Measurement of Gas-Accessible NAPL Saturation in Soil Using

Nov 23, 2006 - 0.16, implying more NAPL was accessible to the mobile gas phase at lower water saturation. Introduction. The volume, spatial distributi...
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Environ. Sci. Technol. 2007, 41, 235-241

Measurement of Gas-Accessible NAPL Saturation in Soil Using Gaseous Tracers H E O N K I K I M , * ,† K Y O N G - M I N C H O I , † A N D P. SURESH C. RAO‡ Department of Environmental Sciences and Biotechnology, Hallym University, Chuncheon, Gangwon-do, 200-702, Korea, School of Civil Engineering, Purdue University, West Lafayette, Indiana 47907-2051

In this laboratory study, a new experimental method involving the use of a set of four gaseous tracers, was developed for measuring the NAPL saturation directly accessible to the mobile gas as well as the total NAPL saturation in unsaturated sand. One tracer with low water solubility (n-pentane) was used as the tracer that partitions into NAPL directly accessible to the mobile gas, and another (chloroform) tracer with moderate water solubility and NAPLpartitioning, was selected for detecting total NAPL saturation. Helium and difluoromethane were used as the nonreactive and water-partitioning tracers, respectively. A saturated hydrocarbon, n-decane, was used as NAPL. Column experiments were conducted at two water saturations (Sw ) 0.68 and 0.16). The total NAPL saturation and NAPL saturation not directly accessible to the mobile gas were also successfully measured using the combined results of tracer experiments. At Sw ) 0.68, only 28% of the total NAPL was detected by n-pentane, whereas 87% of the total NAPL was accessible to n-pentane at Sw ) 0.16, implying more NAPL was accessible to the mobile gas phase at lower water saturation.

Introduction The volume, spatial distribution, and chemical composition of nonaqueous phase liquid (NAPL) in the subsurface are important components of source-zone characterization for risk assessment and designing remedial strategies at NAPLcontaminated sites. Selecting an appropriate technology for the assessment of NAPL saturation in soils and aquifers is also an important issue for evaluating the performance of a remedial technology implemented at a NAPL-contaminated site. Soil-core sampling and analysis, a typical site characterization technique, may provide useful information for the chemical composition and saturation of the NAPL. However, this method is limited to discrete spatial locations where the core samples were taken, the results of the core sample analysis are often highly variable and subject to errors of interpolation in delineating the spatial distribution of the NAPL. As an alternate, Jin et al. (1) proposed the concept of a NAPL partitioning tracer technique for evaluation of the saturation and spatial distribution of NAPL in source-zones. This technique is less invasive and covers a relatively larger volume of the target source-zone with appropriate design of * Corresponding author phone: (+82)33-248-2155; fax: (+82)33-256-3420; e-mail: [email protected]. † Hallym University. ‡ Purdue University. 10.1021/es060992u CCC: $37.00 Published on Web 11/23/2006

 2007 American Chemical Society

well installation than other site characterization technologies including core sampling, cone penetrometer test, and monitoring well sampling. The NAPL-partitioning tracers may be injected into the target zone either in dissolved form in an aqueous solution (aqueous tracers) or in a mixed gas (gaseous tracers). Aliphatic alcohols have been used as aqueous NAPL-partitioning tracers in both laboratory-scale (1, 2) and field-scale partitioning inter-well tracer tests (PITTs) (3-7). Gaseous partitioning tracers have been used for the quantitative evaluation of liquids in unsaturated porous media. Suitable gaseous tracers for NAPL detection in the vadose zone have been screened and tested (8-10). Fluorocarbons have been used as gaseous tracers in PITTs for NAPL detection. Gaseous partitioning tracers were used with nonreactive tracers such as argon (8), methane (9), or sulfur hexafluoride (10), along with exclusively water-partitioning gaseous tracers (e.g., difluoromethane). The concept of a gaseous partitioning tracer technique was expanded for evaluating the water saturation in unsaturated soils (11-13). The NAPL saturation measured using a gaseous PITT in the vadose zone was for the “total” NAPL, because the partitioning tracers detect both gas-accessible NAPL and the NAPL isolated from the mobile gas by water. Since fluorocarbons (e.g., perfluoro-1,3,5-trimethylcyclohexane) used in previous studies (9, 10) as gaseous tracers partition into both water and NAPL, the effect of water-partitioning for the tracers had to be corrected using an exclusively water-partitioning tracer (e.g., difluoromethane). Although the “total” NAPL saturation measured using a PITT in a source-zone is very useful information, it would be more helpful for understanding the fate of NAPL if the ratio of gas-accessible NAPL saturation was known. The ratio of NAPL saturation in an unsaturated porous medium that is directly accessible to the mobile gas phase to the total NAPL saturation may range from 0 to 100%. This ratio, fna, may be one of the parameters that control the mass transfer (vaporization) rate from NAPL to the gas phase. Removal efficiency of a soil vapor extraction (SVE), implemented at a NAPL-contaminated vadose zone, is best when all the NAPL is accessible to the mobile gas phase. Groundwater air sparging uses the same mass removal mechanism for NAPL as SVE. Thus, information about fna may be a critical factor that controls the performance of groundwater air sparging. The objective of this study was to develop a gaseous PITT technique for the evaluation of fna and total NAPL saturation in NAPL-contaminated soils. This paper presents the rationale of the “dual gaseous NAPL-partitioning tracer technique” coupled with water-partitioning and nonreactive tracer application to achieve the objective of this study.

Theory Partition Coefficients. Consider a porous medium containing three fluid phases, water, gas, and NAPL, and a chemical, used here as the gaseous tracer, is introduced into the gas phase of the medium (Figure 1). When equilibrium is achieved, three partitioning processes (Figure 1, p1∼p3) are completed, and the following relationships are established:

γgxg ) γwxw ) γnwxnw ) γnaxna

(1)

where γ is the activity coefficient, x is the mole fraction of the tracer, the subscripts g, w, nw, and na represent the gas phase, aqueous phase, NAPL that is separated from the mobile gas phase by water, and NAPL that is in contact with VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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respectively, S is the volumetric fluid saturation (cm3-fluid/ cm3-total fluid: the subscripts are the same as in eq 1), and Kn ()Knw)Kna) is the partition coefficient of the tracer between NAPL and gas phase. Estimation of Snw and Sna requires at least two different NAPL-partitioning tracers, one that partitions into water and NAPL so that it detects total NAPL (Sn ) Snw + Sna), while the other tracer dominantly partitions into NAPL phase (Sna) that is directly accessible to the mobile gas phase. Note that in eq 5 other sorption processes (e.g., soil sorption) were assumed not to significantly contribute to Rt. Method of Moments. Fluid saturations in porous media may be estimated using partitioning tracers Rt values (eq 5). And the Rt for a tracer can be calculated by analyzing the breakthrough curves (BTCs) of the tracer and a nonreactive tracer obtained experimentally. The method of moments was used to calculate the Rt values (14): FIGURE 1. Schematic diagram of the fluid configurations in soil system with air, water and NAPL phases coexist; p1, waterpartitioning process, p2, NAPL-partitioning process for gasaccessible NAPL(na), and p3, NAPL-partitioning process for NAPL(nw) not in direct contact with the mobile gas phase. the mobile gas phase, respectively, and a pure liquid state of the chemical is set as the reference state. The partition coefficients for tracer distribution between NAPL and gas phases are expressed in terms of the ratio of concentrations:

Kna )

Cna xnaVg ) Cg xgVna

Cnw xnwVg Knw ) ) Cg xgVnw

Cg xgVw ) Cw xwVg

(3)

where Cw is the aqueous concentration (mol/L) of the tracer, and Vw is the average molar volume (L/mol) of the aqueous phase. Retardation Factor. The transport velocity of a partitioning tracer through gas phase depends on the quantity of liquids (NAPL and water) and the partition coefficients. The retardation factor, Rt (dimensionless) of a partitioning tracer is related to the linear velocities of the mobile gas phase and the tracer, or the residence times of gas phase and the tracer, and is also a function of the partition coefficient and fluid saturations:

Rt )

vg ht i Sw Kn(Sna + Snw) ) )1+ + vi ht g KHSg Sg

(5)

where vi and vg are the linear velocities (cm/min) of the tracer and the gas phase, hti and htg are the mean residence times (min) of the tracer in the porous medium and the gas phase, 236

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(6)

∫ C(t)t dt ) ht C(t) dt ∫ ∞

µ′ )

(4)

µ′i µ′g

where µ′i and µ′g are the first normalized temporal moments (min) of the BTC of a partitioning tracer and a nonreactive tracer, respectively. Note that µ′ is equivalent to the mean residence time (th) of a tracer during transport through a porous medium, if a small tracer pulse (i.e., Dirac input) is displaced. Based on the concentration-time relationship (BTC) of the tracer, µ′ is determined:

(2)

where Kna and Knw are the partition coefficients (dimensionless) of the tracer between NAPL and gas phases, Cna, Cnw, and Cg are the molar concentrations (mol/L) of the tracer, Vna, Vnw, and Vg are the average molar volumes (L/mol) of fluids, and the subscripts denote the same as those used in eq 1. The Knw can be assumed to be the same as Kna, based on the assumption that the chemical composition of NAPL is identical regardless of the location of the NAPL with respect to the mobile gas, resulting in γnw ) γna. The partition coefficient of the tracer between gas and aqueous phases, known as the “Henry’s law constant”, KH (dimensionless), is

KH )

Rt )

0 ∞

(7)

0

where C(t) is the tracer concentration (mol/L), and t is time (min).

Materials and Methods Materials. Quartz sand (200-500 µm diameter) was used to pack the columns. Saturated hydrocarbons, n-pentane and n-decane (>99%), supplied by Aldrich Chemical Co., were used as the NAPL-partitioning tracer and as the NAPL, respectively. Chloroform and difluoromethane (DFM) (both reagent-grade, 99.7%) were also purchased from Aldrich Chemical Co., and used as the NAPL/water-partitioning and water-partitioning tracers, respectively. Reagent-grade sodium dodecylbenzene sulfonate (SDBS) was purchased from Tokyo Kasei Kogyo Co. Ltd., and used as received. SDBS was used for reducing the surface tension of water to produce lower water saturations during gas flow experiments. Helium (He) (>99.99%) was used as a nonreactive gaseous tracer. Compressed air was used as the mobile gas for the column experiments. Double distilled (DI) water was used in all experiments. Chemical properties relevant to this study are listed in Table 1. Experimental Set-Up. A schematic diagram of the experimental setup is shown in Figure 2. All experiments were conducted at room temperature (25 °C). The length and inner diameter of the sand column were 78.0 and 5.25 cm, respectively. The bottom 3 cm of the column was packed with coarse sand (0.1-0.2 cm diameter) for better distribution of injected air during gas displacement experiments and for preventing the sand particles from clogging the orifice where air was introduced. Above the coarse sand layer, the NAPL (n-decane) contaminated sand was packed with no open space to the top of the column. Sand was well mixed with a known quantity of n-decane in a closed flask and was shaken vigorously before packing in the column. More information for the experimental conditions is shown in Table 2.

TABLE 1. Properties of Chemicals Used in This Study chemical

use

Mw

He DFM n-pentane chloroform n-decane

nonreactive tracer water-partitioning tracer NAPL-partitioning tracer NAPL/water- partitioning tracer NAPL

4.00 52.02 72.15 119.40 142.28

a

Ref 18.

b

Experimentally determined in this study. cRef 13.

d

vapor pressurea (kPa)

KH 0.505c 51.7d 0.166d 19.3d

68.4 25.6 0.175

Knb for n-decane 0.71 208 434

Ref 19.

FIGURE 2. Schematic diagram of the experimental setup; flask 1 was filled with n-decane; flask 2 was filled with DI water.

TABLE 2. Conditions for Column Experiments

description porosity bulk density pore gas velocity (cm/min) total volume of column (cm3) total volume of water (cm3) mean fluid saturationsa Sn Sw Sg a

Expt I

Expt II

Expt III

Kn measurement

tracer test at low water saturation (SVE)

tracer test at high water saturation (SEAS)

0.404 1.58 1.78 1689 0

0.397 1.60 1.10 1689 107

0.404 1.58 2.50 1689 473

0.019 0.000 0.981

0.015 0.159 0.826

0.018 0.676 0.307

Estimated based on the weight of fluids.

Three sets of column experiments were conducted: (1) Expt I, gaseous tracer experiments for determining the Kn values of reactive tracers for n-decane, using a dry-packed NAPL-contaminated (coated) sand column with no water, (2) Expt II, gaseous tracer experiments for sand column at low water saturation during air flow, and (3) Expt III, gaseous tracer experiments at high water saturation during air flow in the presence of surfactant solution. For Expts II and III,

dry-packed NAPL-contaminated sand columns were purged with carbon dioxide before water saturation for better removal of the entrapped gas. The dry-packed sand column was then saturated with water by injecting degassed DI water (Expt II), or an aqueous solution of SDBS (100 mg/L) (Expt III) through a stainless steel tubing (0.10 cm i.d., 0.318 cm o.d.) installed at the bottom of the column. For Expt II, water was drained from the bottom of the column before gas displaceVOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ment experiments. In addition to gravity drainage, water was extracted from the column using a pump. For Expt III, water was extracted from the column and collected in a flask at the column outlet during air flow. Resultant fluid saturations, estimated based on the fluid weights, are listed in Table 2. For Expt I, a flask filled with n-decane was installed before the tracer injection port to saturate the incoming dry air with n-decane vapor, to avoid decreasing the NAPL saturation in the column during experiment due to vaporization. For Expts II and III, an additional gas- washing bottle filled with DI water was installed to saturate the influent gas with water vapor to maintain water saturation during experiments. Outflow from the sand column was connected to a gas chromatograph (GC, Yonglin Model M600D) for tracer analysis during displacement experiments. GC conditions for He analysis were oven 35 °C, thermal conductivity detector (TCD) 140 °C, column Haysep Q80/100, Alltech Co. GC conditions for other tracers were as follows: oven 180 °C, flame ionization detector (FID) 250 °C, column Carbopack 60/80, Supelco Co. Experimental Procedure. Constant gas (air) flow through the column was achieved by controlling the metering value (Table 2). For Expts I and II, the effluent tubing was connected to the GC gas sampling valve (Figure 2, v2) to monitor the tracer concentration in the column effluent, whereas for Expt III the column effluent tubing was connected to a water collecting flask for the extracted water. When no more water was extracted due to air displacement (or air sparging), the column effluent was then connected to the GC gas sampling valve as for Expt I. When the gas flow rate was stable, a gas tracer was introduced into the sand column using a four-way valve (Figure 2, v1) and a 20 cm-long brass loop (0.43 cm i.d.). A single tracer was injected for each experiment to avoid interaction between tracers. Tracers in gaseous state (He and DFM) at room temperature were transferred to the loop by connecting a compressed gas cylinder, and were injected to the column by switching the four-way valve. Since other tracers (n-pentane and chloroform) are liquids under ambient condition, it was necessary to vaporize the tracer before introducing it into the column. The injection loop was detached from the system, and about 50 µL of liquid tracer was injected into the loop using a micro-pipet of 100 µL capacity. The brass loop containing a tracer in liquid state was then plugged, placed in an oven, and heated at 105 °C for at least 30 min for vaporization. After cooling to room temperature, the loop was installed at the switching valve and the vaporized tracer was injected. The four-way valve was at the injection position for only 5 min to avoid a constant tracer bleed from the loop. After tracer injection, column effluent gas was analyzed for the tracer using a GC. The tracer concentration profiles, or the BTCs, were processed for the calculation (eqs 6 and 7) of tracer Rt values. Based on the results of moment analysis, the tracer Rt values and corresponding information (Kn, Sw, Snw, Sna) were calculated based on eq 5. Batch Experiments. A set of batch experiments was also conducted to verify the Kn value of n-pentane measured from Expt I. Glass bottles (160 cm3 capacity) sealed with Teflonlined caps were used. A 4 µL aliquot of n-pentane was placed in each of the bottles, followed by vaporization in an oven at 35 °C for 30 min. After the bottles were cooled down, known amount of n-decane was injected into each bottle. The quantity of n-decane injected into the bottles ranged from 0.2 to 2.0 cm3. The gas phase in the bottles was analyzed for n-pentane using a GC (Agilent 6890, injector 150 °C, oven 180 °C, FID 250 °C, HP-1 column). A set of control experiments using bottles without NAPL (n-decane) was also conducted. 238

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FIGURE 3. BTCs of tracers; measured in Expt II, (low water saturation, Sw ) 0.16), normalized concentrationsconcentration normalized to the total area of BTC.

Results and Discussion Expt I and Batch Experiments for Kn Measurement. The Rt values (Table S-1) and the tracer BTCs (Figure S-1) for Expt I are shown in the Supporting Information. Using moment analyses (eqs 6, 7), the Rt for DFM was estimated to be 1.01, implying that the travel velocity of DFM was close to that of He, or that of mobile gas. DFM was selected as the exclusive water-partitioning tracer in this study, as used in previous studies (9, 10, 13). The Kn value for DFM for n-decane was estimated to be 0.71 using eq 5 with Sw ) 0, and Sn ) 0.0195. The Kn values of n-pentane and chloroform were estimated to be 208 and 434, respectively, and used for Expts II and III. The partition isotherm of n-pentane for n-decane, measured by the batch experiments, is shown in Fig. S-2, Supporting Information. In batch studies, the Kn value (the slope of the partition isotherm) was found to be 207 (with r2 value of 0.99) which was in good agreement with the value of 208 estimated from Expt I. This implies that a local partitioning equilibrium for n-pentane between NAPL and gas phases was established during the gas displacement experiment through the sand-packed column under the experimental conditions used in this study. No batch experiments for chloroform partition in n-decane were conducted, and the partition coefficient measured by column experiment was used for Expts II and III. Expts II and III: Partitioning Tracer Experiments for Unsaturated Sand. Water saturation (Sw ) 0.16) of the sandpacked column used for Expt II was typical residual water saturation in the vadose zone and may be representative of conditions during SVE process; however, compressed air was injected instead of applying negative pressure to draw air from the medium. Expt III was a modified air sparging process (SEAS, surfactant-enhanced air sparging) that the surface tension of groundwater was lowered to 40 dyn/cm (by dropweigh method; 15) by adding 100 mg/L SDBS to achieve lower water saturation during air sparging compared to that with no surfactant addition (16). Tracer BTCs measured in Expts II and III are shown in Figure 3 and Figure 4, respectively. The Rt values of the partitioning tracers are shown in Table S-1, Supporting Information. Truncated tails of the BTCs were extrapolated to avoid underestimation of the first temporal moment (mean residence time) (17). The first temporal moments of He, the nonreactive tracer, were 602 cm3 and 203 cm3 (mean residence times were converted to volumes using the average gas flow rates) for Expt II and Expt III, respectively, which are in good agreement with those (564 cm3 and 209 cm3) estimated based on the weights of water. Note that the temporal moment of He represents the volume of gas phase in the column. The first temporal moments for partitioning tracers were larger than that of He due to partitioning into liquid phases.

large KH and small interfacial adsorption coefficient for n-pentane. Since n-pentane has very limited water solubility (or large KH value), the NAPL into which the vaporized n-pentane can partition should be either in direct contact with the mobile gas phase or otherwise readily accessible to the mobile gas phase (i.e., close enough that the partitioning mass transfer between gas and NAPL phases is not significantly diffusion rate-limited). Thus, the NAPL saturations (Sna in Table 3) estimated using the Rt values of n-pentane can be regarded as the “gas-accessible” NAPL saturations. The Sna values were calculated using eq 10, assuming Sg ) 1 - Sw, where Sw was from DFM experiment, and partitioning into the gas-accessible NAPL was the only process responsible for retardation of n-pentane during gaseous transport:

Rt ) 1 +

FIGURE 4. BTCs of tracers; measured in Expt III. (high water saturation, Sw ) 0.68), normalized concentrationsconcentration normalized to the total area of BTC. Water saturation of the sand-packed column was measured using DFM, assuming that partitioning into aqueous phase dominantly contributes to the Rt of DFM. However, as found in Expt I, DFM partitions slightly into n-decane (Table 1), and the contribution of NAPL-partitioning (KnSn/Sg) to Rt was estimated to be 0.013 for Expt II and 0.041 for Expt III, respectively. This is negligible compared to its Rt values, 1.38 (Expt II) and 4.80 (Expt III). This is due to the small magnitudes of both the partitioning coefficient (0.71) of DFM for n-decane and the NAPL saturations, which provide a basis for discarding the effect of NAPL-partitioning for DFM on Rt in this study. However, this assumption is only valid when the product of KH, Kn, and Sn (that is KHKnSn) is much smaller than Sw () 1 - Sn - Sg), as shown in following eq 8. For the Sw calculation, it was also assumed that Sn is negligible compared to 1 - Sg, resulting in Sw ) 1 - Sg, as shown in eqs 8 and 9:

Rt ) 1 +

Sw KnSn 1 - Sn - Sg + KHKnSn + )1+ KHSg Sg KH S g Rt ) 1 +

Sw KH(1 - Sw)

(8) (9)

The Sw values estimated using eq 9 with DFM as the tracer were found to be close to the actual value measured gravimetrically (Table 3). More than 90% of the water in the column was detected by DFM. Although DFM was used successfully in this study for measurement of total quantity of water in the system, care has to be taken for applying this technique for porous media with higher water saturations. Higher pore gas velocity may also introduce errors in Sw measurement because of the possibility for mass transfer constraints or flow bypassing. The retardation of n-pentane is considered to be primarily the result of partitioning into NAPL directly in contact with mobile gas, and the contribution of adsorption at the airwater or NAPL-water interfaces (11) and partitioning into water was evaluated to be negligibly small because of the

KnSna Sg

(10)

The resultant Sna values for Expt II (Sw ) 0.16) was 87% of total NAPL (estimated based on weight), which was about 3 times larger than that measured in Expt III (28%), where Sw ) 0.68 (Table 3). This result indicates that the ratio of gasaccessible NAPL saturation to the total NAPL saturation (fna) is inversely proportional to the water saturation although the exact functional relationship remains to be established. Draining (or displacing) more water from NAPL-contaminated soil (or aquifer) will result in more NAPL being directly accessible to the mobile gas phase. Since groundwater air sparging and SVE processes depend primarily on the mass transfer from NAPL to the mobile gas (air) phase, more gasaccessible NAPL would enhance the remedial performance of these air-driven subsurface restoration technologies. Chloroform was used as the NAPL/water-partitioning tracer that partitions not only into the gas-accessible NAPL but also into the NAPL isolated from the gas phase by the aqueous phase. This type of NAPL/water-partitioning tracer has been used along with a water-partitioning tracer and a nonreactive tracer, in previous studies for estimation of “total” NAPL saturation in soils (8-10) using following eq 11:

Sn )

(RtKH - KH + 1)Sg - 1 KHKn - 1

(11)

The total NAPL saturations estimated from chloroform Rt values using eq 11 and the Sg value estimated from DFM experiment (Sg ) 1 - Sw) were in good agreement with the actual NAPL saturation (Table 3). The Rt values for chloroform during gaseous transport were used to estimate not only the “total” NAPL saturation, but the NAPL saturation (Snw) that is not in contact with the mobile gas by the difference between Sn and Sna. The fraction of gas-accessible NAPL is shown in Figure 5 along with the fraction of air and water; note that the total NAPL estimated using chloroform was set to 100%, and the combined volume of water and air was set to 100%. The fna value follows the relative volume of air. This ratio, however, was found to be slightly smaller than that of air to the combined volume of air and water. More study is needed to verify this proportional relationship between air saturation and gas-accessible NAPL saturation. Using eq 11 (or eq 10) for estimating Sn, (or Sna) requires all parameters in the equation have to be known including Sg, unless Sn is much smaller than both of Sw and Sg (so that Sg can be approximated as 1 - Sw) as in this study. If Sg cannot be approximated as 1 - Sw due to relatively large Sn, using eq 11 becomes problematic with two unknowns (Sn and Sg) requiring two sets of data (two Rt data sets with corresponding partitioning coefficients) for calculating Sn. This requires the use of another NAPL/water-partitioning VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Estimated Mean Fluid Saturations Compared to Gravimetrically Measured Values Sw Expt II (SVE)

Expt III (SEAS)

actual (gravimetrically determined) estimated by Rt of DFM chloroform n-pentane combining Rts of DFM and chloroform % of actual Sw, Sn % of measured Sn by chloroform actual (gravimetrically determined) estimated by Rt of DFM chloroform n-pentane combining Rts of DFM and chloroform % of actual Sw, Sn % of measured Sn by chloroform

0.16

Sn(total)

Sna

Snw

0.015

0.15 0.016 0.013 0.0033 91

109 100

0.68

0.018

87 80

22 20

0.64 0.018 0.0051 0.013 95

104 100

28 27

75 73

method, however, may be greatly limited (particularly for field cases) due to several factors interfering the partition processes of tracers (e.g., sorption by soil), mass transfer constraints, or flow bypassing. Factors that could affect the tracer Rt values have to be carefully evaluated and incorporated into the procedure for successful application of this method.

Acknowledgments This work was supported by Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (2004202-D00456).

Supporting Information Available Breakthrough curves of tracers for Expt I, the sorption (partition) isotherm for n-pentane in NAPL (n-decane), the retardation factors measured for tracers, and an equation which may be used for estimating the saturation of gas phase (Sg) using two different NAPL/water-partitioning tracers. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited FIGURE 5. Schematic diagrams for the ratios of fluids measured using tracers. tracer in addition to one such as chloroform with different Kn/KH ratio, and then Sg can be calculated without aforementioned approximation (see eq S-1 in the Supporting Information). Note that parameters for DFM including Rt value were found to be inadequate for Sg estimation using eq S-1 in the Supporting Information because of very small Kn value which resulted in large error in the estimated Sg value. The first moment of a nonreactive tracer (He in this study), with other information available related to the system such as porosity, may be used for estimating the Sg. The experimental procedure presented in this study offers a new application of the gaseous NAPL-partitioning tracer technique for assessing the NAPL contamination in the vadose zone (during SVE) and aquifers (during SEAS). The method, proposed in this study, may provide more detailed information about NAPL saturations than that can be acquired using conventional tracer techniques, which can be very useful for understanding contaminant mass transfer processes between fluid phases. The applicability of this 240

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(1) Jin, M.; Delshad, M.; Dwarakanath, V.; McKinney, D. C.; Pope, G. A.; Sepehrnoori, K.; Tilburg, C. E. Partitioning tracer test for detection, estimation, and remediation performance assessment of subsurface nonaqueous phase liquids. Water Resour. Res. 1995, 31, 1201-1211. (2) Nelson, N. T.; Oostrom, M.; Wietsma, T. W.; Brusseau, M. L. Partitioning tracer method for the in situ measurement of DNAPL saturation: Influence of heterogeneity and sampling method. Environ. Sci. Technol. 1999, 33, 4046-4053. (3) Nelson, N. T.; Brusseau, M. L. Field study of the partitioning tracer method for detection of dense nonaqueous phase liquid in a trichloroethene-contaminated aquifer Environ. Sci. Technol. 1996, 30, 2859-2863. (4) Hunkeler, D.; Hoehn, E.; Ho¨hener, P.; Zeyer, J. 222Rn as a partitioning tracer to detect diesel fuel contamination in aquifers: Laboratory study and field observations. Environ. Sci. Technol. 1997, 31, 3180-3187. (5) Annable, M. D.; Rao, P. S. C.; Hatfield, K.; Graham, W. D.; Wood, A. L.; Enfield, C. G. Partitioning tracers for measuring residual NAPL: Field-scale test results. J. Environ. Eng. 1998, 124, 498503. (6) Annable, M. D.; Jawitz, J. W.; Rao, P. S. C.; Dai, D. P.; Kim, H.; Wood, A. L. Field evaluation of interfacial and partitioning tracers for characterization of effective NAPL-water contact areas. Ground Water 1998, 36, 495-502. (7) Young, C. M.; Jackson, R. E.; Jin, M.; Londergan, J. T.; Mariner, P. E.; Pope, G. A.; Anderson, F. J.; Houk, T. Characterization of

(8) (9)

(10)

(11) (12)

(13)

a TCE DNAPL zone in alluvium by partitioning tracers. Ground Water Monit. Rem. 1999, 19, 84-94. Whitley, G. A., Jr.; McKinney, D. C.; Pope, G. A.; Rouse, B. A.; Deeds, N. E. Contaminated vadose zone characterization using partitioning gas tracers. J. Environ. Eng. 1999, 125, 574-582. Deeds, N. E.; Pope, G. A.; McKinney, D. C. Vadose zone characterization at a contaminated field site using partitioning interwell tracer technology. Environ. Sci. Technol. 1999, 33, 2745-2751. Mariner, P. E.; Jin, M.; Studer, J. E.; Pope, G. A. The first vadose zone partitioning interwell tracer test for nonaqueous phase liquid and water residual. Environ. Sci. Technol. 1999, 33, 28252828. Kim, H.; Rao, P. S. C.; Annable, M. D. Gaseous tracer technique for estimating air-water interfacial areas and interface mobility. Soil Sci. Soc. Am. J. 1999, 63, 1554-1560. Nelson, N. T.; Brusseau, M. L.; Carlson, T. D.; Costanza, M. S.; Young, M. H.; Johnson, G. R.; Wierenga, P. J. A gas-phase partitioning tracer method for the in situ measurement of soilwater content. Water Resour. Res. 1999, 35, 3699-3707. Keller, J. M.; Brusseau, M. L. In-situ characterization of soilwater content using gas-phase partitioning tracer tests: Fieldscale evaluation. Environ. Sci. Technol. 2003, 37, 3141-3144.

(14) Valocchi, A. J. Validity of the local equilibrium assumption for modeling sorbing solute transport through homogeneous soils. Water Resour. Res. 1985, 21, 808-820. (15) Harkins, W. D.; Brown, F. E. The determination of surface tension (Free surface energy), and the weight of falling drops: The surface tension of water and benzene by the capillary height method. J. Amer. Chem. Soc. 1919, 41, 499-524. (16) Kim, H.; Soh, H. -E.; Annable, M. D.; Kim, D. -J. Surfactantenhanced air sparging in saturated sand. Environ. Sci. Technol. 2004, 38, 1170-1175. (17) Jawitz, J. W. Moments of truncated continuous univariate distributions. Adv. Water Resour. 2004, 27, 269-281. (18) Yaws, C.; Yang, H. -C.; Pan, X. Henry’s law constants for organic 362 compounds in water. Chem. Eng. 1991, 98, 179-185. (19) Mackay, D.; Shiu, W. Y. A critical review of Henrys’s law constants for chemicals of environmental interest. J. Phys. Chem. Ref. Data 1981, 10, 1175-1199.

Received for review April 25, 2006. Revised manuscript received September 20, 2006. Accepted September 26, 2006. ES060992U

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