Location

Eastern Tucson, Ariz. The northern boundary gradually descends to the south from Golf Links Road to Irvington Road. The eastern boundary is Harrison Road and the western boundary is Alvernon Way. The southern-most boundary is Valencia Road on the east side of the site.

Contaminants of Concern

Contaminants of concern for sites where petroleum was released include petroleum hydrocarbons and BTEX in soils. The capped landfill contaminants of concern includes methane gas, volatile organic compounds (VOCs) and metals in soil. Per and polyfluorinated alkyl substances (PFAS) at concentrations above the EPA Health Advisory level of 70 nanograms per liter were found in groundwater.

Public Health Concerns

PFAS at concentrations above the EPA Health Advisory level of 70 nanograms per liter are present in groundwater. Investigations are on going to assess the nature and extent of PFAS in the environment. All other COC exposure pathways have been eliminated through remediation or restricted access/use.

Site Hydrogeology

Davis-Monthan Air Force Base (DMAFB) is located within the Tucson basin, a northwest trending alluvial valley. The Santa Cruz River flows northwesterly and provides the main drainage for the Tucson basin and is located approximately 4.6 miles west of the base. DMAFB is located in the Sonoran Desert which has an arid climate with a low precipitation rate of 12 inches per year and a high evaporation rate of 65 inches per year.

Groundwater occurs within the unconsolidated alluvial deposits consisting of interfingering sand, gravel, silt, and clay. The saturated thickness of these sediments is extremely variable, being thin (less than 200 feet) toward the mountains and thickening (greater than 5,000 feet) toward the center of the basin. These deposits were distributed laterally over time by a constantly changing stream course.

In the vicinity of DMAFB, the Pantano Formation, Tinaja Beds, and Fort Lowell Formation are the primary water bearing units. Groundwater is encountered at approximately 350 feet below ground surface near DMAFB, and groundwater generally flows in a northwest direction.

Action Taken

Petroleum contaminated sites ST-35, ST-36, and ST-52 were treated with soil-vapor extraction (SVE). An additional petroleum contaminated site ST-55 was proposed for risk-based closure. The low-level radioactive material waste storage area RW-16 was excavated and remediated. Geophysical surveys and limited excavations were conducted at the three Munitions Response Sites (MRS) TM553, TM553a, and TM554a. The closed base landfill LF-01 had a methane control system and landfill cap installed in 1999. A focused Remedial Investigation (RI) of PFAS is in-progress.

Status

The following is the status on the sites that action is currently being conducted:

• Final Record of Decisions (RODs) for sites ST-35 and ST-52 were completed in December 2018 so remediation for these two sites are complete.
• Site ST-36, the United States Air Force (USAF) published a Final Proposed Plan in April 2020 which proposed a preferred alternative of “No Further Action.” The ROD is being finalized and expect to be completed early 2021.
• Excavation and disposal was complete at site RW-16 in 2018. In July 2020 a Draft Final ROD was prepared with a selected remedy for RW016 of NFA, based on completion of a Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM) Site Characterization Investigation and Interim Removal Action. The finalized ROD is expected to be available early 2021. 
• In April 2019, the USAF published a Draft ROD for MRS sites TM553, TM553a, and TM554a with Land Use Controls (LUCs) as the selected remedies. The finalized ROD is expected to be available early 2021.
• Site LF-01, the USAF continues operation and maintenance of the landfill cap, the methane collection system, and annual inspection reporting which is typically available for review by the middle of the following year.
• In September 2015, the USAF released the Final Preliminary Assessment Report (PA) for Perfluorinated Compounds. The purpose and objective of the PA report was to identify locations at DMAFB where PFCs may have been released to the environment and to conduct an initial assessment of possible migration pathways and receptors of potential contamination. The findings from the PA report recommended identified locations to proceed to a Site Inspection (SI).
• In 2019, USAF released the Final SI report for PFAS releases at DM. The SI report identified areas of interest (AOI) that were recommended to proceed to a Remedial Investigation (RI).
• In August 2020, the USAF published a Relative Risk Site Evaluation (RRSE) for PFAS releases at DM. The RRSE identified the Storm water Outfall Canal along the north side of the base with an overall site category of High. Five other AFFF release areas had overall site categories of medium or low.
• In 2020, USAF is in the process of preparing a focused RI to continue to investigate PFAS possible migration pathways and receptors of potential contamination.

Community Involvement Activities

DMAFB is currently investigating the nature and extent of PFAS contamination on the base. Tucson Water supply wells just north of the base property have been impacted with PFAS at concentrations above the EPA Health Advisory level of 70 nanograms per liter.