Mobilization and Rupture of LNAPL Ganglia during Freeze-Thaw: Two

Environ. Sci. Technol. 2008, 42, 5467–5472

Mobilization and Rupture of LNAPL Ganglia during Freeze-Thaw: Two-Dimensional Cell Experiments R O B E R T K . N I V E N * ,†,‡ A N D KAMALJIT SINGH† School of Aerospace, Civil and Mechanical Engineering, The University of New South Wales at ADFA, Northcott Drive, Canberra, ACT, 2600, Australia, and Niels Bohr Institute, University of Copenhagen, Copenhagen Ø, Denmark

Received September 28, 2007. Revised manuscript received April 14, 2008. Accepted May 5, 2008.

Experiments were conducted on dodecane at residual saturation (21-26%) in a two-dimensional water-saturated glass bead cell (0.5 mm diameter)sto simulate light nonaqueous phase liquid (LNAPL) trapped below the water tablessubject to controlled freeze-thaw cycles. The experiments reveal substantial remobilization and rupture of LNAPL ganglia during freeze-thaw, especially during the first few cycles. This includes the detachment and upward mobilization of LNAPL from larger ganglia during upward propagation of the freezing front; the formation of numerous subsinglet ganglia during this transport process, and their entrapment in ice; and the coalescence of such small ganglia during thawing, to form larger singlets. Theoretical calculations suggest that the LNAPL redistribution is caused by large freezing-induced pressure gradients, of up to 6 orders of magnitude higher than the waterLNAPL interfacial (capillary) pressure. The findings have important implications for the understanding and remediation of LNAPLs in cold climate regions.

1. Introduction Subsurface contamination by light nonaqueous phase liquids (LNAPLs), due to vehicular, industrial, military, and scientific activities, is a significant and increasing problem worldwide. If spilled on an unfrozen soil, LNAPL percolates downward through the unsaturated zone until it reaches the water table, where it collects to form a mobile free product zone containing highly concentrated LNAPL (1, 2). Due to water table fluctuations, such LNAPL can become entrapped beneath the water table as so-called residual saturation, consisting of disconnected LNAPL globules or ganglia; these are held in place by extremely strong interfacial tension forces, making them almost impossible to remobilize by an applied hydraulic gradient (1, 2). LNAPL residual saturations typically range from 14 to 35% in saturated bead packs and sandy soils (3–7). Chatzis and co-workers (8, 9) distinguished four types of ganglia in oil reservoir sandstone cores and glass bead packs: subsinglets, singlets, doublets, and branched, respectively occupying less than one, one, two, or multiple pore pockets. Since the pioneering experiments by Schwille (10) on the behavior of dense nonaqueous phase liquids (DNAPLs) in two-dimensional (monolayer) and three-dimensional glass * Corresponding author e-mail: [email protected]. † The University of New South Wales at ADFA. ‡ University of Copenhagen. 10.1021/es702442j CCC: $40.75

Published on Web 06/27/2008

 2008 American Chemical Society

bead packs, many studies have been conducted on the porescale behavior of NAPL ganglia in porous media. Glass-bead cells have been widely used for this purpose to provide a simplified but realistic representation of the pore structure of natural porous media (6, 11–14). Of interest is the soil in cold regions, here defined as those which experience temperatures sufficiently low to form ice in the soil pores. This can be divided into two horizons: an upper active layer, which may undergo diurnal or seasonal freeze-thaw cycles depending on the temperature gradient and profile (15), and a lower permafrost, defined as the zone which remains at 1000 mg/kg) caused high consolidation (23). Chuvilin and co-workers (24, 25) showed that controlled freeze-thaw of an LNAPL-contaminated sandy soil can cause significant LNAPL (oil) mobilization, due to release and propagation ahead of the freezing front. Freezing experiments on soil cores open to water entry, by Grechishchev et al. (26), showed that LNAPL (oil) inhibits ice segregation and frost heave. Other studies have examined LNAPL spills on frozen soils and/or permafrost in the field (27–31), LNAPL percolation into partly frozen unsaturated soils (32), downward LNAPL dispersion (“smearing”) due to freeze-thaw cycles (33), and LNAPL migration along fissures or LNAPL-melted tubes (34). The LNAPL permeability decreases with increasing ice content, but does not reach zero (20, 24, 35–37). Further studies examined the spreadability of LNAPL on ice or snow (24, 38, 39) and the freezethaw behavior of organic solutes not involving NAPL (40–42). A mass balance model of dissolved benzene in soil subject to freeze-thaw indicates that its concentration ahead of the freezing front can exceed its aqueous solubility (43). Apart from the works of White and coauthors (20, 23), the above studies mainly concern bulk effects and did not examine the soil microstructure (at pore scales). To the authors’ knowledge, no studies have been reported on the effect of freezethaw on the pore-scale structure of LNAPL residual saturation, i.e. on the entrapment and mobility of LNAPL ganglia. The aim of this study was to experimentally examine the pore-scale behavior of LNAPL ganglia in a two-dimensional, water-saturated porous mediumsto represent LNAPL residual saturation below the water tablessubject to controlled freeze-thaw cycles. The interactions between LNAPL ganglia and the water/ice phases were observed during and after each freeze-thaw cycle and examined by a theoretical model. The effect of successive freeze-thaw cycles on the microstructure and spatial distribution of LNAPL ganglia, and the implications for LNAPL contamination in cold regions, are discussed. This work follows a preliminary study by the authors (44). While the two-dimensional model does not fully VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Physicochemical Properties of Dodecane parameter chemical formula molecular weight (g/mol) freezing point (°C) boiling point (°C) density (kg/m3) at 20 °C interfacial tension (N/m) at 0 °C solubility in water (µg/L) vapor pressure (N/m2) at 25 °C dynamic viscosity (Pa s) heat of fusion (kJ/mol) isothermal compressibility (GPa)-1 contact angle, dodecane-water-glass, measured in water (degrees)

value C12H26 170.34a,b -12a,b 214-216a,b 748.7a,c 0.0537d 3.7c 15.7c,e 0.001219f 35.85c 0.839g 14.8 to 15.6h

a Ref (50). b Certified values reported by manufacturer (Sigma-Aldrich). c Ref (51). d Extrapolated to 273.15 K from data in ref (52), using fit curve σ ) 3.0 × 10-8T3 - 2.738 × 10-5T2 + 0.00825T - 0.76805 (with R2 ) 0.9991). e Ref (53). f Ref (54). g Extrapolated to 273.15 K from data in refs (55–57), using fit curve log10κT ) 4.5127(log10T)2 20.316log10T + 19.634 (with R2 ) 0.9994). h Measured by authors.

reproduce the three-dimensional structure of a natural porous medium, such two-dimensional cells (“micromodels”) are a standard tool for the investigation of immiscible fluid movement and pore networks, especially in early studies of new phenomena (e.g., 10, 45–48). The disadvantage of lack of representativity is outweighed by the advantage of direct observation of the phenomenasespecially when the mechanisms are not knownsas well as the simplicity, rapidity, and low cost of experiments compared to those using threedimensional systems.

2. Materials and Methods Materials. Dodecane (reagent plus grade, g99%, SigmaAldrich, MO), representing the saturated hydrocarbons found in gasoline and jet fuel (49), was selected as a suitable LNAPL on the basis of its low aqueous solubility and low volatility. Its physicochemical properties are listed in Table 1. The dodecane was dyed with organic-soluble Oil-Red-O dye (Sigma-Aldrich, MO) at a concentration of 0.04 g/L to facilitate observation. The aqueous phase was deionized and deaired water (electric conductivity e1.0 µS/cm, Purite Ltd., Oxon). The porous medium consisted of a monolayer of 0.5 mm diameter soda lime glass beads (Biospec Products, Inc., OK), which had been passed through a 600 µm sieve and retained on a 425 µm sieve. Before each experiment, the beads were washed as per the manufacturer’s instruction manual. Methodology. The experiments were conducted in a 210 mm (vertical) × 100 mm (horizontal) two-dimensional cell to enable observation of the behavior of LNAPL ganglia and the water/ice phases in a water-saturated porous medium. The cell contained a monolayer of glass beads held between two glass plates, separated by stainless steel (316) spacers and sealed with a viton O-ring, as shown in Figure S1 in the Supporting Information (pore volume 5.46 mL). The cell was designed so that the upper section could be detached after establishment of residual LNAPL saturation to allow room for ice expansion during freezing. Residual LNAPL saturations of 21-26% were established in the cell, following which it was subjected to freeze-thaw cycles with upward propagation of the freezing front. Further details of the methodology, including the establishment of LNAPL residual saturation and the freeze-thaw cycles, are discussed in the Supporting Information; this includes illustrations of the apparatus (Figure S1), typical temperature profiles (Figure S2), and 5468

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recorded time-of-nucleation measurements (Figure S3). Three experiments are reported here: experiments 1 and 3, in which the cell was insulated and photographs were recorded only during the thawed cycles, and experiment 2, in which the cell was photographed in situ in the freezing chamber.

3. Observations and Results Photographs of one 13 mm × 13 mm photographic window, located at 57 mm above the base of the cell in experiment 1, taken after the establishment of residual saturation and after freeze-thaw cycles 1, 4, 7, 9, and 10, are shown respectively in Figure 1a-f. All photographs in Figure 1 are of the thawed state. As evident: • After cycle 1 (Figure 1b), a significant increase in LNAPL volume, consisting predominantly of small ganglia, can be seen on the left (A, marked by black arrows), while a substantial loss of LNAPL volume (B, marked by dotted lines) from a large branched ganglion is evident on the upper right. • After cycle 4 (Figure 1c), a further substantial loss of LNAPL volume is evident from a large branched ganglion near the base of the window (B), with the formation of minor small ganglia (A). • After cycles 7 (Figure 1d), 9 (Figure 1e), and 10 (Figure 1f), further significant changes are observed, including a reduction in volume of most multipore ganglia (B) throughout the window, accompanied by the formation of numerous singlet and subsinglet ganglia (A). Comparing Figure 1a and f, it can be seen that the ten freezethaw cycles had a substantial effect on the LNAPL distribution, including the destruction of most branched and multipore ganglia and transport of this LNAPL out of the frame, as well as the formation of numerous small ganglia. Such changes were especially prominent throughout the lower part of the cell (including the region shown in Figure 1) where the freezing commenced, and in the upper right part of the cell (not shown). In contrast, the upper left part of the cell experienced a net inward transport of LNAPL, leading to the formation of a LNAPL pool. Photographs of a smaller window from experiment 2, at 62 mm above the base of the cell, at residual saturation and various stages of freezing or thawing are shown in Figure 2a-h. As evident: • After freezing of cycle 1 (Figure 2b), numerous tiny ganglia (marked by A) can be observed entrapped in polycrystalline ice in several pores, including within a long pore corridor. Reductions in ganglion volume can be observed in some instances (B), as well as grain relocation due to freezing (E). • After the thaw of cycle 1 and freezing of cycle 2 (Figure 2c), many of the tiny ganglia have disappeared; in their place, a number of singlet ganglia (C) can be observed in several pores. Some cracks in the ice and/or ice grain boundaries (D) were also observed. • After subsequent freeze-thaw cycles (Figure 2d-h), further changes can be observed, including the formation of small ganglia entrapped in ice (lower left, Figure 2e), their coalescence on thawing (Figure 2f), and minor changes in grain positions. • In this experiment, several ganglia entrapped within large pore cavities were largely unaffected by the freezethaw cycles. Photographs of experiment 3, from 60 mm above the base of the cell, are shown in Figure S4a and b in the Supporting Information, at residual saturation and after the sixth freezethaw cycle. The effect of freeze-thaw cycles on the residual LNAPL saturation in the lower part of the cell in experiment 1 is illustrated in Figure 3, both in terms of the fractional area of

FIGURE 1. Photographs of experiment 1 at 57 mm above base: (a) at residual saturation; and after (b) freeze and thaw cycle 1; (c) cycle 4; (d) cycle 7; (e) cycle 9; and (f) cycle 10. “A” indicates new ganglion formation or migration, and “B” indicates former LNAPL zones (for discussion see text). LNAPL (Aor), and the LNAPL area normalized against that before freezing (Aor/Aor*); these may be taken as crude approximations to the LNAPL volume fraction (voidage) and relative saturation. The LNAPL area was calculated by thresholding of the image using the analySIS software package. While these results were sensitive to the thresholding scheme, in all cases the LNAPL volume diminished substantially with successive freeze-thaw cycles. However, as evident in Figures 1 and 2, the surface area of the remaining LNAPL-and hence its potential for dissolution-remains significant.

4. Discussion From Figures 1 and 4 and the Supporting Information, it is evident that successive freeze-thaw cycles, with upward propagation of the freezing front, caused the following changes in the systems examined: (i) the substantial upward transport of LNAPL during freezing, accompanied by the rupture (fragmentation) of larger, branched ganglia; (ii) the formation of numerous subsinglet ganglia during this transport process, and their entrapment in ice; and (iii) the

coalescence of such small ganglia during thawing, to form larger singlets. The observations may be interpreted using a simple model of the change in pressure induced on the LNAPL during freezing due to the water-ice volumetric expansion. Consider a cell of fixed total volume, containing water of volume Vw and LNAPL of volume VN. On freezing, the water expands in volume ∆Vw > 0, forcing an equivalent contraction of the LNAPL. By conservation of water mass mw, the volumetric expansion is

(

∆Vw ) Vw

Fw -1 Fice

)

(1)

where Fw and Fice are the densities of liquid water and ice at the freezing point. For an almost incompressible LNAPL such as dodecane, the variation of volume with pressure is given by ∂VN ) -VNκT ∂P VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

(2)

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FIGURE 2. Photographs of experiment 2 at 62 mm above base: (a) at residual saturation; (b) frozen, cycle 1; (c) frozen, cycle 2; (d) unfrozen, after cycle 2; (e) frozen, cycle 3; (f) unfrozen, after cycle 3; (g) frozen, cycle 4; and (h) unfrozen, after cycle 4. “A” indicates ganglion detachment from larger pools and entrapment in ice; “B” indicates ganglia of reduced volume; “C” indicates ganglia formed by coalescence during thawing; “D” indicates cracks or grain boundaries in the ice; and “E” indicates grain relocation. where κT is the isothermal compressibility. Equation 2 can be integrated between LNAPL volumes VN and VN′ ) VN ∆Vw and corresponding pressures PN and PN′ ) PN + ∆PN, giving the change in pressure imposed on the LNAPL due to compression: ∆PN )

(

VN 1 ln κT VN - ∆Vw

)

(3)

Substituting for ∆Vw from eq 1 and using the LNAPL saturation SN ) VN/(VN + Vw) gives ∆PN )

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1 ln κT

(

SN SN Fw (1 - SN) 1 - S - F + 1 N ice

(

))

(4)

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A plot of eq 4 against SN, for water and ice densities 999.8 and 916.7 kg m-3 at 0 °C (50, 51) and dodecane compressibility (Table 1), is shown in Figure 4. As evident, the maximum change in pressure experienced by the LNAPL is of the order of 350-500 MPa, for the experimental LNAPL saturations of 21-26% (shown as points in Figure 4). Now consider the interfacial pressure (sometimes termed the capillary pressure (14)) associated with entrapment of ganglia in porous media. This is of the order of (2, 14) ∆Pint ) 2σH ≈

2σ r

(5)

where H is the mean interfacial curvature, σ is the interfacial tension, and r is the radius of curvature of a pseudospherical interface. In these experiments, for known σ (Table 1) and

mental support by David Sharp, Jim Baxter, and Wayne Jealous, and the comments of the three anonymous reviewers.

Supporting Information Available Greater detail of the experimental methodology and several additional results. This information is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited

FIGURE 3. Calculated fractional residual LNAPL area (Aor) and normalized area (Aor/Aor*) from the lower part of the cell, experiment 1, against the freeze-thaw cycle.

FIGURE 4. Calculated maximum change in pressure on the LNAPL due to water-ice expansion (eq 4). for r of the order of the glass bead radius, the interfacial pressure is approximately 215 Pa. In consequence, the maximum freezing-induced pressure is 6 orders of magnitude higher than the pressure associated with ganglion entrapment, and so can easily effect ganglion movement and/or rupture at local scales. Of course, the very high pressure gradients suggested by eq 4 (Figure 4) will not actually be realized, as LNAPL ganglia will be expelled from pores at much lower pressures; however, eq 4 does provide an upper bound for the change in pressure induced by pore water freezing. Equation 4 also explains the dramatic changes in soil fabric and stratigraphy-which act against the overburden pressure-in soils subject to “frost heave” (15, 18–22). The experimental results and analysis reveal a considerable dynamicism-not usually considered to apply to LNAPL ganglia-within a LNAPL-water-solid system subject to freeze-thaw. A caveat to the findings is that a two-dimensional micromodel does not completely reproduce the structure of a three-dimensional porous medium, for example the presence of pore pockets to accommodate lateral rather than vertical LNAPL transport. Furthermore, in natural soils, freezing will usually occur from the soil surface downward, which might be expected to cause downward rather than upward propagation of LNAPL; a limitation of the twodimensional apparatus was the inability to control the direction of the freezing front. These effects require further investigation in a three-dimensional setting. Finally, whether LNAPL-contaminated porous media of lower permeability (such as silts and clays) exhibit such dramatic changes with freeze-thaw also warrants further investigation.

Acknowledgments This work forms part of a research program supported by two UNSW Faculty Research Support Program grants, and K.S. is supported by RTS scholarship and international travel grant by UNSW@ADFA. This work benefited from experi-

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