Environ. Sci. Technol. 2005, 39, 317-324
Solvent Release into a Sandy Aquifer. 2. Estimation of DNAPL Mass Based on a MultipleComponent Dissolution Model K I M B R O H O L M , * ,† STANLEY FEENSTRA, AND JOHN A. CHERRY Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
A chlorinated solvent mixture (2.0 L of trichloroethylene, 0.5 L of chloroform, and 2.5 L of tetrachloroethylene) was released into a sandy aquifer to create a heterogeneously distributed DNAPL (dense nonaqueous-phase liquid) source. The dissolution and dissolved-phase plume development from this source were studied in detail along a cross-section downgradient of the source for a period of approximately 1 year. At the conclusion of the experiment, the site was excavated to map the actual distribution of solvent residuals in the subsurface. Multiple-component dissolution theory provides a tool for the estimation of the mass of a multiple-component DNAPL source present in the groundwater. Concentration ratios between the compounds change with time, and those changes can be used to estimate the mass of DNAPL upgradient of the monitoring point(s) or well(s). The method is independent of the dilution occurring in the groundwater and only requires observations of time series of the contaminants in one or more monitoring points. For the field experiment, the method was applied using the measured concentrations of individual sampling points, the depth-integrated concentrations, the area-integrated concentrations, and the effluent concentrations of the cell. The experiment showed that multiple-component dissolution theory may be a valuable tool for the estimation of the mass of multiplecomponent DNAPL residuals in the saturated zone.
Ci ) xiSi
Introduction This paper is the second of a series of three describing the results from a field experiment at the Canadian Forces Base Borden, Ontario, Canada, involving a release of 5 L of a chlorinated solvent mixture (0.5 L of chloroform (TCM), 2 L of trichloroethylene (TCE), and 2.5 L of tetrachloroethylene (PCE)) into a sandy aquifer to create a heterogeneous DNAPL (dense nonaqueous-phase liquid) source zone. The first paper (1) described the experimental site, the field instrumentation, the tracer test, the DNAPL release, the monitoring of the dissolved phase, and the concluding excavation of the source zone. The first paper examined the interpretation of the spatial distribution of DNAPL residuals in the aquifer by monitoring dissolved concentrations downgradient of the * Corresponding author phone: +45 45169200; fax: +45 45169292; e-mail:
[email protected]. † Present address: DHI Water and Environment, Agern Alle 5, DK-2970 Hørsholm, Denmark. 10.1021/es0306462 CCC: $30.25 Published on Web 12/02/2004
source. This paper will focus on the application of multiplecomponent dissolution theory to estimate the mass in the source zone. The third paper will describe the enhanced dissolution caused be methanol injection initiated in the last part of the experiment. The mass of contaminant is rarely known at contaminated sites. However, an estimate of the mass present in DNAPL source zones and an estimate of the distribution is important to choose the optimal remedial technique(s) at the site and to estimate the cleanup time and cost. The most frequently used technique is collection and analysis of soil samples from soil cores. However, due to spatial variability in chemical concentration, numerous soil samples are necessary and the cost will be high. Furthermore, the collection of soil samples close to and in the source zone brings the risk of downward spreading of the DNAPL during drilling. Using partitioning tracers is one method to estimate the amount of chlorinated solvent residuals and other compounds in the subsurface. One tracer is naturally occurring radon-222 in groundwater (2-5). Because radon-222 partitions into solvent residuals, by measuring radon-222 up- and downgradient of the suspected contaminated zone, the amount of solvent residuals can be estimated. Another method uses SF6 that also partitions into solvent residuals, but otherwise it behaves as a conservative tracer, and the background concentration of SF6 is very low (6, 7). Thus, applying a dual tracer experiment with SF6 and, for example, bromide, the retardation of SF6 relative to bromide is an expression of the amount of solvent residuals. Partitioning tracer tests have been used at field experiments, demonstration projects, and at real sites in the unsaturated and saturated zones (e.g., 8-13) and have achieved some degree of success at real sites but are relatively costly to perform. Geophysical methods may also be applied to locate the DNAPL residuals and estimate the source mass (14-16). In general, those methods are not very sensitive and are limited to shallow depth and homogeneous geology. When a multiple-component DNAPL is dissolved, the most soluble components are depleted fastest and the composition of the DNAPL changes, thereby changing the subsequent dissolved concentrations. The temporal pattern of the changes in dissolved concentrations may be a characteristic fingerprint that may be used to estimate the DNAPL mass. The estimation of the mass of solvent residuals is based on the theory of dissolution from a multiplecomponent mixture of organic compounds, for which the dissolved concentrations are described by Raoult’s law (17):
2005 American Chemical Society
(1)
where Ci is the aqueous equilibrium concentration of compound i (called the effective solubility), xi is the mole fraction of compound i in the organic phase, and Si is the aqueous solubility of compound i. Raoult’s law expressed by eq 1 has been shown to be valid for binary and ternary mixtures of chlorinated solvents relevant for this study (17). Raoult’s law has also been shown to be valid for mixtures of various hydrocarbons (18-21). The aqueous solubility and other relevant physical/chemical properties for the chlorinated solvents used for this study are given in Table 1. The dissolved concentrations resulting from continued dissolution of a multiple-component source can be calculated using a multibatch approach (22). Assuming an initial composition of the organic phase, the dissolved equilibrium concentrations can be calculated using Raoult’s law. The changed composition can then be calculated assuming a VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Physical and Chemical Properties of the Relevant Chlorinated Solvents, the Initial Volume Fraction of the Solvent Mixture Used in the Field Experiment, and the Corresponding Aqueous Equilibrium Concentration According to Eq 1 compound chloroform (TCM) trichloroethylene (TCE) tetrachloroethylene (PCE) c
aqueous initial initial density solubility vol concn mol wt (g mol-1)a (g cm-3)a (mg L-1)b fraction (mg L-1)c 119.4 131.4
1.48 1.46
8700 1400
0.1 0.4
1016 597
165.8
1.62
240
0.5
112
a Data at 20 °C, from ref 38. Calculated according to eq 1.
b
Data at 23-24 °C, from ref 17.
FIGURE 1. Theoretical dissolution curve for a DNAPL mixture consisting of, by volume, 10% of TCM, 40% of TCE, and 50% of PCE. Refer to the text for details of these calculations. certain water to DNAPL ratio. Finally, the new dissolved equilibrium concentrations can be calculated and so on according to progressive dissolution of the DNAPL source. Figure 1 shows the theoretical dissolution profile for a DNAPL source consisting of 10 vol % of TCM, 40 vol % of TCE, and 50 vol % of PCE. This profile shows the dissolved concentrations versus the ratio of water volume passing through the DNAPL to the volume of DNAPL. The most soluble compound in the mixture, in this case TCM, is depleted fastest, and the less soluble one, in this case PCE, is depleted slowest. The magnitude of the dissolved concentrations shown in Figure 1 will seldom be observed in the field, because the DNAPL saturated water will be diluted by less contaminated water between the source and the monitoring point(s). Another reason is that DNAPL residuals can be distributed on a very small scale (centimeters) (1, 23, 24). Consequently, if water samples are taken from monitoring wells screened over meters or more, the DNAPL saturated water is diluted in the monitoring wells. Both effects result in observed concentrations much less than the saturated concentration. However, based on the theoretical dissolution curves (Figure 1), the concentration ratios between TCE and TCM, PCE and TCM, and PCE and TCE as a function of the source depletion can be constructed (Figure 2). These ratio relationships will not be altered by dilution/dispersion in the aquifer and mixing in the monitoring well, assuming that the dissolved components are affected to the same degree by these processes. This assumption will be valid for compounds, such as chlorinated solvents, having generally similar coefficients of molecular diffusion and sorption properties. The assumption may not be valid for the component pairs of DNAPL, such as coal tar, which may exhibit large differences in diffusion and sorption properties, which are reflected in differences in dispersion. The multiple-component dissolution theory has been applied successfully on several column experiments (25, 26) and a field experiment involving a homogeneous distributed 318
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FIGURE 2. Ratios of TCE and TCM, PCE and TCM, and PCE and TCE as a function of source depletion. Those ratios are calculated using Figure 1. source consisting of TCM, TCE, and PCE (27, 28). In general, the multiple-component dissolution theory seems to be valid for the above-mentioned experiments. However, the laboratory experiments were carried out in homogeneous porous media, and the residuals were homogeneously distributed in the porous media. Furthermore, in most of the column dissolution experiments, the water was forced through the columns ensuring optimal contact between the water and the residuals. There are situations where the multiple-component dissolution theory may not be valid. The theory is based on the assumption of chemical equilibrium to allow the use of Raoult’s law. Nonequilibrium dissolved concentrations in groundwater may be caused by factors such as the following: high flow rates result in insufficient contact time (29); heterogeneously distributed residuals with respect to location and residual concentration (1, 29); high residuals in some zones results in bypassing of the water due to the low relative permeability (30-32); and large zones of very high residual concentration may result in diffusion-limited dissolution, which means that the concentration at the surface is different than the concentration inside. During dissolution, the physical properties of the DNAPL source (i.e., density, viscosity, interfacial tension) may change and alter the magnitude of the dissolved concentrations emitted from the source. Although any of these factors may cause the magnitudes of the dissolved concentrations to be less than equilibrium concentrations, these factors should not affect the ratios of components that have similar diffusion and sorption properties. There may be cases where complex chemical mixtures in the DNAPL, or the presence of surfactant compounds in the groundwater, may cause Raoult’s law to be invalid (eq 1) (33-35), and consequently the multiple-component dissolution theory described here will not apply. The use of multiple-component dissolution theory to evaluate dissolved concentrations and ratios in groundwater assumes also that none of the components undergo significant chemical or biological degradation during transport from the DNAPL source zone to the monitoring locations. The objective for this field experiment was to create a heterogeneous multiple-component DNAPL source with respect to spatial distribution and residual saturation, from which the dissolved concentrations in groundwater could be monitored over time to evaluate the applicability of multiple-component dissolution theory to estimate DNAPL source mass. The groundwater velocity at the site was approximately 13 cm d-1, which was expected to allow equilibrium dissolution except for very small residual zones (i.e., less than a few cm) for which contact times may be too short to achieve chemical equilibrium. Furthermore, Raoult’s law has been shown to be valid for the solvents used in this study (17).
FIGURE 3. Plan view of the cell and the instrumentation. SN 1-9 are the locations of sampling nests 1-9; C 10, 24, and 38 are the locations of the three soil cores used for measurement of the hydraulic conductivity.
One hundred ninety-five stainless steel sampling points were installed with a vertical spacing of 10 cm and a horizontal spacing of 50 cm to allow very detailed monitoring. The sampling points were located in a cross section 0.7 m upgradient of the extraction wells. The first sampling point at a depth of 17 cm bgs is named the sampling nests number 01, and the second at a depth of 27 cm bgs is named the sampling nests number 02, etc. Thus, sampling point 504 is located a sampling nest number 5 at a depth of 47 cm bgs. Approximately 7000 water samples were collected during the experiment and analyzed for TCM, TCE, and PCE. The sampling and analytical techniques have been described in ref 1. The tracer experiment (sodium chloride) was carried out prior to the solvent release to characterize the hydraulic conditions of the site. The groundwater velocity was estimated for each of the 195 sampling points. The hydraulic conductivity of the aquifer ranged from 2.4 × 10-5 to 1.1 × 10-4 m s-1 based on the tracer experiment and from 1.1 × 10-5 to 8.0 × 10-5 m s-1 based on hydraulic conductivity measurements of soil samples obtained from three soil cores (1). To create a heterogeneously distributed source of DNAPL residuals in the subsurface, a mixture of 5 L of chlorinated solvents (10% of TCM, 40% of TCE, and 50% of PCE by volume) was released into a tube located approximately 66 cm bgs. This was approximately 5 cm below the water table (see Figure 3 for location of the tube). Groundwater flow was sustained through the cell for 291 d. As part of an enhanced dissolution experiment, a mixture of 30% (vol/ vol) methanol in water was injected for a period of 5.5 d initiated at day 220 (36). Because the methanol injection was initiated at day 220, all of the observations presented in this paper are from the experimental period before the methanol was injected. The site was excavated at the end of the experiment to determine the actual distribution of solvent residuals in the subsurface. The excavation revealed that most of the solvent residuals were located in a layer 0.55 m bgs.
Results and Discussion FIGURE 4. Cross section through the cell along the flow direction. The illustration of DNAPL in the figure does not represent the real distribution.
Description of the Field Experiment A detailed description of the experimental conditions of this field experiment is presented in ref 1. The field experiment was performed in a sandy, unconfined, shallow aquifer at Canadian Forces Base Borden, east of Toronto, Ontario, Canada. The experiment was conducted in a 4.5 m × 5.5 m steel sheet pile cell. The aquifer is approximately 2.3 m in thickness. Figure 3 shows a plan view of the cell and the instrumentation, and Figure 4 shows a cross section through the cell along the direction of flow. Groundwater flow through the cell was controlled by means of five injection wells and five extraction wells. All of the wells were fully screened. Uncontaminated groundwater from the same aquifer was introduced into the injection wells to maintain a constant head 8.7 cm bgs (below ground surface). Water was pumped from each extraction well at a constant rate of 0.05 L min-1 (in total 0.25 L min-1), creating an average groundwater velocity of 0.13 m d-1 (as determined by a tracer experiment within the cell), approximately the natural groundwater velocity at this site. Although the flow rate was constant, the water level in the extraction wells varied during the experiment, which caused the water table in the cell to fluctuate. The average hydraulic head in the middle of the cell was 0.48 m bgs.
Effluent Data. Figure 5A shows the concentrations of TCM, TCE, and PCE in the effluent from the cell. The cell effluent is the combined discharge from the five extraction wells, and its concentration would be equivalent to the measured concentration in a pumping well or a pumping well system that captured the groundwater plume emitted from a DNAPL source zone. Initially, concentrations of all components were low and increased as the dissolved plumes arrived at the extraction wells. TCM arrived first slightly ahead of TCE and peaked in concentration at about 50 d. TCE reached a peak in concentration at about 60 d. PCE continued to increase in concentration until about 100 d. The peak concentrations of TCE and PCE represented about 4% of their effective solubilities (597 000 and 112 000 µg/L, respectively) calculated on the basis of the initial DNAPL composition as released and Raoult’s law. The cell effluent concentrations are much lower than the effective solubility values due to dilution because the area of groundwater flow through the cell is much larger than the area of the DNAPL source zone. The peak concentration of TCM was about 2.5% of its effective solubility (1 016 000 µg/L). The fact that TCM peak was a lower portion of its effective solubility than TCE or PCE suggested some preferential depletion of TCM in the early stages of the experiment. The gradual increase in PCE concentrations from 50 to 100 d is likely due in part to greater sorption of PCE relative to TCM and TCE. In addition, PCE concentrations would have to increase due to the preferential depletion of TCM in the first 100 d, which would have increased the mole fraction of PCE in the DNAPL source zone and its effective solubility. VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The mass removed at time t was calculated as the sum of the three compounds of the time-integrated concentrations from time 0 to time t multiplied by the total water flow rate from the cell: 3
M(t) )
This initial depletion in TCM may have been a result of evaporation of TCM from the DNAPL during injection or from the subsurface when the water table in the cell declined. For some time during the experiment, the water table declined to the degree that a portion of the DNAPL source zone may have been above the water table. TCM has the highest vapor pressure of the three components and would have been lost preferentially. Preferentially loss of TCM would not be expected by volatilization of dissolved TCM from the groundwater because it has the lowest Henry’s constant of the three components. It is unlikely has TCM would have been removed preferentially by anaerobic biodegradation because conditions in the shallow Borden aquifer have been shown in other studies to be unfavorable. After about 50 d, the TCM concentration decreased over time, whereas the concentration of TCE fluctuated but remained at relatively high concentrations. PCE concentration continued to increase over time until day 220, but at a slower rate than in the first 100 d as PCE arrived at the extraction wells. This behavior is in accordance with the early part of the theoretical dissolution curve shown in Figure 1, specifically, a decline in TCM and gradual increase in PCE. The observations (shown in Figure 5A) were used to construct the observed relationships between source depletion and concentration ratios for the experiment that would be equivalent to the theoretical relationships as shown in Figure 2. The source depletion was calculated as the cumulative mass removed determined from monitoring of the cell effluent at time t divided by the initial source mass:
source depletion )
M(t) Mtotal
(2)
where M(t) is the mass removed up to time t, and Mtotal is the total mass. 320
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3
t
i)1
0
i
i)1
FIGURE 5. (A) Effluent concentration of TCM (9), TCE (]), and PCE ([) as a function of time. (B) Ratios of TCE and TCM (9), PCE and TCM ([), and PCE and TCE (2) as a function of source depletion. The solid lines show the theoretical curves for the three ratios.
t
∑ ∑ QC (t) dt ) Q∑ ∑ C (t) dt 0
i
(3)
where Q is the total effluent flow rate from the cell, Ci(t) is the concentration of compound i at time t, and i ) 1, 2, and 3 correspond to TCM, TCE, and PCE. In this case, the total mass, Mtotal, is known (7.7 kg equal to 5 L). The source depletion was calculated for each sampling time at which the dissolved concentration ratios were determined in the experiment. Figure 5B shows experimental data of concentration ratios versus source depletion (shown as data points) as compared to the theoretical relationship for concentration ratios versus source depletion (shown as curves). The theoretical curves were based on the initial DNAPL composition, uncorrected for any initial depletion of TCM. The observations are in reasonable accordance with the expected behavior. TCE/TCM and PCE/TCM ratios in the groundwater increased by about a factor of 5-10 times over the period of monitoring. The TCE/TCM and PCE/TCM ratios tend to be generally higher than the predicted curves, likely due to the apparent initial loss of some TCM by evaporation. Despite the apparent initial loss of TCM, the estimated source depletions based on the final TCE/TCM and PCE/TCM ratios were about 0.23-0.25 in comparison to the actual source depletion of about 0.23 obtained from the theoretical curve. The change in PCE/TCE ratio was much less, as predicted, because TCE concentrations did not decline and PCE concentrations did not increase substantially during the course of the experiment. The total DNAPL mass and initial DNAPL composition (or total mass of each component) were known in this case, but at a real site, this information would not be known. The DNAPL composition is needed to permit the calculation of the theoretical concentration ratio versus source depletion curves. DNAPL composition can be back-calculated using Raoult’s law from the initial dissolved concentrations measured in the groundwater. The total mass would be determined by iteratively specifying the total mass values used to calculate the source depletion for the different sampling times. The best estimate of the total mass would be that which yielded the best match between the observed data and the theoretical curves. Individual Sampling Points. Figure 6A shows the concentrations of TCM, TCE, and PCE as a function of time for a specific sampling point (SP 506). The individual sampling points are equivalent to monitoring where samples are obtained with, for example, a GeoProbe or from a multilevel sampling device. The concentrations exhibited a pattern similar to the effluent concentrations (Figure 5A), but the peak concentrations are higher and approach the calculated effective solubility values for each of the components. This illustrates that on a scale of a single sampling point, the DNAPL source upgradient undergoes dissolution under a condition of chemical equilibrium. Concentrations of similar magnitude and pattern were observed in a small number of other sampling points. In this case, the mass removed at the sampling point was not measured directly, as in the extraction wells, and must be estimated on the basis of an estimated mass flux associated with the monitoring location. Equation 3 is slightly modified (see eq 4) (the flow rate is replaced with the groundwater velocity multiplied by the porosity and an
FIGURE 6. (A) Concentration for sampling point 506 of TCM (9), TCE (]), and PCE ([) as a function of time. (B) Ratios of TCE and TCM (9), PCE and TCM ([), and PCE and TCE (2) as a function of source depletion. The solid lines show the best fit of the theoretical curves for the three ratios. area), so it can be used to calculate the mass removed during the monitoring period. 3
M(t) )
t
∑∑ i)1
0
t
nvACi(t) dt ) nvA
∑ ∑ C (t) dt 3 i)1
i
(4)
0
where n is the porosity, v is the groundwater velocity, and A is the (control) area. In this case, the area for a single sampling point is assumed to be 10 cm × 50 cm. Ten cm corresponds to the vertical distance between the sampling points, and 50 cm corresponds to the horizontal spacing. The groundwater velocity for each sampling point was determined from the tracer test prior to the solvent release. However, the velocities were corrected slightly because the average hydraulic conditions differed prior to the solvent release and after. Furthermore, the velocities were assumed to be constant after the solvent release, but of course they changed as a result of the changing water table and hydraulic gradient. Previously, the porosity of the aquifer has been measured to 0.33 (37). For each sampling time, the mass removed during the monitoring interval was calculated using eq 4. Thus, for each sampling time, the concentration ratios were plotted versus the source depletion calculated using the mass removed and an initial estimated of the total mass. Thus, the resultant data points were compared visually to the theoretical curves by adjusting the estimate of total mass (Mtotal). Figure 6B shows the best fit of the observations to the theoretical curves. The data and curves agree reasonably well for this specific sampling point. The curve matching in Figure 6B yielded a source depletion somewhat lower than the 0.23 determined from the cell effluent data. In general, it was not possible to match curves and estimate the mass upgradient the individual sampling points
FIGURE 7. (A) Concentration for the depth-integrated observations of sampling points 502-511 of TCM (9), TCE (]), and PCE ([) as a function of time. (B) Ratios of TCE and TCM (9), PCE and TCM ([), and PCE and TCE (2) as a function of source depletion. The solid lines show the best fit of the theoretical curves for the three ratios. for this field experiment, because there was too much scatter in the observations. The scatter was probably caused by the fluctuating water table and resultant changes in hydraulic gradient and small-scale changes in groundwater flow pattern. Such flow variations would probably cause the sampling points to reflect the concentrations and flow from different parts of the upgradient source. As a result, the mass flux calculated for a specific sampling point could reflect the flux from different parts of the source zone at different time. To overcome the effects of the data scatter for individual sampling points, depth-integrated and area-integrated dissolved concentrations were evaluated. Depth- and Area-Integrated Sampling Points. The observations of the individual sampling points in each sampling nest were depth integrated (averaged) from the first sampling point below the shallowest location of the water table to the deepest sampling point where dissolved concentrations were observed. The depth-integrated sampling points presented here would be equivalent to monitoring in a fully screened well. In a fully screened monitoring well, the resulting concentration is a mixture of groundwater contaminated to different degrees. Figure 7A shows the averaged concentrations for sampling points 502-511. The mass removed was calculated for the monitoring nest as previously described, and the observations were compared to the theoretical curves resulting in an estimate of the total mass upgradient of the sampling points 502-511. Figure 7B shows the best fit of the observations to the theoretical curves and reveals that reasonably good matches were achieved. The depth-integrated concentrations for sampling points 402-413 and for sampling points 602-611 resulted in breakthrough curves similar to the breakthrough curves for sampling points 502-511, which could be used to obtain VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Mass Estimation Upgradient of the Sampling Pointsa estimated sampling Vaverage “mass” points (cm d-1) (g L-1 d-1) 302-314 402-413 502-511 602-611 702-710 sum all
11.3 10.4 10.3 9.4 9.0
17b 140 360 200 1.5c
10.2
120
area (cm2) 50 × 88 50 × 78 50 × 58 50 × 58 50 × 48
total mass (kg)
0.28 1.9 3.5 1.8 0.011 7.5 50 × 330 6.7
(L) 0.18 1.2 2.3 1.2 0.007 4.9 4.3
(%)
DNAPL mapping (%)
3.7 13.9 25.4 26.9 46.7 40.6 24.0 14.5 0.15 4.0 100 100
a
Vaverage is the average groundwater velocity for the sampling points based on the tracer experiment. The calculations are explained in the text. The total mass was converted from kg to L via the initial density of the solvent mixture of 1.54 kg L-1. b The best mass estimate was obtained for an initial composition of 0.3% of TCM, 24.9% of TCE, and 74.8% of PCE. c The best mass estimate was obtained for an initial composition of 5.3% of TCM, 42.1% of TCE, and 52.6% of PCE.
FIGURE 8. (A) Concentration for the depth-integrated observations of sampling points 302-314 of TCM (9), TCE (]), and PCE ([) as a function of time. (B) Ratios of TCE and TCM (9), PCE and TCM ([), and PCE and TCE (2) as a function of source depletion. The solid lines show the best fit of the theoretical curves for the three ratios. reasonable mass estimates. These three sampling nests are located directly downgradient from the DNAPL source zone. Figure 8A shows the depth-integrated concentrations for sampling points 302-314 located at the lateral margin of the DNAPL source zone. The breakthrough curves do not look like the expected dissolution curves shown in Figure 1. The observed TCM concentration is relatively low, and the concentrations of both TCE and PCE seem to increase significantly after about 175 d. Figures 8B shows the best matches of the observations to the theoretical curves to obtain a mass estimate. To obtain even approximate matches, the initial composition of the solvent mixture required to calculate the theoretical curves needed to be adjusted to 0.3% of TCM, 24.9% of TCE, and 74.8% of PCE to match the initial measured dissolved concentration ratios. Despite this adjustment, the concentration ratios between TCE and TCM, and PCE and TCM, do not follow the theoretical curves, whereas the concentration ratio between PCE and TCE does at least at the part of the curve where the source depletion exceeds 0.3. These discrepancies are likely due to the relatively low TCM concentration as compared to the concentrations of TCE and PCE, so the uncertainty of the ratios including TCM is relatively high. The depth-integrated concentrations for sampling points 702-710, located also at the lateral margin of the DNAPL source zone, behaved similarly to the sampling points 302-314, and to obtain even approximate matches the initial composition of the mixture needed to be adjusted to 5.3% of TCM, 42.1% of TCE, and 52.6% of PCE. The increases in concentrations at later time suggest that DNAPL may have moved into the areas upgradient of sampling locations 302-314 and 702-710 during the course of the experiment. The fact that the initial composition of the solvents had to be changed to obtain reasonable matches suggests that TCM may been depleted during this migration 322
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of the DNAPL relative to its initial composition. This depletion may have resulted from dissolution during migration of the DNAPL to its final location at the margins of the DNAPL zone, or by volatilization to the vadose zone at times when the water table elevation was low. The estimated masses (in units of g L-1 d-1) for the five sampling nests downgradient of the DNAPL source zone and for which significant dissolved concentrations were observed are listed in Table 2. In the same table, the corrected average groundwater velocity for the representative sampling points and the areas are listed. The areas begin at the depth of the average water table after the solvent release and end at the deepest sampling point where dissolved concentrations were observed. The total mass was then calculated using eq 4 (the estimated mass shown in Table 2 corresponds to the sum of the integrals in eq 4). The total mass upgradient the five sampling nests was estimated to be 7.5 kg or 4.9 L of solvent (using an average density of the solvent mixture of 1.54 kg L-1), which is close to the released 5 L. Finally, the concentrations observed at all of the contaminated sampling points (302-314, 402-413, 502-511, 602-611, and 702710) were averaged. The resulting average concentration data are shown in Figure 9A, and the best match of the ratios as a function of source depletion is shown in Figure 9B. The total mass was estimated to 6.7 kg or 4.3 L (see Table 2). The total source zone mass estimates using the depth-integrated and area-integrated data are both close to the known total initial source mass. Any error in the estimated total mass that might be associated with the poor matches of the data from sampling locations 302-314 and 702-710 is small because the source mass estimates for those locations represented less than 5% of the total estimated source mass. Comparison of the Estimated Masses and the Location of the Solvent Residuals. The result of the mapping of the solvent residuals has been presented in ref 1. On the basis of the observed volume of solvent residuals contaminated soil, the relative distribution of the solvent residuals upgradient of the sampling nests were estimated. The numbers are shown in Table 2 as a comparison to the distribution based on the multiple-component dissolution theory. The dissolution theory results in mass estimates representing the initial conditions, whereas the DNAPL mapping represents the solvent residual distribution at the time of the excavation. The numbers based on the dissolution theory compare reasonably well to the numbers based on the DNAPL mapping, except for sampling points 302-314 and sampling points 702-710. A better comparison may have been obtained with more detailed measurements of the solvent residuals in the subsurface, because the solvent residuals were not evenly distributed in the soil. However, as discussed previously (1), the number of soil samples obtained during the
FIGURE 9. (A) Concentration for the area-integrated observations of all of the sampling points of TCM (9), TCE (]), and PCE ([) as a function of time. (B) Ratios of TCE and TCM (9), PCE and TCM ([), and PCE and TCE (2) as a function of source depletion. The solid lines show the best fit of the theoretical curves for the three ratios. excavation was not sufficient for precise mass calculations, particularly for the pool located 55 cm bgs. The reasonable accordance between the actual distribution of solvent residuals and the estimated relative masses confirms that the dissolution theory results in reasonable mass estimates. Application. The experiment has shown that multiplecomponent dissolution theory has the potential be a valuable tool to estimate the mass of multicomponent DNAPL residuals in the saturated zone. It is the only method that only requires periodic measurement of the dissolved concentrations of the contaminants for a sufficient time period, in which there are observed changes in dissolved concentration ratios. For the period over which changes in dissolved concentrations are observed, there must be an estimate of the cumulative mass flux from an array of monitoring wells or mass removed from an extraction well. The necessary time period depends on the amount and the composition of the DNAPL source but is probably several years in many circumstances. Suitable groundwater monitoring data for application of this technique may already exist at many DNAPL sites, particularly from groundwater pumping wells within or near suspected source zones. Specific knowledge of the initial composition of the DNAPL source by direct sampling is not necessary. The dissolved phase composition at the time of the first observation can be used to estimate the composition at that time, and the remaining calculations are with respect to that time. Theoretical dissolved concentration ratios versus source depletion curves are prepared using the DNAPL composition and are compared to the field data. The field data include the dissolved concentration ratios and cumulative mass fluxes or mass removed. The cumulative mass flux or mass removed is expressed as estimated source depletion values using an estimate of the total source mass, and these data are then compared to the theoretical curves.
The estimated total source mass is adjusted to provide the best fit between the field data and the theoretical curves. The method is not applicable generally for degradable contaminants. However, if the degree of degradation in the groundwater between the source zone and the monitoring location could be quantified adequately, for example, using a first-order decay rate, then the mass flux calculated, the mass removed, and the concentration ratios at the monitoring location would have to be corrected to account for the mass lost by degradation. From this field experiment, it is evident that the cell effluent concentration ratios resulted in data that provided the best match to the theoretical relationships predicted by multiple-component dissolution theory. Because the cell effluent data correspond to data from a pumping well which capture the dissolved plumes emitted from a DNAPL source zone, it is expected that data obtained from pumping wells would be most suitable to fit with the theoretical relationships because most or all of the mass flux emitted from the source zone will be captured and the mass removed will be determined directly from concentration and pumping rate data. Although dissolved concentrations will be diluted in a pumping well, dissolved concentration ratios will not be influenced by dilution effects so that the technique is applicable. However, the monitoring data from a pumping well do not give any additional information of the vertical or horizontal distribution of the DNAPL, which may be valuable in source zone characterization. Another possibility is the estimation of the source mass from arrays of monitoring points, for example, GeoProbe or multilevel sampling points. Individual sampling points may give more detailed knowledge of the spatial distribution of DNAPL residuals and of the corresponding mass. However, as the results presented in this paper have indicated, it may be more difficult to obtain appropriate data because the observations obtained in monitoring wells may be influenced by changing directions and rates of groundwater flow both vertically and horizontally. In addition, many sampling points might be required and much groundwater flow data would be necessary for a large DNAPL source zone to determine the mass flux for application of this technique. As shown for the depth-integrated data presented in this paper, it results in suitable data because the data are less influenced by temporal fluctuations in groundwater flow.
Acknowledgments This work was supported by the University Consortium Solvents-in-Groundwater Research Program which is sponsored by the Boeing Co., Ciba-Geigy, Eastman Kodak Co., General Electric, Laidlaw Environmental Services, Mitre Corp., Motorola, PPG Industries, United Technologies, the Ontario Research Incentive Fund, and the Natural Sciences and Engineering Research Council of Canada. The fieldwork by Paul Johnson, Bert Habicher, Andre Unger, Scott Vales, Sam Vales, and Mette M. Broholm, and the laboratory work by Dave Baerg and Jeff Murphy, Waterloo Centre for Groundwater Research, is greatly appreciated.
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Received for review September 29, 2003. Revised manuscript received September 29, 2004. Accepted October 13, 2004. ES0306462
