Why is a Vapor Mitigation System Required?
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 contaminated footprint is left behind. Environmental problems such as leakage of methane and other volatile chemicals into breathable air, water and soil systems are continuing problems.
Oil Wells are just one source of contamination that now requires mitigation. Phase 1 and Phase 2 environmental site assessments will provide complete environmental history and current subsurface contamination reports for your property. Contamination that results from previous work can be found and outlined in these reports, examples of this include mechanic shops, industrial manufacturing, and dry cleaning businesses.
Due to various soil contamination, and recommendations that are made from environmental reports, vapor mitigation systems are becoming a more frequent requirement for new developments around the United States.
Role of a DTSC Vapor Mitigation Consultant
The implementation of 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. If a phase 2 environmental report recommends a vapor mitigation system, it is highly recommended to hire a vapor Mitigation consultant to help distinguish what the DTSC requirements will be for your project.
The vapor mitigation requirements can 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 along with the imposed contaminants that are present in the property.
Not all vapor mitigation consultants have credibility, considering this is a new field, it is common to witness extremely inexperienced vapor Mitigation consultants advertising their professionalism and experience. Due to this, it’s important to ask for references from other vapor mitigation consultants that can attest to the experience that was faced for that particular consultant.
Outlined in the Assembly Bill 422 (AB 422) is the amendment of the California Health and Safety Code with the addition of the Water Code. This bill requires that an exposure assessment including the response action be made. The assessment and response action must also include the development of reasonable maximum estimates of exposure to volatile chemicals that might enter the project and will cause exposure for the occupants due to the accumulation in the indoor air. VI mitigation is the response action commonly done to 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 a 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 proposed 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 that the risk is low and there might have been no building yet, a developer might still install a VI mitigation as a preemptive solution. Another case is if a project is near an area with subsurface contamination, 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 and mitigation is not required by the DTSC, it might still be installed. If your project is not really affected by a groundwater plume as of the moment but might be, a preventative VI mitigation might still be installed. In these cases, since DTSC does not necessarily require vapor mitigation construction, they will neither approve nor enforce the proposal.
But if the project does require a VI mitigation, the Vapor Intrusion Mitigation Advisory provides guidelines for the selection of the appropriate mitigation approach. Given that Sub-slab Depressurization (SSD) and Sub-slab Venting Systems (SSV) are the most commonly used vapor mitigation techniques, these are more emphasized over other technologies. If one of the two is selected, there is no need to provide an in-depth analysis for the other mitigation systems.
A 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 in outside air to the sub-slab area, which lowers the volatile chemical concentrations. One material used in a SSV system is a venting material such as sand or pea gravel placed below the slab. A SSD system creates a lower pressure under the building floor which results to sub-slab negative pressure. The negative pressure field prevents volatile gases from entering the building by collecting and piping them to the atmosphere. A blower is utilized in this process to draw air away from the soil below the building.
However, site-specific characteristics (such as building type and use, receptor type, and volatile chemical concentrations) might require alternative technologies other than SSD or SSV. Some of these are 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, Sub-slab pressurization (SSP) systems are a specific type of SSV system, and Podium-Style Buildings.
These approaches will be compared to each other or might even be combined for a project. To determine what should be applied, 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, 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, Community acceptance. If an alternative will be applied, a detailed evaluation on it must be provided as well. 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 applicable criteria. As you can see, the DTSC consultant would have to consider site-specific conditions when selecting the most appropriate technology for the 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 and 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.
Building 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 fieldstone. These might become entry points for volatile chemicals. The discovered entry points must then be sealed off. Doing this will also strengthen the negative pressure field created through a SSD system, if that approach was applied. A diagnostic testing should also be done on the air flow characteristics of the material under the slab to verify if a negative pressure field below the slab will be created and developed. Due to the movement of the soil 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 will be required. This plan generally includes the system’s performance goals and measures and monitoring requirements. Performance goals and measures should be established to ensure that the system is operating correctly and preventing volatile chemical migration into the structure. Monitoring requirements include the establishing of baseline conditions, routine vapor and pressure monitoring, routine monitoring of system operations, indoor air quality monitoring, soil vapor monitoring, adjacent buildings, and monitoring for combustible gases.
All of these VI mitigation requirements might not be fully met if inexperienced independent subcontractors will be solely tasked for the installation. This is why a DTSC consultant who’s fully aware of and experienced with these requirements must be consulted. This is to ensure that the proper vapor intrusion mitigation system will be installed. Partnered with the VI vapor intrusion contractor, the system will be ensured of its integrity and quality.