This study focused on improving our ability to confidently assess the groundwater to indoor air vapor intrusion (VI) pathway at chlorinated solvent-impacted groundwater plume sites. Federal, state, and local agency guidance has evolved toward multiple-lines-of-evidence-based approaches that involve indoor air, sub-slab soil gas, deeper soil gas, groundwater, and soil sampling in combination with empirical analysis and screening-level modeling. Of these, indoor air data tend to be most heavily weighted in decision-making.

Prior to this study, it was recognized that there could be temporal variability in pathway assessment data, but little was known about this topic. This project focused on collecting the first high-frequency and long-term data set at a VI site, with the hope that it would be useful for advancing our understanding of temporal variability and determining how it should be accounted for in designing sampling plans and interpreting data.

This project was primarily conducted at a house overlying a dilute chlorinated hydrocarbon groundwater plume. The house was outfitted with sensors and automated systems to facilitate monitoring of indoor air and ambient and building conditions as well as groundwater and soil gas. Monitoring was conducted under both natural and controlled building conditions, and both chlorinated hydrocarbons and radon were quantified in indoor air and soil gas. Sampling was conducted under natural conditions for about 2.5 years. During that time trichloroethene (TCE) concentrations in groundwater were relatively constant (10 – 50 μg/L) while indoor air concentrations varied by three orders of magnitude (<0.01 to 10 ppbv).

Two recurring behaviors were observed with the indoor air data. The temporal behavior prevalent in Fall, Winter, and Spring involved time-varying impacts intermixed with sporadic periods of inactivity. In Summer, VI activity was more dormant, having long periods of inactivity combined with sporadic VI impacts. Subsurface concentrations were less temporally variable than indoor air and the variability increased in moving from the source to indoor air. Indoor air data were used to predict the likely outcomes of three sampling plans representative of conventional practice. The analysis showed a significant potential for poor characterization of long-term mean concentrations and exposures (both false negative and false positive outcomes) and a need for further investigation into the robustness of VI assessment paradigms. It also suggested the need for other long-term high-frequency indoor air data sets to better understand the range of behaviors that might be observed at other sites.

In contrast to indoor air concentrations under natural conditions, the long-term (9 month) controlled pressure method (CPM) test results were relatively constant with time across all seasons, indoor air concentrations were similar to the maximum values measured under natural conditions, and false-negative results were not obtained. This suggests that CPM tests might reliably detect VI occurrence and worst-case impacts regardless of day or time of year of the CPM test. The results also showed that CPM testing might reveal alternative VI pathways not detectable through routine monitoring under natural conditions.

It was also observed during this study that indoor air sources can create subsurface soil gas plumes, and that these can persist for weeks following removal of the indoor source. This is of note because some regulatory guidance allows for VI pathway assessment within days of indoor source identification and removal. If that happens, and the soil gas plume created by the indoor source persists, then the VI pathway assessment data might lead to false-positive conclusions concerning the potential for VI impacts.

Field, laboratory, and modeling studies were also conducted to improve the understanding of the groundwater table elevation changes on vapor emissions from dissolved groundwater sources. The results suggested long-term average emission increases due to groundwater table elevation changes are likely to be less than two times for most site conditions. The practical implication is that groundwater table elevation movement should not be considered a major factor in VI pathway assessment plan design at dissolved plume sites, unless the groundwater table is shallow and elevation fluctuations are frequent.

The project provides valuable information for improving the current VI assessment paradigm. It has also raised a number of questions that should be considered in future research projects. These include:

1. Further study of controlled pressure method testing,
2. Mitigation system effectiveness at sites with significant alternative VI pathways, and
3. Groundwater table elevation fluctuations and their effect on VI impacts.