EPA Comments OU 15 2016-2017 letter work plan.pdf

EPA DRAFT COMMENTS ON THE HILL AFB OU-15 LETTER WORK PLAN FOR

VI GRADUATION CRITERIA

1. Operable Unit-15 (OU-15) is one of 15 OUs at Hill AFB and it encompasses all of the on- and off-base areas managed under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) where vapor intrusion into residences and other structures is known to occur. The affected medium considered in OU-15 is indoor air. The sources of vapor intrusion are contaminated soil and groundwater which can lead to a preferential pathway termed “sewer gas”.

USEPA guidance for vapor intrusion (VI) investigation and mitigation suggests that multiple lines of evidence be used to base risk management decisions concerning VI. Factors affecting vapor intrusion might include the level of soil gas in the vadose zone, the depth of the contamination in soil/groundwater, the concentration of contaminants in the groundwater, the underlying geology [gravels, sands, clay, etc.], the concentration of contaminant in the indoor air, the type of indoor air sample [SUMMA canister, passive sampler, Hapsite, number of replicate(s)],the modeled concentration, of contaminant indoors from the underlying soil/groundwater, the building construction [slab-on-grade, perimeter foundation, basement, multiple floors], the season of sampling, the presence of preferential pathways [source of “sewer gas”], the type of heating/cooling in the building, building pressurization, presence of indoor sources, meteorological conditions, etc.

The proposed plan for graduating structures out of the Vapor Intrusion Monitoring [VIM] plan is based on two [2] lines of evidence: indoor air concentrations below the RBAL [longer term passive and multiple SUMMA canister sampling events] and building pressure cycling.

Since there are several groundwater plumes emanating from on-site sources with varying VOCs epand VOC concentrations that affect both on-site structures and off-site structures, it would be helpful if the supporting criteria for graduation out of the VIM program be increased. This might include the presence of a clean water layer above the contaminated zone, a clay layer between contamination and structure, depth to groundwater from bottom of structure, the presence or absence of preferential pathways [i.e., sewer gas source] and subsurface geology that might influence VI. The rational for graduation should not be based solely on two lines of evidence.

2. Additional data collection and additional criteria may warrant inclusion in the proposed work plan for ceasing indoor air monitoring (i.e., graduating buildings from the indoor air monitoring program) in buildings that overlie an area of subsurface contamination wherein cleanup goals for vapor intrusion have not yet been attained.

a. With the primary and occasional exception of invoking “building pressure cycling”, the proposed criteria for “building graduation” are based almost exclusively on vapor concentrations in indoor air, whereas the OSWER Technical Guide For Assessing And Mitigating The Vapor Intrusion Pathway From Subsurface Vapor Sources To Indoor Air (OSWER Publication 9200.2-154; “OSWER VI Guide”) recommends that risk management decisions generally be based upon a consideration of multiple lines of evidence.

b. Site-related documents identify that, for some buildings found to be subject to vapor intrusion, sewer/conduit gas intrusion is the more predominant mechanism of vapor intrusion, compared to soil gas intrusion. The final work plan and criteria should appropriately reflect consideration of sewer/conduit gas intrusion as a mechanism of vapor intrusion. So, for example, vapor concentrations in sewers and other conduits may warrant measuring and evaluation in certain areas with known subsurface contamination and perhaps outside the known area of groundwater contamination, depending upon site/OU history, infrastructure locations and condition, and other factors. By contrast, EPA understands that “a specific investigation has not been performed to assess all preferential pathways”.

c. Additional lines of evidence warranting consideration (and incorporation into the criteria and work plan) may be identified as a result of updating the conceptual site model to explain why so few buildings have been found so far to be subject to actionable vapor intrusion, notwithstanding the building’s presence over an area of subsurface contamination with vapor-forming chemicals. So, for example, as recommended in the OSWER VI Guide, near-source soil gas samples should be collected and analyzed for vapor-forming chemicals in areas, if any, where a hydrogeologic explanation is offered or hypothesized to account for the unexpected findings of indoor air testing.

3. EPA anticipates including in the final Record of Decision for OU15 specific subsurface cleanup goals to be attained by the remedies for the various operable units, which will ensure that the vapor intrusion pathway will not pose an unacceptable human health risk and which will establish criteria for termination of indoor air monitoring in all buildings and deletion of the respective operable unit (OU) as the published termination criteria are met.

a. This may require merely collating the cleanup goals established in Records of Decision (RODs) for the respective OUs, which ensure protection from vapor intrusion by both mechanisms of VI, if each ROD is complete.

b. To the extent that any previous ROD did not consider vapor intrusion when setting cleanup goals and remedial action objectives, it would be appropriate to address that shortcoming(s), either through a ROD Amendment for the specific OU and/or an implicit amendment via the ROD for

OU15.

4. To the extent that sewer or conduit gas intrusion is a cause of unacceptable vapor exposures in any building, specialized response actions (e.g., sewer depressurization and/or rehabilitation) would generally warrant consideration (i.e., subslab depressurization, a typically applied method for “mitigation” of soil gas intrusion, would not be effective for addressing sewer gas intrusion).

5. Certain representations and language in the 2017 Letter Work Plan may be outdated (and warrant correction in a revised workplan for 2020), in light of data and other site-specific information developed since its publication (e.g., reportedly, there have been recent discussions about the draft ROD for OU15 and an “enhanced public outreach approach” that merit consideration for incorporation into a revised workplan for 2020.

6. EPA requests information which documents that the contractors performing the indoor air monitoring have obtained and produced reliable and representative data. Although certain researchers have demonstrated that certain passive samplers can produce reliable data (e.g., data comparable to those obtained via a time-series of Summa canisters, for example), such demonstration may not be sufficient, by itself, for showing that the methods chosen by the Hill AFB team yield reliable data when implemented by their contractors. A problem with data quality, if any exists, might explain why so few buildings have been found so far to be subject to actionable vapor intrusion, notwithstanding the building’s presence over an area of subsurface contamination with vapor-forming chemicals.