Sewer Gas: An Indoor Air Source of PCE to Consider During Vapor Intrusion Investigations

Environmental Engineering Science

2013

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Mathematical models have been widely used in analyzing the effects of various environmental factors in the vapor intrusion process. Soil moisture content is one of the key factors determining the subsurface vapor concentration profile. This manuscript considers the effects of soil moisture profiles on the soil gas vapor concentration away from any surface capping by buildings or pavement. The ''open field'' soil gas vapor concentration profile is observed to be sensitive to the soil moisture distribution. The van Genuchten relations can be used for describing the soil moisture retention curve, and give results consistent with the results from a previous experimental study. Other modeling methods that account for soil moisture are evaluated. These modeling results are also compared with the measured subsurface concentration profiles in the U.S. EPA vapor intrusion database.

Section: Resultsmentioning

confidence: 99%

Influence of Soil Moisture on Soil Gas Vapor Concentration for Vapor Intrusion

Shen

Pennell

Environmental Engineering Science

2013

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“…When c ss is less than 100~1000 ug/m 3 , most of the c in data fall in the range between 0.1 and 10 ug/m 3 , with no real dependence on c ss . As discussed elsewhere, such data strongly suggest the role of background concentration [12, 15, 26]. When c ss is above about 100-1000 ug/m 3 , a trend emerges, showing that c in increases with c ss .…”

Section: Resultsmentioning

confidence: 57%

“…Comparison between the distribution of indoor air concentration with subslab concentration ( c ss ) above 500 μg / m 3 in the EPA database, with other background studies [3, 26-30]. …”

Section: Figurementioning

confidence: 99%

Vapor intrusion attenuation factors relative to subslab and source, reconsidered in light of background data

Yao

Journal of Hazardous Materials

et al. 2015

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The basis upon which recommended attenuation factors for vapor intrusion (VI) have been derived are reconsidered. By making a fitting curve to the plot showing the dependence of observed indoor air concentration (cin) on subslab concentration (css) for residences in EPA database, an analytical equation is obtained to identify the relationship among cin, css and the averaged background level. The new relationship indicates that subslab measurements may serve as a useful guide only if css is above 500 μg / m3. Otherwise, cin is independent of css, with a distribution in good agreements with other studies of background levels. Therefore, employing this screening value (500 μg / m3), new contaminant concentration attenuation factors are proposed for VI, and the values for groundwater-to-indoor and subslab-to-indoor air concentration attenuation factors are 0.004 and 0.02, respectively. The former is applied to examining the reported temporal variations of cin obtained during a long-term monitoring study. The results show that using this new groundwater-to-indoor air concentration attenuation factor also provides a reasonably conservative estimate of cin.

“…The higher attenuation factor (10 -3 ) was detected in a Property B, which was later mitigated and the basement floor was sealed. Several years after mitigation, Pennell et al (2013) reported evidence that a faulty sewer connection in Property B was a source for elevated PCE concentrations in the indoor air on the first floor during the field study included in this research. It is not known how long the sewer connection may have been influencing indoor air concentrations in that property.…”

Section: Resultsmentioning

confidence: 91%

Field data and numerical modeling: A multiple lines of evidence approach for assessing vapor intrusion exposure risks

Pennell

McClean

Science of The Total Environment

et al. 2016

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USEPA recommends a multiple lines of evidence approach to make informed decisions at vapor intrusion sites because the vapor intrusion pathway is notoriously difficult to characterize. Our study uses this approach by incorporating groundwater, soil gas, indoor air field measurements and numerical models to evaluate vapor intrusion exposure risks in a Metro-Boston neighborhood known to exhibit lower than anticipated indoor air concentrations based on groundwater concentrations. We collected and evaluated five rounds of field sampling data over the period of one year. Field data results show a steep gradient in soil gas concentrations near the groundwater surface; however as the depth decreases, soil gas concentration gradients also decrease. Together, the field data and the numerical model results suggest that a subsurface feature is limiting vapor transport into indoor air spaces at the study site and that groundwater concentrations are not appropriate indicators of vapor intrusion exposure risks in this neighborhood. This research also reveals the importance of including relevant physical models when evaluating vapor intrusion exposure risks using the multiple lines of evidence approach. Overall, the findings provide insight about how the multiple lines of evidence approach can be used to inform decisions by using field data collected using regulatory-relevant sampling techniques, and a well-established 3-D vapor intrusion model.