4.10.0 In-well aeration (Cell 2)

 The University of Arizona, in cooperation with the U.S. EPA, operated the in-well aeration test cell.  This cell had leakage at the northeast corner of the test cell.  Attempts to reduce the leakage were only minimally successful.  After discussion with the regulatory community it was decided to continue with the use of this test cell since only small amounts of tracers would be introduced into the cell and the tracers were considered to have little or no environmental impact.   To maintain a constant head throughout the experiment, water was added to the cell at 0.035 lpm.  This effectively acted as a "pump-and-treat" system.  This would be equivalent to one pore volume every 175 days.  The losses from leakage can, thus, be assumed to be negligible. The remedial operation should only introduce air into the system which should have a positive influence and no known potential negative influences.The Cell layout  is shown in the attached figure.  The inwell aeration well is labeled 2271iwa which is in the approximate center of the test cell.  The cell was instrumented with dissolved oxygen sensors and VOC loops that could measure the VOC at points in the soil profile.  The injection wells were along the south side of the cell and labeled 224x.  The extraction wells were along the north side of the cell and labeled 225x.  The numbers in red in Figure 4.6.0.1 are the elevation of the top of casing.  The locations labeled 229x are the locations where post-remediation cores were collected.  Multilevel samplers are designated 221x, 222x, and 223x.  Objects with the designation 226x are small diameter screened wells installed to monitor the head during the experiments.

Seven tasks were performed as a part of this technology demonstration.  These tasks are shown in Table 4.10.0.1.  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 forced gradient flush or water flood, created the dynamic flow 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 4.10.0.1 and key operational parameters during the remediation demonstration and tracer tests are listed in Table 4.10.0.2.

The objective of this remedial approach is to extract contaminants by enhanced volatilization.  This is accomplished by circulating contaminated groundwater through a double screened well and sparging the groundwater with air as it passes through the well bore.  The system is designed to establish a vertical recirculation flow field in order to continually sweep contaminants from the formation surrounding the well and stripping the contaminants from the water when it is resident in the well.
 

Table 4.10.0.1.  Study Sequence
Test Activity Fluid injected * Total flux rate (lpm)- volume (l) Duration
Core and install instrumentation Collect soil samples None    1/19/96 - 1/22/96
Ground-water sampling Collect water samples None    7/23/96
Pre-remediation tracer Establish flow field Water  3 (lpm)  
  Inject tracer suite 1  br, dmp, eth, pen, hex  3 (lpm) 
950 (l)		  6/28/96
  Maintain flow field Water  3 (lpm)  6/28/96 - 7/8/96
 Remedial technology Inject remedial fluid  Air 1170 (lpm)  Note: no gas fluid chemistry data in data base
Post-remediation tracer Establish flow field  Water  3 (lpm)  
  Inject tracer suite 2  br, eth, pen, hex, hep, dmp, methep, pfba  3 (lpm) 
900 (l)		  9/28/96
  Maintain flow field  Water  3 (lpm)  9/28/96 - 10/6/96
Ground-water sampling Collect water samples None    9/26/96
Core  Collect soil samples None    11/13/96 - 11/14/96
* br = bromide, dmp=2,2,-dimethyl-3-pentanol, eth=ethanol, pen = pentanol, hex = hexanol, hep = heptanol, methep= 6-methyl-2-heptanol, pfba=pentafluorobenzoic acid
 

Table 4.10.0.2. Operating Conditions for the Experiments
Parameter
Activity
Pre-remediation Tracer Demonstration Post-Remediation Tracer
Average Saturated Thickness (m)    3  
Average Head Across Cell (m)      
Average Influent Flow rate water (lpm)    0.035 *  
Average Effluent Flow rate (lpm)  3.0  n.a.  3.03
* flux required to maintain head in test cell.
 
 

4.10.1    Core Analysis

Core data are plotted by constituent, in Figures 4.10.1.1 through 4.10.1.10, 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 4.10.1.1, dichlorobenzene; Figure 4.10.1.2,  decane; Figure 4.10.1.3,  ethylbenzene; Figure 4.10.1.4,  m-xylene; Figure 4.10.1.5,  naphthalene; Figure 4.10.1.6,  o-xylene; Figure 4.10.1.7,  1,1,1-trichloroethane; Figure 4.10.1.8,  trimethylbenzene; Figure 4.10.1.9,  toluene; Figure 4.10.1.10,  undecane.  These core data suggest that contaminant concentrations decreased somewhat in the upper portion of the treatment zone and increased lower in the profile.  Table 4.10.1.1 contains a summary of core data results obtained using the boxcar averaging technique (see Section 4.1.1).

During this demonstration, air was injected  at 1170 lpm stp from 7/26 to 9/23 generating a fluid flux of 5 lpm through the sparging well.  The air injection created a significant water flux.  If uniform flow is assumed, the flux would be equivalent to one pore volume every 1.2 days or about 47 pore volumes during the duration of the remedial activities.  Although contaminant removal was relatively low for this technology it exceeded removal rates expected for a pump-and-treat system.   The enhanced removal is likely due to accelerated biodegradation as a result of oxygen introduced into the profile through the air lift and stripping activities.
 
 

Table 4.10.1.1 .Summary of Core data based on boxcar averaged results.
Chemical Pre-remediation concentration (mg/kg) Post-remediation concentration (mg/kg) Fraction removed
dichlorobenzene 0.84 N.D.*  
1,1,1-trichloroethane 0.024 0.013 0.46
toluene 0.17 0.08 0.53
o-xylene 1.7 0.83 0.48
m-xylene 0.51 N.D.  
naphthalene 1.4 1.2 0.14
trimethylbenzene 3.6 2.9 0.18
decane 4.6 3.5 0.24
undecane 82 69 0.16
ethylbenzene 0.25 N.D.  
* Not determined.

4.10.2 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 4.10.2.1).  To maintain internal consistency, the partition coefficients listed in Table 4.1.1 were used  for interpretation of results from all the studies.  This was done even though the NAPL composition was variable across the site.  Results from one reactive tracer (2,2-dimethyl-3-pentanol) are summarized in Tables 4.10.2.2, 4.10.2.3, and 4.10.2.4.  Results for the other tracers are included in the hyperlinked images.

Based on tracer results, one would conclude that NAPL mass removal by in-well aeration was insignificant 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, and the second is a stochastic one-dimensional model which assumes dispersion is a result of the heterogeneity in the flow field.  The stochastic modeling approach assumes the flow field is log-normally distributed and that the contaminant is uniformly distributed within the flow field.  See Section 4.1.2 for additional discussion of the methods used for analyzing tracer data.

A summary of the tracer activities is shown it the following table

Table 4.10.2.1. Summary of Tracer Results
Tracer Pre-Remediation Post-Remediation
Concentration (mg/l) Total Mass Injected (g) Mass extracted (g) Concentration (mg/l) Total Mass Injected (g) Mass extracted (g)
bromide
370
117
234
307
92
186
2,2-dimethyl-3-pentanol
395
125
274
406
121
304
hexanol
946
300
  
991
297
  
pentanol
988
313
  
522
157
  
Tracer Volume (l)
950
910
Injection Time (min)
317
300

The first test for internal consistency is to evaluate the tracer's performance based on conservation of mass.  The summary data in Table 4.10.2.1 suggest that mass was not conserved during the tracer test.  Injected mass is typically one-half of what was extracted.  This makes the validity of the tracer data suspect.  Under these circumstances, the data can only be used for qualitative assessment of technology performance.  Data  for the conservative tracer and one of the retarded tracers (2,2-dimethyl-3-pentanol) are reported in Tables 4.10.2.2, 4.10.2.3, and 4.10.2.4.  In the pre-remediation test, approximately the same mass was extracted from each of the extraction wells and the travel times are approximately the same, suggesting that there is little heterogeneity perpendicular to the flow lines in the plan view.  During post-remediation, somewhat more flow was directed toward the west side of the test cell as can be seen from well 3 data compared to the other two wells.  The tracer results for extraction well 1 suggest an increase in NAPL content during the course of this demonstration.  This interpretation is independent of the method used to analyze the data.  Well 2 results are similar but not quite as consistent.  One might surmise from the model fit parameters for well 3 that there was some removal.  This would be consistent with an apparent increase in the permeability of this region.  The moment analysis, however, still suggests an increase in NAPL content.  Increased NAPL mass could occur if NAPL was transported from surrounding contaminated zones into the treatment area.  Because of the containment, this is unlikely unless NAPL was transported from higher zones in the cell.  Another potential explanation is that NAPL composition or accessibility changes because of the recirculation and aeration activities.  Since this technology will likely selectively remove the more soluble and volatile contaminants, the residual NAPL could have a significantly larger partitioning coefficient for the tracers.  In addition, as with the sparging/SVE demonstration, enhanced biodegradation could generate biomass for which tracers have an affinity.

Table 4.10.2.2. Summary of Extraction Well 1 Tracer Analysis
Extraction Well 1

 
Pre-Remediation
Post-Remediation
conservative (bromide) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation conservative (bromide) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation
Pulse duration (days)  0.220      0.208    
zero moment data  53.472  60.641    39.215  65.467  
zero moment extrapolated  53.586  61.258    39.259  67.459  
first moment / zero moment data (days) 0.762   1.603  0.173  0.679  2.104  0.333
first moment extrapolated / zero moment (days)  0.766  1.668 0.185   0.661  2.229  0.361
Convective Dispersive Model      0.110      0.186
dispersivity (m)  1.290  4.059    1.809  3.335  
est. initial conc. (mg/l)  259.879  313.650    200.025  299.487  
mean time of travel (days)  0.603 1.097    0.487  1.161  
Stochastic model      0.111      0.190
variance in travel time  0.992  1.008    0.988  1.002  
travel time (days)  0.617  1.125    0.483  1.165  
est. initial conc. (mg/l)  275.678  279.498    204.188  269.530  

Figure 4.10.2.1  Extraction Well 1 Pre-remediation tracer analysis 2-2,dimethyl-3-pentanol
Figure 4.10.2.1.b  Extraction Well 1 Pre-remediation tracer analysis 2-2,dimethyl-3-pentanol log
Figure 4.10.2.2  Extraction Well 1 Post-remediation tracer analysis 2-2,dimethyl-3-pentanol
Figure 4.10.2.2.b  Extraction Well 1 Post-remediation tracer analysis 2-2,dimethyl-3-pentanol log
Figure 4.10.2.3   Extraction Well 1 Post-remediation hexanol
Figure 4.10.2.3.b   Extraction Well 1 Post-remediation hexanol log
Figure 4.10.2.4   Extraction Well 1 Post-remediation methylheptanol
Figure 4.10.2.4.b  Extraction Well 1 Post-remediation methylheptanol log

Table 4.10.2.3. Summary of Extraction Well 2 Tracer Analysis
Extraction Well 2
Pre-Remediation
Post-Remediation
  conservative (bromide) 2,2-dimethyl
-3-pentanol		 NAPL Saturation conservative (bromide) 2,2-dimethyl 
-3-pentanol		 NAPL Saturation
Pulse duration (days)  0.220      0.208    
zero moment data  55.620  61.226    38.189  63.443  
zero moment extrapolated  55.831  61.818    38.245  65.294  
first moment / zero moment data (days)  0.839  1.811  0.179  0.776  2.020  0.249
first moment extrapolated / zero moment (days)  0.848  1.864  0.185  0.778  2.245  0.292
Convective Dispersive Model      0.150      0.157
dispersivity (m)  0.604  0.490    0.772  0.705  
est. initial conc. (mg/l)  248.996  270.117    188.080  279.583  
mean time of travel (days)  0.685  1.477    0.634  1.373  
Stochastic model      0.152      0.109
variance in travel time  0.981  0.999    0.799  1.062  
travel time (days)  0.683  1.453    0.763  1.383  
est. initial conc. (mg/l)  290.636  276.428    233.385  299.514  

Figure 4.10.2.5  Extraction Well 2 Pre-remediation tracer analysis 2.2-dimethyl-3-pentanol
Figure 4.10.2.5.b  Extraction Well 2 Pre-remediation tracer analysis 2.2-dimethyl-3-pentanol log
Figure 4.10.2.6  Extraction Well 2 Post-remediation tracer analysis 2,2,-dimethyl-3-pentanol log
Figure 4.10.2.7   Extraction Well 2 Post-remediation hexanol
Figure 4.10.2.7.b  Extraction Well 2 Post-remediation hexanol log
Figure 4.10.2.8   Extraction Well 2 Post-remediation methylheptanol
Figure 4.10.2.8.b  Extraction Well 2 Post-remediation methylheptanol log

Table 4.10.2.4. 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)  0.220      0.208    
zero moment data  53.084  66.877    50.298  74.072  
zero moment extrapolated 53.161   67.649    50.334  75.930  
first moment / zero moment data (days)  0.676  1.471  0.189  0.616  1.547  0.245
first moment extrapolated / zero moment (days)  0.679  1.533  0.202  0.617  1.769  0.302
Convective Dispersive Model      0.181      0.089
dispersivity (m)  0.857  1.649    1.454  2.037  
est. initial conc. (mg/l)  243.868  326.603    248.963  324.427  
mean time of travel (days)  0.467  1.095    0.435  0.723  
Stochastic model      0.130      0.094
variance in travel time  0.831  0.999    0.990  0.981  
travel time (days)  0.556  1.094    0.432  0.733  
est. initial conc. (mg/l)  296.863  322.849    261.460  325.761  

Figure 4.10.2.9  Extraction Well 3 Pre-remediation tracer analysis 2,2-dimethyl-3-pentanol
Figure 4.10.2.9.b  Extraction Well 3 Pre-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.11.2.10  Extraction Well 3 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol
Figure 4.11.2.10.b  Extraction Well 3 Post-remediation tracer analysis 2,2-dimethyl-3-pentanol log
Figure 4.10.2.11   Extraction Well 3 Post-remediation hexanol
Figure 4.10.2.11.b  Extraction Well 3 Post-remediation hexanol log
Figure 4.10.2.12   Extraction Well 3 Post-remediation methylheptanol
Figure 4.10.2.12.b   Extraction Well 3 Post-remediation methylheptanol log