Draft 1 – Regulatory Framework Workgroup for MassDEP Indoor Air Guidance
2.1 When Should the Indoor Air Pathway Be Investigated?
There has been an increased interest in and concern about vapor intrusion as a pathway
for human exposure to contaminants from groundwater originating from sites of oil and
hazardous material releases. MassDEP developed an Indoor Air SOP (August 2007) for
use by its staff, which offers valuable, detailed information regarding this pathway and
has been released for use outside the agency. Most actions described in the MassDEP
Indoor Air SOP start with evidence of a potential indoor air pathway: indoor air data,
elevated contaminant levels in soil, and/or contaminant levels in groundwater above the
GW-2 Standards. Other state and federal guidance listed in the appendix of the SOP
offers “how to” approaches to vapor intrusion, once the decision has been made to pursue
the possibility of human exposure through this pathway. This section addresses the
question of “when” it is necessary to investigate vapor intrusion to indoor air as an
exposure pathway under the Massachusetts Contingency Plan (MCP).
2.1.1
Interpreting the MCP Requirements
The MCP includes some specific requirements regarding notification that relate to the
indoor air pathway. 310 CMR 40.0321(1)(d) requires 2-hour notification to MassDEP for
releases that pose an Imminent Hazard. Some sites with indoor air contamination meet
this notification requirement. 310 CMR 40.0313(4) requires 72-hour notification for “a
release to the environment indicated by measurement within the groundwater of equal to
or greater than five milligrams per liter of total volatile organic compounds at any point
located within 30 feet of a school or occupied residential structure, where the
groundwater table is less than 15 feet below the surface of the ground.” 310 CMR
40.0006 defines another 72-hour notification requirement for a Condition of Substantial
Release Migration (SRM), including “releases to the groundwater or to the vadose zone
that have resulted or are within one year likely to result in the discharge of vapors into
school buildings or occupied residential dwellings.” 310 CMR 40.0006 defines Critical
Exposure Pathways (CEP) related to the indoor air pathway as “those routes by which oil
and/or hazardous material(s) released at a disposal site are transported, or are likely to be
transported, to human receptors via: (a) vapor-phase emissions of measureable
concentrations of oil and/or hazardous materials into the living or working space of a preschool, daycare, school or occupied residential dwelling...” The presence of CEP triggers
specific mitigation requirements. SRM and CEPs will be discussed in more detail at 2.2
below.
The reporting and investigation requirements in the MCP triggered by these specific
situations are relatively straightforward. More challenging are the decisions about the
direction and extent of investigation at a site which may lead to obtaining the data for
evaluating the indoor air pathway. Evaluating the indoor air pathway should be
approached in the context of characterizing the nature and extent of contamination at a
site. It would be impossible to list all situations in which the indoor air pathway needs to
be addressed under the MCP. The role of good professional judgment cannot be
overstated. The evaluation of potential indoor air pathways should be approached in the
same way as other pathways; that is, new site information and data could indicate the
need for further investigation, even if the pathway had previously been removed from
consideration.
Consideration of the indoor air pathway at a site may start from one of many points, such
as:
 A specific MCP reporting condition
 Site use and history
 Groundwater data
 Soil data
 Soil gas data
 Indoor air data
 Odors or screening data within a building
 Presence of Non-Aqueous Phase Liquid
 Building-specific concerns, such as an earthen floor, fieldstone foundation, and/or
groundwater sump.
Other relevant factors which may reinforce consideration of an indoor air exposure
pathway based on an initial concern from data and/or property use/history include:
 Site-specific concerns related to contaminant transport, such as utility corridors or
high hydraulic conductivity
 Sensitivity of receptors
 Volatility, toxicity, persistence and/or concentration of the contaminants
This information may be obtained as part of Preliminary or Comprehensive Response
Actions. The requirements for a Phase I Report relevant to this pathway include: general
disposal site information including description of land uses surrounding the disposal site,
structures such as building, floor and storm drains, subsurface utilities, OHM storage and
disposal structures, disposal site history, nature and extent of contamination, migration
pathways and exposure potential (see 310 CMR 40.0483).
The Phase II requirements at 310 CMR 40.0835 include Environmental Fate and
Transport of Oil and/or Hazardous Material including “an evaluation of the potential for
groundwater to be a source of vapors of oil and/or hazardous material to indoor air of
occupied structures as described in 310 CMR 40.0900.”
A well-designed, iterative approach to site characterization would indicate consideration
of the indoor air pathway at several points within an investigation. For example, a
decision not to investigate the pathway may be made based on an initial single round of
depth to groundwater measurements. As additional measurements are taken on a
seasonal basis, the annual average depth to groundwater may result in the decision to
investigate indoor air. Another example could be contaminated soil; it may not be
considered as a potential source of indoor air contamination until a review of site history
indicates that a location adjacent to a building at the property was used as a hazardous
waste storage area.
A common sequence of investigative actions related to the indoor air pathway is to
collect data from groundwater first, followed by soil gas, and finally indoor air sampling.
However, these actions may be taken out of order if others are involved in the site; for
example, a building tenant may perform indoor air sampling early in the process. A soil
investigation near a building (for example, removal of floor drains or a dry well) may
trigger concerns about the indoor air pathway, even before any groundwater monitoring
wells are installed.
It is also important to note that the need for iterative data collection, analysis and
evaluation during the course of a site investigation is not unique to the indoor air
pathway. For example, if new analytical data from surface soil indicated contamination
in an area with the potential for runoff to a surface water body, a new potential exposure
pathway would need to be investigated.
The decision to evaluate a potential indoor air pathway should be revisited continually
during the characterization of a release of VOCs at a site. This is an iterative process, in
the same way that new data obtained during site characterization is continually evaluated
for new reporting requirements and for determining whether the current Conceptual Site
Model needs revision.
2.1.2
Using the Method 1, GW-2 Standard to Rule out the Vapor Intrusion Pathway
The use of a Method 1 Risk Characterization under the MCP is restricted to sites at which
the contamination is limited to soil and groundwater. The GW-2 Standards were
developed to address the potential for exposure through the indoor air pathway when
contaminants volatilize from groundwater beneath or near buildings. Groundwater is
classified as GW-2 when it is located within 30 feet of an existing or planned building or
structure that is or will be occupied, and the average annual depth to groundwater in that
area is 15 feet or less. The GW-2 Standards were developed based on back-calculations
from the lowest of: (1) the one in one million Excess Lifetime Cancer Risk
concentration, (2) 20% of the Reference Concentration, or (3) the 50% odor recognition
threshold, unless those values were less than the expected indoor air “background”
concentrations. In those cases, the acceptable indoor air concentration defaulted to the
background conditions.
The GW-2 Standards have often been used by LSPs as the sole determining factor in a
decision whether to evaluate the potential for an indoor air pathway. While tempting in
terms of convenience, this approach may not be protective for sites with some volatile
and persistent contaminants where soil contamination near an occupied structure is the
source of indoor air contamination. Using the GW-2 Standards as a screening tool for
these contaminants could result in the failure to identify indoor air exposures related to
the site and subsequently in an underestimate of the risk posed by contamination at the
site. A prudent approach for LSPs would be to obtain additional lines of evidence (such
as sub-slab soil gas samples and/or indoor air samples) to rule out the potential for an
indoor air pathway. By obtaining additional lines of evidence before ruling out the indoor
air pathway, LSPs could lower the probability of making an inaccurate conclusion that
No Significant Risk exists at a site.
For some well-characterized sites, the comparison of groundwater contaminant levels to
the GW-2 Standards may be appropriate as a screening tool. Note that groundwater
contamination below GW-2 Standards on its own would not be adequate to rule out the
potential for the indoor air pathway, if other lines of evidence indicate the potential for
exposure through the indoor air pathway.
For adequately characterized sites where groundwater contamination does not exceed the
GW-2 standards, certain other factors would indicate that the indoor air pathway should
be evaluated, notwithstanding low levels of contamination in groundwater:
1) Indoor air contamination and/or odors potentially attributable to groundwater
contaminants have been reported within the structure of concern.
In this situation, the indoor air pathway would need to be evaluated to determine
if the indoor air contamination and/or odors are related to a disposal site. When
indoor air contamination could possibly be attributed to facility operations at or
near the structure of concern, the collection of sub-slab soil gas data would be
valuable in determining whether soil or groundwater contamination near the
building could be contributing to contaminant concentrations in indoor air. It
would be premature to rule out the vapor intrusion pathway in this situation based
solely on the GW-2 standards without additional lines of evidence.
2) The structure of concern has an earthen floor, fieldstone foundation, significant
cracks, and/or a groundwater sump.
These conditions could allow an unusually direct connection between the interior
of the structure and the groundwater contamination beneath the structure. They
are not consistent with the assumptions used in the Johnson and Ettinger model
used to calculate GW-2 Standards, and so would indicate the need for evaluating
the indoor air pathway at levels in groundwater below the GW-2 standards.
3) Non-Aqueous Phase Liquid (LNAPL or DNAPL) is present or is likely to be
present within 30 feet (horizontally) of the potentially impacted structure.
Again, these conditions are not consistent with the assumptions used in the
Johnson and Ettinger model used to calculate some GW-2 Standards, and so
would indicate the need for evaluating the indoor air pathway even if
concentrations in groundwater are less than the GW-2 standards.
4) VOC contamination is present in soil or soil gas near the structure of concern.
A contaminant source such as a dry well, leaking floor drain or spill location,
could allow contaminant volatilization near the structure of concern without
contamination of the underlying aquifer. Evidence of contamination could come
from screening results or analytical data from soil or soil gas samples. The
MassDEP Policy #WSC-02-411 Implementation of the MADEP VPH/EPH
Approach lists soil gas screening levels for evaluating indoor air impacts from
petroleum contamination (see Tables 4-9 and 4-10). Pursuant to 310 CMR
40.0942(1)(d), "If one or more Volatile Organic Compounds is present in vadose
zone soil adjacent to an occupied structure (e.g., within six feet,
measured horizontally from the wall of the structure, and within ten feet,
measured vertically from the basement floor or foundation slab) then the soil has
the potential to result in significant indoor air concentrations of OHM and Method
1 alone cannot be used to characterize the risk at the disposal site." However, the
presence of contaminated soil or soil gas at greater distances from an occupied
structure does not necessarily indicate the lack of an indoor air pathway. In this
situation, additional characterization would be advised, in order to determine the
nature and extent of the contamination and determine whether there is a
substantial likelihood that an indoor air exposure pathway is complete.
5) Specific site conditions exist which indicate that the assumptions used in the
Johnson and Ettinger model used to calculate some GW-2 Standards do not apply to the
site under investigation.
Summary Table – Any suggestions for making this a flow chart or checklist?
Site Conditions Indicating a Need to Evaluate the Indoor Air Pathway
A. MCP Reporting Conditions
1) Potential Imminent Hazard – 310 CMR 40.0311(7)
2) Groundwater with greater than 5mg/L VOCs within 30 feet of a
school/residence [(310 CMR 40.0313(4)]
3) Substantial Release Migration [310 CMR 40.313(5)]
B. Critical Exposure Pathway exists
C. GW-2 Standard exceeded
D. GW-2 Standard not exceeded, but one or more of the following conditions exist:
5) Indoor air contamination and/or odors potentially attributable to groundwater
contaminants have been reported within the structure of concern.
6) The structure of concern has an earthen floor, fieldstone foundation, and/or
groundwater sump.
7) Non-Aqueous Phase Liquid (LNAPL or DNAPL) is present or is likely to be
present within 30 feet (horizontally) of the potentially impacted structure.
8) VOC contamination is present in soil or soil gas near the structure of concern