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Main TCE Plume 

 The Main TCE Plume consists of two plumes: the Main TCE Plume, originating at the former auto hobby shop and Building 1432 in the north central portion of the former base; and the tributary Outdoor Firing Range (OFR) TCE Plume, which originates in the old firing range facilities near the eastern base boundary.   The origin of the Main TCE Plume is likely associated with activities at the auto hobby shop and several underground storage tanks (USTs) located at the 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.  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.

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 OFR TCE plume is traceable northwestward along the course of a buried paleochannel to an area north of Bonfils Blood Center 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 miles northward to a point beneath the western part of the former Stapleton International Airport.

Carbon tetrachloride (CT) has been detected in the bedrock waterbearing zone in the center of the Main TCE Plume in the area of well ETMW03 (approximately 500 feet northwest of the Bon Fils Blood Center).  The source of CT is unknown; however, aerial photographs and lithologic cores from well ETMW03 indicate possible localized disposal during the 1940’s. CT could have infiltrated through the saturated alluvium and into the bedrock waterbearing zones. In 2004 and 2005, LAC conducted investigations to assess the extent of and concentrations of CT in the source area.   The data also suggest the CT mass is very localized and there is limited communication between the bedrock and saturated alluvium. 

Since privatization of OU5, the focus of the remediation program has been to eliminate contaminant mass in the source area at the OFR and the Outfall/Building 1432 area in order to reduce or cut-off the downgradient transport of contaminants within the groundwater. The results of this approach have yielded overall TCE concentration reduction in the OFR and Outfall/Building 1432 area that range from approximately 18 to 96 percent and 14 to 94 percent, respectively.

The Main TCE Plume continues downgradient of the northern Lowry boundary to approximately 24th Avenue and Syracuse Street.  In the off-base portion of the plume, the percent reductions of TCE in the alluvium range up to 72 percent.  TCE in 11 of 13 off-base alluvial wells being monitored have decreased.  Concentrations of TCE in two of four off-base bedrock wells, being monitored, have decreased.  As explained in the discussion on in situ groundwater treatment, a recent KMnO4 injection event was conducted  in the offbase portion of the plume in November of 2007 and concentrations have generally shown additional decreases.

In the CT Plume, targeted remediation of CT in the bedrock was performed in 2006 using in situ injections of sodium persulfate.  A significant decrease in CT concentrations was observed for the bedrock wells inside the CT plume boundary.  Within the source area at well ETMW03, the CT concentration initially dropped 81 percent but has rebounded to the pre-injection level.  Just beyond the downgradient edge of the source area, the CT concentration dropped from 270 ug/L to 9.5 ug/L.  Approximately 430 feet downgradient of the source area, the CT concentration dropped from 94 ug/L to 33 ug/L.  In order to address the remaining CT mass within the source area, LAC is currently evaluating alternative remedial options to optimize the remediation in this area.  A bench-scale treatability study to assess the effectiveness of BOS-100® Trap and Treat technology as a potential in situ treatment was initiated in December 2007.  The results of the bench-scale study will be used to design and implement a field-scale treatability study in the first half of 2008.

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