Should I Hire a DTSC Vapor Mitigation Consultant?
California used to be one of the world’s biggest oil producers. Many of those high-output wells are now abandoned, but the contamination footprint remains. Vapor mitigation deals with environmental problems from methane and other volatile chemicals leaking into breathable air and water. Contaminated soil systems continue to cause problems.
Oil wells are just one contamination source that now requires mitigation. Phase 1 and 2 environmental site assessments provide your property’s complete ecological history and current subsurface contamination reports. These reports outline contamination from previous uses like mechanic shops, industrial manufacturing, and dry cleaning businesses.
Because of soil contamination and recommendations from environmental reports, vapor mitigation systems became a requirement for new developments around the United States.
Role of a DTSC Vapor Mitigation Consultant
Implementing vapor mitigation systems requires understanding EPA’s DTSC vapor mitigation code. A vapor mitigation consultant ensures code requirements are implemented correctly. If a Phase 2 environmental report recommends a vapor mitigation system, hiring a vapor mitigation consultant is highly recommended. This helps determine what DTSC requirements apply to your project.
Vapor mitigation requirements vary from passive sub-slab depressurization systems to active monitoring and ventilation systems. The cost varies greatly depending on the architectural design and the contaminants present on the property.
Not all vapor mitigation consultants have credibility. Since this is a relatively new field, inexperienced consultants often advertise professionalism they don’t possess. Ask for references from other vapor mitigation projects. These references can confirm the consultant’s actual knowledge and experience.
DTSC Code Requirements
Assembly Bill 422 (AB 422) amended the California Health and Safety Code and added provisions to the Water Code. This bill requires an exposure assessment, including a response action. The assessment and response action must include reasonable maximum estimates of exposure to volatile chemicals that could enter the project and expose occupants through indoor air accumulation.
Vapor intrusion (VI) mitigation is the most common response action because it blocks the pathway between the vapor source and occupants until the vapor source can be removed from the subsurface. In most cases, remediation isn’t practical, so VI mitigation is the most achievable solution.
Sometimes DTSC won’t require a vapor mitigation system, but VI mitigation might be proposed as a preventative solution for a perceived threat. For example, if a site has a risk of less than 1 x 10-6 or a hazard index (HI) of 1, a developer might still install VI mitigation as a preemptive solution.
Examples of DTSC Applications
Another case: if a project is near an area with subsurface contamination, mitigation might still be installed even if the calculated risk level is less than or equal to 1 x 10-6 or a HI less than or equal to 1. Even when DTSC doesn’t require mitigation, if your project isn’t currently affected by a groundwater plume but might be in the future, preventative VI mitigation might still be installed. Since DTSC doesn’t necessarily require vapor mitigation construction in these cases, they won’t approve or enforce the proposal.
But if the project requires VI mitigation, the Vapor Intrusion Mitigation Advisory provides guidelines for selecting the appropriate mitigation approach. Sub-slab Depressurization (SSD) and Sub-slab Venting Systems (SSV) are the most commonly used vapor mitigation techniques. If you select one of these two systems, you don’t need to analyze other mitigation systems as thoroughly.
How DTSC-Compliant Systems Work
An SSV system vents sub-slab soil gases to direct the migration of soil gas to the exterior of the building, preventing it from entering the structure. The system draws outside air to the sub-slab area, lowering volatile chemical concentrations. One material used in an SSV system is a venting material like sand or pea gravel placed below the slab.
An SSD system creates lower pressure under the building floor, resulting in sub-slab negative pressure. The negative pressure field prevents volatile gases from entering the building by collecting and piping them into the atmosphere. A blower draws air away from the soil below the building.
However, site-specific characteristics (building type and use, receptor type, and volatile chemical concentrations) might require alternative technologies other than SSD or SSV. These include sealing cracks and openings, sub-slab liners (passive membranes or vapor barriers), submembrane depressurization (SMD), building pressurization, and indoor air treatment. Some projects might require other variations of SSD systems, such as aerated floor systems, block-wall suction systems, drain-tile suction systems, and sub-slab pressurization (SSP) systems, which are specific types of SSV systems for podium-style buildings.
Selecting the Right Mitigation Approach
These approaches can be compared to each other or combined for a project. To determine what’s applicable, a detailed analysis and evaluation with specific criteria must be executed: overall protection of human health and the environment, compliance with federal/state/local requirements, long-term effectiveness and permanence, reduction of toxicity, mobility or volume through treatment, short-term effectiveness, implementability based on technical and administrative feasibility, cost, state and local agency acceptance, and community acceptance. If an alternative approach is applicable, you’ll need to provide a detailed evaluation of it as well.
Evaluation for Mitigation Systems
The evaluation should include establishing site-specific performance objectives for the VI mitigation system, recording land use covenants, recognizing long-term responsibilities in maintaining financial assurance and compliance with the five-year review requirement, identifying applicable federal/state/local requirements, and evaluating the mitigation alternatives and the no-action alternative against the relevant criteria. The DTSC consultant must consider site-specific conditions when selecting the most appropriate technology for VI mitigation.
Once the evaluation of the approaches is done, the building design process takes place. If the project involves constructing a new building, several considerations must reduce VI risks, such as varying the basements or location of elevator shafts. If VI mitigation is done on an existing building, the building foundation, possible entry points, sub-slab permeability, flow characteristics, and future inspections should all be considered in the design process. Depth to water and labeling might also be considered depending on the severity of volatile chemical concentrations.
Foundation Considerations
Building foundation analysis must be one of the design process considerations for vapor intrusion mitigation design. This verifies the presence of cracks in concrete slabs, construction joints between slabs and walls, and gaps in the fieldstone. These can become entry points for volatile chemicals. Identified entry points must then be sealed. If this approach is applicable, doing this also strengthens the negative pressure field created through an SSD system.
Diagnostic testing thoroughly examines the airflow characteristics of the material under the slab to verify if a negative pressure field below the slab can be created and developed. Because of soil movement and other factors, future inspections should be expected and accommodated during the design process.
For the operation and maintenance of the actual vapor mitigation system, an O&M plan is required. This plan generally includes the system’s performance goals, measures, and monitoring requirements. Performance goals and actions are established to ensure the system operates correctly and prevents volatile chemical migration into the structure. Monitoring requirements include establishing baseline conditions, routine vapor and pressure monitoring, regular monitoring of system operations, indoor air quality monitoring, soil vapor monitoring, adjacent buildings, and monitoring for combustible gases.
Why You Need an Experienced DTSC Consultant
These VI mitigation requirements won’t be met if independent subcontractors lacking experience are solely tasked with installation. A DTSC consultant fully aware of and experienced with these requirements must be involved. This ensures the proper vapor intrusion mitigation system is installed. Partnering with an experienced VI vapor intrusion contractor provides assurance of the system’s integrity and quality.
Sway Features specializes in DTSC vapor mitigation design and construction. Our team of licensed engineers and contractors has a proven track record of receiving plan-check approval for vapor intrusion systems with LADBS and working closely with DTSC. Contact us at 888.949.7929 to discuss your project’s vapor mitigation requirements.