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South ramp air‑quality modeling shows net emissions declines; commission narrows sound‑barrier choice to perforated PVC with mineral wool
Summary
Commissioners reviewed a modeling report showing modeled emission reductions from the South Apron expansion under worst‑case activity assumptions, and consultants recommended a perforated PVC absorptive sound wall with mineral‑wool backing as the preferred noise barrier.
Commissioners reviewed an air‑quality technical report and a consultant sound‑barrier study for the south apron expansion at the Feb. 11 meeting. Staff described the air‑quality study as a baseline, worst‑case modeling run that compared pre‑ and post‑expansion conditions using locally recorded meteorology and aircraft activity patterns.
Consultants reported that the model showed reductions in total emissions across modeled criteria pollutants in the scenarios run. The greatest modeled reductions came from decreased taxi time and more efficient aircraft routing to the new apron configurations; receptors near some portions of the extended ramp would see modest increases in worst‑case scenarios, but the magnitude of modeled increases remained far below EPA criteria pollutant thresholds, staff and consultants said.
On noise mitigation, McFarland Johnson and HMMH consultants presented four wall options: perforated PVC with mineral‑wool absorptive fill, fiberglass absorptive panels, precast absorptive concrete, and timber panels. The presenters explained the difference between reflective walls, which bounce sound back to the airfield, and absorptive walls that capture and dissipate sound energy. Consultants said an absorptive wall with a sloped top and side returns would perform better than a simple reflective barrier.
Consultant Rich Lasdun recommended a perforated PVC panel system with mineral‑wool absorption as the preferred solution based on modeled performance, anticipated noise reduction, maintenance considerations and modular construction benefits. Lasdun said manufacturers describe a service life up to 40 years and that some installations exist in comparable northeastern climates; he said the PVC option and fiberglass panels met the minimum 0.9 decibel reduction threshold the team used as a reference.
Commissioners asked about durability in Nantucket’s salt‑spray environment, foundation and attachment options, and whether a sloped 45‑degree top return could be used; the consultant said manufacturers offer those details and that foundation options would include drilled shafts or caissons sized for wind loads. Commissioners requested ballpark cost estimates and said they would like a follow‑up in the March meeting to discuss cost ranges and bid timing. Staff said the team aims to issue bids in March to obtain firm pricing ahead of annual town meeting, noting the tradeoff that state procurement rules typically require contractors to hold prices only for a limited period (approximately three months), which could make early bids stale by the time contracts are executed.
The commission did not take a formal vote on a final wall material at the meeting but directed staff and consultants to return with cost estimates and bid‑ready documents.
