Lower Issaquah Valley
Issaquah, Washington
Farallon Consulting, L.L.C.
April 14, 2021
Executive Summary
Farallon has prepared this Summary Report on behalf of Eastside Fire and Rescue to summarize the work performed and analytical results for the Lower Issaquah Valley Per- and Polyfluoroalkyl Substances Additional Characterization Study performed from March through October 2020 (Study).
Previous investigation work performed from 2016 through 2018 identified five areas of interest in the Lower Issaquah Valley with confirmed historical use of aqueous film-forming foams (AFFF) in firefighting training exercises. This Study was designed to address a series of data gaps previously identified in the Lower Issaquah Valley PFAS Characterization Study performed in 2018 and summarized in the Per- and Polyfluoroalkyl Substances Characterization Summary Report prepared by Farallon (2019) and to develop a preliminary conceptual site model for confirmed Per- and Polyfluoroalkyl Substances (PFAS) releases.
The Study evaluated five areas of interest previously identified where AFFF had been released primarily through training exercises. The scope of work for the Study included:
- Evaluate PFAS impacts to soil and groundwater, subsurface conditions, and migration pathways at previously identified areas of interest
- Evaluate PFAS impacts to soil and groundwater and subsurface conditions at 175 Newport Way Northwest sufficiently to develop and evaluate feasible source remediation alternatives
- Further refine the nature and extent of PFAS impacts in shallow and intermediate groundwater and associated seasonal variations at areas of interest and down-gradient locations that may affect drinking water production wells through additional sampling of new and existing monitoring wells
- Collect adequate hydrostratigraphic and analytical data to support development of a preliminary conceptual site model and support development of a groundwater model that can be used to evaluate potential source remedial action performance
- Review and document the history of use, including historical business listings and otherpublicly available information for additional potential up-gradient sources
- Collect initial data that can be used to evaluate potential interaction between surface water and groundwater along the primary axis of the Lower Issaquah Valley
Additional subsurface investigation performed in this Study at the areas of interest confirmed existing stratigraphy and lithologies that were previously observed in 2018 and further delineated key stratigraphic contacts and hydrogeologic units. Study analytical results identified the primary PFAS constituents of concern as perfluorohexane sulfonic acid (PFHxS), perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and perfluorononanoic acid (PFNA). PFOS and PFOA were detected at concentrations exceeding Investigatory Levels developed by the Washington State Department of Ecology in shallow and intermediate groundwater samples collected from areas of interest on the western portion of the Lower Issaquah Valley, west-valley down-gradient locations, and in shallow groundwater at Memorial Field on the eastern portion of the Lower Issaquah Valley. PFHxS, PFOA, PFOS, and PFNA were also detected in shallow and intermediate groundwater samples collected from multiple locations at concentrations that exceed proposed Washington State Department of Health State Action Levels (SALs) for drinking water. Exceedances of the SALs were generally co-located with exceedances of Investigatory Levels, but also occurred at additional locations that were either up-gradient or cross-gradient of impacted areas, expanding their overall extent.
To date, reported concentrations of PFAS in soil are less than the Investigatory Level for direct human contact (residential and industrial exposure scenarios) at all locations evaluated. Confirmed releases of PFAS to soil at concentrations exceeding the Investigatory Levels for protection of groundwater (unsaturated and/or saturated soil), and associated migration and confirmed exceedances of the Investigatory Level for groundwater were confirmed for the West Playfield, Dodd Field, and 175 Newport Way Northwest areas of interest. Confirmed releases at the Memorial Field area of interest AFFF training area have impacted shallow groundwater only. Confirmed soil impacts at the Rainier Trail area exceed Investigatory Levels for unsaturated and saturated soil, and groundwater sampling indicates the migration pathway from soil to shallow groundwater is complete; however, impacts to shallow groundwater have not exceeded either Investigatory Levels or proposed State Action Levels.
Potential human exposure pathways for groundwater include the ingestion pathway. Treatment of raw groundwater prior to entering the City of Issaquah service water system continues to be effective at removing PFAS down to concentrations that are less than analytical laboratory method reporting limits, making the ingestion pathway through service water incomplete. Ingestion of impacted groundwater is possible if groundwater is extracted from the affected areas and used without treatment or is encountered through subsurface excavation work that results in incidental ingestion of untreated groundwater. Both exposure scenarios have limited potential impact to public health and can be mitigated with appropriate planning and corrective actions.
Farallon performed a preliminary screening of currently available remediation technologies for PFAS in soil and groundwater. Recommended remedial technologies were proven, commercially available technologies, including direct excavation, engineering and institutional controls, solidification, and in-situ amendment with activated carbon additives for soil; and activated carbon treatment, subsurface barriers, and localized hydraulic control for groundwater.
Additional characterization of the nature and extent of contamination and identification of source material is still necessary prior to development of source remediation alternatives for the West Playfield, Dodd Field, and Memorial Field areas of interest. Characterization of the 175 Newport Way Northwest area of interest is more complete than at other areas of interest in the Lower Issaquah Valley and was considered sufficient to begin development of a Source Remediation Plan for PFAS-impacted soil and groundwater.