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. Author manuscript; available in PMC: 2021 Dec 1.
Published in final edited form as: Chemosphere. 2020 Jul 8;260:127562. doi: 10.1016/j.chemosphere.2020.127562

NAPL-water Interfacial Area as a Function of Fluid Saturation Measured with the Interfacial Partitioning Tracer Test Method

ML Brusseau 1,*, H Taghap 1
PMCID: PMC7654436  NIHMSID: NIHMS1640478  PMID: 32683025

Abstract

The presence of organic immiscible liquids such as chlorinated solvents and fuels continues to be a primary source of risk for many hazardous waste sites. In this study, the standard miscible-displacement interfacial partitioning tracer test (IPTT) method was used for the first time to measure NAPL-water interfacial areas for a range of saturations. Multiple measurements were conducted for a natural quartz sand, with tetrachloroethene as the representative NAPL. The interfacial areas increased with decreasing water saturation. The measurements compared well to interfacial areas measured for the same sand with two alternative tracer methods, the mass-distribution batch method and the two-phase flow method. Measurements obtained with all three tracer-based methods exhibit a relatively large degree of variability. Thus, it is important to employ replication when using these methods. In contrast, interfacial areas measured with x-ray microtomography exhibit very small variability. However, the measured interfacial areas do not capture the contribution of surface-roughness to film-associated interfacial area. Each method has associated advantages and disadvantages, and it is important to be cognizant of them during their application.

Keywords: interface, mass transfer, TCE, PCE, oil

1. Introduction

The presence of organic immiscible liquids such as chlorinated solvents and fuels, often referred to as NAPL (nonaqueous-phase liquids), at hazardous waste sites continues to be a major impediment to site cleanup and closure. Mass-transfer processes play a key role in the mass removal of OIL and the evolution of contaminant discharge from OIL source zones. It is now well-established that the magnitude, configuration, and accessibility of OIL-water interfaces mediates NAPL dissolution and other mass-transfer processes. Hence, the development and testing of methods to measure NAPL-water interfacial area in porous media remains a critical research topic.

Characterizing NAPL-water interfacial area has taken on new relevance in light of the potential impact of NAPL source zones on the transport of per and polyfluoroalkyl substances (PFAS) in soil and groundwater. PFAS are emerging contaminants of critical concern, and they can co-occur at sites with NAPL source zones (e.g., Moody et al., 2003; McGuire et al., 2014). Recent research has shown that PFAS can adsorb at NAPL-water interfaces, which can impact their retention and transport in the subsurface (Brusseau, 2018, 2019; Brusseau et al., 2019).

Saripalli et al. (1997a, 1998) introduced the standard tracer-test method used to measure NAPL-water interfacial areas in porous media. Typically referred to as the interfacial partitioning tracer test, the method is based on measuring the retention of a tracer that adsorbs preferentially at the NAPL-water interface. The test is implemented via a miscible-displacement column experiment in which a residual saturation of NAPL is established within the packed column. Since its development, this method has been used successfully for a number of applications (see Table 1).

Table 1.

Prior laboratory studies employing tracer-test methods to measure OIL-water interfacial areas.

Author Year Range of Sw Replicates Method
Saripalli 1997a No No Standard
Saripalli 1997b Noa No Standard
Saripalli 1998 No No Standard
Kim 1999 No No Standard
Setarge 1999 No No Standard
Noordman 2000 No No Standard
Dai 2001 No No Standard
Piepenbrink 2002 No No Standard
Cho 2005 Nob No Standard
Dobson 2006 No Yes Standard
Brusseau 2008 No Yes Standard
Schaefer 2009 No Yes Standard
Brusseau 2010 No Yes Standard
Narter 2010 No Yes Standard
Christensen 2015 No No Standard
McDonald 2016 No Yes Standard
Schaefer 2000 Yes Yes Mass Distribution
Jain 2003 Yes No Two-phase Flow
Zhong 2016 Yes No Two-phase Flow
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a

Measurements were conducted after multiple mobilization steps, but not for different initial saturations

b

Measurements were conducted after multiple dissolution steps, but not for different initial saturations

One major limitation of the prior applications is that the method has been used only for measuring interfacial areas at one given fluid saturation (Table 1). None of the prior studies characterized interfacial area for different initial saturations. Determining such relationships is important for understanding and quantifying mass-transfer and interfacial-adsorption processes. In addition, only a few of the prior studies incorporated measurement replication, which is necessary to fully evaluate the robustness of the method. The objective of this study is to examine the efficacy of the standard IPTT method for measuring NAPL-water interfacial areas across a range of saturation. This is accomplished by establishing different initial NAPL saturations and conducting replicate tracer tests for each one. The results of the IPTT measurements are compared to measurements obtained with alternative methods.

2. Materials and Methods

The present study builds upon the study of Brusseau et al. (2008). Therefore, the same natural quartz sand and organic liquid (tetrachloroethene, PCE) as used in the prior study are employed in this work. The sand (Accusand, Unimin Corp) has a median diameter of 0.35, and a total organic carbon content of 0.04%, negligible clay content, and Fe, Mn, and Al oxide contents of 14, 2.5, and 12 mg/g, respectively.. The same compound, pentafluorobenzoate, was used for the nonreactive tracer (NRT). It has been used in numerous prior studies. In addition, the same compound, sodium dodecylbenzene sulfonate (SDBS), was used as the interfacial tracer. This surfactant is the most commonly used interfacial-adsorption tracer for IPTT studies.

The experiments are conducted identically to Brusseau et al. (2008). The column is packed with air-dried sand and then saturated with solution. The solution is injected at a low flow rate into the bottom of the vertically oriented column. Prior tests have shown that the procedure produces uniform water saturation of the packed column with no measurable air present (e.g., Brusseau et al., 2008). Once saturated, a residual phase of PCE is established in the packed column under secondary imbibition conditions. This is done by first injecting a certain volume of PCE into the bottom of the column (drainage step), after which solution is injected into the top of the column to displace the PCE (imbibition step). Different PCE saturations were achieved by varying the volume of PCE injected during the drainage step and the flow rate used for the imbibition step.

Sequential tracer tests are conducted with a nonreactive tracer and with SDBS. The tracer solutions are saturated with PCE to prevent dissolution during the tests. Standard moment analysis is applied to the breakthrough curves obtained from the SDBS tests to determine retardation factors (R). The NAPL-water interfacial area (Anw, cm−1) is then determined from:

Anw=[R−1−(Kdρb/θw)]θw/Knw (1)

where Kd is the adsorption coefficient (cm3/g), ρb is bulk density of the porous medium (g/cm3), θw is volumetric water content (−), and Knw is the NAPL-water interfacial adsorption coefficient (cm). Knw is obtained from measurement of the interfacial tension function. Additional details of the methods are provided in Brusseau et al. (2008).

Eight IPTT experiments were reported in the prior study. Seventeen new experiments are conducted to obtain replicate Anw measurements at different saturations. Furthermore, seven additional experiments are conducted in the present study to provide additional Kd measurements. These additional data are used to further test the variability of Kd and its potential impact on Anw measurement uncertainty.

3. Results and Discussion

The nonreactive tracer exhibited ideal transport behavior, as illustrated by the sharp, sigmoidal breakthrough curve (see Figure 1). The breakthrough curves for SDBS, the interfacial adsorptive tracer, exhibited some degree of tailing due to rate-limited adsorption by the sand. Mass recoveries were greater than 98%. Overall, the tests produced a robust set of breakthrough curves from which to determine retardation factors and NAPL-water interfacial areas.

Figure 1.

Figure 1.

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Example breakthrough curves for nonreactive tracer (NRT) and interfacial adsorbing tracer (SDBS) transport in a column containing PCE. Relative concentration [-] represents the measured effluent concentration divided by the input concentration; pore volumes [-] represents the effluent volumetric discharge divided by the resident water-filled pore volume of the column. The solid line shown for the NRT is a simulation produced with a one-dimensional solute-transport model incorporating advective-dispersive transport in a homogeneous domain.

Measured NAPL-water interfacial areas as a function of water saturation are presented in Figure 2. The interfacial areas are observed to increase linearly with decreasing water saturation, increasing from 35 to 106 cm−1 over the measured range. This behavior is consistent with prior measurements of NAPL-water interfacial areas employing alternative methods (Schaefer et al., 2000; Brusseau et al., 2009; Zhong et al., 2016).

Figure 2.

Figure 2.

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Measured NAPL-water interfacial areas for different water saturations. Each data point represents the mean of multiple tests and the error bars represent the 95% confidence intervals. The number of measurements comprising each mean point are as follows (reported in order from lowest to highest water saturation--- left to right): 3, 4, 4, 9, and 5. The regression is: Anw = X(1-Sw), where X = 230 (214-236) cm−1, r2 = 0.997.

Inspection of Figure 2 shows that the measurements have a relatively large degree of variability (large 95% confidence intervals, CI). It is noteworthy that the CI for the lowest water-saturation data set is significantly smaller than for the other four data sets. The three tests conducted for the lowest water saturation were all conducted with the same packed column, whereas the other four sets include tests conducted with different column packs. The greater variability observed when employing multiple packs suggests that changes in configuration of the immiscible liquid from pack to pack may have contributed to the observed variability. This issue will be revisited in the discussion below.

Brusseau et al. (2008) discussed the various sources of uncertainty in IPTT data, and highlighted solid-phase adsorption as one factor of potential significance. They reported adsorption coefficient (Kd) data for SDBS adsorption by the sand, based on 5 independent measurements, with a mean of 0.038 cm3/g. Seven additional experiments were conducted in the present study to provide additional Kd measurements. The mean Kd for all 12 measurements is 0.040, with a 95% CI of 0.036-0.043. The new analysis produces a similar mean value, with a smaller CI, compared to the original data. Thus, the measured Kd value used for determining Anw values through equation 1 appears to be robust, and solid-phase adsorption likely has minimal impact on Anw measurement uncertainty for this system.

The results obtained in the present study are compared in Figure 3 to NAPL-water interfacial areas measured for the same sand in three prior studies. Dobson et al. (2006) used the standard IPTT method to measure NAPL-water interfacial areas for a hexadecane-water system. Five tests were conducted. Inspection of Figure 3 shows that their measurements are consistent with those reported herein. This indicates good reproducibility of the IPTT method among different investigators.

Figure 3.

Figure 3.

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NAPL-water interfacial areas measured at different water saturations for the same sand. Data obtained with different tracer-based methods. The regression is based only on the data labelled as “measured”. Dobson 2006 employed the standard IPTT method. Schaefer 2000 used a mass-distribution batch method. Zhong 2016 used the two-phase flow method.

Schaefer et al. (2000a) used an alternative tracer-test method to measure NAPL-water interfacial areas for a two-phase decane-water system. The method, referred to as the mass-distribution method, is based on determining retention via analysis of tracer mass distribution in a segmented column after a period of equilibrium. The method differs from the standard IPTT method in that it is a batch-type approach that is not based on miscible displacement. They conducted 6 experiments to generate a data set spanning a wide range of Anw-Sw. The data comprising the higher water-saturation range are presented in Figure 3.

The slope obtained for their full Anw-Sw data set is 228, which is statistically identical to the slope of 230 obtained for the standard IPTT data reported in Figure 1. This consistency is remarkable given that different methods were used by different investigators. Inspection of the original data and of Figure 3 shows that the mass-distribution method exhibits very large variability in the individual measurements. Some points deviate by 100% or more from the mean regression line. Such high variability appears to be endemic to the method, as similar degrees of variability are observed for applications of the method to air-water systems (Schaefer et al., 2000b; Araujo et al., 2015).

Zhong et al. (2016) used a two-phase flow tracer-test method to measure NAPL-water interfacial areas for a PCE-water system. The data, presented in Figure 3, are consistent with the standard IPTT data reported herein. In addition, the regression determined for the standard IPTT data set provides a reasonable representation of the two-phase data. These data also show a degree of variability, but to a considerably smaller magnitude than the mass-distribution data.

Inspection of Figure 3 reveals that NAPL-water interfacial areas measured with all three methods are consistent for the same porous medium. This is especially noteworthy given that different liquids were used for the NAPL, and that the tests were conducted by different research groups. This indicates that all three methods are characterizing the same interfacial domain, and that interfacial areas are consistent among different NAPL-water pairs. This is despite the fact that the different methods differ in terms of NAPL configuration and potential constraints to tracer transport. For the standard IPTT method, the NAPL exists in a discontinuous state established under secondary imbibition conditions. Tracer accessibility to NAPL-water interfaces in this case is influenced by advection, dispersion, and diffusive mass transfer. Conversely, the NAPL is distributed in a continuous state established under drainage conditions for the mass-distribution method. Tracer accessibility in this case is influenced only by diffusive mass transfer. The two-phase flow method involves concurrent flow of the two immiscible liquids, and the data reported in Figure 3 represent both primary drainage and secondary imbibition conditions.

Measurements made with the three tracer-based methods are compared in Figure 4 to NAPL-water interfacial areas measured with x-ray microtomography (XMT) for the same sand. To our knowledge this represents the first comparison of NAPL-water IPTT and XMT data for a range of saturations. Comparison of the XMT and tracer-derived data sets shows a significant disparity in their respective magnitudes. For example, the interfacial areas are approximately 15 and 45 cm−1, respectively, at a water saturation of 0.8. They are approximately 40 and 115 cm−1 at a water saturation of 0.5. This difference is attributed to the inability of the XMT method to characterize film interfacial area associated with surface roughness, as discussed in prior studies (Brusseau et al., 2007, 2008; Jiang et al., 2020).

Figure 4.

Figure 4.

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NAPL-water interfacial areas measured at different water saturations for the same sand. Data obtained with different tracer-based methods compared to measurements obtained with x-ray microtomography (XMT). Tetrachloroethene was the NAPL for the Brusseau et al. (2009) data, whereas trichloroethene was the NAPL for the McDonald et al. (2016) data. The two regressions are based only on the data labelled as “measured” and “XMT”. Note that the McDonald 2016 data set comprises measurements for 7 separate packed columns, and that the 95% CI error bars are the same size as the data symbol.

The XMT data exhibit a very small degree of variability. For example, the McDonald et al. (2016) data set comprises measurements for 7 separate packed columns. It is observed that the 95% CI error bars are of the same dimension as the data symbol. This high degree of reproducibility has been observed in prior XMT applications for both NAPL-water and air-water systems (Brusseau et al., 2008; Araujo and Brusseau, 2020).

As discussed above, the standard IPTT data exhibit a relatively large degree of variability when different column packs are included. Conversely, the XMT data exhibit very small variability even when multiple column packs are used. The small variability for the latter system indicates that differences in NAPL configuration between column packs have minimal impact on XMT-measured NAPL-water interfacial area. The XMT method is assumed to image all NAPL-water interfaces for this coarse, monodisperse medium. In comparison, the standard IPTT method measures the interfaces that are accessible via advective and diffusive transport of the tracer. Thus, the greater uncertainty observed for the tracer-test method may be related in part to the impact of differences in NAPL configuration from pack to pack on tracer accessibility, and not solely to the inherent variability of NAPL configuration itself. Another potential reason for the disparity in degrees of variability is related to the inability of the XMT method to measure interfacial area associated with surface roughness, which is captured to some degree by the IPTT method. The contribution of roughness-associated interfacial area is very sensitive to fluid configuration and film thickness (Jiang et al., 2020). Hence, it is possible that changes in fluid configuration from one column pack to another could influence the magnitude of roughness-associated interface and therefore contribute to variability in the measured interfacial area.

The XMT data presented in Figure 4 comprises a set of data obtained under a sequential drainage process as well as a number of measurements for NAPL established under secondary imbibition conditions. There is no apparent difference in the drainage and imbibition values, consistent with air-water interfacial areas measured for a soil under drainage and imbibition by XMT (Brusseau et al., 2007). As noted above, the tracer-test data reported in Figures 3 and 4 represent both drainage and imbibition conditions, with no apparent differences. The combined results indicate similarity of NAPL-water interfacial areas irrespective of drainage vs secondary imbibition conditions. However, Zhong et al. (2016) observed differences in interfacial areas obtained under secondary drainage conditions. The relative insensitivity to drainage status likely reflects the fact that total interfacial area is comprised primarily of film-associated area (Brusseau et al., 2007).

4. Conclusion

The standard IPTT method was used for the first time to measure NAPL-water interfacial areas for a range of saturations. The interfacial areas increased with decreasing water saturation as anticipated. The measurements compared well to interfacial areas measured with alternative tracer methods, the mass-distribution batch method and the two-phase flow method. Measurements obtained with all three tracer-based methods exhibit a relatively large degree of variability. Thus, it is important to employ replication when using these methods.

The three methods each have associated advantages and disadvantages. The standard IPTT method is the simplest to conduct. However, the range of saturation that can be achieved is the smallest, and is constrained to the higher water-saturation range. The mass-distribution method can characterize a much wider saturation range. However, this method exhibits the greatest degree of measurement variability. The two-phase flow method can also achieve a wider saturation range; additional testing is needed to evaluate its relative degree of variability. In contrast, the XMT method exhibits very small variability. However, the measured interfacial areas do not capture the contribution of surface-roughness to film-associated interfacial area. Each method has associated advantages and disadvantages, and it is important to be cognizant of them during their application.

Acknowledgements

This research was supported by the NIEHS Superfund Research Program (P42 ES04940). We thank the reviewers for their constructive comments.

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