Identification of Alternative Vapor Intrusion Pathways Using Controlled Pressure Testing, Soil Gas Monitoring, and Screening Model Calculations

Abstract

Vapor intrusion (VI) pathway assessment and data interpretation have been guided by an historical conceptual model in which vapors originating from contaminated soil or groundwater diffuse upward through soil and are swept into a building by soil gas flow induced by building underpressurization. Recent studies reveal that alternative VI pathways involving neighborhood sewers, land drains, and other major underground piping can also be significant VI contributors, even to buildings beyond the delineated footprint of soil and groundwater contamination. This work illustrates how controlled-pressure-method testing (CPM), soil gas sampling, and screening-level emissions calculations can be used to identify significant alternative VI pathways that might go undetected by conventional sampling under natural conditions at some sites. The combined utility of these tools is shown through data collected at a long-term study house, where a significant alternative VI pathway was discovered and altered so that it could be manipulated to be on or off. Data collected during periods of natural and CPM conditions show that the alternative pathway was significant, but its presence was not identifiable under natural conditions; it was identified under CPM conditions when measured emission rates were 2 orders of magnitude greater than screening-model estimates and subfoundation vertical soil gas profiles changed and were no longer consistent with the conventional VI conceptual model.

US residential building air exchange rates: new perspectives to improve decision making at vapor intrusion sites

“…One source of IA variability has been linked to preferential pathways, in particular the unintentional entry of sewer gas entering indoor spaces e.g. 12,27 . However, experimental results have also shown that induced building-pressure variations influence the temporal and spatial variability of both radon and VOC concentrations in sub-slab and IA 7 .…”

Three‐Dimensional Simulation of Land Drains as a Preferential Pathway for Vapor Intrusion into Buildings

“…Measured indoor–outdoor pressure differences and exchange rate were used in the numerical modeling to predict temporal variations of contaminant emission rate and indoor air concentration, which were then compared with field observations to validate the model. Readers interested in other details of that field experiment are directed to the original references (Holton et al, 2013, 2015; Guo et al, 2015). …”

Air exchange rates and alternative vapor entry pathways to inform vapor intrusion exposure risk assessments

“…Guo et al (32) conducted a long-term VI continuous monitoring study at a house overlying a groundwater plume contaminated by 1,1-dichlorethylene (1,1-DCE), 1,1,1-trichloroethane (1,1,1-TCA), and TCE near Hill Air Force Base in Layton, Utah (US). By applying controlled-pressure-method testing (which includes whole house pumping and IA sampling), soil gas sampling, and screening-level emission calculations, the study concluded that subsurface pipe networks, including sewer mains and land drains, may be significant alternative VI pathways.…”

“…The purpose of the pipe was presumed to be a foundation drain. As part of the study, researchers installed a valve so that the land drain could be shut and vapors could be prevented from being released (32). …”