GSI Publishes Study on False Positive Detections in Vapor Intrusion Investigations

Over the years, we have observed that there is greater variability in field duplicate results for vapor samples than for groundwater samples. To better understand the differences between these types of samples, we mined data from the California GeoTracker database.

This research project was conducted by Dr. Thomas McHugh, Ms. Carlyssa Villarreal (former GSI intern), Ms. Lila Beckley, and Ms. Sharon Rauch. Findings from the project include:

  • Vapor duplicates are more variable than water duplicates with median percent difference in concentration of 25% compared to 7% for water samples.
  • Large differences in concentration between normal and duplicate pairs are more common in vapor samples. For vapor, 20% of paired data had a percent difference in concentration of >300% while, for groundwater, only 3% had a percent difference of >300%.
  • Contamination of samples during collection or analysis is more significant for vapor samples. For water samples, sample contamination appears unlikely to result in false positive exceedances of drinking water standards; however, for vapor samples, sample contamination may result in false positive exceedances of indoor air screening values. For vapor samples, the use of reusable canisters and flow controllers is likely an important source of sample contamination.
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For more information, please contact Tom McHugh at 713-522-6300 or Lila Beckley at 512-346-4474.