monitoring lateral transport of ethanol and dissolved gasoline compounds in the capillary fringe

NGWA.org Ground Water Monitoring & Remediation 31, no. 3/ Summer 2011/pages 95–102 95

© 2011, The Author(s)Ground Water Monitoring & Remediation © 2011, National Ground Water Association.doi: 10.1111/j1745–6592.2011.01338.x

Monitoring Lateral Transport of Ethanol and Dissolved Gasoline Compounds in the Capillary Fringeby Juliana G. Freitas and James F. Barker

IntroductionWhen gasoline is released in the unsaturated zone it

migrates downwards. Depending on the volume released it might reach and, due to its lower density, accumulate in the capillary fringe, the region above the water table that is virtually fully water saturated (Parker 1989; Pantazidou and Sitar 1993; Schroth et al. 1995). Organic compounds in the gasoline (a lighter-than-water, nonaqueous phase liquid or LNAPL) are slowly dissolved into the groundwater, threat-ening groundwater quality. Similarly, ethanol has a lower density than water and, therefore, if a significant volume of ethanol-blended fuel is released, ethanol will also accumu-late on top of the capillary fringe. This behavior has been shown in two-dimensional (2D) laboratory tests (McDowell et al. 2003; Capiro et al. 2007; Yu et al. 2009). If ethanol and gasoline are accumulating in the capillary fringe, it is fun-damental to evaluate if they are being dissolved and trans-ported in the capillary fringe.

Lateral flow within the capillary fringe has been recog-nized for decades (Wickoff et al. 1932; Ronen et al. 1997;

Silliman et al. 2002; and others). As long as there is a con-tinuous aqueous phase, flow in the capillary fringe occurs as it does below the water table (Ronen et al. 1997; Berg and Gillham 2010). Recently, Berg and Gillham (2010) showed, through point measurements using a point velocity probe, the continuity of lateral groundwater velocity from the satu-rated zone to the top of the capillary fringe. In a labora-tory experiment using Borden sand, groundwater velocity profile was continuous from below the water table up to 44 cm above the water table, corresponding to the air-entry pressure, when the velocity decreased sharply (Berg and Gillham 2010).

In the same way that there is continuity in groundwa-ter velocity across the water table, horizontal transport of solutes happens in the capillary fringe and below the water table. This has been shown in laboratory tests using 2D boxes (Silliman et al. 2002; Henry and Smith 2003; Berkowitz et al. 2004). Ethanol transport in the capillary fringe was demonstrated in 2D lab tests (Capiro et al. 2007; Yu et al. 2009) and also in pilot-scale tests (Capiro et al. 2007; Stafford et al. 2009). For example, Stafford et al. (2009) found ethanol concentrations in the capillary fringe two orders of magnitude higher than just below the water table.

Compounds dissolving from a gasoline LNAPL will travel in the capillary fringe (Stafford et al. 2009). Dissolved

AbstractFuel mixtures composed of gasoline and ethanol are lighter than water and, if enough volume is released into the unsatu-

rated zone, they accumulate in the capillary fringe, acting as a source for dissolved plumes. To evaluate different sampling techniques and transport in the capillary fringe, two controlled releases of gasoline and ethanol mixtures were conducted in the unsaturated zone at the CFB Borden aquifer. Lateral flow and transport in the capillary fringe is well documented, but this is the first field documentation of transport of organic compounds in the capillary fringe following fuel spills. Transport of both ethanol and hydrocarbon compounds in the capillary fringe was significant, ethanol being transported exclusively above the water table. Significant concentrations of benzene were found above the water table up to 6 m downgradient from the source. The groundwater sampling techniques evaluated were fully screened monitoring wells; multilevel wells constructed with ceramic porous cups located in both the capillary fringe and below the water table; and soil coring. The fully screened monitoring well was unable to draw water from the capillary fringe and so failed to adequately describe the contaminant distribution. Pore water concentrations obtained by sampling the multilevel porous cups and calculated based on analysis of soil core yielded similar results.

96 J.G. Freitas and J.F. Barker/ Ground Water Monitoring & Remediation 31, no. 3: 95–102 NGWA.org

table to full depth, porous suction samplers, and estimate of pore water composition derived from analysis of soil core. Finally, we present field evidence of organic compound transport in the capillary fringe following two controlled releases of ethanol-blended fuels. A more complete discus-sion of ethanol and hydrocarbon fate after the releases is presented by Freitas and Barker (in revision), Freitas et al. (in revision) and in manuscripts in preparation.

MethodsTwo releases of ethanol-blended fuels into the unsatu-

rated zone were performed in the unconfined sand aquifer at CFB Borden, Ontario, Canada, in the same test cell that was used in previous research (Molson et al. 2008; Freitas et al. 2010). The site was covered with a roof, and therefore there was no recharge on the release area. However, the depth to the water table was not controlled and oscillated from ground surface to around 90 cm depth during the approximately 600 d of monitoring. The first release was of 200 L of API 91-01 gasoline (Prince et al. 2007) with 10% ethanol and 4.5% MTBE (methyl-tert-butyl alcohol), which was con-ducted on August 21, 2008 (Freitas and Barker, in revi-sion). The second release was approximately 1 year later, on September 02, 2009, on top of the gasoline that remained from the first release. The second release was of 184 L of E95—95% ethanol—and the remaining a mixture of selected compounds simulating gasoline: 82% hexane, 11% fluoro-benzene (FB), and 7% 4-bromo-fluorobenzene (4-BFB). In the second release, gasoline was not used to allow the dis-tinction of the compounds from the first and second release. FB and 4-BFB have physical–chemical properties similar to benzene and trimethylbenzenes (TMBs), respectively.

The mixtures were released into a trench 20 cm deep, with dimensions of 1.5 m × 0.8 m. The position of the trench was the same for both releases. A plastic liner was initially placed on the sides and bottom of the trench, the ethanol-blended fuels were released and then the liner was removed. The trench was covered with plywood to mini-mize volatilization, and was packed with clean Borden sand after the infiltration was complete. The water table was at 52 cm below ground surface (bgs) at the time of the first release and 58 cm bgs during the second release.

Three groundwater sampling techniques were evaluated at the site. The first was porous suction samplers (or suction lysimeters), which were constructed with ceramic porous cups attached to 3.18 mm (1/8 inch) OD Teflon tubing arranged around a PVC pipe in multilevel wells (Figure 1). The porous cups are described by Freitas and Barker (2008). They are high flow, 2.858 cm long, with 0.635 cm OD and 0.160 cm ID (Soil Moisture Equipment Corp. 2006). The vertical spacing between the centers of the porous cups ranged from 6 to 10 cm. The volume of bubbles in the sam-pling tubing was monitored and the methodology proposed in Freitas and Barker (2008) to account for volatilization losses due to bubble formation was applied when air bub-bles occupied more than 10% of the sampling tubing. In addition, a minimum purge volume of 25 mL was estab-lished to ensure the removal of the compounds that could be retained in the porous ceramic (Freitas and Barker 2008).

plumes of hydrocarbons derived from gasoline will have essentially the same density as the water because of the lim-ited solubility of the hydrocarbons. Vertical gradients, such as those created by recharge, can cause the dissolved plumes to sink as it travels away from the source. However, as etha-nol is completely miscible with water, high concentrations in the capillary fringe are anticipated. As ethanol–water mix-tures are less dense than water, buoyancy effects are likely to be significant (Molson et al. 2008). So, when ethanol is pres-ent, the dissolved plumes are likely to remain in the capillary fringe for longer distances due to density effects.

Even though transport of solutes in the capillary fringe has been recognized, the capillary fringe is neglected in most field studies. One tracer test at field scale to evalu-ate transport in the capillary fringe was conducted by Abit et al. (2008), releasing bromide in the unsaturated zone and using tension samplers for groundwater monitoring. Concentrations in the capillary fringe were one order of magnitude higher than below the water table (Abit et al. 2008). Precipitation events (2 cm total) transported some of the bromide to below the water table, but bromide was still found in the capillary fringe 3.2 m away from the source after 58 d (Abit et al. 2008).

The lack of field investigations of transport in the cap-illary fringe is likely due to the difficulties in sampling (Berkowitz et al. 2004), which are amplified when dealing with volatile organic compounds, such as monoaromatics common in gasoline. Groundwater sampling in gasoline contaminated sites is commonly performed using monitor-ing wells with screens from 1.5 to 6 m long (Einarson and Mackay 2001). One issue when using this type of well is that the sample will represent an average of the concentration over the screened interval, and the maximum concentration in the groundwater cannot be quantified. When comparing concentrations from multilevel wells to that obtained from monitoring wells, Hutchins and Acree (2000) observed that the concentration in the monitoring well agreed with the average concentration from multilevel wells. This is a particular problem as organic contaminants plumes can be highly stratified (Einarson and Mackay 2001). Further, as the water from above the water table does not flow into the well, any contaminants present in the capillary fringe will not even be included in the composite sample. One option for sampling the capillary fringe is to use porous suction samplers (Everett and McMillion 1985; Grossmann and Udluft 1991; Wilson et al. 1995). However, suction might exacerbate degassing during sampling, resulting in lower concentrations of volatile organics (Everett et al. 1988; Smith et al. 1992). Retention and sorption of compounds in the porous sampler material and the development of bio-films in the surface of the porous cups are also the issues associated with the use of suction samplers (Silkworth and Grigal 1981; Grossmann and Udluft 1991; Lewis et al. 1992; Freitas and Barker 2008).

In this study, a controlled field test was conducted to evaluate different sampling techniques and to document, for the first time under well-defined field conditions, the transport of organic compounds in the capillary fringe. Three alternative sampling techniques were investigated in the field test: monitoring wells screened across the water

NGWA.org J.G. Freitas and J.F. Barker/ Ground Water Monitoring & Remediation 31, no. 3: 95–102 97

ethanol, and MTBE. The chemical analyses were performed at the Organic Chemistry Laboratory, Department of Earth and Environmental Sciences, University of Waterloo, follow-ing the procedures described in Freitas and Barker (2008).

Concentrations measured in the groundwater (above and below the water table) were used to assess the transport of dissolved contaminants in the capillary fringe and below the water table. In addition, laboratory falling head perme-ameter testing (Reynolds 2008) was done on a 1.5 m core collected 30 cm upgradient the trench on June 22, 2009, to assess the vertical distribution of saturated hydraulic con-ductivity (K).

Results and Discussion

Evaluation of the Monitoring TechniquesIn general, field duplicates agreed well. Samples with

low concentrations occasionally show a high percentage dif-ference; however, the absolute difference was small and not significant for our interest in defining where most of the mass is. The opposite is true for high-concentration sam-ples. For example, even though duplicate samples differed by 5000 µg/L, the measured concentration was about 85,000 µg/L, the error was only 6%, and therefore, the samples are considered to be in agreement.

The good agreement between duplicates indicates preci-sion, but not accuracy. The accuracy cannot be directly eval-uated from field samples, as the real concentration values are unknown. However, it indicates that the 25-mL purging removed any contaminants retained in the porous ceramic, otherwise significant differences would be seen between duplicates. In addition, laboratory tests have been used pre-viously to evaluate the accuracy of samples obtained from porous suction samplers (Freitas and Barker 2008).

The presence of bubbles in the sampling could be avoided in most cases by minimizing the vacuum pressure applied. Only when sampling the top of the capillary fringe (more than 30 cm above the water table) was a significant volume of bubbles seen in some of the sampling lines. This indicates that losses by volatilization were probably mini-mal, and when there was indication of degassing, the loss was estimated by applying the method of Freitas and Barker (2008).

To evaluate the performance of the fully screened moni-toring well, the concentrations obtained from this well were compared to the concentrations at the multilevel well located 15 cm upgradient, using samples obtained on the same day (November 22, 2009). The water table was located at 65 cm bgs. The concentrations measured in the fully screened well were lower than the maximum concentra-tions measured at the multilevel well for all compounds; for example, the results for FB and ethylbenzene are shown in Figure 3. This was expected as the sample obtained from the monitoring well is a composite sample from the screened interval (Einarson and Mackay 2001).

The concentrations obtained in the fully screened monitoring well agree reasonably well with the average concentration in the multilevel well when only sampling ports below the water table are considered (filled circles

“Duplicate” samples were collected sequentially after the purging.

The second monitoring technique was a 2.5 cm OD monitoring well, screened from ground surface to full depth (1.2 m) which was installed 15 cm downgradient of a mul-tilevel well (Figure 2).

The third technique was the collection and analysis of soil cores and estimation of pore water concentrations based on equilibrium between phases, using the method of Feenstra et al. (1991). Cores were collected in aluminum tubes of 2.8 cm ID, the tube walls were drilled every 3 or 4 cm and the sample collected with plastic syringes with the tip cut. The samples were transferred to preweighed vials containing a solvent, water for ethanol and MTBE analysis and methylene chloride for hydrocarbon analysis (Freitas and Barker, in revision). Soil samples were also collected for water content analysis. The location of wells and cores are presented in Figure 2.

The samples were analyzed for BTEX (benzene, toluene, ethylbenzene, xylenes), TMBs, naphthalene, FB, 4-BFB,

F igure 2. Site plan view. The release trench was the same for both releases. Multilevel wells (filled circles) were constructed with 15 sampling ports spaced 10 cm apart. Sampler nest (filled diamonds) had only four sampling ports spaced 20 cm apart.

F igure 1. Detail of multilevel well constructed with porous suction samplers.


directing flow into the well screen. However, recent stud-ies at Borden aquifer by Bunn et al. (submitted for publi-cation) have shown that during a 24-h pumping test there was no significant dewatering of the aquifer despite the drawdown in the water table. Also, no significant down-ward vertical gradients were measured in the capillary fringe (Bunn et al. in submission). Therefore, even during

in Figure 4). However, when considering all the sampling points (represented by the open circles in Figure 4), includ-ing those above the water table, the average concentration in the multilevel well differ significantly from the concen-tration in the fully screened monitoring well for some com-pounds. The average concentration in the multilevel well is always higher than the concentration in the fully screened monitoring well.

The concentrations measured in the multilevel well were used to estimate how much of the mass of each compound was traveling above the water table. For example, 100% of the ethanol mass was above the water table, but only 32% of ethylbenzene was above it. These values of percentage mass above the water table were compared to the percentage mass observed at the fully screened well, calculated assuming that samples collected at these wells are representative of the entire saturated profile. When more mass was above the water table, the efficacy of the fully screened well to detect the total mass decreased, while when most of the mass was below the water table the fully screened well efficacy tended to be 100% (Figure 5). This indicates that the fully screened well was not capturing the mass above the water table. Therefore, if a contaminant is traveling only above the water table (like ethanol in this case), the fully screened well will not indicate the presence of the contaminant.

It could be argued that more mass could be withdrawn from the capillary fringe into the fully screened monitoring well if a drawdown is maintained at the sampling well for a longer period, creating a significant vertical gradient

F igure 3. Fluorobenzene and ethylbenzene concentrations in the multilevel well (solid lines) and fully screened monitoring well (dashed lines over the screened depth).

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F igure 5. Efficacy of the fully screened well sample compared to the proportion of each dissolved plume that was above the water table. Each point corresponds to a different compound.

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where flow is more significant, while in soil samples water present in all pores is analyzed (Wilson et al. 1995).

Transport in the Capillary FringeCompounds present in the mixtures released were found

in the capillary fringe downgradient of the release trench following both the E10 and E95 release. For example, 47 d after the E10 release (October 07, 2008) one soil core collected 1 m downgradient of the trench had high con-centrations measured in the capillary fringe (Figure 6). Calculations assuming equilibrium between phases

extensive pumping in the Borden aquifer there is minimal flow from the capillary fringe downwards into the well.

Groundwater concentrations in the capillary fringe were also estimated based on soil cores (Figure 6). The results agree reasonably well with values from groundwa-ter samples obtained with suction cups, particularly con-sidering that the samples were collected 3 d apart. The good agreement is perhaps unexpected, given the expec-tation that soil core analysis and suction cup sampling sample different pore water. With wells or suction cups groundwater is withdrawn from more permeable regions

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Soil data from October 07, 2008 (47 days after E10 release); groundwater data from two 4 level sampler nests, obtained on October 10, 2008 (Figure 2).

Soil data from April 24, 2010 (611 days after E10 release and 234 days after E95 release); groundwater data from multilevel well RA-W07, April 27, 2010 (Figure 2).

F igure 6. Comparison of pore water concentration determined by soil sampling (solid line) and groundwater sampling (open circles, dashed line). Depth (m bgs) is in the y-axis and pore water concentration (mg/L for ethanol and µg/L for hydrocarbon) is presented in the x-axis. Depth to the water table is also indicated.


(Feenstra et al. 1991) indicated that all mass was present in dissolved or sorbed phases, without the presence of NAPL. The ratio of hydrocarbon in the samples was also signifi-cantly different than the ratio in the NAPL phase, indicating that NAPL was not present (Freitas and Barker, in revi-sion). In addition, surface ground penetrating radar surveys indicated that the gasoline phase from the E10 release had not migrated more than 30 cm away from the trench sides (McNaughton, in preparation). Therefore, the compounds found in this core were most likely transported by lateral flow in the capillary fringe and not by NAPL spreading. Groundwater concentrations obtained in multilevel wells also illustrate the organic compound transport in the capil-lary fringe (Figure 7). Despite water table oscillations, etha-nol was found mainly above the water table following the E95 spill. Breakthrough curves constructed using data from periodic sampling at selected wells also confirmed this behavior, with most of the ethanol being above the water table (Freitas et al., in revision). Hydrocarbon compounds were also detected above the water table up to 6 m down-gradient of the source (Figure 7).

Groundwater velocities at different depths were esti-mated based on MTBE breakthrough curves. MTBE is not

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Fi gure 7. Ethanol, fluorobenzene, and benzene concentration 68 d after the E95 release. Groundwater concentration in the source was calculated from soil core data collected 58 d after E95 release (October 30, 2009). Groundwater flows from the left to the right.

expected to be affected by sorption and its biodegradation rate in the Borden aquifer under aerobic conditions is low (Schirmer et al. 2003) and unlikely to impact its transport within the distance and time frame being evaluated here. Therefore, MTBE is assumed to behave conservatively. Velocities were estimated considering transport from the source to a center well in the first transect (source to RA-W07) and from this well in the first transect to one well in the second transect at the same position (RA-W07 to RB-W07) (Figure 8). The good agreement between velocity estimates considering transport from the source to Row A and transport from Row A to Row B indicates that the pres-ence of NAPL in the source is not interfering significantly with the time of travel. Although groundwater flow appears slower in the capillary fringe, it is consistent with the increasing saturated hydraulic conductivity (K) with depth (Figure 8), suggesting that K controls the horizontal trans-port of chemicals. The correlation coefficient (R²) between the groundwater velocity (source to RA-W06) and K is 0.87. Therefore, other factors that could potentially decrease the effective hydraulic conductivity in the capillary fringe, such as air entrapment and increased tortuosity, are not playing a major role.


be used to directly sample the soil water. The advantage of suction samplers over soil coring is that it allows the collection of samples at the same position multiple times. Overall, both methods were shown to be adequate for sam-pling groundwater from the capillary fringe, even for vola-tile compounds.

Transport of the organic compounds in the capil-lary fringe was verified in these field tests (Freitas et al., in revision). Following the E10 then E95 release, ethanol was transported downgradient almost exclusively above the water table. Significant transport of gasoline hydrocarbons also happened above the water table. Advective velocity in the capillary fringe was controlled mainly by the saturated hydraulic conductivity of the porous medium.

Although different spill scenarios, dispersion and recharge, may move some ethanol and gasoline hydrocar-bon below the water table and make them available for careful monitoring (i.e., short [<1 m] wells screened at the water table), there remains a likelihood that groundwater monitoring using only conventional wells will either miss or underestimate the occurrence of ethanol and fuel hydro-carbons. Therefore, other available monitoring techniques, such as extraction or analysis of core samples and suction samplers, should be used to monitor the dissolved contami-nant plumes migrating in the capillary fringe.

AcknowledgmentsThe authors would like to acknowledge the financial sup-

port of the American Petroleum Institute, the NSERC CRD program, the Canadian Petroleum Products Institute, Water and Earth Science Associates Ltd. (WESA), Conestoga Rovers and Associates (CRA), and the Ontario Ministry of Environment. Juliana Freitas was supported by a schol-arship from the Brazilian Government (CAPES, Brazil). Analyses were performed by Marianne Vandergriendt and Shirley Chatten at UW.

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Biographical SketchesJ.G. Freitas, PhD, corresponding author, is at Department of

Earth and Environmental Sciences, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, Canada N2L 3G1; (519) 888-4567 ext. 35372; fax: (519) 746-7484; [email protected].

J.F. Barker, PhD, Professor, is at Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, Canada N2L 3G1; [email protected].

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monitoring lateral transport of ethanol and dissolved gasoline compounds in the capillary fringe

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