Main TCE Plume

The Main TCE Plume includes the tributary Outdoor Firing Range (OFR) TCE Plume, which originated in the old firing range facilities near the eastern base boundary, and the Main TCE Plume that originated 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 commingled with the Main TCE Plume.  From the intersection of the Main TCE Plume and the OFR TCE Plume, the Main TCE Plume once extended 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 indicated that the original chlorinated solvent impacts were greater in the bedrock waterbearing zone than in the saturated alluvium. 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 higher 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. 

With active remediation efforts, initiated in 2004, the extent of the Main TCE Plume has been reduced greatly  with concurrent decreases in dissolved-phase TCE concentrations in the alluvium.  The 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 July 2013, and in July 2018.  

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 July 2018 is 60 µg/l (MWCA03) on-base and 17 µg/l (MWHE05) in the off-base portion of the Main TCE Plume.  As discussed in the Site-Specific Standard page, in July 2014 the WQCC promulgated site-specific standards for TCE; 11 µg/l on-base (south of 11th Avenue) and 12 µg/l off-base (north of 11th Avenue).  Large areas of the Main TCE Plume , both on- and off-base, are now at or below the applicable site-specific standard.

On-Base Plume Area

In the on-base portion of the Main TCE Plume (i.e., south of 11th Avenue), the remediation program was biased toward the elimination of contaminant mass at the OFR and Building 1432 source areas in order to reduce or cut-off the downgradient transport of contaminants within the dissolved-phase TCE plume.  As documented in the project record, remediation was successful in reducing the TCE concentrations in the source areas such that the contaminant mass had been confined and was no longer contributing to downgradient groundwater impacts.  Also, TCE concentrations in alluvial groundwater between 2001 and 2013 were reduced by as much as 78 percentand the plume had disaggregated with a plume area reduction since 2001 by over 68 percent.  The overall reductions in concentrations and extent reflect the cumulative effects of the on-base KMnO4 injections in alluvium and bedrock and ongoing physical attenuative processes of dispersion, diffusion, and volatilization. 

At the OFR source area, the groundwater monitoring data accumualted largely reflect continued attenuation of the dissolved-phase plume with only localized areas exhibiting TCE concentrations greater than the on-base site-specific standard of 11 µg/l.  There is no indication that the remaining TCE mass in the bedrock source area, inferred by detected TCE concentrations at monitoring well IMOFR01, is affecting concentrations in the downgradient alluvium. 

In the Building 1432 source area, the bedrock interval has been characterized by the residual presence of KMnO4, an effect of the prior focused remedial injections in the bedrock.  Monitoring well MWMF02D has exhibited the presence of KMnO4 (i.e., purge water was purple) since at least July 2009.  With no visual indication of KMnO4 present in July 2018, the well was sampled and the reported TCE concentration was 220,000 µg/l.  When compared to pre-remediation concentrations of 220,000 µg/l in December 2004 and 310,000 µg/l in January 2008, the July 2018 result for MWMF02D shows little or no reduction in TCE concentrations within the bedrock of the source area.  The lack of visible KMnO4 in well MWMF02D suggests the final depletion of residual KMnO4 in the source area bedrock. 

In the absence of residual KMnO4 in the bedrock at the Building 1432 source area, the desorption of some TCE mass from within the bedrock matrix is likely to occur.  The significant increases observed in the July 2018 TCE concentrations for the three alluvial monitoring wells immediately adjacent to the bedrock source area (MWMF02A, MWMF07A, and IRAMW17) suggest that some TCE source mass has mobilized as a result of that process.  Based on the fairly static July 2018 monitoring results for wells located farther downgradient toward the northern base boundary (i.e., 11th Avenue), the effects of any mass release do not appear extensive at this time.  However, the rise in TCE concentrations in the alluvial monitoring wells is indicative of the need for additional remedial action in the Building 1432 source area to eliminate further migration of TCE mass away from the source area.  Additional remedial measures for the Building 1432 source area were evaluated and a Phase 2 CAP addendum was submitted to CDPHE for review and approval.  The implementation of additional remedial measures is anticipated to begin in late 2019.

Because the immediately surrounding land use in the Building 1432 source area is open space/park land, there is no increase to short-term exposure or risk anticipated.  The current conditions do not represent an adverse effect to the long-term protectiveness of the remedy as the use restrictions conveyed in the deed and imposed by the State Environmental Covenant for this area remain in full effect.

Off-Base Plume Area

The Groundwater 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.  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.  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.  TheOU5 groundwater monitoring program through July 2018 documents the nearly complete extinguishing of the off-base plume.  Except for monitoring well MWHE05, all wells located in the off-base portion of the Main TCE Plume have achieved the applicable site-specific ground water standard of 12µg/l set forth in Regulation 42.7(54).The TCE concentration at MWHE05 continues to trend toward achieving the off-base site-specific standard.

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 were 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. 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-July 2015) - 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-July 2015).

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.  Since the 2010 follow-on injections, the Colorado Basic Groundwater Standard for CT (5 µg/l) has been achieved in all but one monitoring well associated with the CT Source Area.  Bedrock monitoring well MWCT01 exhibits a stable to gradually attenuating concentration of CT slightly above the standard.