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Articles

Optimizing Vapor Intrusion Mitigation: Green Remedies in Practice

April 10, 2000

Application of Green Remedies to Vapor Intrusion Mitigation

Some states require a responsible party to consider green remediation and sustainability in remedy selection and periodically during remedy implementation. For example, the New York State Department of Environmental Conservation (NYSDEC) has a Green Remediation Policy (DER-31) which states “Green Remediation…the practice of considering all environmental effects of remedy implementation and incorporating options to minimize the environmental footprint of cleanup actions.” An example listed in this policy document is “Focus on optimization to reduce energy use or time to closure.” This example can be directly applied to vapor intrusion mitigation methods. Not only is Green Remediation good for the environment, but it is also typically good for the economics of the project.

Vapor intrusion can be an ongoing liability and operation and maintenance (O&M) costs can erode profits for property owners. We continually encounter commercial property owners with overdesigned, oversized, and inefficient sub-slab depressurization systems (SSDS) to mitigate vapor intrusion issue at hand. Even if not required by applicable regulations, it is important to periodically evaluate ongoing O&M protocols and proactively apply more sustainable methods to remediate the site and reduce energy use and costs via operational optimization, waste stream reduction, and conservation of resources.

What is Vapor Intrusion?

According to the United States Environmental Protection Agency (USEPA), vapor intrusion occurs when vapor-forming chemicals migrate from a subsurface source into an overlying building. Volatile chemicals from a contaminated groundwater or soil source can enter a building through cracks or holes in the foundation slab or walls, utility penetrations, and/or can permeate through building materials (i.e., concrete slabs and walls). Vapor-forming chemical compounds typically include:

Volatile organic compounds (VOCs), such as perchloroethylene and benzene;
• Select semi-volatile organic compounds, such as naphthalene;
• Inorganic chemicals, such as elemental mercury; and
• Explosive and Toxic Gases, such as methane and hydrogen sulfide.

State regulations often require the removal of the contaminant source; however, a source often cannot be identified or is emanating from an offsite property, and a SSDS may be required to ensure short-term or long-term protection of building occupants against breathing the vapors that contaminate indoor air. SSDSs should be designed and installed by experienced professionals with the use of site-specific information (e.g., pilot testing) to ensure efficient performance. There are various types of SSDSs that can be designed to ensure that the operation achieves the lowest energy use possible while effectively mitigating vapor intrusion. Existing SSDSs can also be altered or retrofitted to improve efficiency and performance and reduce O&M costs.

What Type of System do You Have?

Passive:

For passive systems, the goal is simple; prevent the buildup of contaminant vapor concentrations below the building slab with the implementation of passive ventilation via natural forces. Porous media, collection piping, and vertical risers may be all that are required for a system to reduce sub-slab vapor concentrations sufficiently to prevent vapor intrusion. Consider that the effectiveness of these systems depends and varies greatly on weather (temperature, pressure, wind, etc.), indoor ventilation (i.e., operation of exhaust fans) and other design factors such as the number and location of risers and size of the piping. Hourly, daily, and seasonal weather changes can influence the efficiency of passive systems. As relating to Green Remediation, passive systems utilize no energy, minimize waste streams, and require little to no O&M, which minimizes site visits and lowers the overall impact to the environment.

If your passive system is not doing the job, consider the addition of wind-driven syphon or turbine ventilation fans (sometimes referred to as a semi-active system) which can increase the effectiveness of passive systems during what may be times of quiescence. The wind-driven fans induce flow and create a negative pressure gradient at times when there may be none under passive conditions. The costs for the addition of these fans are relatively low and they use no electricity. They serve as a rain and animal cap and can provide additional ventilation at times when a purely passive system may be lacking. They also can prevent the potential future need for an active system that uses electrically powered fans.

Active:

Active systems utilize electrically powered fans to induce a continuous flow of air and a negative pressure gradient across the building slab. The initial design of a SSDS should be based on the results of a pilot study and should consider long-term O&M costs, which include electrical, consumables, equipment lifespan, etc. VERTEX has encountered systems designed with no site-specific operational data, that if obtained from a pilot study, would have prevented the installation of a more energy intensive system than what is required to achieve the desired results. These systems tend to utilize high vacuum and horsepower blowers that are inefficient for the application at hand. It should be noted that air permits may also be required in certain jurisdictions for the operation of an active SSDS and may require off-gas treatment, while passive SSDS typically do not require an air permit.

VERTEX was recently tasked to take over the design of a SSDS that originally planned to utilize two 5.5-horsepower (HP) regenerative blowers. Based on the project location, likely soil type, and prior experience, the design was thought to be excessive. VERTEX performed a pilot study to obtain site-specific operational parameters, including pressure and flow, and calculated a radius of influence. Based on this data, the two blower 11 HP system was redesigned with a single 1 HP centrifugal blower. This resulted in an approximate 90% reduction in energy usage that offset the cost of the pilot study in the SSDS’s first year of operation. The resulting design was not only more efficient and cost-effective, but was quieter and required less O&M, all components of a “green” remedy.

As outlined in NYSDEC DER-31, responsible parties may periodically need to complete a Remedial Site Optimization Process to identify opportunities to reduce energy and other impacts on the environment. Although the example above may be an extreme case, depending on electrical rates and motor load, a relatively small horsepower (<5 HP) motor running continuously can cost upwards of $4,000/year in electricity. Costs to run the blower for years along with the added costs of other O&M add up quickly and cut into the bottom line.

Changing the blower type or adding a variable frequency drive may incur upfront capital costs but will provide savings over the life of the remediation project. A change in the flow, number of extraction points, and piping location, may allow for the use of a more efficient blower. There is of course always a tradeoff, and each blower type has its pros and cons as they vary in flow rate, induced vacuum, noise, lifespan, and capital cost. Understanding the limitations for each blower type and application is key, and the overall evaluation should consider the breakeven timeframe of the equipment.

Upon installation of a SSDS, it should be confirmed that the system is operating efficiently and effectively. It must also be confirmed that the system is balanced as an increased flow rate at a single extraction point may limit the system performance elsewhere. Regulators are typically looking for confirmation of the induced pressure field below the entire building slab. This is a conservative approach to show the system is creating a slight vacuum beneath the entire sub-slab region to prevent vapor intrusion. The overall mass and concentration of sub-slab vapors will be reduced upon startup and continued operation of the system and indoor air testing would generally be required to confirm the effectiveness of the system at preventing vapor intrusion.

How can VERTEX help?

Whether or not you are required to consider green remediation practices, if your SSDS is not operating as designed, is expensive to operate, generates an unnecessary waste stream, or the sub-slab pressure field is significantly more than what is needed to prevent vapor intrusion, a VERTEX expert can perform a periodic evaluation or a review of your existing SSDS to determine if efficiency can be increased and if cost savings can be gained. The evaluation will take into account the size, type, and operation of the blower, ancillary systems, and waste streams generated. The earlier into the remediation the evaluation is performed, the better for increased returns. Whether you are in need of services for a single asset or a full portfolio, VERTEX’s Environmental and Engineering Experts partner with you to resolve complex challenges across the remediation lifecycle.

To learn more about VERTEX’s Environmental Consulting services or to speak with an Environmental Expert, call 888.298.5162 or submit an inquiry.

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