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Oyster Pond watershed plan shows IA upgrades alone likely won’t meet TMDL; committee debates credits and sewers
Summary
Town watershed staff presented a proposed IA geoboundary for Oyster Pond and said using the conservative 10 mg/L credit for advanced systems would yield roughly 608 kg nitrogen reduction but fall short of MEP/TMDL targets (≈1,071 kg); committee members and residents discussed whether higher‑performing ‘enhanced IA’ approvals or sewering would be required.
Town watershed staff (Kristin) presented a draft geoboundary and nitrogen accounting for Oyster Pond, explaining why different data sources (MEP, TMDL summary table and the Cape Cod Commission’s MVP tool) show divergent loading estimates for the watershed.
Using a conservative planning assumption — that approved IA systems receive credit at the regulatory performance level of 10 mg/L — the town’s analysis shows upgrading IA systems across the watershed would reduce nitrogen by a minimum estimated 608 kilograms per year, after crediting existing systems. That figure falls short of the MEP/TMDL reduction requirement (presenter cited ~1,071 kg). The Cape Cod Commission’s estimate (731 kg) was lower than the MEP’s but higher than the town’s conservative IA estimate.
Kristin emphasized the exercise is intended to show scale and feasibility: if the state continues to require credit at the approved performance level (10 mg/L), meeting Oyster Pond’s TMDL without sewering is unlikely. Committee members raised the possibility that enhanced innovative alternative systems (EIA) with monitored performance under provisional approval (some products showing median performance below 10 mg/L) could change the calculation if DEP grants general‑use credit at a lower number in the future.
Public members and committee members asked whether sewers or package plants (common for clustered developments like Treetops) could be more cost‑effective for multifamily parcels. Staff said watershed plans will examine parcel‑level loading and options (sewer, IAs, package plants) and that the geoboundary is a planning step that would be refined in a formal CWMP or watershed plan.
No formal policy decision was made; the presentation frames the scale of nitrogen removal needed and flagged that, absent lower IA crediting or sewering, Oyster Pond’s TMDL will be difficult to meet with conventional IA upgrades alone.
