Why is Vapor Mitigation System a Requirement?

California was once considered one of the largest contributing states to the world’s oil output.  Many of these high-output oil sources are now abandoned, but the contamination footprint is left behind. Vapor Mitigation involves environmental problems from methane and other volatile chemicals leakage into breathable air and water. More contributions to the contaminated impression include soil systems are continuing problems.

Oil Wells are just one source of contamination that now requires mitigation.  Phase 1 and 2 environmental site assessments will provide your property’s complete ecological history and current subsurface contamination reports.  Contamination from previous work can be found in an outline in these reports. Examples include mechanic shops, industrial manufacturing, and dry cleaning businesses.

Due to various 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 navigation through EPA’s DTSC‘s vapor mitigation code. Understanding this and implementing the code requirements correctly is the duty of a vapor mitigation consultant. Suppose a phase 2 environmental report recommends a vapor mitigation system. In that case, hiring a vapor Mitigation consultant is highly recommended. This helps distinguish what the DTSC requirements will be for your project.

The vapor mitigation requirements vary from passive sub-slab deep pressure GPD pressurization systems to active monitoring and ventilation systems. The cost associated with these will vary greatly depending on the architectural design and the imposed contaminants in the property.

Not all vapor mitigation consultants have credibility. Considering this is a new field, it is common to witness inexperienced vapor Mitigation consultants advertising professionalism and experience. Due to this, it’s essential to ask for references from other vapor mitigation consultants. Whom can attest to the knowledge faced by that particular consultant.

DTSC Code

Outlined in Assembly Bill 422 (AB 422) is the California Health and Safety Code amendment. Along with the addition of the Water Code. This bill requires an exposure assessment, including the response action, to occur. The assessment and response action must include the development of reasonable maximum estimates of exposure to volatile chemicals that enter the project. Which will cause exposure for the occupants due to the accumulation in the indoor air. VI mitigation is the response action commonly done in most projects because it essentially blocks the pathway between the vapor source and occupants until the vapor source removal from the subsurface is done. In most cases, remediation is not the most practical solution, so VI mitigation seems to be the most achievable solution.

In some cases, DTSC will not require a vapor mitigation system. But a VI mitigation might be  a proposition as a preventative solution for a perceived threat. For example, the site has a risk of less than a 1 x 10-6 risk level or a hazard index (HI) of 1. Given the low risk and there might have been no building. A developer might still install a VI mitigation as a preemptive solution.

Examples of DTSC

Another case is if a project is near an area with subsurface contamination. It might still  install even if the calculation risk level is less than or equal to 1 x 10-6 or a HI less than or equal to 1. Also the DTSC does not require mitigation. If your project is not affected by a groundwater plume as of the moment. But might be, a preventative VI mitigation might still be installed. Since DTSC does not necessarily require vapor mitigation construction, they will neither approve nor enforce the proposal in these cases.

But if the project requires a VI mitigation, the Vapor Intrusion Mitigation Advisory provides guidelines for selecting the appropriate mitigation approach. Given that Sub-slab Depressurization (SSD) and Sub-slab Venting Systems (SSV) are the most commonly in use vapor mitigation techniques, these emphasize over other technologies. You don’t need to analyze the other mitigation systems thoroughly if one of the two is selected.

DTSC Code Cont’d

An SSV system essentially vents sub-slab soil gases to direct the migration of soil gas to the exterior of the building to prevent it from entering a structure. The system does this by drawing outside air to the sub-slab area, lowering the volatile chemical concentrations. One material used in an SSV system is a venting material such as 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 is utilized in this process to draw 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) and Submembrane Depressurization (SMD). As well as  Building Pressurization, and Indoor Air Treatment. Some projects might require other variations of SSD Systems. Such as Aerated floor systems, Block-wall suction systems, and Drain-tile suction systems. As well as Sub-slab pressurization (SSP) systems, which are specific types of SSV systems and Podium-Style Buildings.

Notes on DTSC Code

These approaches will  compare to each other or might even  combine for a project. To determine what is  applicable, a detailed analysis and evaluation with the following criteria will be executed. Overall protection of human health and the environment, Compliance with federal/state/local requirements, and Long-term effectiveness and permanence. Also protection in  Reduction of toxicity, mobility or volume through treatment, Short-term energy, and Implementation based on technical and administrative feasibility. Such as Cost, State and local agency acceptance, and Community acceptance. If an alternative is applicable, you’ll need to provide a detail evaluation of it as well.

Evaluation for Mitigation Systems

The evaluation should include the establishment of site-specific performance objectives for the VI mitigation system, recordation of land use covenants, recognition of long-term responsibilities in maintaining financial assurance and compliance with the five-year review requirement, identification of applicable federal/state/local requirements, and evaluation of the mitigation alternatives and the no-action alternative against the relevant criteria. As you can see, 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 will take place. If the project involves the construction of a new building, several considerations must be made to reduce VI risks, such as varying the basements or location of elevator shafts. If VI mitigation is to be 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 the concentration of volatile chemicals.

Consideration for Foundations

Building a foundation must be one of the considerations in the design process of a vapor intrusion mitigation design to verify the presence of cracks in concrete slabs, construction joints between slabs and walls, and gaps in the fieldstone. These might become entry points for volatile chemicals. Then discovery entry points must then seal off. If this approach is applicable, doing this would also strengthen the negative pressure field created through an SSD system. Diagnostic testing thoroughly on the airflow characteristics of the material under the slab to verify if a negative pressure field below the slab will  create and develope. Due to the movement of the soil and other factors, future inspections should be expect and accommodate during the design process.

For the operation and maintenance of the actual vapor mitigation system, an O&M plan is a requirement. This plan generally includes the system’s performance goals, measures, and monitoring requirements. Performance goals and actions  establish to ensure the system operates correctly and prevent 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.

These VI mitigation requirements won’t be meet if independent subcontractors lacking experience are solely at task for installation. A DTSC consultant fully aware of and with experience on these requirements must be in comunication. This ensures that the proper vapor intrusion mitigation system  installs. Partnering with the VI vapor intrusion contractor, the system meets  assurance of its integrity and quality.