4.9.0 Sparging and Venting (Cell 1)

Michigan Technological University, in cooperation with the U.S. EPA, operated this cell.  The layout of this approximately 3m by 5m cell is shown in  Figure 4.9.0.1.  Air was injected through two sparge wells having screened intervals at approximately 7m to 7.3m below surface.  These sparging wells are denoted as 2174 and 2175.  Wells 2171, 2172, 2173, 2176, 2177, and 2178 were used to extract vapors from the formation and were screened from approximately 1.8m to 4.3m below ground surface.  Injection wells along the south end of the cell and extraction wells at the north end are labeled 214x and 215x, respectively.   Multilevel samplers are designated 211x, 212x, or 213x, and post-remediation cores were taken at the locations denoted by 219x.  Numbers in red are elevations of the top of casing or tubing for wells and samplers, respectively.  Instruments for measuring DO (dissolved oxygen) and VOC's (volatile organic compounds) were provided by CSIRO, Australia and were installed in the locations shown in the figure.

Seven tasks were performed as a part of this technology demonstration.  These tasks are shown in Table 4.9.0.1.  Initially, the formation was sampled by coring the formation during the installation of wells and multilevel samplers.  Descriptive logs of the wells cored are available in the database in the table labeled BoringLogs.  The core samples were analyzed for a group of chemicals believed to be representative of the site NAPL and of regulatory concern.  The selected contaminants represented several classes of chemicals. Following installation of the  wells and monitoring equipment, 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 natural gradient at the site, created the dynamic conditions.  Static samples were collected to assess 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 and distribution of NAPL in the formation and to characterize hydrodynamic performance.  Once the pre-remediation tracer study was completed, the remedial technology was implemented and its performance monitored.  After termination of the remedial demonstration, another tracer test was conducted, followed by post-demonstration ground-water sampling and core sampling.  A time line for these activities is shown in Table 4.9.0.1 and key operational parameters during the remediation demonstration and tracer tests are listed in Table 4.9.0.2.  Analysis of these data permits an evaluation of the technology.
 

Table 4.9.0.1.  Sequence and Description of Study Tasks
Test Activity Fluid injected  Total flux rate 
volume 		 Duration
Core and install instrumentation Collect soil samples None  NA  10/19/95 - 10/23/95
Ground-water sampling Collect water samples None  NA  3/17/96 - 4/3/96 *
Pre-remediation tracer Establish flow field Water  5.25 (l/m)  ? - 3/19/96
  Inject tracer suite 1  meth, br, dmp, hex** 5.25 (l/m) 
945 (l)		  3/19/96
  Maintain flow field Water 5.25 (l/m)   3/19/96 - 3/28/96
Remedial technology SVE  Air  22.5 m3/hr 
10767 m3		  8/26/96 - 9/15/96 
  Sparg and vent  Air  45 m3/hr 
34082 m3		  9/20/96 - 10/22/96
Post-remediation tracer Establish flow field  water  5.25 (l/m)  ? - 10/28/96
  Inject tracer suite 2 meth, br, dmp, hex, methep  5.25 (l/m) 
 955 (l)		  10/28/96
  Maintain flow field water   5.25 (l/m)  10/28/96 - 11/6/96
Ground-water sampling Collect water samples None  NA  10/29/96 - 11/17/96 *
Core  Collect soil samples None  NA  11/11/96 - 11/12/96
* Static and dynamic ground-water samples were collected
** meth = methanol, dmp = 2,2-dimethyl-3-pentanol, hex = hexanol, methep = 6-methyl-2-heptanol.

Table 4.9.0.2.  Operating Conditions for the Experiments
Parameter
Activity
Pre-Remediation Tracer Demonstration Post-Remediation Tracer
Average Saturated Thickness (m)  2.74 2.74  2.74 
Average Head Across Cell (m)      
Average Influent Flow rate (lpm)      
Average Effluent Flow rate (cubic meters per hr) during SVE    2.25  

The objective of this remedial approach is to extract contaminants by enhanced volatilization.  This is accomplished by moving air through the formation using soil vapor extraction wells in the vadose zone and air sparging wells in the saturated zone.  The surface of the cell was sealed with an impermeable membrane in order to restrict flow of air at the top of the formation.  The water table was positioned at 15.5 feet below ground surface and the SVE system was initially operated without air sparging in order to reduce the level of contamination in the vadose zone above the water table.  Upon reduction of vadose zone contaminant concentrations, the combined air sparging/soil vapor extraction system was placed into operation.

4.9.1    Core Analysis

Core data are plotted by constituent in Figures 4.6.1.1 through 4.6.1.10 as a function of depth without regard for spatial location within the test cell.  Black data points represent samples collected prior to remediation and the blue data points represent samples collected after remediation.  The vertical bars show mean concentrations for each depth zone.  Confidence limits at the 95% level are denoted by the small + at the center of the bar.  As described in Section 4.1, several methods were used to interpret the core data.  Total mass of contaminant in the treatment zone estimated by each of these methods is displayed at the top of each figure.  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. Core data are displayed in the following figures:  Figure 4.9.1.1 dichlorobenzene, Figure 4.9.1.2  decane, Figure 4.9.1.3  ethylbenzene, Figure 4.9.1.4  m-xylene, Figure 4.9.1.5  naphthalene ,Figure 4.9.1.6  o-xylene, Figure 4.9.1.7  1,1,1-trichloroethane, Figure 4.9.1.8  trimethylbenzene , Figure 4.9.1.9  toluene,  Figure 4.9.1.10  undecane and Figure 4.9.1.11  benzene  The fractions of monitored NAPL constituents removed by this technology, as determined from core data, are shown in Table 4.9.1.1.  The mean fraction removed for chemicals that had initial concentrations greater than 0.1 was 0.34.  The low initial concentration data are not included because of the uncertainty associated with differences between very small  values close to the detection limits.  Removal efficiencies appear to be relatively consistent for the NAPL monitored constituents.  If the SVE or sparging had changed the phase of the chemicals and removed them in the offgas one would  anticipate significantly higher removal rates for the more volatile chemicals.  As discussed in Section 4.9.3, very small fractions of the contaminants were removed in the offgas.  Apparently much of the removal is a result of increased biological activity due to the oxygen that was brought into the system.

Table 4.9.1.1.  Summary of Core Data Based on Boxcar Averaged Results
Chemical Pre-Remediation Concentration (mg/kg) Post-Remediation Concentration (mg/kg) Fraction Removed
dichlorobenzene 0.37 0.37 0
1,1,1-trichloroethane 0.015 0.013 0.13
toluene 0.042 0.018 0.57
o-xylene 0.95 0.72 0.24
m-xylene 0.42 0.28 0.33
naphthalene 1.1 0.55 0.50
trimethylbenzene 2.6 1.6 0.38
decane 24 14 0.42
undecane 55 28 0.49
ethylbenzene 0.22 0.14 0.36

 
 
 

4.9.2 Tracer Analysis

Partitioning tracer tests (PTT) were performed on all cells at OU1, Hill AFB prior to and after completion of each technology demonstration.  The purpose of the tracer tests was to estimate the pre- and post-treatment NAPL content and distribution within the saturated zone of the cell.  Changes in NAPL content were used to assess the effectiveness of the technology.  For the partitioning tracer tests, a flow field was established within the saturated zone by pumping water into and out of the cell at opposite ends.  After steady-state flow was established, a pulse of tracers was introduced through the injection wells.  Breakthrough curves for the tracers were measured at extraction wells and intermediate multilevel samplers.  A suite of both partitioning and non-partitioning tracers were used (Table 4.9.2.1), and the differences in arrival times were used to estimate NAPL content. Four different approaches were used to interpret the tracer data. Two of the approaches evaluate the data using moment analysis calculating the first and second moments.  The first approach used all of the data as it was available in the database.  The second approach extrapolated the final 25% of the data assuming it followed a log-linear relationship until the concentration was 0.001 mg/l.  The remaining two methods fit the data to simple one-dimensional models.  The first model is a simple convective dispersive model as described in Section 4.1.2.  The second model, also described in 4.1.2, is a stochastic one- dimensional model, which assumes dispersion is a result of the heterogeneity in the flow field.  This approach assumes the flow field is log-normally distributed and that the contaminant is uniformly distributed within the flow field.

For Extraction Well 1 data, all four approaches gave essentially the same saturation when based on the bromide- dimethylpentanol tracer pair.  This was the only well in this cell for which the model solutions converged.  The initial NAPL saturation (fraction of voids filled with NAPL) in the swept volume of Well 1 was approximately 0.09.  Based on the method of moments approach, NAPL saturation in the swept volumes of Wells 2 and 3 were 0.095 and 0.08, respectively.  These NAPL saturation estimates appear to be reasonable when compared to data across the test area.  Comparison of pre-and post-remediation data suggests a slight increase in the NAPL.  This is in contrast to the results of core sample analysis that suggested some contaminant removal.  It should be noted that these two performance assessment approaches measured different things.  The tracers provide a measure of the bulk NAPL while only specific NAPL constituents were monitored by the core analysis.  Decreases in constituent concentrations may not correspond to equivalent reduction in bulk NAPL content.  Alternatively, the losees of the more volatile and biodegradable compounds could alter the NAPL in such a way as to increase tracer partitioning into it.  Also, biomass generated as a result of enhanced biological activity could contribute to retardation of partitioning tracers.

The NAPL mass estimated from tracer breakthrough curves is inversely proportional to the NAPL:water partition coefficient.  To maintain internal consistency, the partition coefficients listed in Table 4.1.1 were used  for all the studies.  This was done even though the NAPL composition was quite variable across the site.  Results from one reactive tracer (2,2-dimethyl-3-pentanol) are summarized in Tables 4.9.2.2, 4.9.2.3, and 4.9.2.4.  Results for the other tracers are included in the hyperlinked images.  Based on tracer results, one would conclude that NAPL mass removal by sparging/SVE was insignificant.

A summary of the tracer activities is shown it the following table:
Table 4.9.2.1 Tracer Summary
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
1049
992
1114
714
680
 
methanol
983
930
 
885
845
 
2,2-dimethyl-3-pentanol
1039
983
 999
889
884
  
hexanol
970
    
925
 883
  
6-methyl-2-heptanol      
918
 877
  
Tracer Volume (l)
945
 955
Injection Time (min)
180
 180
* Based on zero moment of extrapolated data.

Table 4.9.2.2. 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    0.150  0.150  
zero moment data  141.994  128.371    123.625  117.555  
zero moment extrapolated  153.501  145.419    143.513  134.228  
first moment / zero moment data (days)  0.733  1.152 0.086  0.995  1.497  0.073
first moment extrapolated / zero moment (days)  0.851  1.450 0.104   1.471  1.845 0.036 
Convective Dispersive Model      0.092      0.130
dispersivity (m)  1.427  1.256    0.809  1.104  
est. initial conc. (mg/l)  1014  923    827  945  
mean time of travel (days)  0.560  0.943    0.749  1.473  
Stochastic model      0.089      0.132
variance in travel time  0.717  0.679    0.572  0.645  
travel time (days)  0.583  0.968    0.766  1.519  
est. initial conc. (mg/l)  1006  914    833  943  

Figure 4.9.2.1  Extraction Well 1 Pre-remediation tracer analysis 2-2,dimethyl-3-pentanol
Figure 4.9.2.1.b  Extraction Well 1 Pre-remediation tracer analysis 2-2,dimethyl-3-pentanol log
Figure 4.9.2.2 Extraction Well 1 Post-remediation tracer analysis 2-2,dimethyl-3-pentanol
Figure 4.9.2.2b  Extraction Well 1 Post-remediation tracer analysis 2-2,dimethyl-3-pentanol log
Figure 4.9.2.3  Extraction Well1 Post-remediation tracer analysis hexanol
Figure 4.9.2.3b  Extraction Well1 Post-remediation tracer analysis hexanol log
Figure 4.9.2.4  Extraction Well1 Post-remediation tracer analysis methylheptanol
Figure 4.9.2.4b  Extraction Well1 Post-remediation tracer analysis methylheptanol log
 

Table 4.9.2.3. Summary of Extraction Well 2 Tracer Analysis
Extraction Well 2
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    0.150    
zero moment data  143.087  129.415    131.717  118.788  
zero moment extrapolated  155.445  160.764    151.491 133.979   
first moment / zero moment data (days)  0.760  1.211  0.088  0.933  1.483 0.086
first moment extrapolated / zero moment (days)  0.884  1.763 0.146  1.391  1.799 0.042 
Convective Dispersive Model            0.140
dispersivity (m)  7.763*  14.976*    0.921 1.541  
est. initial conc. (mg/l)  1797*  1115*    847  1007  
mean time of travel (days)  0.929  3.101*    0.742  1.516  
Stochastic model      0.062      0.145
variance in travel time  1.272  1.192    0.610  0.744  
travel time (days)  1.159  1.192    0.765  1.592  
est. initial conc. (mg/l)  1613  1287    858  1006  
* large standard deviation
Figure 4.9.2.2  Extraction Well 2 Pre-remediation tracer analysis 2.2-dimethyl-3-pentanol
Figure 4.9.2.2.b  Extraction Well 2 Pre-remediation tracer analysis 2.2-dimethyl-3-pentanol log
Figure 4.9.2.3  Extraction Well 2 Pre-remediation tracer analysis hexanol
Figure 4.9.2.4  Extraction Well 2 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol
Figure 4.9.2.4.b  Extraction Well 2 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.9.2.5  Extraction Well 2 Post-remediation tracer analysis methylheptanol
Figure 4.9.2.5.b  Extraction Well 2 Post-remediation tracer analysis methylheptanol log

Table 4.9.2.4. Summary of Extraction Well 3 Tracer Analysis
Extraction Well 3
Pre-Remediation
Post-Remediation
  conservative (bromide) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation conservative (bromide) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation
Pulse duration (days)  0.150      0.150    
zero moment data  134.944  114.394    108.580  110.159  
zero moment extrapolated  146.966 137.571     115.201  124.092  
first moment / zero moment data (days)  0.969  1.555 0.088  0.997  1.721  0.106
first moment extrapolated / zero moment (days)  1.143  2.178 0.130   1.101  2.043  0.123
Convective Dispersive Model          
dispersivity (m)  7.203  4.336  0.082  1.375  6.451*  
est. initial conc. (mg/l)  1452  1138    834  797*  
mean time of travel (days)  1.016  1.635    0.808  3.687*  
Stochastic model      0.061      0.216
variance in travel time  1.244  1.067    0.721  0.883  
travel time (days)  1.260  1.848    0.847  2.207  
est. initial conc. (mg/l)  1324  1077    843  1103  

Figure 4.9.2.6  Extraction Well 3 Pre-remediation tracer analysis 2,2-dimethyl-3-pentanol
Figure 4.9.2.6.b  Extraction Well 3 Pre-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.9.2.7  Extraction Well 3 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol
Figure 4.9.2.7.b  Extraction Well 3 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.9.2.8  Extraction Well 3 Post-remediation tracer analysis hexanol
Figure 4.9.2.8.b  Extraction Well 3 Post-remediation tracer analysis hexanol log
Figure 4.9.2.9  Extraction Well 3 Post-remediation tracer analysis methylheptanol
Figure 4.9.2.9.b  Extraction Well 3 Post-remediation tracer analysis methylheptanol log
 

4.9.3    Extraction Analysis

The SVE offgas during this demonstration was monitored for the following NAPL constituents:  1,2-dichloroethene; 1,1,1-trichloroethane; trichloroethene; ethylbenzene; benzene; toluene; o-xylene; m-xylene; 1,3,5-trimethylbenzene; decane; undecane; 1,2-dichlorobenzene, and naphthalene.  The masses of eight of these constituents removed through the offgas during sparging and venting are shown in Table 4.9.3.1.  In order to assess the effectiveness of this remedial technology for contaminant removal, estimates of initial masses of the constituents are needed.  Based on the data available from this study, two methods can be used to obtain these estimates.  First, if NAPL composition was constant throughout the cell and there was a complete analysis of the NAPL, one could estimate the initial mass of a particular contaminant based on NAPL saturations from tracer data and the mole fraction of the individual contaminant in the NAPL.  This would likely give the best estimate if the flow field during the tracer experiments and the extraction test were the same since the formation volume "seen" by the tracer is likely very similar to the volume "seen" by the remedial fluid. Samples of NAPL were collected and analyzed from several locations around the site.  Samples collected in this manner represent readily mobile NAPL.  It is necessary to assume that this NAPL is in equilibrium (i.e., having the same composition) as with the rest of the NAPL in the immediate area.  If there is a "pitch" portion to the NAPL, this fraction may not be in equilibrium with the energetically free NAPL creating a potential bias to the results.
 

Second, core data from the pre-remediation sampling can be used to estimate initial contaminant mass.  Using core data creates a different set of  biases.  The core samples that were analyzed represent only a sub-sample of the total core removed from the formation.  Much of the formation is occupied by cobbles that were too large to be included in the subsample; therefore, the estimated mass is likely larger than the actual mass.  This is not a significant problem when comparing core data to core data but may be a significant problem when using core data as an estimate for the starting mass in the individual test cell.  In addition to this problem, the geometry of the test cell creates dead zones or at least zones that are difficult to access.

Using core data to estimate initial mass, the effectiveness of sparging/SVE for removal of selected NAPL constituents is shown in Table 4.9.3.1.  Less than 10% of most of these contaminants were extracted during this demonstration.  The exceptions are o- and m-xylene for which removal estimates approached 16% and 45%, respectively.  In the case of 1,2-dichlorobenzene, mass removal exceeded estimates of initial mass by more than a factor of 6, suggesting a problem with initial mass estimates or offgas measurements or both. If this approach were used to assess performance, it would be concluded that sparging/SVE is a relatively inefficient method of remediation for this site, but that substantial quantities of selected contaminants were removed.

Table 4.9.3.1.  Estimates of Fractions of Selected Contaminants Removed in Sparging/SVE Offgas.
Chemical Initial mass * (g) Mass removed (g) Fraction removed
naphthalene 110 5 0.05
ethylbenzene 22 2 0.09
o-xylene 79 13 0.16
m-xylene 29 13 0.45
decane 2800 152 0.05
1,3,5-trimethylbenzene 410 14 0.03
undecane 5800 553 0.10
1,2-dichlorobenzene 46 300 6.52