Groundwater - 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 (OU5). OU5 is defined as all areas within and adjacent to Lowry AFB where groundwater has been impacted by past Air Force activities. The OU5 groundwater contamination occurs in five distinct, mapable contaminant plumes. These include:

• Main Trichloroethene (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 OU5, 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-valet iron across a portion of the Main TCE Plume just north of the SARS; and
• the installation of active subslab depressurization systems.

In August 2002, the Air Force privatized the environmental cleanup efforts for OU5, turning management and implementation over to the LRA and its contractor, LAC. An alternatives analysis was performed for OU5 and the remedy chosen by LAC 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 offered 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 (Administrative Record),and include the injection of oxidizing reagents for in situ destruction of chlorinated solvents and ongoing groundwater monitoring.  

Active remediation of the TCE plumes began in October 2004 and continued through 2010.  As a result of the aggressive remediation efforts, TCE concentrations in the OU5 plumes haven ben significantly reduced.  LAC continues to monitor the attenuation of groundwater concentrations in accordance with a schedule approved by CDPHE.  Surplus monitoring wells that are no longer needed for the approved monitoring program and those wells that meet the applicable site-specific standard for TCE are abandoned.  The most recent Addendum to the Phase 2 CAP for OU5 sets forth the current groundwater monitoring program and criteria for well abandonment. 

While aggressive groundwater treatment was the best way to mitigate the long-term concerns associated with TCE, CDPHE also required the installation of active subslab depressurization systems in all new residential construction over the areas of impacted groundwater to provide a barrier to any potential exposure.  State Environmental Covenant No. HMCOV00022  for OU5 was put in place in January 2006 requiring the installation of these systems in known areas of impacted groundwater at the Former Lowry AFB.