Phase I Remedial Investigation for PFAS

Trihydro conducted comprehensive Remedial Investigations (RI) and Site Inspections (SI) for Per- and Polyfluoroalkyl Substances (PFAS) at various sites at Eareckson Air Station (AS) in Alaska. The air station is located on Shemya Island near the western edge of the Aleutian Islands, approximately 1,500 miles southwest of Anchorage and 400 miles from Russia. Despite logistical challenges, the team successfully collected over 1,500 samples, developed a digital 3D conceptual site model, and completed a 60,000-page Remedial Investigation Report.

The project faced unique logistical challenges, including mobilizing and transporting drilling rigs, sampling equipment, UTVs, field vehicles, supplies, materials, and personnel to this remote Alaskan island via limited charter beach landings, ocean-going barges, air cargo, and passenger flights. Logistics were further complicated by the nature of PFAS sampling, which requires strict sampling protocols, samples needing to be shipped to Lower 48 laboratories, and decontaminated water needing to be shipped to the site. 

The initial objective was to complete a PFAS RI at four Eareckson AS sites and a PFAS SI at one site. A limited PFAS dataset for the project sites, combined with high costs and significant time required to plan and mobilize heavy equipment to Eareckson AS, made it critical for the team to develop and execute a detailed and flexible investigative strategy. This plan aimed to expand the PFAS dataset in the early stages of the project fieldwork, while still reserving enough project funding for later stages of fieldwork needed to fill data gaps, so project objectives could be achieved. The project could not have been completed within the project schedule and budget without using the detailed and flexible investigative strategy, as additional mobilizations of drilling equipment to the island would have been required.

Tetra Tech and Trihydro shared the workload, with both contributing to each task. Tetra Tech was the lead on planning and reporting documents, and Trihydro led the field efforts. Field work included:

• Installing, developing, and sampling groundwater monitoring wells at 29 locations for PFAS.
• Sampling over 100 existing groundwater wells for PFAS.
• Collecting approximately 75 surface water/sediment samples to assist in delineating PFAS.
• Collecting and analyzing 140 surface soil samples for PFAS.
• Advancing over 200 soil borings to groundwater or bedrock and collecting soil samples for PFAS analysis.
• Installing 12 lysimeters to sample pore water.
• In total, over 1,500 samples were collected and submitted for PFAS analysis for up to 40 separate PFAS analytes in soil, groundwater, surface water, sediment, and pore water via Draft EPA Method 1633 and other applicable analytical methods.

The data collected was validated in accordance with the U.S. Air Force Civil Engineer Center (AFCEC) and Alaska Department of Environmental Conservation (ADEC) requirements and reviewed by licensed/qualified environmental professionals. A digital 3D conceptual site model (CSM) was created using quantitative and qualitative data collected as part of this RI and supplemented with data from past studies to visualize the relationship between contaminant sources, site hydrogeology, and receptor exposure pathways. Data components of the digital 3D CSM include regional and site-specific geologic and hydrogeologic data, high-resolution orthoimagery, historical aerials, digital elevation models, physical site features, past and current chemical data, and physical data. Environmental visualization system (EVS) software served as the foundation of this model. The results of this evaluation were then summarized in the RI Report. The RI Report was reviewed by both the Air Force and ADEC. ADEC had only 23 comments on the 60,000-page Draft RI Report. The Final RI Report was submitted in 2025.

Major Project Accomplishments

• The team completed multiple Uniform Federal Policy for Quality Assurance Project Plans (UFP-QAPP) and multiple UFP-QAPP Addendums and obtained early conditional approval to execute the first groundwater, surface water, and soil sampling stages of the project prior to final approval of the QAPP. This allowed for a more focused drilling program utilizing additional data collected during the first stage.
• The regulator assigned to this site by ADEC commented that the PFAS RI strategy proposed “looked very aggressive” and that ADEC “really liked the use of the staged approach for the investigation.”
• The team executed a staggered field approach to utilize data collected in the early stages, to field-adjust and optimize later-stage sampling and specifically drilling locations, and to minimize data gaps while having a fixed number of field sampling assets and field mobilizations available. This best management practice (BMP) was received so positively by the Air Force Remedial Project Manager that she recommended its adoption and implementation at other Air Force installations in Alaska.
• Field work in 2021 and 2023 was completed safely, on time, and within budget, though site access is difficult and subject to extreme delays, and weather can severely impact field performance. Contingency plans were required to promote seamless work when difficulties occurred. Despite these challenges, the project team completed the work on time and within budget.