Assessment

Upcoming Meetings

Each sub-group meeting will be chaired in either Boston or Worcester (as indicated below) with video-conferencing available at the other location.

There are no Assessment Sub-Group meetings scheduled at this time.


DEP Leader:
Andy Friedmann, Northeast Regional Office
andrew.friedmann@state.ma.us,
978.694.3217

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3 Responses

  1. Section 2.4.2 Site Receptors, Activities and Uses, under Ongoing Permitted Commercial Operations

    Specifically:
    “The above approach applies only to ongoing and active business, commercial, and/or industrial operations that are actively using the chemicals of concern in a licensed and permitted manner. Under these circumstances it is generally not possible to achieve a Permanent Solution because it is not possible ascertain the significance of the vapor intrusion pathway. However, it may be possible to achieve temporary solution.”

    This suggests that a Permanent Solution cannot “generally” be obtained in facilities utilizing chemical of concern. Please consider the usage of any chemical raising dilution factors of IA samples, ROS as a Permanent Solution, not to mention the ability to remediate all environmental media to NSF levels. I recommend the following “it is more difficult to achieve a Permanent Solution because ascertaining the significance of the vapor intrusion pathway is complicated by on site chemical usage, and achieving a temporary solution or ROS would typically be a simpler process.”

  2. I think we need to address the key mechanism for indoor air impacts from vapor intrusion, which is the pressure difference between the sub-slab soil gas and the indoor air.

    Transport of contaminants from the soil vapor to indoor air is primarily due to flow of the soil vapor/soil gas into the indoor air through openings in the foundation, such as cracks, joints, drains, sumps and the like. This flow of gas is due to pressure difference, and only occurs if there is a positive pressure gradient between the soil gas and the indoor air, i.e. the pressure in the soil gas is higher than the indoor air. If there is no pressure gradient, there is no flow. If the pressure gradient is in the opposite direction (indoor air pressure is higher than soil gas pressure) the flow will also be in the opposite direction, i.e. indoor air will flow through the same cracks, joints, drains, sumps and the like, into the soil gas.

    A number of natural and man-made phenomena can create these pressure differentials. Perhaps the most often cited is the “chimney effect”, where the relative density difference between heated indoor air and cold outdoor air causes indoor air to exit the building, creating a negative pressure (analogous to the draft in a chimney from an operating stove or fireplace) in indoor air relative to the soil gas, thus creating a driving force for flow of soil gas into indoor air.

    Other phenomena that can cause this pressure gradient are wind across the building, which can create a lower pressure in the building on the downstream side, barometric pressure, such as when rain is approaching, which cause ambient outside air, and, if there is connection between outside air and indoor air, can cause indoor air pressure to drop faster than soil gas, and other thermal and physical mechanisms.

    The important point in all this is that the mechanism by which the pressure gradient is created are relatively unimportant. The flow of soil gas into indoor air, or vice versa, is solely a function of pressure gradient, regardless of what causes that pressure gradient.

    This has three key implications for the workgroup:

    1.

    • Continuing:

      1. we need to establish what the “worst case” “conservative” pressure gradient is. Various values have been proposed. John Fitzgerald/DEP has presented 0.2 inches of water or 50 Pascals (Pa), which is the value typically used to define whether a building is “tight” for energy efficiency purposes. Values as high as 1 inch of water (250 Pa) and as low as 0.01 inch of water (2.5 Pa) have been suggested. Whatever the number is, we need to determine this number, because it is key to BOTH assessment and mitigation.

      2. Measurements of soil gas concentrations MUST be accompanied by simultaneous measurement of the pressure gradient, to make sure that the measurement occurs under “worst case” or “conservative” conditions. Measurement of soil gas without the simultaneous measurement of pressure differential is meaningless, regardless of the length of measurement, because if the air flow is from indoor air to soil gas, measuring soil gas beneath a slab is just measuring indoor air. How the pressure differential is accomplished is not important. It may be accomplished by using the “chimney effect” during the heating season, or it may be accomplished using exhaust fans to depressurize indoor air with regard to soil gas in the middle of summer. The important fact is that while the soil gas sample is collected, the pressure gradient, and thus the potential flow of soil gas, is from soil gas to indoor air.

      3. This pressure gradient is the foundation of successful mitigation techniques, whether passive or active. Until the value of this “worst case” or “conservative” gradient is established, mitigation technique effectiveness cannot be evaluated. However, once established, the effectiveness can be demonstrated. For example. If we use the (negative) 0.2 inches of water (-50Pa) as a “worst case” gradient, then we have several options. We can “artificially” depressurize the underside of the building foundation using vacuum blowers, to create a “vacuum” of say -0.5 inches of water (-125 Pa). Then air flow from both the soil gas and the indoor air will be toward this depressurized (“vacuum”) zone, and there can be no flow of soil gas into the building.

      Establishing this pressure gradient value is critical. If the value is 0.2 inches of water (50 Pa), significant vacuum blower horsepower is required to maintain this under a building slab. However, if the number is 0.02 inches of water (5 Pa), this can be created by the fractional horsepower fan, such as is used for radon mitigation. At 0.01 inch of water or less, wind powered ventilators may be able to create the required suction.

      In summary, the measurement of the pressure gradient across the floor slab is critically important in assessing the potential for soil gas to impact indoor air. Digital micromanometers with logging capability are relatively inexpensive and available. A four-hour SUMMA canister sample, collected when the pressure gradient between soil gas and indoor air is maintained at 0.2 inches of water (higher in soil gas than indoor air) for the entire sampling period, is much more representative of “worst case” than three or ten 24-hour samples collected when the pressure gradient is unknown, and air may actually be flowing from indoor air into soil gas. How the pressure gradient is created is immaterial. The soil gas really doesn’t know or care.

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