In 2006, a “passive” non-aqueous phase liquid (NAPL) barrier was installed at New York State Electric & Gas Corporation’s (NYSEG’s) Court Street Former MGP Site in Binghamton, New York, USA (the Site). The term “passive” means that no pumping of groundwater is required for NAPL collection. The purpose of the NAPL barrier is to prevent on-site NAPL from migrating to the Susquehanna River, which is located less than 30 meters (m) downgradient of the site. The site had several complicating features including multiple active natural gas mains, a 170-centimeter diameter storm sewer, a major roadway, and a flood wall along the Susquehanna River. The construction was the culmination of nine years of work involving site characterization, conceptual design development, site-specific multi-phase fluid-flow modeling, and remedial design. The design methods and techniques used to construct the NAPL barrier were innovative and selected specifically to address the complex site conditions. The NAPL barrier was constructed using a “design–build” approach where the engineer and the contractor were the same entity. The remainder of this document describes site conditions, data requirements for designing the NAPL barrier, NAPL barrier design, and construction methods.