4.8 Steam (Cell 7)

Applied Research Associates, in cooperation with Praxis Environmental, operated the steam extraction cell. The cell layout is shown in Figure 4.8.0.1.  The test cell was approximately 4.3 m by 3.5 m with four injection wells labeled 274x, three extraction wells labeled 275x and twelve multilevel-samplers labeled 271x, 272x, and 273x. The injection wells were along the south side of the cell and the extraction wells were along the north side of the cell. The well labels match the location information in the database attached in the supporting data section. The elevations, indicated in red, are the elevation of the top of the device.  In addition to the four injection wells that were used during the tracer studies, there was an additional injection well (2745) near the center of the cell used to inject steam and small diameter wells, labeled 276x, to measure the head during the experiments.

Essentially, seven studies were performed. Initially, the formation was sampled by coring the formation 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 groundwater 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 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 objective of the study was to inject steam at a sufficient rate as to create an oil bank at the steam condensation front.  The oil bank would move to the extraction wells where it could be pumped to the surface. After moving the oil bank to the extraction well, the remainder of the volatile and semi-volatile contaminants would be removed by distillation.  During the remedial activities the water table was lowered to near the clay interface.  Initially, the system was operated as an SVE system with the injection well intermittently either closed or open. After initiation of steaming, it took from 11 to 19 hours of injecting 100 kg steam per hr for steam to reach the extraction wells. The formation was sufficiently permeable that the formation of an "oil bank" was not observed. Steam and liquid were extracted from these wells through-out the steaming cycle.  By observing the temperature front, it appeared that the steaming essentially operated effectively as a distillation column volatilizing chemicals and moving them from the injection point to the extraction points.  The removal efficiency should thus be best near the center of the cell.   A  total of 7600 kg of steam was injected during the test and 2540 kg of water removed  during the steam test in the form of condensate.  The condensate included approximately 8 l of NAPL.

 Table 4.8.0.1 Operating Conditions for the Experiments
Parameter
Activity
Pre-Remediation Tracer Demonstration Post-Remediation Tracer
Average Saturated Thickness (m)  3  2  3
Average Head Across Cell (m)    N.D.  0.3
Average Influent Flow rate (lpm)  3.5  1.6  3.5
Average Effluent Flow rate (lpm)      

Table 4.8.0.2 Study Sequence
Test Activity Fluid injected  * Total flux rate (l/m)- volume (l) Duration
Core and install instrumentation Collect soil samples None    11/11/95 - 11/14/95
Ground-water sampling Collect water samples None    no date in data base
Pre-remediation tracer Establish flow field Water    
  Inject tracer suite 1  ? ?   4/25/96
  Maintain flow field Water  ?  4/25/96 - 5/4/96
Remedial technology Inject steam  Steam  No flux in data base  9/11/96 - 10/3/96
Post-remediation tracer Establish flow field  Water    ?
  Inject tracer suite 2  meth, dmp, methep  4.59  10/9/96 
  Maintain flow field  Water  4.59  10/9/96 - 10/17/96
Ground-water sampling Collect water samples None    no date in data base
Core  Collect soil samples None   11/7/96 - 11/8/96 

* meth = methanol, dmp = 2,2-dimethyl-3-pentanol, methep = 6-methyl-2-heptanol

4.8.1  Core Analysis

The steam remediation reduced the level of the selected contaminants by more than an order of magnitude in the upper 40% of the target zone but very little removal in the lower portion of the target zone and some accumulation of the less volatile compounds near the bottom of the targeted region. Dichlorobenzene is typical of the results for very volatile compounds with 67% overall removal but essentially no removal near the bottom of the aquifer.  The figures plot the concentration of the individual contaminants as a function of depth without regard for spatial location within the test cell.  Black data were the data collected prior to remediation and the blue data were the observations collected after remediation.  The vertical bars are the means for the values over the bar's depth zone;  95% confidence limits are plotted, where possible, with the limits represented by the small + at the center of the bar.  At the top of the figure, several different methods are reported to calculate the total mass in the target zone of the test cell.  The two magenta lines depict the vertical extent of the target zone and the surface area within the cell is printed at the top of the figure.  M-xylene showed a 84% removal, and 79% of the o-xylene was removed.  The core data suggest that 98% of the  1,1,1-trichloroethane  initially in this cell was removed.  Approximately 85% of the naphthalene  was removed. Approximately 78% of  trimethylbenzene and 92% of the  toluene were removed.  No post-remediation data is available for the ethylbenzene. Decane results are typical for the semi-volatile substances. In this study, approximately 72% of the decane was removed.  Much of the decane was removed from the upper half of the profile with little removal  near the center and accumulation in the lower sampling interval of the target zone. Undecane another semi-volatile compound had 73% removal with a removal pattern similar to decane. The overall removal effectiveness was quite good, however, the removal was much better at the top of the profile. Removal efficiency decreases in the lower portion of the profile.  Due to the density effects of the steam and the variability in water saturation it is likely that the steam was overriding the contamination  near the bottom of the profile.  The steam likely had to heat the lower portion of the aquifer by conduction rather than actual mass flux of steam.  To be considered a top performing technology, one would want to remove the contamination from the bottom of the profile as well as the top of the profile.  As demonstrated, steam was ineffective at removing the contamination from the bottom of the profile.  To be considered for implementation one would either have to consider steam as a part of a treatment train or significantly more steam would be required to remove contaminants lower in the profile.  Since it appears that the very volatile compounds were being removed in the lower portion of the profile, it is likely that additional steaming might have been effective in sufficiently increasing the temperature in the lower portion of the profile to volatilize these compounds as well. Careful analysis of the temperature profiles and elution curves would be required to adequately evaluate this potential.

Table 4.8.1.1 Summary of Core Data Based on Boxcar Averaged Results
Chemical Pre-Remediation Concentration (mg/kg) Post-Remediation Concentration (mg/kg) Fraction Removed
dichlorobenzene 11 3.6 0.67
1,1,1-trichlorethane 0.77 0.017 0.98
toluene 2.3 0.18 0.92
o-xylene 3.8 0.79 0.79
m-xylene 1.8 0.28 0.84
naphthalene 1.8 0.52 0.71
trimethylbenzene 2.4 0.54 0.78
decane 25 7 0.72
undecane 59 16 0.73
ethylbenzene 0.82 N.D.*  
* N.D. Not determined

4.8.2 Tracer Analysis

A first step in the evaluation of tracer data is to evaluate the conservation of mass.  Significant questions arise in the validity of the data when there is a significant difference in the mass injected versus the mass extracted.  Data is not yet available in the database to evaluate the mass of tracer injected in the pre-remediation study.  There are, however, data available to evaluate mass balance closure in the post remediation tracer study.  For the post-remediation study, 99% of the retarded tracer 2,2-dimethyl-3-pentanol (DMP), was recovered while only 60% of the nonretarded methanol was recovered.  The closure for DMP was excellent showing a carefully performed test, however, a significant portion of the methanol was lost.  Methanol was probably a poor selection for a tracer post-remediation due to the elevated temperature and the injection of oxygen with the steam during remediation potentially stimulating biological activity.   In general, the tracer data showed no mass removal of the NAPL and possibly an increase in NAPL mass.  The mass detected by the tracer at the end of the study was actually more than the mass prior to remediation.  Since the analysis was performed based on the assumption of a true conservative tracer with no transformation or loss, the moment analysis significantly underestimates the moments for the conservative component while the moment analysis for the retarded tracer appears to be conservative (mass recovered equal to the mass injected).  The result is overestimating the NAPL saturation in the post-remediation analysis.  It is possible to reanalyze the data assuming a transformation function such as first order such that the mass removed is equal to the mass injected, however, there was no data collected to verify an appropriate transformation function.  It would be necessary to assume something like a first-order transformation.  It should be noted that bromide was the non-retarded tracer on two out of three pre-remediation tracer breakthrough curves that should behave conservatively. The shape of the breakthrough curve for extraction well 3 does not follow the shape of the simple models used for regression analysis and the models are not likely to give results that are appropriate. The initial estimates, based on the moment analysis of mass, are likely more accurate than the post-remediation estimates of mass based on moment analysis.  Even with well 2, where the same tracers were used, both pre- and post-remediation moment analysis suggests an increase in mass by a factor of two.  The average saturated thickness used in the tracer experiments was the same 3 m.  Based on the target zone from the core data, the tracer experiments should have been run with a saturated thickness of 6 m.  As indicated in the discussion of the core data, it appears that the removal was from the upper portion of the profile with potential accumulation of NAPL in the lower portion of the profile.  The apparent increase in NAPL saturation, based on tracer analysis, may be a reality in the portion of the profile where the tracer experiment was performed.  There was a total net removal of contaminant mass but simultaneously there appears to have been an accumulation of NAPL mass near the clay interface where the tracer tests were performed.
 

Table 4.8.2.1 Summary of the Tracer Activities
Tracer Pre-remediation Post-Remediation
  Concentration (mg/l) Total Mass Injected (g) Mass recovered 
(g) *		 Concentration (mg/l) Total Mass Injected (g) Mass recovered 
(g) *
Bromide            
2,2-dimethyl-3-pentanol        364  367  363
Methanol        1241  1220  734
6-methyl-2-heptanol        210    
Tracer Volume (l)    980
Injection Time (min)    279
* Based on zero moment of extrapolated data

Table 4.8.2.2 Summary of Extraction Well 1 Tracer Analysis
Extraction Well 1
Pre-Remediation
 Post-Remediation
  Conservative (bromide) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation Conservative (methanol) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation
Pulse duration (days)  0.150  0.150    0.194  0.194  
zero moment data  46.088  110.815    143.003  65.790  
zero moment extrapolated  47.266  120.230    151.959  72.507  
first moment / zero moment data (days)  2.054  2.993  0.064  1.615  3.015  0.124
first moment extrapolated / zero moment (days)  2.199  3.315  0.090  1.800  3.743 0.153
Convective Dispersive Model      0.068      0.106
dispersivity (m)  1.815  0.695    0.801  0.933  
est. initial conc. (mg/l)  276  599    770  343  
mean time of travel (days)  1.211  1.823    1.306  2.337  
Stochastic model      0.064      0.107
variance in travel time  0.767  0.519    0.562  0.603  
travel time (days)  1.229  1.814    1.325  2.376  
est. initial conc. (mg/l)  261  586    763  342  

Figure 4.8.2.1  Extraction Well 1 Pre-remediation 2,2-dimethyl-3-pentanol
Figure 4.8.2.1.b  Extraction Well 1 Pre-remediation 2,2-dimethyl-3-pentanol log
Figure 4.8.2.2  Extraction Well 1 Post-remediation 2,2-dimethyl-3-pentanol
Figure 4.8.2.2.b  Extraction Well 1 Post-remediation 2,2-dimethyl-3-pentanol log

Table 4.8.2.3 Summary of Extraction Well 2 Tracer Analysis
Extraction Well 2
Pre-Remediation
 Post-Remediation
  Conservative (methanol) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation Conservative (methanol) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation
Pulse duration (days)  0.150  0.150    0.194  0.194  
zero moment data  152.925  141.071    191.049  74.245  
zero moment extrapolated  154.751  145.045    192.310 76.573  
first moment / zero moment data (days)  1.205  1.729  0.062  0.890  1.577 0.117 
first moment extrapolated / zero moment (days)  1.265  1.981  0.081  0.905  1.728 0.137 
Convective Dispersive Model      0.143      0.064
dispersivity (m)  0.724  1.091    0.930  0.826  
est. initial conc. (mg/l)  805  901    877  267  
mean time of travel (days)  0.562  1.160    0.498  0.734  
Stochastic model      0.146      0.060
variance in travel time  0.546  0.644    0.588  0.548  
travel time (days)  0.576  1.199    0.498  0.721  
est. initial conc. (mg/l)  811  901    855  257  

Figure 4.8.2.3  Extraction Well 2 pre-remediation 2,2-dimethyl-3-pentanol
Figure 4.8.2.3.b  Extraction Well 2 pre-remediation 2,2-dimethyl-3-pentanol log
Figure 4.8.2.4  Extraction Well 2 Post-remediation 2,2-dimethyl-3-pentanol
Figure 4.8.2.4.b  Extraction Well 2 Post-remediation 2,2-dimethyl-3-pentanol log

Table 4.8.2.4. Summary of Extraction Well 3 Tracer Analysis
Extraction Well 3
Pre-Remediation
 Post-Remediation
  Conservative (bromide) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation Conservative (methanol) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation
Pulse duration (days)  0.150  0.150    0.194  0.194  
zero moment data  44.733  116.368    109.752  60.732  
zero moment extrapolated  45.913  120.176    116.131 63.864  
first moment / zero moment data (days)  1.891  2.703  0.060  1.507  2.996 0.142 
first moment extrapolated / zero moment (days)  2.048  2.943  0.061  1.646  3.349 0.148
Convective Dispersive Model           0.082 
dispersivity (m)  0.646  N.C*.    1.462  1.046  
est. initial conc. (mg/l)  262      679  318  
mean time of travel (days)  1.278      1.469  2.362  
Stochastic model      0.070     0.077
variance in travel time  0.513  0.448    0.721  0.629  
travel time (days)  1.295  1.967    1.528  2.407  
est. initial conc. (mg/l)  262  691    669  316  
* N.C. not converged.
Figure 4.8.2.5  Extraction Well 3 Pre-remediation 2,2-dimethyl-3-pentanol
Figure 4.8.2.5.b  Extraction Well 3 Pre-remediation 2,2-dimethyl-3-pentanol log
Figure 4.8.2.6  Extraction Well 3 Post-remediation 2,2-dimethyl-3-pentanol
Figure 4.8.2.6.b  Extraction Well 3 Post-remediation 2,2-dimethyl-3-pentanol log

During the post-remediation test, an average of 3.5 lpm were injected through the cell.  Assuming an effective flow path width of 3.5 m and an average saturated thickness of 3 m, the saturated hydraulic conductivity of the formation was approximately 2 cm/sec.