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How to mitigate vapor migration thru an existing (1920s) slab on potentially contaminated soil ?

anarchitect | Posted in General Questions on

A client is considering purchasing an existing storefront building built in the 1920s.
A gas station with underground storage tanks existed 50 feet away from 1940-1985.
A Phase-I report has declared the tanks as potential hazards, with potentially contaminated soils and groundwater migrating to nearby properties, including ours.
To avoid the $10k Phase-II, is there a way to protect against the potential of harmful vapors from petroleum contaminants below the existing slab that likely has no vapor barrier by doing some kind of treatment above the slab?
Would any sealers mitigate the potential issue?
It’s a great building and location but should we walk simply because of the possibility of contaminated soil and groundwater on-site?
…Nobody wants to spend the $$$ for soil remediation, which would be impossible anyway since we’re not demolishing the building.
I look forward to your input.

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  1. GBA Editor
    Martin Holladay | | #1

    You need to hire a consultant who specializes in soil contamination from gas stations -- in other words, a hazardous waste expert. This isn't the kind of problem that can be addressed on the Q&A pages of a web site.

    Get professional help. The health of the occupants of the building require the developers of this project to take a responsible, professional approach to this problem.

  2. Stephen Sheehy | | #2

    Have you done test borings through the slab to see if anything of concern is below the slab? That would also tell you how far down groundwater is.
    Has the gas station site been remediated? As a general rule, if your site is contaminated, you own the cost of cleanup, subject to the possibility of proving the contamination came from somewhere else and being able to collect costs from that owner/operator.

    Will your client ever want to dig below the surface for any reason, such as a foundation for an addition?
    Is there any history of leaks from the tanks? If so, groundwater flow direction will determine the direction any leaked gasoline took.

    In other words, I'd make sure you aren't buying an environmental problem, before taking steps to isolate the (potential) problem.

    The Phase I report typically is just an overview of a site, checking history of the site and surrounding property, but not doing any testing. Every gas station will raise red flags, but not every gas tank has ever leaked. That said, tanks installed in the 40s and not removed until the 80s certainly warrant a test boring or two. Contacting a geotechnical engineer would be prudent.

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