DTSC, Water Board outline ‘path to closure’ findings: robust conceptual models, early scoping speed closure

Department of Toxic Substances Control and State Water Board staff described a range of site outcomes and practical steps that have helped move vapor-intrusion sites to closure since the agencies issued supplemental guidance in 2023. Ben Stanfill of DTSC summarized the CalEPA "path to closure" review, which surveyed case managers and project managers on sites that had reached closure or entered long-term stewardship after the guidance. He said closure can take multiple forms: unrestricted "no further action," media-specific closures (for example, soil closed while groundwater remains under management), certified remedies with land-use controls and ongoing duties, or "suitable for occupancy" letters used by local governments. Stanfill said common features at sites that achieved endpoints included robust conceptual site models (CSMs), site-specific data with good temporal and spatial coverage, evidence that subsurface concentrations were stable or decreasing, targeted source removal (for example, soil excavation or soil-vapor extraction) and, in many cases, some form of long-term stewardship. He emphasized that subsurface concentrations above screening levels sometimes remained at sites that nonetheless reached practical endpoints if other lines of evidence supported the decision. On best practices, staff recommended scoping meetings early and often, front-loading investigations to build datasets that anticipate regulator questions, quick and direct issue resolution (for example, in-person or regular calls rather than multiple written comment-response cycles), and clear communication on project timelines so agencies and consultants can coordinate resources. Stanfill and panelists said interim remedies and mitigation can and should proceed where appropriate instead of waiting for every investigative question to be resolved. Why it matters: The guidance’s stated goal is to protect occupants earlier in the process; practitioners at the workshop argued that pragmatic, site-specific approaches and early regulator-consultant coordination shorten delays that otherwise threaten redevelopment, including housing projects under time-bound funding deadlines. Ending: CalEPA staff said they will continue to develop case studies for future technical workshops to show examples of successful closures and to provide guidance on when alternative attenuation factors, mitigation and monitoring approaches have supported site endpoints.