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Residential Soil Vapor Intrusion: An Independent Evaluation

Excerpts From a Report orginally Submitted To Client

By: Timothy Minnich, MS, QEP
Tel: (908) 409-9900
Email Mr. Minnich


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This report is in response to a request by McCallion & Associates, LLC for an independent evaluation of the likelihood of periodic, residential soil vapor intrusion (SVI) arising from contaminated groundwater beneath the Mousis property, 58 North Clinton Avenue, Bay Shore, New York. The contaminated groundwater is attributable to past disposal practices of the nearby former Bay Shore/Brightwaters manufactured gas plant (MGP) currently being remediated by National Grid (formerly KeySpan). Injection of oxygen (O2) into the contaminated groundwater – which has adversely affected the Mousis property – began in earnest during January 2010. Also referred to as oxygenation, the goal of oxygen injection is to enhance the aerobic processes in the groundwater plume, with the end result being a reduction in its contaminant loading.

We have based our evaluation on: (a) all relevant, publicly available groundwater and oxygenation data; (b) the SVI sampling performed at the Mousis residence by National Grid on October 9-10, 2013; and (c) relevant State and Federal guidance concerning SVI sampling and oxygenation as a means to reduce groundwater contamination.


The following conclusions and recommendations were reached based on detailed analyses of the above-identified data.

  • A highly contaminated groundwater plume was shown to be present directly beneath the Mousis property at all times during (and before) the entire span of the remediation (2008 until present).

  • To a reasonable degree of scientific certainty, the oxygenation program – which presumably continues today – caused a significant increase in the upward mobilization of contaminants through the soil, which adversely impacted the Mousis property and house, starting in 2010.

  • Relatively warm ambient air temperatures observed during the 2013 SVI sampling campaign, together with results of a barometric pressure analysis (performed herein), evidenced that indoor sampling was performed under conditions which were not reasonably worst-case, thus indicating that further sampling was warranted (yet apparently never performed).

  • National Grid failed to follow current New York State Department of Health mitigation guidance, based on the presence of unacceptably high sub-slab contaminant levels during the 2013 SVI sampling campaign.

We strongly recommend that a vapor intrusion mitigation system be immediately implemented on the Mousis property to prevent any further exposure to harmful indoor air contaminants.


Groundwater Contamination and Oxygenation

The Mousis property was shown to lie atop a contaminated groundwater plume based on: (a) the Remedial Investigation (RI) findings and (b) results of quarterly groundwater monitoring which spanned the remediation, as presented in National Grid’s “Fact Sheets” and/or quarterly or annual Operation & Maintenance (O&M) Reports, beginning in 2008 and continuing until June 2018. The RI and O&M reports can be found here, and the Fact Sheets here.

Figures 1 through 8 (all figures begin on Page 9), respectively, are selected quarterly “snapshots” depicting both the contaminated groundwater plume with respect to the Mousis property, as well as the location of all oxygenation systems – either planned or in operation – upon completion of the following milestones: the RI in 2003 (Figure 1); Quarter 3, 2009 (Figure 2); Quarter 2, 2010 (Figure 3); Quarter 4, 2010 (Figure 4); Quarter 1, 2012 (Figure 5); Quarter 4, 2012 (Figure 6); Quarter 2, 2013 (Figure 7); and Quarter 2, 2015 (Figure 8).

Each quarterly map marks the initial time an updated plume configuration was depicted by National Grid’s consultants. These figures clearly evidence that the contaminated plume remained beneath the Mousis property during this entire period (and likely still today).

Several oxygenation systems were installed over the course of the remediation. Two of these, installed in 2009, had the potential to enhance the upward contaminant mobility in the vicinity of the Mousis property. These systems were just upgradient of the property (i.e., to the north-northwest) on the downgradient edge of Operable Unit 1 (OU-1). They are known as the 66 N. Clinton Avenue system and the Union Boulevard system, and are depicted by the green and/or orange lines in the quarterly maps (labeled in Figures 2 through 8). Oxygenation has been shown to increase the risk of fugitive vapors entering buildings, and for this reason has been deemed by the U.S. Environmental Protection Agency (U.S. EPA) to be a disadvantage compared to other enhanced aerobic bioremediation technologies employed for mitigating contaminated groundwater plumes (for example, see here, PDF page 10 of 74).

Table 1 (following the figures) presents a summary of the monthly oxygen injection data for the two systems discussed above (66 N. Clinton Avenue and Union Boulevard) for the five-year period spanning 2009 through 2013. It should be noted that operation of each system went beyond 2013 – to 2018, at least – and that, to the best of our knowledge, both systems are still operating today. This data can be found in the O&M Reports (link provided above), with the relevant PDF page number for each month and system included in the table.

Figure 9 graphically depicts the monthly combined oxygen injection rates for the two systems, based on the data presented in Table 1. Oxygenation increased markedly beginning in January 2010, with the highest rates from the middle of 2011 through 2013.

SVI Sampling

Appendix A (following Table 1) presents the first several pages of a November 8, 2013 National Grid letter report sent to Mrs. Mousis, which summarized results of their soil vapor intrusion assessment. From this report, “Based on the sampling conducted at the 58 North Clinton Avenue property, it does not appear that the indoor air at the property is being impacted by MGP site-related chemicals through soil vapor intrusion [emphasis provided in original report].” As discussed below, this conclusion is misleading and clearly not supported by the facts.

Beginning at approximately 10:40 am, October 9, 2013, a total of six, 24-hour-averaged air samples were collected as follows:

  • four indoor samples (basement, kitchen, living room, and living room duplicate);
  • one outdoor sample; and
  • one sub-slab sample.

In general, for the indoor and outdoor samples, those contaminants associated with MGP sites were either not detected or were shown to be present at low levels. Conversely, the sub-slab sample showed significantly elevated levels of these “fingerprint” contaminants (discussed below). That vapors originating from the contaminated groundwater plume are emanating upward through the soil under the house is indisputable. National Grid’s conclusion that the indoor air is not being impacted by soil vapor intrusion must, therefore, assume that the basement slab (floor) provides an effective barrier to the toxic vapors.


We reviewed existing SVI sampling guidance, focusing on two documents for preparation of this portion of our report. They present recommended approaches and environmental conditions to consider when designing investigations to determine whether a given subsurface source poses a potential SVI health threat to building occupants. Further, they provide comprehensive discussion on the myriad factors governing soil vapor intrusion and, despite their publication dates, are the most recent versions – still widely considered to be state-of-the-art reference material. They are:

  • Final Guidance for Evaluating Soil Vapor Intrusion in the State of New York,” New York State Department of Health, Center for Environmental Health, Bureau of Environmental Exposure Investigation, October 2006. This is subsequently referred to as the New York State Department of Health (NYSDOH) guidance, and can be viewed here.

  • Conceptual Model Scenarios for the Vapor Intrusion Pathway,” U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington D.C. 20460, EPA 530-R-10-003, February 2012. This is subsequently referred to as the U.S. EPA guidance, and can be viewed here.

Based on these documents, the following questions should generally be asked in order to determine the need for either mitigation, or at least intensive investigation (involving multiple sampling campaigns):

  • Are there high concentrations of vapors in the sub-slab soil?

  • Is there a correlation between the indoor and/or sub-slab vapors detected and the volatile compounds present in the subsurface source?

  • If previous sampling was limited to a single round, was it conducted under reasonably worst- case conditions?


An analysis of the SVI sampling is discussed first, which shows that the October 2013 campaign was not performed under reasonably worst-case conditions; further, had the sampling been performed under such conditions, there would have assuredly been significant indoor concentrations. The second component of this analysis discusses the oxygenation and its contribution to the myriad indoor odor complaints since at least 2013.

SVI Sampling Results

The October 9-10, 2013 data set is discussed in terms of the above questions:

Sub-Slab Concentration

In general, the concentration of vapors in the sub-slab sample was very high, ranging up to 12,300 micrograms per cubic meter (ug/m3) for m/p xylene (see Table 1 of Appendix A – the National Grid letter report). This alone provides justification for mitigation, as is indicated in the NYSDOH guidance which essentially states that if sub-slab concentrations are above 1,000 ug/m3, mitigation is called for even if all indoor concentrations are below the detection limits during the time of measurement (see PDF page 64 of 241).

Correlation with Subsurface Source

From the universe of compounds analyzed for, the NYSDOH guidance identifies a total of eight typically associated with MGP waste (see PDF page 41 of 241). These are: trimethylbenzene isomers (three), tetramethylbenzene isomers (one), thiopenes (one), indene, indane, and naphthalene.

High concentrations of seven of these fingerprint compounds were detected in the sub-slab sample, confirming that the contaminated groundwater plume was the source (see Table 1 of Appendix A – the National Grid letter report)...

Download PDF to continue reading report, footnotes, figures, tables, and references.

Timothy R. Minnich, President, MS, QEP, is a Meteorologist and Atmospheric Scientist with over 40 years experience in the design and management of a wide range of ambient air and meteorological investigations under CERCLA and the Clean Air Act. He is a recognized technical expert on high-profile legal cases, with assignments involving forensic meteorology and reconstruction of inhalation scenarios in relation to community exposure to hazardous air pollutants (HAP). He is a nationally recognized expert in the application of optical remote sensing (ORS) for hazardous waste site remediation. He has designed and managed more than 25 ORS field investigations and air dispersion model validation studies since the promulgation of U.S.EPA (EPA) Method TO-16 for open-path FTIR (Fourier-transform infrared) spectroscopy in 1988.

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