Essentially, seven studies were performed in this test cell. Initially, the formation was sampled by soil coring during the installation of wells and multilevel samplers. The core samples were analyzed for a group of chemicals initially believed to be representative of the site and of regulatory concern. The selected chemicals represented several classes of contaminants. Following the installation of the instrumentation, the formation was allowed to re-equilibrate before any additional measurements were made. Ground-water samples were collected under both static and dynamic conditions. Injecting uncontaminated ground-water into the formation simulating a forced gradient flush or water flood created the dynamic flow conditions. The static samples would be representative of the maximum concentration that would be anticipated in water samples from this location. Ground-water sampling was followed by a pre-remediation tracer study. The pre-remediation tracer study was designed to estimate the total mass of NAPL in the formation. Once the pre-remediation tracer study was completed, the remedial technology was implemented and its performance monitored. At the termination of the remedial demonstration, the three characterization steps were repeated in reverse order. Analysis of this data permits an evaluation of the technology.
The cosolvent flush took place over a ten-day period followed by 20 days of flushing with water. The flow rate through the cell was approximately 3 l/min. This was equivalent to pumping approximately one pore volume through the cell per day. This flow rate produced an average hydraulic gradient of 0.05 or 0.2 m across the cell. The properties of the flushing fluid are shown in Table 4.2.0.1. Two different solution compositions were used initially ethanol (72% by volume) was injected for approximately two days by gradually increasing the percentage of remedial fluid over the first 24 hr period in an attempt to minimize the effects of density. After two days of injection the mixture was modified such that it was approximately 70% ethanol, 12% n-pentanol, and 18% water by volume. After approximately 9.5 days, the pentanol was removed from the flushing solution and the original mixture was used to remove the n-pentanol. During the final day of remedial fluid injection, the concentration of water was ramped up to 100% water. The ramping was performed in an effort to minimize the unstable flow conditions generated by the viscosity and fluid density differences.
Table 4.2.0.1 Properties of Cosolvents
| Property |
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| Density (g/cm3) |
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| Viscosity (cP @ 25oC) |
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|
| Boiling Point (oC) |
|
|
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| Flash point (oC) |
|
|
|
| Molecular weight (g/mol) |
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| Aqueous solubility (wt % @ 25oC) |
|
|
|
Table 4.2.0.2 Operating conditions for the experiments
| Parameter |
|
||
| Pre-Remediation Tracer | Demonstration | Post-Remediation Tracer | |
| Average Saturated Thickness (m) | 1.5 | 1.5 | 1.5 |
| Average Gradient Across Cell (m/m) | 0.05 | 0.07 | 0.05 |
| Average Influent Flow rate (lpm) | 3.2 | 3 | 3.3 |
| Average Effluent Flow rate (lpm) | 3.2 | 3 | 3.3 |
Table 4.2.0.3 Study sequence
| Test | Activity | Fluid Injected | Total Flux Rate (l/m)- Volume (l) | Duration |
| Core and install instrumentation | Collect soil samples | None | 6/29/94 - 7/8/94 | |
| Ground-water sampling | Collect water samples | None | 7/8/94 | |
| Pre-remediation tracer | Establish flow field | Water | 3.2 | 9/21/94 |
| Inject tracer suite 1 | Water with tracers | 3.2 | 9/23/94 | |
| Maintain flow field | Water | 3.2 | 10/4/94 | |
| Remedial technology | Establish flow field | Water | 3 | 4/11/95 |
| Inject remedial fluid | Ethanol-pentanol-water | 3 | 4/12/95-4/23/95 | |
| Remove pentanol | Ethanol-water | 3 | 4/23/95-4/25/95 | |
| Remove ethanol | Water | 3 | 4/25/95-5/20/95 | |
| Post-remediation tracer | Establish flow field | Water | 3.3 | 6/1/95 |
| Inject tracer suite 2 | Water with tracers | 3.3 | 6/2/95 | |
| Maintain flow field | Water | 3.3 | 6/13/95 | |
| Ground-water sampling | Collect water samples | None | 6/16/95 | |
| Core | Collect soil samples | None | 6/15/95-6/18/95 |
The decane removal pattern is quite similar to that for dichlorobenzene (Figure 4.2.1.2 decane). After remediation, decane was present at a couple of locations within the upper portion of the profile but most samples were at or below the detection limits. Again, there was a significant amount of decane left near the bottom of the aquifer. There are two possible explanations for having difficulty in removing contaminants near the clay. The ethanol/pentanol flushing fluid is less dense than water. One potential explanation would be that the remedial fluid was not effectively delivered to the lower portion of the aquifer because of the density. Another explanation would be that undulations in the clay/sand interface created stagnant pools or regions that were not perfused by the remedial fluid. The removal percentage is in the range of 96.1% to 98.8%. The geostatistical approach gave the lowest removal fraction. Four out of five methods suggested 98% or greater removal.
Naphthalene was also effectively removed through the majority of the profile (Figure 4.2.1.3 naphthalene). The percentage of samples that were below detection limits and also below the clay interface appeared to increase. However, one should not conclude that this was actual removal because the pattern with the other chemicals would suggest that the remedial fluid did not sweep this volume of the profile. The removal percentage is in the range of 83.4% to 88.2%. Only two of the samples contained naphthalene in quantifiable concentrations. The low initial concentration and the way we assumed the concentration was 0.5 times the quantification limit if no quantifiable contaminant was present biases the reporting of removal efficiency for naphthalene.
Only small amounts of 1,1,1-trichloroethane (TCA) were present in the profile initially, as can be seen in Figure 4.2.1.4 1,1,1-trichloroethane. Statistically, one would say that there was good removal. The variation in the numbers plotted is due to the size of the samples used in the analysis. Since none of the samples have quantifiable 1,1,1trichloroethane at the end of the remediation, one might suggest 100% removal. This is not likely correct, but the amount remaining is very small. Based on detection limits the TCA removal efficiency is probably greater than 99%.
Toluene was not as effectively removed as the other chemicals, as can be seen in Figure 4.2.1.5 toluene. There were several occasions where toluene concentrations in the post-remediation samples were in the same range as the concentrations in the pre-remediation samples. All of the core analyses were performed on the same set of core samples. Toluene should be no more difficult to remove than the other chemicals included in the discussion. No hypothesis is proposed to explain this lower removal efficiency. The observed removal percentage ranges from 94.5% to 99.7%. The geostatistical approach gave the highest removal fraction.
The only contaminant that quantitatively did not remove 95% was naphthalene and there was not sufficient naphthalene in the system to suggest that 95% had been removed. Generally the naphthalene was below the quantification limit or there did not appear to be a significant change in the concentration. This would suggest that where the remedial fluid came in contact with the contamination sufficient remedial fluid was used to remove the NAPL.
Table 4.2.1.1 Summary of core data based on boxcar averaged results
| Chemical | Pre-Remediation Concentration (mg/kg) | Post-Remediation Concentration (mg/kg) | Fraction Removed |
| dichlorobenzene | 30 | 1.2 | 0.96 |
| 1,1,1-trichlorethane | 0.34 | 0.000054 | 0.99+ |
| toluene | 1.7 | 0.15 | 0.91 |
| o-xylene | N.D.* | N.D | |
| m-xylene | N.D. | ||
| naphthalene | 2.2 | 0.25 | 0.89 |
| trimethylbenzene | N.D | N.D | |
| decane | 72 | 2.3 | 0.97 |
| undecane | N.D | N.D | |
| ethylbenzene | N.D | N.D |
Table 4.2.2.1 Summary of tracer activities
| Tracer |
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Total Mass Injected (g) |
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| Bromide |
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| 2,2-dimethyl-3-pentanol |
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| n-hexanol |
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| n-pentanol |
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| Tracer Volume (l) |
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| Injection Time (min) |
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* Based on the zero moment of extrapolated data.
A suite of chemicals was injected, both pre-remediation and post-remediation,
to characterize the mass and distribution of the contaminants. The experimental
tracer breakthrough curves (BTCs) collected prior to remediation from the
extraction wells are shown in Figures 4.2.2.1 to 4.2.2.12. Since
the partition coefficient for the 2,2-dimethyl-3-pentanol is the largest
of the tracers used, this tracer provides the greatest separation
between the retarded and non-retarded tracer and should provide the most
reliable data. All of the results are summarized in Table 4.2.2.2 for extraction
well 1, Table 4.2.2.3 for extraction well 2, and Table 4.2.2.4 for extraction
well 3. The pre-remediation NAPL saturation (volume fraction) based on
2,2-dimethyl-3-pentanol data, ranges from 0.052 to 0.087 for the volume
sampled by well 1, 0.069 to 0.121 for well 2, and 0.085 to 0.131 for well
3. Moment analysis consistently gives the lowest saturation of the three
methods presented. The one-dimensional-connective-dispersive model (CDM)
consistently gave the lowest value. The mean residence time for bromide
in extraction well 1 ranged from 1.4 days using the method of moments and
the extrapolated data (note: the mean residence time = first moment/zero
moment minus 0.5*the pulse duration) to 0.72 days for an unconstrained
fit to the CDM. The modeling results suggest that the residence time
for bromide is approximately 1/2 the time suggested by the method of moments.
Post-remediation residence time for methanol was less than the pre-remediation
residence time. It is not reasonable to make conclusions about remediation
based on changes in residence times of the non-retarded tracers since minor
changes in the flow volume or flow rate could significantly influence
the interpretation. The modeling estimates of the NAPL saturation
are consistent with each other but the pre-remediation data gives a greater
estimate of NAPL saturation than the moment method. Qualitatively
looking the fit of the model to the data (Figures 4.2.2.1 and 4.2.2.2)
the one-dimensional-convective-dispersive model (CDM) appears to describe
the data better than the stochastic model. The stochastic model suggests
an earlier rise in concentration than is predicted by the CDM. Post-remediation
estimates are quite similar, however, the stochastic model still gives
a poor fit to the data. The apparent problem is the assumed distribution
function for the flow times. A single log normal distribution does
not appear to be correct.
Table 4.2.2.2. Summary of Extraction Well 1 tracer analysis
| Extraction Well 1 |
|
|
|||||
| conservative (bromide) | 2,2-dimethyl
-3-pentanol |
NAPL Saturation | conservative (methanol) | 2,2-dimethyl
-3-pentanol |
NAPL Saturation | ||
| Pulse duration (days) |
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| zero moment data |
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| zero moment extrapolated |
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| first moment / zero moment data (days) |
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| first moment extrapolated / zero moment (days) |
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| Convective Dispersive Model |
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| dispersivity (m) |
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| est. initial conc. (mg/l) |
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| mean time of travel (days) |
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| Stochastic model |
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| variance in travel time |
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| travel time (days) |
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| est. initial conc. (mg/l) |
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Figure 4.2.2.1 Extraction
Well 1 Pre-remediation Tracer analysis 2,2-dimethyl-3-pentanol
Figure 4.2.2.1.b Extraction
Well 1 Pre-remediation Tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.2.2.2 Extraction
Well 1 Post-remediation tracer analysis
2,2-dimethyl-3-pentanol
Figure 4.2.2.2.b Extraction
Well 1 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.2.2.3 Extraction
Well 1 Post-remediation tracer analysis hexanol
Figure 4.2.2.3.b Extraction
Well 1 Post-remediation tracer analysis hexanol log
Figure 4.2.2.4 Extraction
Well 1 Post-remediation methylheptanol
Figure 4.2.2.4.b Extraction
Well 1 Post-remediation methylheptanol log
Table 4.2.2.3. Summary of Extraction Well 2 tracer analysis
| Extraction Well 2 |
|
|
|||||
| conservative (bromide) | 2,2-dimethyl
-3-pentanol |
NAPL Saturation | conservative (methanol) | 2,2-dimethyl
-3-pentanol |
NAPL Saturation | ||
| Pulse duration (days) |
|
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| zero moment data |
|
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| zero moment extrapolated |
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| first moment / zero moment data (days) |
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| first moment extrapolated / zero moment (days) |
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| Convective Dispersive Model |
|
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| dispersivity (m) |
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| est. initial conc. (mg/l) |
|
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| mean time of travel (days) |
|
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| Stochastic model |
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| variance in travel time |
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| travel time (days) |
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| est. initial conc. (mg/l) |
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Figure 4.2.2.5 Extraction
Well 2 Pre-remediation tracer analysis 2,2-dimethyl-3-pentanol
Figrue 4.2.2.5.b Extraction
Well 2 Pre-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.2.2.6 Extraction
Well 2 Post-remediation tracer analysis
2,2-dimethyl-3-pentanol
Figure 4.2.2.6.b Extraction
well 2 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.2.2.7 Extraction
Well 2 Post-remediation tracer analysis hexanol
Figure 4.2.2.7.b Extraction
Well 2 Post-remediation tracer analysis hexanol log
Figure 4.2.2.8 Extraction
Well 2 Post-remediation methylheptanol
Figure 4.2.2.8.b Extraction
Well 2 Post-remediation methylheptanol log
Table 4.2.2.4. Summary of Extraction Well 3 tracer analysis
| Extraction Well 3 |
|
|
|||||
| conservative (bromide) | 2,2-dimethyl
-3-pentanol |
NAPL Saturation | conservative (methanol) | 2,2-dimethyl
-3-pentanol |
NAPL Saturation | ||
| Pulse duration (days) |
|
|
|
|
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| zero moment data |
|
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| zero moment extrapolated |
|
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| first moment / zero moment data (days) |
|
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| first moment extrapolated / zero moment (days) |
|
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| Convective Dispersive Model |
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| dispersivity (m) |
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| est. initial conc. (mg/l) |
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| mean time of travel (days) |
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| Stochastic model |
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| variance in travel time |
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| travel time (days) |
|
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|
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| est. initial conc. (mg/l) |
|
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Figure 4.2.2.9 Extraction
Well 3 Pre-remediation tracer analysis
2,2-dimethyl-3-pentanol
Figure 4.2.2.9.b Extraction
Well 3 Pre-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.2.2.10 Extraction
Well 3 Post-remediation tracer analysis
2,2-dimethyl-3-pentanol
Figure 4.2.2.10.b Extraction
Well 3 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.2.2.11 Extraction
Well 3 Post-remediation tracer analysis hexanol
Figure 4.2.2.11.b Extraction
Well 3 Post-remediation tracer analysis hexanol log
Figure 4.2.2.12 Extraction
Well 3 Post-remediation methylheptanol
Figure 4.2.2.12.b Extracton
Well 3 Post-remediation methylheptanol log
Data collected during the remedial activity included flux through each of the wells and the concentration of the contaminants and precursors (ethanol and pentanol) in the effluent stream. This information was used to quantify the mass of contaminant removed from the system. Six figures are available to display the results (Figures 4.2.3.1 through Figure 4.2.3.6). The upper portion of the figure is the concentration versus time for each of the wells for a given chemical. The lower portion of the figure is the fluid flux passing through the extraction well. By multiplying the flux by the concentration one can calculate the mass of chemical extracted for each of the wells. This is printed at the bottom of each of the figures. Figure 4.2.3.1 (Extraction well data for dichlorobenzene) suggests that 1330g of dichlorobenzene were removed during the enhanced extraction process. This can be compared to the mass originally contained in the test cell which ranges, depending on the estimation technique from 1200g to 3300g. The removal fraction using the initial mass based on core data would be 60% to 110%. The inverse distance squared estimate of mass appears to be giving a much greater mass estimate than the other four methods. If this were not included in the estimate, the worst performance would be changed to range from 89% to 110%.
Decane removal is shown in Figure 4.2.3.2 (Extraction well data for n-decane ). The data suggest that 1860 grams of decane were removed. The core data suggest that originally there was 2800 to 3700 g of decane in the test cell. This ignores the mass estimated using inverse distance squared, which gives a much higher estimate and does not appear to be consistent with the other estimation methods.. Based on core data for an initial estimate of contaminant mass and elution data for the mass removed, the percentage removal appears to be 50% to 66%. The volume used to calculate the initial mass of contamination in the cell includes the volume in the irregularities along the cell sides. This volume was not likely well swept by either the remedial fluid or the tracers. All of the core samples were collected within the swept volume. The data could be made to appear better if one only selected a portion of the total volume that was believed to be swept by the remedial fluid. However, it would be difficult to verify what volume should actually be used since there were no samples taken to evaluate movement at the edge of the test cell.
The remaining figures are presented since the data were collected. There is no core data collected for the initial soil mass of undecane or trimethylbenzene. The only method for approximating the initial soil mass would have to be based on an estimate based on a ratio of undecane or trimethylbenzene to decane that was quantified using samples of NAPL collected prior to the remediation. This information is available in Rao et al, 1997. Figure 4.2.3.4 Extraction well data for n-undecane Figure 4.2.3.6 Extraction well data for trimethylbenzenene shows the elution of these two chemicals.
Figure 4.2.3.5 Extraction well data for pentanol and Figure 4.2.3.3 Extraction well data for ethanol allows one to estimate the mass of the remedial fluids collected.
Table 4.2.3.1 Table Summary of the extraction of contaminant from the profile
| Chemical | Initial mass* (g) | Mass extracted (g) | Fraction removed |
| dichlorobenzene | 1300 | 1334 | 1.03 |
| n-decane | 2800 | 1861 | 0.66 |
| n-undecane | N.D. | 3611 | - |
| trimethylbenzene | N.D. | 385 | - |
| pentanol (injected) | 3889000 | ||
| ethanol (injected) | 31991000 |
*Initial mass estimate based on geotatistical analysis
