Operable Unit 5

Before discussion of contamination in groundwater, it is important to remember that no one at Lowry is drinking groundwater that underlies Denver. Lowry's drinking water, as well as the rest of the Metro Denver area, comes from surface water reservoirs replenished annually by high mountain snowmelt. For more information about Denver drinking water, visit Denver Water or call 303.628.6510.

As discussed in Site History, the activities that occurred at the former Lowry AFB were similar to other communities at that time. However, there were some contaminant releases to the environment as a result of these activities. Groundwater contaminants were initially released into the environment at several locations on the Lowry AFB. The primary origin of contaminant release was into the storm sewers from several on-base facilities including an auto hobby shop. Other likely sources of contamination included accidental spills or leaks of solvents around firing range facilities, disposal of solvents into septic systems at the firing range, leakage from a storm sewer in the Headquarters area, accidental spills or leaks at the fuel storage area in the vicinity of Uinta Street, releases related to fire training activities, and leakage from fuel tanks at the Yosemite Street Gate Site. The specific hydrogeology at Lowry AFB strongly affected the infiltration and movement of the contaminant releases to the groundwater. Infiltration of solvents and fuels from these source areas transported contaminants through the vadose (unsaturated) zone, into the alluvial water-bearing zone, and in some cases, the underlying bedrock water-bearing zone. 

To facilitate the investigation and evaluation of remediation alternatives for the groundwater contamination, all of the impacted groundwater was grouped into one unit identified as Operable Unit 5(OU 5).  OU5 is defined as all areas within and adjacent to Lowry AFB where groundwater has been impacted by Air Force activities. The OU 5 groundwater contamination occurs in five distinct, mappable contaminant plumes. These include:

• Main Trichloroethane (TCE) Plume (including onsite and offsite portions)
• Headquarters TCE Plume
• Fire Training Zone TCE Plumes
• Yosemite Street Gate Plume

Yosemite Street Gate Plume was closed in May 2007 and is discussed in more detail as part of the NFA sites.

After contamination was delineated in OU 5, several steps were taken to begin cleanup.  Initial remediation activities conducted by the Air Force included:  

•  Source Area Reduction System (SARS) at the Outfall Source Area of the Main TCE Plume;
• the Boundary Area Hydraulic Containment Systems (BAHCS) across the Main TCE plume along the northern base boundary (11th Avenue);
• the installation of a permeable reactive barrier (PRB) containing zero-valence iron across a portion of the Main TCE Plume just north of the SARS;
• and the installation of active subslab depressurization systems. 

In 2002, the Air Force privatized the environmental cleanup efforts for OU5, turning management and implementation over to the LRA and its contractor, LAC. Since 2002, an alternatives analysis was performed for OU5 and the remedy chosen was in situ chemical oxidation.  This technology was favored because it can enhance the desorption and destruction of chlorinated solvents in low groundwater flow areas and it offerred the better alternative for more rapid TCE removal. Remedies proposed to accelerate groundwater cleanup are discussed in detail in the Final Phase 2 Corrective Action Plan (CAP) for Groundwater Cleanup at Lowry, and include the injection of oxidizing reagents for in situ destruction of chlorinated solvents and ongoing groundwater monitoring. 

Cleanup of the TCE began in October 2004. While the ongoing, aggressive groundwater treatment is the best way to mitigate the long-term concerns, CDPHE has required the installation of active subslab depressurization systems in all new residential construction built over the Main TCE plume at Lowry to provide a barrier to any potential exposure. 

Since monitoring began, concentrations in the OU5 plumes have been significantly reduced and it appears that mass transfer from the source areas to the downgradient plumes has been limited.  These concentration reductions are likely the result of a combination of all remedial efforts to date as well as natural attenuation processes.  The reduction of TCE concentrations in the Main TCE Plume, Headquarters TCE Plume, and FTZ TCE Plume, are shown on the map. 

 In 2008 LAC began a Remedial Project Assessment for OU5.  The objective of the RPA is to develop cleanup criteria to close OU5 and includes the evaluation of the current groundwater treatment and the development of future steps required to reach no further action for OU5.   LAC and CDPHE are currently discussing what these future steps will be.