Main TCE Plume

The Main TCE Plume includes the tributary Outdoor Firing Range (OFR) TCE Plume, which originates in the old firing range facilities near the eastern base boundary, and the Main TCE Plume originating near the former Auto Hobby Shop (AHS) and Building 1432 in the north-central portion of the base.  The historical outline of the OFR TCE Plume (>5 micrograms per liter [µg/l]) is traceable northwestward along the course of a buried paleochannel to an area north of the Bonfils Blood Center (Building 640, formerly the Base Commissary) near Westerly Creek, where it commingles with the Main TCE Plume.  From the intersection of the Main TCE Plume and the OFR TCE Plume, the Main TCE Plume extends approximately two (2) miles northward to a point beneath the western part of the former Stapleton International Airport (Location Map).

The source of the OFR plume likely included accidental spills or leaks of solvents around the firing range facilities and disposal of solvents via the septic system at the firing range.  The released solvents entered weathered and fractured bedrock beneath a relatively thin interval of unsaturated alluvium.  The origin of the Main TCE Plume is likely associated with past activities at the AHS and several former underground storage tanks (USTs) located at former Building 1432.   In 2002, four USTs were excavated and removed from the north side of former Building 1432, approximately 300 feet upgradient of the Outfall Source area.  In the Outfall Source Area at the AHS, solvents apparently passed through the drain system and into the subsurface adjacent to Westerly Creek (Versar, 2001 OU5 RI).  Groundwater sampling in the area of Building 1432 indicated that leaking USTs had contributed to groundwater contamination in alluvium and in bedrock waterbearing zones. Evaluation of the data indicates that the chlorinated solvent impacts are greater in the bedrock waterbearing zone than in the saturated alluvium and it is possible that a dense non-aqueous phase liquid (DNAPL) may still be present in the bedrock, at the source area. Because the bedrock waterbearing zones are discontinuous and that there is limited communication between the bedrock and saturated alluvium, TCE moves into the groundwater primarily through molecular diffusion. Therefore, the TCE remains at high concentrations within the bedrock without migrating away from the source area. 

As an interim action at the Outfall Source Area, the AF installed the SARS.  The SARS encloses a remnant source area where elevated concentrations of chlorinated solvents were present in the soil column overlying bedrock and in both alluvial and bedrock water-bearing zones. 

Since active remediation efforts were initiated in 2004, the extent of the Main TCE Plume has contracted with concurrent decreases in dissolved-phase TCE concentrations in the alluvium.  Tthe concentration decreases within and the contraction throughout the Main TCE Plume are illustrated in a time sequence figure beginning with October 2001 (pre-remediation extent), then in January 2008, and in January 2013.  

The reductions of dissolved-phase concentrations in the alluvium are interpreted to reflect elimination of mass transfer from the source areas as well as from residual TCE in shallow weathered bedrock throughout the plume.  This has resulted in the Main TCE Plume separating into distinct segments with overall lower TCE concentrations in alluvium relative to those observed in 2003 and earlier.  The highest detected TCE concentration along the axis of the alluvial portion of the plume outside of the source areas in January 2013 is 82 µg/l (TWOFR-01) on-base and 24 µg/l (IM-7A) in the off-base portion of the Main TCE Plume.  Each of the segments is separated by areas where dissolved-phase concentrations of TCE in groundwater are less than 5 µg/l. 

On-Base Plume Area

In the on-base portion of the Main TCE Plume (i.e., south of 11th Avenue), the overall TCE concentrations in alluvial groundwater have been reduced by 76% when comparing the maximum concentrations from February 2001 to January 2013.  Since 2001, the overall extent of the on-base portion of the Main TCE Plume has been reduced to approximately 68% of the original plume area (Main TCE Plume map January 2013).  The following table illustrates the comparative reductions in average TCE concentrations for this time period. (Note: The averages cited are a calculated arithmetic mean of detected concentrations.)  

On-base Alluvium

Minimum Concentration (µg/l)

Maximum Concentration (µg/l)

Average Concentration (µg/l)

February 2001

29

1100

234

January 2008

<5

630

87

January 2013

<2

260

43

 The decreases in concentrations realized over the time period summarized in the table above reflect the cumulative effects of the on-base KMnO4 injections in alluvium and bedrock and ongoing physical processes of dispersion, diffusion, and volatilization.  The following figure illustrates the overall decline in the average TCE concentration in the alluvium within the on-base plume area.   

The remediation program focused on the elimination of contaminant mass in the source areas at the OFR and the Building 1432/Outfall area in order to reduce or cut-off the downgradient transport of contaminants within the dissolved-phase plume.  The results of this approach have yielded overall TCE concentration reductions in the OFR and Building 1432/Outfall area.  In the Outfall Source area, relatively high concentrations of TCE remain in the bedrock matrix.  However, data from downgradient bedrock and alluvial wells demonstrate reductions of TCE concentrations indicating that the source mass has been confined by the KMnO4 injections and is no longer contributing to downgradient groundwater impacts.  These data imply that there is no transfer from the shallow bedrock to the saturated alluvium; hence the concentration decreases observed in the alluvium are directly linked to a decrease in potential risk exposure.  The highest residual concentrations in the alluvium are adjacent to the contaminant source areas for the Main TCE Plume, specifically the OFR Source Area and the Building 1432/Outfall Source Area.  

Off-Base Plume Area

In the off-base portion of the Main TCE Plume (i.e., north of 11th Avenue), the overall TCE concentrations in alluvial groundwater have been reduced by 89% when comparing the maximum concentrations from December 1999 to January 2013.  Since 2001, the overall extent of the Northern Offsite Plume has been reduced to approximately 73% of the original plume area (Main TCE Plume map-January 2013).  The following table illustrates the comparative reductions in average TCE concentrations for this time period. 

Off-base Alluvium

Minimum Concentration (µg/l)

Maximum Concentration (µg/l)

Average Concentration (µg/l)

December 1999

9.5

220

86

January 2008

2.3

72

21

January 2013

3.5

24

13

 The decrease in off-base concentrations realized since 2004 reflect the cumulative effect of the off-base KMnO4 injections and ongoing physical processes of dispersion, diffusion, and volatilization.  It has been four years since active treatment occurred in off-base portions of the plume and average alluvial concentrations and individual well concentrations of TCE continue to attenuate.  The following figure illustrates the overall decline in average TCE concentration in the alluvium in the off-base plume area. 

 

The remediation program in the off-base plume area was focused on aggressively treating groundwater in order to reduce TCE concentrations to levels that would effectively eliminate vapor intrusion via the groundwater-to-indoor air pathway.  The data indicate that significant concentration reductions were achieved through the remedy implementation with little or no apparent rebound.  LAC evaluated the groundwater-to–indoor air pathway in 2007 and 2010 and found a direct correlation between decreasing groundwater concentrations and decreasing indoor air concentrations.  By the 2010 evaluation (2010 Indoor Air Report), the groundwater concentrations had sufficiently decreased to levels that were considered protective of human health.  CDPHE issued a concurrence letter to LAC on August 6, 2012.  

Carbon Tetrachloride Source Area

Carbon tetrachloride (CT) was detected in the bedrock water-bearing zone in the center of the Main TCE Plume in the area of monitoring well ETMW03 (in the vicinity of 8th Avenue and Uinta).  The CT source area is located adjacent to Uinta Way and East 8th Avenue within the Wetlands Park open-space above the Kelly Road Dam.  In 2004 and 2005, LAC conducted an investigation to assess the extent of CT and to quantitatively assess the source mass.  The investigation indicated that the original source was likely the result of localized disposal in the 1940’s, and the continued detections are due to the presence of the compound in discontinuous bedrock fractures.  In 2006, additional delineation of the area was performed to complete the characterization of the extent of the CT in groundwater and soil (Location Map).

Targeted remediation of CT in bedrock in the vicinity of well ETMW03 was initiated in 2006 (LAC, March 2007).  Using a direct push rig, a focused injection of pH activated sodium persulfate solution was conducted in the CT source area.  A significant decrease in CT concentrations was initially observed for the bedrock wells inside the pre-injection CT plume boundary (CT Source map-January 2013 - wells ETMW03, MWCT01, and MWCT04).  Within the source area at well ETMW03, the CT concentration dropped 81 percent, from 7,800 ug/L to 1,500 ug/L.  Just beyond the downgradient edge of the source area at well MWCT01, the CT concentration dropped from 270 ug/L (January 2004) to 9.5 ug/L.  Approximately 430 feet downgradient of the source area, the CT concentration in well MWCT04 dropped from 94 ug/L (April 2005) to 33 ug/L.  Follow-up groundwater monitoring indicated rebound of CT suggesting the remedial technology or the injection methodology was not adequate to reduce contaminant mass effectively. 

In 2008, BOS-100® trap and treat carbon-based media was injected in the CT source area followed by a second, localized injection in 2010.  The 2008 injections consisted of 15,000 pounds of BOS-100® injected in 81 points in the bedrock source area and 83 points in the overlying saturated alluvium.  In 2010, the follow-up injection event consisted of 600 pounds of BOS-100® delivered into 17 bedrock points in the vicinity of well MWCT08 to address localized residual concentrations of CT still above the CBGWS (CT Source map-January 2013).

ETMW03 was the most impacted point in bedrock and is used to illustrate the positive effect of LAC’s remedial efforts.  The CT concentration in ETMW03 prior to the 2008 BOS-100® treatment was as high as 5,856 µg/l; within six months after the injections CT was non-detect and remained so for four consecutive monitoring events over a nearly two year period.  As illustrated in the CT Source map-January 2013 , the other impacted bedrock wells in the CT area display similar results with concentrations either stable or below the CBGWS.

ETMW03 was the most impacted point in bedrock and is used to illustrate the positive effect of LAC’s remedial efforts. The CT concentration in ETMW03 prior to the 2008 BOS-100® treatment was as high as 5,856 µg/l; within six months after the injections CT was non-detect and remained so for four consecutive monitoring events over a nearly two year period. As illustrated in the CT Source map-January 2013 , the other impacted bedrock wells in the CT area display similar results with concentrations either stable or below the CBGWS.