1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS INSATURATED HOMOGENEOUS AND STRATIFIED POROUS MEDIA

JAIME DIAZ1, MANUEL RENDUELES2 and MARIO DIAZ1,∗1Department of Chemical Engineering and Environmental Technology, University of Oviedo;

2Project Management Area, University of Oviedo(∗author for correspondence, e-mail: mariodiaz@uniovi.es; Tel.: +34-985-10-34-39,

Fax: +34-985-10-34-34)

(Received 25 May 2004; accepted 18 October 2005)

Abstract. Subsurface contamination by nonaqueous phase liquids, such as 1,2,4-trichlorobenzene,

is a widespread problem at numerous sites. In this work, the behaviour of 1,2,4-trichlorobenzene in

the saturated zone was investigated using two natural porous media. To this end, cylindrical columns

were packed with different materials and the initial water saturation was displaced by loading a

constant level of trichlorobenzene on top of the porous bed. The experiments were carried out with

different configurations to simulate natural media with several stratigraphic units. Also, the effect of

pressure gradient on nonaqueous phase liquid flow was investigated. Both the pressure gradient and

the configuration of the layers of porous media have shown a strong effect on water recoveries and

fluxes through the column.

Keywords: drainage and imbibition experiments, NAPL flow, relative permeabilities, soil columns,

trichlorobenzene, NAPL-water displacement, bench scale studies

1. Introduction

Nonaqueous phase liquids (NAPLs) such as hydrocarbons and chlorinated sol-vents are a common source of contamination of many subsurface sites. They mayreach the subsurface as a result of spills, leaks in storage tanks or direct disposal.These compounds are typically found in the following wastes and waste-producingprocesses: solvents, petroleum products, wood preserving wastes, coal tars andpesticides. NAPLs are generally classified into light and dense nonaqueous phaseliquids. Dense nonaqueous liquids (DNAPLs) are denser than water and have verylow solubility in water. They can thus exist as independent phases and, after a spillor leak occurs, may migrate into the subsurface below the water table. Once in thesubsurface, DNAPLs slowly diffuse into the groundwater generating a long-termsource of contamination. DNAPL movement into the subsurface is hardly affectedby lateral groundwater flow, but it is affected by hydrogeological heterogeneity,which may cause lateral migration. In addition to this, dissolved substances fromDNAPL will migrate in the direction of the flow generating a contamination plume.

Some of the DNAPLs found at U.S. Superfund Sites (U.S. EPA. Subsur-face Contamination Reference Guide, EPA/540/2-90/011, October, 1990) include

Water, Air, and Soil Pollution (2006) 177: 3–17

DOI: 10.1007/s11270-005-9019-4 C© Springer 2006

4 J. DIAZ ET AL.

halogenated semi-volatiles (dichlorobezenes, tetrachlorophenol. . .), halogenatedvolatiles (trichloroethylene, carbon tetrachloride. . .) and nonhalogenated semi-volatiles (anthracene, naphtalene. . .). Among these, the transport of halogenatedvolatiles such as TCE, PCE and carbon tetrachloride has been extensively stud-ied (Hofstee et al., 1998; Guigard et al., 1996; Oostrom et al., 2003). All thesecontaminants are characterized by a higher density than water and a low viscos-ity. In contrast, the transport of DNAPLs with higher viscosities, such as 1,2,4-trichlorobenzene, has received less attention.

1,2,4-Trichlorobenzene (TCB) is a semivolatile compound with high viscosityused as a solvent in chemical manufacturing, dielectric fluid, heat transfer medium,dye carrier, herbicide intermediate and degreaser. According to EPA’s Toxic Chemi-cal Release Inventory, releases to land in the U.S.A. during 2001 were approximately3497 kg and total releases 53023 kg. These releases were principally produced bytextile finishing industries.

The present work is a study of factors affecting the displacement of water froma saturated medium by a DNAPL, 1,2,4-trichlorobenzene, such as the presence oflow permeability layers or the piezometric head of nonaqueous liquid. To this end,gravity drainage experiments were carried out on 1-D soil columns saturated withwater. In drainage displacements, a non-wetting invading fluid displaces a wettingfluid. The opposite case, imbibition, occurs when a wetting fluid displaces a non-wetting fluid. Thus, the drainage experiments allow us to know the mechanisms bywhich TCB displaces water when it is released into a saturated porous medium. Theimbibition experiments are useful, on the other hand, in predicting the efficiency of aflushing technology, since the proportion of TCB removed by water injection may bestudied. In the experiments carried out in the present study, water recoveries and flowrates of TCB and water were obtained and compared under different experimentalconditions. In addition, drainage and imbibition processes were compared in oneof the porous media by means of the relative permeability curves extracted fromthe immiscible displacement data.

2. Materials and Methods

2.1. NONAQUEOUS LIQUID

The nonaqueous liquid employed in the experiments (1,2,4-trichlorobenzene) wasobtained from Sigma-Aldrich. 1,2,4,-trichlorobenzene is a nonaqueous liquid thatis denser than water (1460 kg/m3), with low solubility in water (31 mg/L at 25 ◦C)and a high viscosity (1.89 cp at 25 ◦C). 1,2,4-Trichlorobenzene was chosen becauseit frequently appears as a contaminant of sediments and aquifers. Due to its toxic-ity and bioaccumulative properties, contamination by 1,2,4-trichlorobenzene is anissue of major concern (Middeldorp et al., 1997).

1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS 5

2.2. MATERIALS

Two different materials were used in this study, namely sand and natural soil. Sandsamples were collected from a beach in the north of Asturias (Spain). Naturalsoil samples were collected in the central area of Asturias. Before being used, thesamples were air-dried, crushed and sieved through a 2 mm mesh. Samples below2 mm particle diameter were taken for physical and chemical characterization.Organic matter content was determined using a TOC analyzer setup combinedwith a solid sample combustion unit (Shimadzu, SSM-5000A). Particle densitywas determined by pycnometry (Black, 1965). Bulk density was determined bymeasuring the volume of a weight of dried soil (Radojevic et al., 1999). Soil graininternal porosity was determined with an Autopore II 9220 mercury porosimeter(Micromeritics). Hydraulic conductivity for water was determined in each materialvia the constant head method and was employed to estimate the value of the intrinsicpermeability of the sand and the soil. Wettabilities were obtained by the Amottmethod (Amott, 1959). The results of the physico-chemical characterization for thetwo materials are presented in Table I.

Particle size analyses were carried out using a laser diffraction system(Mastersizer S; Malvern Instruments Ltd., UK) and grain size distribution curveswere obtained for both soils (Figure 1).

It can be seen from Figure 1 that particle size distribution is narrower for sand.The width of the distribution may be quantified by the span value, which is calculatedas:

Span = d(0.9) − d(0.1)

d(0.5)(1)

TABLE I

Physico-chemical characterization of soils

Sand Natural soil

Organic matter % 0.12 2.89

Particle density (g/cm3) 2.68 2.63

Bulk density (g/cm3) 1.549 1.003

Apparent porosity (%) 47 62

Grain internal porosity (%) 4.13 20.15

Water hydraulic conductivity (cm/s) 0.0496 0.006

Intrinsic permeability, calculated from 5.06 × 10−7 6.12 × 10−8

water hydraulic condutivity (cm2)

Wettability (Amott index to water) 0.94 0.77

Average particle size (μm) 260.78 342.57

Span 0.7579 2.897

6 J. DIAZ ET AL.

Figure 1. Particle size distribution for sand and natural soil.

where d(0.1), d(0.5) and d(0.9) are the sizes of the particles for which 10%, 50%and 90% of the sample is below this size, respectively. The value of d(0.5) is alsoknown as the “mass median diameter” and will be taken as the average particlesize.

2.3. COLUMN EXPERIMENTS

Transport experiments were carried out in 100 cm long and 10.3 cm diametermethacrylate columns in which the different porous media were packed. Thecolumns were wet packed with the sieved material so as to avoid trapping airbubbles. To fill the columns, layers of 3 cm were flooded and subsequently filledwith the porous material while the columns were shaken. In this way, air trappingis minimized and the formation of segregation fronts is avoided. In the experimentswith natural soil, sand and natural soil were arranged in layers, as shown in Figure 2,whereas in the experiments with sand, only sand was packed in the columns. Thisarrangement in layers simulates a natural porous medium in which a contaminantwould have to move through layers with different permeabilities, porosities andsurface properties.

In all the experiments, the transport of trichlorobenzene was studied by loadinga constant level of nonaqueous phase liquid on top of the initially water-saturatedporous medium to simulate a continuous spill. The level of trichlorobenzene wasmaintained constant by means of a Mariotte bottle.

The column outlet was connected to a graduated container in which the twoimmiscible phases were separated and the cumulative volume of the denser phase

1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS 7

Figure 2. Experimental setup for column experiments.

(trichlorobenzene) could be measured as it displaced the initial water saturationfrom the bed. The cumulative volume of the lighter phase (water) was measured inanother graduated container placed at the same height as the top of the column. Itmust be noticed that the volume changes due to DNAPL dissolution are negligiblebecause of the low solubility of trichlorobenzene in water. All experiments werecarried out by triplicate to test the reproducibility of the results. A diagram of theexperimental device is shown in Figure 2.

2.4. EVALUATION METHODS. ESTIMATION OF RELATIVE PERMEABILITIES

Relative permeabilities at different saturations can be obtained from post-breakthrough cumulative volumes obtained during immiscible displacements. Thestandard procedure used to estimate relative permeabilities from immiscible dis-placement data is the JBN method (Johnson et al., 1959). In the present case,however, relative permeability curves were estimated by the method developedby Toth et al. (2002). This method allows estimation of relative permeabilityvalues from immiscible displacement data obtained not only at constant flowrate but also at constant pressure. In addition, the noise of the experimentaldata is reduced by the interpolation of cumulative volume curves prior to thecalculations.

8 J. DIAZ ET AL.

According to this method, the cumulative injected fluid volume curves can bemodeled by:

Vi (t) = a2tb2 (2)

where Vi is the cumulative injected volume, t is the time, and a2, b2 fitting param-eters.

Furthermore, the cumulative injected volume and produced displaced fluid vol-ume can be correlated by the following linear relationship:

Vi (t)

VK= a + b

(Vi (t)

Vp

)(3)

where Vk is the displaced fluid volume and Vp the pore volume.From Darcy’s law for multiphase flow and the Buckley-Leverett’s (Buckley

et al., 1942) solution for immiscible displacement in one dimension, the followingexpression can be obtained:

d [�p(t)]

dt= �p(t)

qi (t)Vi (t)

d

dt[qi (t)Vi (t)] − q2

i (t)L

KAY(Sd2)Vi (t)(4)

where �p is the pressure drop across the porous medium, qi is the injection flowrate, L is the length of the porous medium, K is the absolute permeability, A is thecross-section area of the porous medium, and Y(Sd2) is the sum of the displacedand displacing fluid phase motilities, given by:

Y (Sd2) = Krd

μd+ Krk

μk(5)

where Krd , Krk are the relative permeabilities of the displacing and displacedphase and μd , μk are the viscosities of the displacing and displaced phase,respectively.

For a constant pressure displacement, the expression is reduced to:

Y (Sd2) = L

�pK A

q3i (t)

ddt [qi (t)Vi (t)]

(6)

Using Equation (2), Y (Sd2) can be expressed as:

Y (Sd2) = La2b22

( Vp

a2

)(1−1/b2)

�pK A(2b2 − 1)

(Vi (t)

Vp

)(1−1/b2)

(7)

1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS 9

Once the sum of the displaced and displacing phase mobilities (Y(Sd2)) havebeen calculated, the single relative permeabilities can be calculated by:

Krk = μk fkY (Sd2) (8)

Krd = μd fdY (Sd2) (9)

where fk and fd are the fractional flows of the displaced and displacing fluids.Considering Equation (3), these can be expressed as:

fk = a(a + b Vi (t)

V p

)2(10)

fd = 1 − fk (11)

The relative permeability is represented versus outlet saturation, which can beobtained from:

Sd2 = Sdi + b

[ Vi (t)Vp

a+bVi (t)Vp

]2

(12)

where Sdi is the displacing fluid saturation of the medium prior to the displacement.

3. Results and Discussion

3.1. MIGRATION OF 1,2,4-TRICHOLOROBENZENE THROUGH SAND

AND NATURAL SOIL

In order to study the movement of the nonaqueous liquid through natural porousmedia, 1,2,4-trichlorobenzene was loaded on top of columns packed with the testedmaterials (sand and natural soil). Since 1,2,4-trichlorobenzene is denser than water,it may be expected to displace the water from the packed material, thus simulatinga spill of liquid reaching the water table.

Two sets of experiments were carried out: in the first set of experiments, a columnwas packed with sand to a bed height of 7 cm. In the second set of experiments, asand layer of 1 cm was placed at the bottom of the column, followed by a layer of4.5 cm of natural soil and finally, a layer of 1.5 cm of sand at the top of the column(Figure 2). In this way, the effect of the stratification of the porous medium couldbe studied.

In both sets of experiments, a constant piezometric head of 6.3 cm oftrichlorobenzene was maintained on top of the packed materials and the cumu-lative volume of trichlorobenzene and water was measured at the column outlet.

10 J. DIAZ ET AL.

Figure 3. Water recovery for soil and sand columns.

The water recovery curves (% of initial water saturation drained from the column)for the sand and the stratified column are presented in Figure 3.

The solid and dashed lines represent the breakthrough of trichlorobenzene (i.e.the time at which the first drop of trichlorobenzene reaches the column outlet)in the soil and sand column, respectively. As can be seen, the breakthrough oftrichlorobenzene is delayed when the liquid has to flow through a lower hydraulicconductivity layer, as happens in the soil column.

Water recovery also changes when different porous media are packed in thecolumn. In the sand column, 78.3% of the initial water is recovered, whereas only47.8% of the initial water is drained from the system in the soil column. The higherretention of water in the soil columns might be explained as a result of the wider grainsize distribution (given by the span value, Table I), which leads to narrower porethroats. The capillary forces in these throats become more important, meaning thathigher pressures of the non-wetting fluid (TCB) are needed to displace the wettingfluid (water) from the pores. The other main factor controlling recovery, wettability,can not explain the differences in water recovery since the more water-wet material(sand) has yielded the higher water recoveries. The differences in wettabilities,however, are not very remarkable (Table I), so the recovery could be controlled bythe structure of the porous media rather than by its wetting characteristics.

In addition, a substantial volume of water is drained from the sand column aftertrichlorobenzene breakthrough. In the soil column, however, only a small amountof water is recovered after trichlorobenzene breakthrough. This behaviour may bedue to the stratification of the porous medium in the soil column. In this column,the trichlorobenzene flow finds an obstacle when it reaches the soil layer, whichhas a lower permeability. This causes an accumulation of trichlorobenzene abovethe soil layer, thus leading to the saturation change through the column, which

1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS 11

Figure 4. Flow rates in sand column.

was continuous before trichlorobenzene reached the soil layer, becoming sharp anddiscontinuous. Due to this sharp change in saturation, only a small percentage ofwater is recovered after breakthrough of trichlorobenzene.

Trichlorobenzene, water and total flow rates were calculated from the cumulativevolumes for the sand and the natural soil column (Figures 4 and 5, respectively).

From Figure 4, it can be seen that the water flow rate in the sand column increasesrapidly to a value of 7.14 ml/min, and remains approximately constant until minute30, when it begins to decrease, being completely stopped at minute 35.

The total flow rate, however, increases from 7 min onwards as a consequence ofthe increasing flow rate of TCB, which reaches the column outlet in only 5 min (seeFigure 4). This is the expected behaviour of a uniformly packed material, in which

Figure 5. Flow rates in soil column.

12 J. DIAZ ET AL.

the relative permeability of a nonaqueous liquid increases as water is drained fromthe system and saturation of the immiscible liquid increases.

In the soil column, however, the flow rates behave differently (Figure 5). Theinitial water flow rate, similar to that obtained in the sand column (Figure 4),experiments a sharp decrease to a value of 0.6 ml/min at minute 15 of the experiment.This decrease may be attributed to the arrival of TCB at the central soil layer, whichhas a lower permeability than the sand layer. In line with this explanation, TCBwould reach the soil layer in few minutes, as can be concluded from the previousexperiment, in which TCB reached the outlet of the sand column in 5 minutes. Fromthe time at which TCB reached the soil layer onwards, TCB would accumulate abovethe sand-soil interface until the pressure of the nonwetting phase is high enoughto exceed the threshold capillary pressure for the soil layer. Subsequently, as TCBdisplaced the water from the soil layer, the water flow rate would start to increaseuntil a maximum is reached. The maximum flow rate is obtained when the firstTCB drop reaches the column outlet. From this moment onwards, the fraction ofwater at the column outlet decreases making the water flow rate decrease.

3.2. INFLUENCE OF NONAQUEOUS LIQUID HEAD ON TRANSPORT

To study the influence of nonaqueous phase pressure on transport, two experimentswere carried out with different heights of trichlorobenzene above the sand beds.In the first experiment, a constant level of 4 cm of trichlorobenzene was loadedonto the column, whereas the second experiment was carried out with a constantlevel of 6.7 cm. In both experiments, 10 cm of sand were packed into the columnsand saturated with water before the organic liquid was loaded. The water recoverycurves are presented in Figure 6.

Figure 6. Water recovery under different TCB heads.

1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS 13

Figure 7. Flow rates in sand column with 6.7 cm of TCB.

As can be seen, the percentage of water drained from the system increases asthe nonaqueous phase level increases. The recovery of water changes from 5.8%to 51.9% of the initial water saturation when the trichlorobenzene level is variedfrom 4 cm to 6.7 cm. This behaviour is similar to that observed when changingthe packing material inside the columns and could have a similar explanation.That is, when the nonaqueous liquid pressure is not high enough, the water fromthe narrowest pores cannot be displaced, forcing trichlorobenzene to flow throughpreferential paths and thus resulting in finger flow.

Trichlorobenzene breakthrough, however, is very similar in both experiments.When the nonaqueous level is 6.7 cm, breakthrough occurs after 0.066 pore volumesof injected trichlorobenzene. When the trichlorobenzene level is 4 cm, this occursafter 0.045 pore volumes are injected. Water, trichlorobenzene and total flow rateswere calculated from the cumulative volumes (Figures 7 and 8).

As can be seen, the flow rates present similar patterns, with an initial increase inthe water flow rate followed by a decrease until no water flows from the columns.After breakthrough, TCB increases in both cases until water flow is arrested. How-ever, the magnitude of the flow rate seems to show a strong dependence on thepressure of the nonaqueous phase applied to the column. Thus, when the level ofTCB on top of the column is 6.7 cm, the total flow reaches a value of 18 ml/min. Incontrast, when the level of TCB is 4 cm, the maximum flow rate is only 0.5 ml/min.Consequently, the decrease in flow rate seems to be caused by the lower TCB satu-ration attained rather than by the pressure gradient across the column, since a smallchange in the pressure gradient causes a reduction in the flow rates of 97%.

3.3. DRAINAGE AND IMBIBITION

Having ascertained how porous medium and nonaqueous phase level changes affectthe drainage process, a comparison was made between drainage and imbibition. In

14 J. DIAZ ET AL.

Figure 8. Flow rates in sand column with 4 cm of TCB.

Figure 9. Water and TCB recovery curves.

order to do so, after primary drainage with a constant level of 6.7 cm of trichloroben-zene, secondary imbibition with 6.7 cm of water was carried out. Subsequently,water and trichlorobenzene recovery curves were obtained for the processes ofdrainage and imbibition, respectively (Figure 9).

As can be seen, recovery at breakthrough is similar in both cases. Final waterrecovery, however, is higher than final TCB recovery (78% vs 66%), suggest-ing that TCB is more efficient in displacing water than water in displacing TCB.The recovery in the imbibition process can be compared with those obtained byother authors for water-oil displacements. For example, Tang et al. (2001) havereported oil recoveries of 65% by water injection in Kansas outcrop chalk samples

1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS 15

(strongly water-wet) with 21% of initial water saturation. Zhou et al. (2000) havealso reported data for water floods, with recoveries of 41% in Berea sandstoneswith 20% of initial water saturations. The differences in recovery might be dueto the different displacement mechanisms in drainage and imbibition processes,although other factors such as initial saturations can also affect recoveries. Thedifferences in drainage and imbibition mechanisms are caused by the wetting char-acteristics of the material: when a pore body or throat simultaneously carries waterand TCB, NAPL appears in the centre of the pores, whereas water flows in filamentsalong the pore corners (the Amott index to water for the sand is 0.94, see Table I).This causes drainage and imbibition to exhibit different pore invasion mechanisms.During drainage, preferential flow does not take place and the main mechanism ispiston-type displacement, since the porous medium is initially saturated with water.In secondary imbibition, part of the pore space has been invaded by NAPL and thedisplacement mechanisms include not only piston-type, but also snap-off and othermechanisms. This differentiation of displacement mechanisms in drainage and im-bibition were first presented by Lenormand et al. (1983) and may be used to explainthe different recoveries. The snap-off mechanism happens when the filaments ofwater along the pore corners swell as the capillary pressure decreases. At a criticalpoint two or more of these filaments fuse together, become unstable and the entirecross-section of the pore fills with water, cutting the NAPL filament into two parts.In a piston-type drainage displacement, both NAPL and water always remain con-nected through the medium. In secondary imbibition, however, mechanisms suchus snap-off break the NAPL phase into different clusters disconnected from the restof the phase. Thus the residual saturation may increase in the imbibition processdue to the existence of these mechanisms.

The calculated relative permeability values vs. saturation are presented inFigure 10.

In this figure Kr (w) is the relative permeability of the wetting fluid (water) andKr (n) is the relative permeability of the non-wetting fluid (TCB). As expected,relative permeability to water increases and that to TCB decreases with water satu-ration. However, the change appears to be more pronounced in relative permeabilityto TCB than to water for both the imbibition and drainage processes. In addition,hysteretic behaviour of the curves can be observed in Figure 10, mainly for waterpermeabilities, which are considerably higher for the imbibition process. The rela-tive permeabilities to TCB, however, seem to be coincident; at least in the range ofwater saturations between 0.55 and 0.75. The low values obtained for the wettingphase relative permeabilities are also noticeable, both for drainage and imbibitionprocesses. The wetting phase relative permeability in the drainage process, for ex-ample, does not exceed the value of 0.016. This low value can be compared withthe results obtained by other authors for oil-water relative permeabilities, who havereported values for water relative permeabilities in primary drainages up to 0.7 atwater saturations of 0.95 (Spiteri et al., 2004), or similar values for water floods(Johnson et al., 1959).

16 J. DIAZ ET AL.

Figure 10. Relative permeability curves for drainage and imbibition.

4. Conclusions

The flow behaviour of a dense nonaqueous liquid, 1,2,4-trichlorobenzene, in satu-rated porous media was studied by means of displacement experiments carriedout on different materials, sand and natural soil, and under different levels oftrichlorobenzene. In the experiments with different materials, it was observed thatthe stratification of the porous beds with layers of sand and soil caused flow fluc-tuations as the nonaqueous liquid moved through layers with different pore distri-butions and permeabilities. Furthermore, the intercalation of a soil layer betweensand layers caused the recovery of water after trichlorobenzene breakthrough to beconsiderably reduced.

The effect of nonaqueous phase pressure on transport was studied by changingthe level of trichlorobenzene on top of the sand bed. It was observed that a slightreduction in trichlorobenzene level caused a substantial decrease in water recovery.The volume of trichlorobenzene injected before breakthrough, however, was verysimilar in all cases.

Moreover, after primary drainage with trichlorobenzene, secondary imbibitionwith water was carried out in the sand column, thus allowing the differences betweendrainage and imbibition to be studied. It was observed that residual saturation after

1,2,4-TRICHLOROBENZENE FLOW CHARACTERISTICS 17

imbibition was higher than after drainage. The relative permeability curves werealso different for the drainage and imbibition processes, mainly for the wettingfluid, and very low values for the wetting phase permeabilities were also observed,both for drainage and imbibition processes.

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