Barnstable County plans reactive barrier to intercept PFAS plume from former Hyannis fire training site

Barnstable County consultants presented plans to install a permeable reactive barrier (PRB) using colloidal activated carbon at the former Hyannis fire training site in central Barnstable to intercept PFAS migrating toward Mary Dunn Pond and nearby public drinking water wells. The team said full‑scale injections are scheduled to begin in mid‑July and are expected to take about 12 weeks to complete, subject to permitting.

The project aims to reduce the volume of PFAS reaching downgradient receptors so the Hyannis Water District’s wellhead treatment systems require less intensive filtration. John Paquin of GZA, a presenter on the call, described the contaminants as “the so‑called forever chemicals… PFAS,” and showed groundwater contours indicating flow from the site to Flint Rock Pond and southeast toward Mary Dunn Pond and the Mary Dunn public wells (identified in presentation materials as MD‑12 and MD‑3).

Consultants emphasized that the county already took two actions: stopping water‑based firefighting training in 2019 and installing a site cap to limit precipitation infiltration. The PRB is the horizontal component of the county’s remedial approach. Dan (senior project manager, GZA) said the technology is passive once installed: "It's a very simple technology… it's a passive technology that just sits there and absorbs the contaminants in situ." He and other presenters said a small pilot injection in October tested the approach at a hot‑spot in the southeast corner of the site and produced encouraging early results.

Design and pilot outcomes: GZA said the pilot test used colloidal activated carbon (branded in the presentation as PloomSTOP) in a roughly 10‑by‑30‑foot area and involved about 24,000 pounds of product. The pilot has been monitored through three rounds of post‑injection sampling; presenters reported concentrations in down‑gradient monitoring wells at or below the regulatory criterion referenced in the presentation (described in the meeting as a PFAS‑6 benchmark of about 20 parts per trillion) or non‑detect.

Full‑scale plan details: The planned PRB alignment runs along the majority of the eastern and southern property boundary and is intended to intercept the site’s natural groundwater gradient. Consultants said the PRB will extend roughly 615 linear feet and be installed by injecting a colloidal activated‑carbon slurry from about 250 injection points spaced roughly 5 feet apart. They estimated approximately 590,000 gallons of slurry will be injected. Subsurface geologic cross sections shown at the meeting indicate a lower‑permeability silt/clay confining layer at about 60–90 feet below grade; presenters said the PRB design targets contact with that layer to limit vertical migration.

Monitoring and schedule: To evaluate performance, the team installed nine multilevel monitoring locations downgradient of the PRB, each with shallow, intermediate and deep ports (27 sample points total). Those wells were developed in April–May and sampled once immediately after installation; additional monthly monitoring is planned during injection activities and quarterly laboratory sampling afterward. Consultants said all monitoring data and IRA (interim remedial action) status reports will be made available to the public.

Permits, risks and next steps: Presenters noted portions of the proposed PRB alignment fall within the 100‑foot wetland buffer; an Order of Conditions from the Barnstable Conservation Commission is pending and was described as a precondition to construction in those areas. The presenters estimated construction would begin in mid‑July and run approximately 12 weeks, with performance monitoring continuing afterward. No formal vote or permitting decision was recorded at the meeting; the session was an informational presentation and public comment period produced no questions.

Background and context: The presenters said the former training site (used by the Hyannis Fire Department from 1959–1983 and by the county beginning in 1987) was frequently used for live‑fire and water‑based training. Because individual departments historically brought their own firefighting foam, the county said it has no records quantifying foam use at the site. Once PFAS was identified, water training ceased in 2019 and the county and town installed wellhead treatment systems on drinking wells to reduce risk to residents.

Public access and reporting: The team said monitoring results from the pilot and from future sampling rounds will be included in forthcoming IRA status reports available to the public. Presenters invited questions at the end of the call; the meeting record indicates no public questions were raised during the session.