No Images? Click here Vapour IntrusionIn this ongoing series of bulletins Mort Schmidt of Cox-Colvin & Associates (US) explores the complex world of vapour intrusion, from the fundamental science through to legislative framework. In this second bulletin we explore a fascinating case study from Cox-Colvin that highlights the necessity for adequate site characterization. The Need for Adequate Site CharacterizationIn 2010, one of Cox-Colvin’s clients expressed the desire to have us locate the source or sources of VOC contamination beneath one of their manufacturing facilities. We knew, on the basis of groundwater contamination and the facility’s history of chemical use, that the solvent tetrachloroethene (aka perchloroethene, PCE) was present in soil. And we knew of several potential sources – two former degreasers, and three above-ground storage tanks (ASTs), one of which had a known PCE spill decades earlier of approximately 50 gallons. But the PCE plume in groundwater didn’t line up with the suspected source areas. Fortunately, we had recently developed the Vapour Pin®, and this was the perfect place to test it. We installed Vapor Pins® in a grid configuration with 20-foot spacing. Two people, equipped with a hand-held rotary hammer, installed and sampled 30 Vapor Pins® each evening. We collected soil-gas data with a hand-held meter, and collected 144 glass vials for laboratory analysis. As it turned out, the correlation between field data and subsequent lab results was excellent. We now collect field parameters at all of the points and submit only a few for lab analysis. Immediately after sampling, we removed the Vapor Pins® for reuse elsewhere, and plugged the holes, leaving little evidence that we had been there. The lab results surprised us. As shown in the figure below, the highest levels of PCE contamination did not coincide with suspected sources. The pictures below show the building configuration through history, based on aerial photographs and Sanborn Fire Insurance maps. In each picture, the current plant outline is shown in the background, with buildings at the time in question outlined in black. In 1924, the area was entirely residential. In 1933, an entrepreneur purchased a building and commenced manufacturing. The business and the associated building (gray) expanded over time, as shown in the photos for subsequent years. The building’s front is on the west (left) side, and its back is on the east side. Interestingly, the areas of maximum contamination aren’t at the degreasers or ASTs. They’re next to former loading docks and back doors, and they probably resulted from what was, many years later, regarded as improper handling. Interestingly, the bulk of contaminant mass was located at a depth of 15 feet, and its footprint was revealed through subslab soil gas. Subsequent investigations conducted with the Vapor Pins® have shown that this pattern is the rule, not the exception. High-level contamination consistently occurs next to former back doors, but building expansion over time obscures the connection. We find Vapor Pins® to be the best way to collect sub-slab soil gas, but whatever approach one uses, soil contamination is extremely variable over space, and one must collect a lot of data to adequately characterize a site. Cox-Colvin has used this approach on a number of facilities, including one which was slated for demolition. We were able to evaluate the presence or absence of VOC contamination at over 140 locations in several days, using hand-held drills and field meters – far less than the cost of collecting and analyzing soil samples after building removal. And of course, sub-slab soil gas is essential to evaluating vapor intrusion. Indoor-air samples are virtually always contaminated with vapors from indoor and outdoor sources, known as “background”. Therefore, sub-slab soil gas is normally evaluated prior to sampling indoor air, and only the constituents present in soil gas are analyzed in indoor air. If it’s necessary to install a vapor-mitigation system (essentially the same as a radon system), system performance can be verified by monitoring the presence of a vacuum at various points in the sub-slab, via Vapor Pin®. Soil gas flows from the area of lowest to highest pressure, and as long as a vacuum is present everywhere beneath the floor, the system is doing its job. Cox-Colvin recently submitted comments to the American Association of Radon Scientists and Technologists on their proposed mitigation standards for radon and vapor intrusion in residences. The proposed standards recommend sampling indoor air at least every two years after installing a mitigation system, to verify system performance. That might make sense for radon, which can be sampled by the building occupant for under $15 per sample, but vapor-intrusion samples cost hundreds of dollars each, and they must be collected and interpreted by a professional. Cox-Colvin made the case that sub-slab vacuum pressures should be allowed to replace at least some indoor air samples for systems installed for vapor intrusion. You can’t fix the problem until you know where it is, and thorough site characterization, early in a site assessment, is essential to efficient, cost effective solutions. About Mort Schmidt Mort Schmidt is a Senior Scientist with Cox-Colvin & Associates, Inc. He received his BS and MS degrees in Geology and Mineralogy from The Ohio State University, and has been a Cox Colvin & Associates employee since 1997. His areas of expertise include vapor intrusion and contaminant investigation and analysis, and he currently serves as Cox Colvin's Practice Leader - Vapor Intrusion Services. Mort is a Certified Professional Geologist with AIPG and is a registered Geologist in Indiana. Vapour Pin by Cox-Colvin
FOR MORE INFORMATION ON OUR PRODUCTS & SERVICES PLEASE CONTACT US ON;(03) 8683 0091info@hydroterra.com.au
|