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Canisters v. Sorbent Tubes: Vapor Intrusion Test Method Comparison

A carefully designed field vapor intrusion investigation consisted of three one-foot deep borings spaced one-foot apart at three locations within the perimeter of a former dry cleaner. One six-liter Summa canister (EPA Method TO-15) and two pump/packed tube assemblies (EPA Method TO-17) were used in each three-boring arrangement for a total of nine sampling locations. A ¼”-diameter length of Teflon®  tubing was lowered into the hole until it was approximately one-inch from the bottom. The tubing had a screen frit (implanted prior to field work) in the end lowered down the hole to prevent the uptake of particle matter. The end of the tubing extending above ground had a stainless steel valve that had been swaged to it prior to sampling. Eight-hour tests were conducted at a gas flow rate of approximately 10 mL/min.

The results of the field comparisons compared exceptionally well (R-squared = 0.98 for PCE) on a relative concentration basis from location to location for PCE, TCE and 1,2-DCE (reported concentration range: 3.7-22,200 ug/m3). A comparison of the concentrations measured by each method revealed a linear relationship between molecular weight and the difference in concentration between the two methods. The TO-17 results were between 0.27 and 0.67 times lower than the TO-15 results on a consistent basis for PCE. The authors believe the major factors contributing to the differences are flow rates of sample collection, moisture effects and dilution procedures with the TO-15 method. Subsequent indoor air testing inside the structure allowed attenuation factor calculations for PCE that varied from 0.05 to 0.002 across the site.