Performance data for enhanced I/A septic systems designed for nitrogen removal that were installed during a demonstration project in Barnstable, MA. Includes 1) field and laboratory measurements of water quality parameters in influent, effluent, and lysimeters below leach fields (where available) and 2) metered flow to each system. The initial release (Version 1) includes data collected from 2021-12-10 through 2023-05-03. The second release (Version 2) adds data through 2023-12-04. The third and final release adds (Version 3) adds data through 2025-07-21. The README file contains additional details, including notes on data limitations and a change log.