces created by the solution having a density of 959.7 kg my3, which

.is lower than water density .Before injection of the solution, DNAPL concentrations in the unsaturated zone were

Žlocally very high. These suspensions of sand in tar lenses up to 64% of DNAPL in. Ž .sand probably correspond to local discharge points of waste oils Fig. 6 . Average

DNAPL concentrations in this zone are four times higher than in the saturated zone,where concentrations are more homogeneous and decrease with depth. Eliminating theupper 90 cm of sand in the unsaturated zone which is not affected by the washingexperiment, the mean DNAPL concentration in the sand of the cell is 5.5 g per 100 g ofdry sand, which represents 1547 kg of DNAPL in 15 m3 of soil or a DNAPL saturation

Ž . Ž .of 0.27 DNAPL volumerpore volume , which is more than residual Table 1 .After circulating the washing solution, DNAPL recovery is observed to increase with

depth. As anticipated, no significant DNAPL recovery was achieved in the lower part ofŽ . Ž .the unsaturated zone 60–90 cm Table 1 . However, displacement of DNAPL may

have occurred in this zone, which suggests that a small quantity of washing solution wasŽ . Ž .spent above the water table Fig. 7 . The intermediate zone 90–120 cm includes soils

Ž .Fig. 6. DNAPL concentrations in soil before the test % wrw .


Ž .Fig. 7. DNAPL concentrations in soil after the test % wrw .

both in the unsaturated and the saturated zones. As the top part is above the silt layerŽ .and above the water table where the washing solution was not circulated , a combined

recovery rate of 48% can be explained by the sampling interval covering two zones. Forthe two lower layers sampled in the saturated zone, DNAPL recoveries of 79% and 93%were observed as anticipated by laboratory sand column tests. The mass balance ofDNAPL in the cell before and after the washing process suggests that more than 980 kg

Ž .Fig. 8. Plan view of DNAPL concentrations in soil layers before and after test % wrw .


Ž .Fig. 9. Recovery of naphthalene, 1,1,2-trichloroethane, and DNAPL in the soil after the washing process % .


or 86% of DNAPL saturation was removed in the cell. The average concentration ofDNAPL remaining in the sediments of the two layers of the saturated zone is 0.45%Ž y1 .4500 mg kg of dry soil , which is less than Criterion C of the Quebec Contaminated

Ž .Site Rehabilitation Policy MEF, 1988 . This DNAPL concentration is low comparedy1 Žwith the overall 55 000 mg kg including the saturated and part of the unsaturated

.zones but it must be decreased to achieve groundwater restoration standards. For thisfield test DNAPL recovery could have been better if a larger volume of the selectedsolution had been injected, or if a different injection–pumping strategy had beenadopted.

Although the washing solution was less dense than water, in spite of some dissolvedŽ y3 .DNAPL from 959.7 to 967.1 kg m no overriding of the washing solution was

Ž . Žobserved. This could be explained by: 1 injection in fully screened wells the whole. Ž . Žsaturated zone was flushed ; 2 anisotropy in soil permeability horizontal greater than

. Ž . Ž .vertical and heterogeneity; 3 the short distance 3 m between injection and pumpingwells. For the purpose of a controlled field-scale experiment, a five-point injection–pumping pattern is suitable, as it minimizes the risk of leaking outside the cell. For thispattern, it is more difficult for the washing solution to reach between the pumping wellsŽ .Fig. 8 , as flowpaths are longer and groundwater velocity is slower. This could be

Žpartially resolved by increasing the number of pumping wells e.g. seven-point injec-.tion–pumping hexagonal pattern . In a larger-scale experiment or in a commercial

application of the technology, another injection–pumping pattern such as a line drivecould be considered.

Ž .Two volatile organic compounds naphthalene and 1,1,2-trichloroethane present inthe Thouin DNAPL were analyzed in the soil before and after the washing process.Below the silt layer, comparable recovery rates are observed for these two compounds

Ž .and for DNAPL Fig. 9 . This is consistent with laboratory experiments using diesel,which showed no preferential extraction of components during washing experimentsŽ .Martel, 1996 . However, apparent minor preferential extraction or chromatographicseparation of DNAPL components may be attributed to the sampling pattern where soilsamples were extracted in adjacent boreholes before and after the test.

Fig. 10. DNAPL and surfactant concentration profiles in soil samples after the test.


Ž y1 .Fig. 11. Residual n-butanol, toluene and D-limonene concentrations in soil after the test mgkg .


Surfactant concentration profiles in the soil confirmed that surfactant flooded part ofŽ .the unsaturated zone, sometimes during the process Fig. 10 . In fact, because polymer

flooding was performed initially at a high flow rate on the whole sand thickness, thewater table rose to the surface of the cell and some of the washing solution may havemigrated upward in the sand where the silt layer is discontinuous. At the end of the final

Ž .water flooding step, the water level was 0.5 m depth originally 1.0 m depth .Because the rinse cycle was stopped after 3.0 PV, significant residual surfactant

Žremains in the soil. Remaining surfactants follow the DNAPL concentrations in.concentrations one order of magnitude smaller because surfactant is lipophilic. Like

DNAPL, residual surfactants decrease with depth. For this field test, residual concentra-Ž . y1tions of solvents from the washing solution range from 4 to 160 mg kg of wet soil

y1 Ž .for toluene and from less than 1 to 500 mg kg of wet soil for D-limonene Fig. 11 .Globally, even if toluene is an ingredient of the washing solution, its concentration in

Ž .soil after the test is about half of its concentration before the test Fig. 12 . Residualconcentrations of n-butanol in soil range from less than 1 to 600 mg kgy1 of wet soil.The maximum residual solvents or n-butanol concentrations correspond to areas wherethe washing solution was located at the end of the rinsing operation. These residual

Žingredient concentrations can be reduced significantly. Laboratory tests Martel et al.,.1998b showed that injection of alcohol–surfactant solution after alcohol–surfactant–

solvent help to recover solvents remaining in the soil. Also, extensive polymer floodingcan decrease residual surfactant and alcohol concentrations. Preliminary laboratory testsalso showed that remaining surfactant, alcohol, and solvent concentrations can bebiodegraded if acclimated bacteria and adequate nutrients are injected into the soil after

Ž .the rinsing operation Roy et al., 1995 .Several important aspects are still being investigated, for instance: effluent treatment;

reuse and recycling of products; waste water elimination; the potential use of bioremedi-ation following the washing experiment by inoculation with acclimated bacteria. Limiteddata show that salting-out procedures help phase separation and facilitate waste water

Ž y1 .Fig. 12. Toluene concentrations in soil before the test mgkg .


treatment by promoting surfactant precipitation. We are conscious that reasonableanswers to these issues are critical for the commercial use of this technology.

4. Conclusions

A field test on DNAPL recovery in a saturated sand aquifer was performed at theThouin Sand Pit using concentrated surfactant solutions to solubilize organic contami-nants and transport them by miscible displacement. The washing solutions weredesigned after several laboratory experiments using phase diagram construction and sandcolumn experiments to optimize the solutions. A five-point injection–pumping wellpattern was used for the test, but with this pattern the DNAPL is more difficult to reachbetween the pumping wells, as flowpaths are longer and groundwater velocity is slow.Increasing the number of pumping wells in the future may partially resolve this problem.

Test results show that for only 0.9 PV of washing solution, the recovery in the sweptzone is 86% of initial residual DNAPL. The usefulness of polymer solutions incontrolling the mobility of a washing solution slug and in improving the vertical sweepefficiency throughout the sand unit has been demonstrated. No gravity separation of thewashing solution was observed in sediments during this test, probably because of the

Žpermeability anisotropy of the sand layers vertical permeability smaller than horizontal.permeability and the small distance that separates the injection from the recovery wells.

DNAPL recovery in the field was higher than in sand column experiments because it iseasier in the field to displace DNAPL parallel to stratification.

Small but significant residual concentrations of surfactant, alcohol, and solvents wereobserved in the soils after the test. However, they may be reduced significantly if analcohol–surfactant solution is injected after the washing solution and if extensivepolymer flooding is performed. Preliminary laboratory tests show that remaining surfac-tant, alcohol and solvents can be biodegraded if acclimated bacteria and adequatenutrients are injected into the soil after the rinsing operation. The average concentration

Ž y1 .of DNAPL remaining in the saturated sand layer is 0.45% 4500 mg kg of dry soil ,which is below Criterion C of the Quebec Contaminated Site Rehabilitation PolicyŽ .MEF, 1988 . This DNAPL concentration is low compared with the initial55 000 mg kgy1, but it must be decreased still further to achieve groundwater restorationstandards. For this field test, DNAPL recovery could have been improved with the

Ž . Ž .following: 1 injection of a larger volume of washing solution; 2 a differentŽ .injection–pumping strategy; 3 a more efficient rinsing procedure.

Acknowledgements

ŽThis project is funded by the DESRT program Development and Demonstration of.Site Remediation Technology of the Quebec Ministry of Environment and Fauna´

Ž .MEF and Environment Canada. Special thanks go to the students who helped inŽlaboratory or field manipulations Annie Larrive, Alicia Moreno, Annie Turgeon, Karl´

´ .Eric Martel, Nathalie Roy, and Alain Hebert . We also thank our research assistants:´


Pierrette Vaillancourt, Catherine Blais, Suzanne Bussiere, Dominique Marceau, Mireille`Lapointe, Julie Simard, and Rene Dufault. We would like to thank Hoechst Gmbh,´Frankfurt, which provided free of charge the surfactant used for this test. Special thanksare due to Yvon Couture and Linda Lecours from the laboratory division of the MEF,who performed many chemical analyses in the field. We are grateful to Rene Lefebvre´from INRS-Georessources for stimulating discussions.´

References

Agriculture Canada, 1977. Manuel de methodes d’echantillonnage et d’analyse des sols. Comite canadien de´ ´ ´pedologie. Soil Research Institute, Ottawa, Ont., 233 pp.´

Desnoyers, J.E., Quirion, F., Hetu, D. and Perron, G., 1983. Tar sand extractions with microemulsions:´1—The dissolution of light hydrocarbons by microemulsion using 2-butoxyethanol and diethylamine ascosurfactants. Can. J. Chem. Eng. 61, 672–679.

Lake, L.W., 1989. Enhanced Oil Recovery. Prentice–Hall, Englewood Cliffs, NJ, 550 pp.Martel, K.E., Martel, R., Lefebvre, R. and Gelinas, P.J., 1997. Laboratory study of solutions used for mobility´

control during in situ NAPL recovery: 1—Laboratory characterization and selection of polymer solutions.Ž .Ground Water Monit. Remediat. 17 4 .

Martel, R., 1996. Developpement de solutions tensioactives pour le recuperation de phases liquides non-´ ´ ´Ž .aqueuses a saturation residuelle dans les aquiferes. These de doctorat Ph.D. , Departement de geologie et` ´ ` ` ´ ´

de genie geologique, Universite Laval, Quebec, 310 pp.´ ´ ´ ´Martel, R. and Gelinas, P.J., 1994. Utilisation de solutions tensioactives pour recuperer les liquides immisci-´ ´ ´

bles a saturation residuelle dans les lieux contamines ‘orphelins’. Soumis au Ministere de l’Environnement` ´ ´ èt de la Faune et Environnement Canada. Rapport d’etape 6, 206 pp.´

Martel, R. and Gelinas, P.J., 1996. Surfactant solutions developed for NAPL recovery in contaminated´Ž .aquifers. Ground Water 34 1 , 143–154.

Martel, R., Gelinas, P.J., Desnoyers, J.E. and Masson, A., 1993. Phase diagrams to optimize surfactant´Ž .solutions for oil and DNAPL recovery in aquifers. Ground Water 5 5 , 789–800.

Martel, R., Gelinas, P.J. and Desnoyers, J.E., 1998a. Aquifer washing by micellar solutions: 1—Optimizationóf alcohol–surfactant–solvent solutions. J. Contam. Hydrol. 29, 317–344.

Martel, R., Lefebvre, R. and Gelinas, P.J., 1998b. Aquifer washing by micellar solutions: 2—Experimental´behavior of an optimized alcohol–surfactant–solvent solution. J. Contam. Hydrol. 30, 1–29.

MEF, 1988. Politique de rehabilitation des terrains contamines. Gouvernement du Quebec, Ministere de´ ´ `l’Environnement, Direction des substances dangereuses, Sainte-Foy, Que., 54 pp.´

Roy, N., Martel, R. and Gelinas, P.J., 1995. Laboratory assessment of surfactant biodegradation in aquifersúnder denitrifying conditions. In: 5th Annual Symp. on Groundwater and Soil Remediation, Toronto, Ont.,Environmental Protection Service, Environment Canada, 2–5 October 1995.

Stegemeier, G.L., 1977. Mechanisms of entrapment and mobilization of oil in porous media. In: D.O. ShahŽ .and R.S. Schechter Editors , Improved Oil Recovery by Surfactant and Polymer Flooding. Academic

Press, New York, 575 pp.Zhen Cao, L., 1978. Dye Stuff Chemicals, 2nd edn. Textile Industry Press, Beijing, pp. 309–317.

Documents

Aquifer washing by micellar solutions:: 3 Field test at the Thouin Sand Pit (L'Assomption, Québec, Canada)