In February 2023, an update to the 2011 DTSC Soil Vapor Intrusion guidance was released with input from DTSC, CalEPA, and Regional and State Water Quality Control Boards.  This comprehensive, long-awaited, revised guidance provides updates on vapor intrusion concepts and practices and will expand their application from regional to statewide in California.  This will impact how stakeholders manage characterization, mitigation, and litigation surrounding VOC exposure from potential sources of building subsurface contamination.  Specifically, changes will be applied to Vapor Intrusion Workplans and Site Modeling, and understanding these updates will be essential for on-site professionals.

Site Specific Attenuation Factor (AF) Has Become Essential for Site Closure/Characterization

The Attenuation Factor is the multiplier by which a slab or barrier prevents sub-surface concentrations from entering the indoor air space. By defining the AF on a site-specific level, the goals for thresholds enter a realm of quantitative uncertainty.  Rather than providing a new AF for California that reflects data from a recent DTSC study, the factors of 0.03 were kept. A method for defining site-specific AF is described, and GeoTracker will ultimately be used to determine a California update. This is contradictory to the findings of the 2022 DTSC study, which compared AF and IA/SS data from sites around the state and found the average AF is typically 3 to 4 orders of magnitude lower. However, it would appear the plan is private sector to fund and develop a database in order to have this recognized (See guidance section 3C.4).

Sub-Slab (SS) Specific Attenuation Factor

By co-locating Sub-Slab with Indoor Air (IA) samples, clients are being given quantifiable tool to develop more evolved models. With reference to indoor residential air Environmental Screening Limits, a Sub-Slab sample with 0.03 AF applied will nearly always trigger a detection above the objective. A prime example of education is needed now.

Applying Indoor Air Attenuation Factor to Sub-Slab Criteria

In applying IA limits to SS, but keeping the AF for SS at 0.03, uncertainty is introduced at the most key point in the process as it pertains to public health. All because of semantics. In most cases, IA samples collected have several secondary chemicals of concern (COC) exceeding their respective residential indoor air screening levels.  Slab sample with 0.03 AF applied will nearly always trigger a detection above the objective if blindly applying IA criteria to sub-surface.

The Consequence of Setting Thresholds Below what is Achievable

We are underestimating how much the slab prevents intrusion into indoor air by 100-1000x, which can mean an overestimation of risk.  This can be quantitatively proven via co-located indoor air and sub-slab samples. When secondary COCs are detected above their method detection limits (MDLs) in the soil gas and sub-slab soil gas samples, the indoor air limits must be applied to the sub-slab.  Assumptions are made here at a level of accuracy and precision that is almost equal to ambient levels in most cases. This makes the ability to achieve site transfer/closure an issue of analysis, not sample or model integrity.

Responsible parties will not be able to transfer a site without an effective model, and the model cannot be created without delineation of risk between ambient and sub-surface thresholds.  The new guidance equips remediation projects with quantitative analytical solutions.  Navigating a site closure will now require increased specialty knowledge, in both field and laboratory practices.  We can help you choose the best data collection options to save time and reduce potential risk mischaracterization.

Will Schoonover Rice

Will Rice has worked as an environmental professional for over 15 years. In his current Business Development role with Enthalpy Analytical, he applies a technical emphasis on vapor intrusion. In his work supporting site investigations, he maintains open communication and collaboration with regulators as well as private and public entities to ensure policies are applied as intended. Will also shares his expertise with several collaborative work groups, including the Interstate Technology and Regulatory Council (ITRC) Vapor Intrusion Committee team and the Education Committee of the Association of Vapor Intrusion Professionals (AVIP), allowing him to participate in public input on best practices and policy. He is part of a design and fabrication team for a patent-pending vapor sampling technology with a unique leak check system. He regularly presents his on work across the country and hopes to aid in nationwide standardization and optimization.