North Carolina Proposes New Guidance for Vapor Intrusion Assessment and Mitigation | Williams Mullen

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The North Carolina Department of Environmental Quality (DEQ) has drafted a new and revised guidance document for the assessment and mitigation of vapor intrusion (VI) in properties undergoing reuse and redevelopment (Draft Guidance). Led by the Brownfield Redevelopment Section of DEQ’s Waste Management Division, new draft guidelines were developed in his February and presented to stakeholders for consideration and comment on March 10, 2023. At that time, DEQ requested comments on the new and revised guidelines for 30 days. Guidance and comments are currently being reviewed by DEQ.

The new Vapor Ingress Guidance contains four draft documents.

  • Revised Brownfield Vapor Penetration Assessment Checklist
  • New minimum relaxation requirements for reuse
  • Revised Brownfield Vapor Ingress Mitigation System Design Checklist
  • New Minimum Sampling Guidelines Summary Table

In developing the draft guidance, DEQ has responded to stakeholder demands for consistency in decision-making, predictability of project budgeting, and achievement of endpoints in steam intrusion assessment and mitigation in site reclamation projects. said that The technical issues that the new guidance seeks to address are not only specific concerns about experience with discrepancies between external soil gas measurements and sub-slab soil vapor data, but also due to changes in the overall technical landscape regarding vapor intrusion issues. identified as compatible. Addressing trichloroethene (TCE) as an immediate exposure threat and addressing methane, an important source of which is present. Ultimately, this guidance promotes the greatest possible consistency and predictability of sampling, the imminent threat of exposure, the distinction between residential and commercial areas (e.g., lower levels of TCE behavior in residential areas), and We aim to achieve a sampling endpoint that balances concerns about TCE with sublevel reflections. – Slab vapor is the best risk predictor compared to external soil gas.

The main components of the draft guidance are:

  • Baseline assessment: Sampling protocol recommendations for external soil gas, subslab soil gas, and indoor air sampling, including: Spatial distribution/sampling frequency

o Minimum initial and final sample canister vacuum

o maximum purge rate during sampling

o Sampling time after probe attachment

o Adding analytes for vapor testing

  • VI Mitigation System Decision Making: Incorporation of several decision matrices determined by the results of the DEQ Risk Calculator, the presence of TCE, and whether the site’s end use is residential or non-residential. Mitigation is recommended and/or additional pre-occupation and post-occupation Inspection requirements now apply.
  • Pre-Occupancy and Post-Occupancy Sampling: Requirements for pre-occupation and post-occupancy vapor sampling for both indoors and sub-slabs, based on the presence of TCE and risk calculation results. Post-occupancy sampling of sites typically defaults to at least one year of sampling, regardless of TCE concentration or mitigation approach.

The draft guidelines aim to establish minimum mitigation and sampling requirements for vapor ingress not only for the brownfield redevelopment sector, but for the entire waste management sector. By describing them as minimum requirements, the draft guidelines could be interpreted as establishing binding standards of conduct for all contaminated sites in North Carolina. As such, this guidance may be relevant to cleaning other areas besides brownfield redevelopment sections, so we invite scrutiny and comments from a wide range of stakeholders.

DEQ is still considering comments, but it is expected that the final version of the draft guidance will be issued in the near future.

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