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Vermont DEC to replace stormwater model as MS4 permits and 3‑acre site inclusions draw local questions
Summary
At the Nov. 4 MS4 Subcommittee meeting, Vermont DEC said it has contracted Geosyntec to replace an outdated stormwater‑impaired‑waters model with $100,000 from the Clean Water Fund; municipalities raised concerns that the new model could change flow‑restoration targets and affect projects included under MS4 permits, especially the inclusion of 3‑acre sites.
Vermont Department of Environmental Conservation officials told the Chittenden County Regional Planning Commission MS4 Subcommittee on Nov. 4 that they have selected Geosyntec Consultants to replace an aging stormwater‑impaired‑waters model and that the project received $100,000 from the Clean Water Fund.
Emily Schelley, representing DEC, said the old model — developed in 2017 and tied to ArcGIS 9.1 — was a bottleneck because only DEC staff could run it. The new platform is intended to be more stable, user‑friendly and based on up‑to‑date, open‑source tools so communities and consultants can run it themselves. Christy Witters said the agency is hiring a new MS4 coordinator to replace Sam Hughes.
Committee members raised technical and practical concerns. Chip Sawyer of the City of St. Albans and others asked whether flow restoration plan (FRP) targets and best management practice designs set under the old model would change once the new model is implemented. Schelley said DEC will try to replicate the old model’s outputs while moving to a more accessible platform but acknowledged additional funds will likely be needed to continue development. "Geosyntec will be wrapping up their final deliverables soon," Schelley said.
The committee also discussed a batch of MS4 permits currently on public notice and differing municipal approaches to including 3‑acre sites. Schelley noted some confusion in communities over whether 3‑acre sites are included; members said Essex incorporated one site, Burlington and the City of St. Albans are incorporating four 3‑acre sites each, and Shelburne and Williston currently do not plan to add new 3‑acre sites. Members requested clearer guidance on water‑quality volume, impervious surface and phosphorus calculations to help municipalities and consultants apply the permits consistently.
DEC staff and committee members said they will coordinate on funding and technical support as the replacement model is finalized. Geosyntec’s deliverables and DEC’s plan for further funding and rollout remain the next steps; committee members expressed readiness to engage once the tool is accessible to local technical staff.
