low as 10 μg/L, and no impacts have been observed in other buildings overlying groundwater concentrations as great as 10–100 mg/L (EPA, 2012b).

A number of commercial products can serve as indoor sources of chlorinated solvent vapors, so that interpreting indoor air quality and sub-slab soil gas data is not always straightforward (Gorder and Dettemmaier, 2011). As a case in point, approximately 3,000 residences overlie chlorinated solvent groundwater plumes originating from Hill Air Force Base, although monitoring has indicated that a very small percentage of the residences have indoor air impacts attributable to groundwater contamination. Detailed study beyond typical pathway assessment monitoring identified numerous indoor air sources of contaminants, including household cleaning products, craft supplies, gun cleaners, and holiday ornaments—leading to a list of 72 household products known to contain TCE and almost another 2,000 products known or suspected of containing chlorinated solvents.

A solid technical basis is lacking for determining which scenarios require indoor sampling and what sampling frequency and duration are appropriate, both over the short term (i.e., daily) and long term (i.e., seasonal). Studies suggest that vapor intrusion emissions into buildings can fluctuate on time scales ranging from days to weeks (Luo, 2009; Luo et al., 2010; Johnson et al., 2012). Research by McHugh et al. (2010) suggests that changes in indoor air concentrations may be different for chemicals emanating from groundwater than those emanating from indoor chemical sources, such that temporal data might be used to distinguish between indoor air impacts from these two sources. However, even with detailed indoor air monitoring data, the issue of temporal variability is further complicated by the dynamics of volatilization from the groundwater plume, which is affected by groundwater table elevation, moisture infiltration rates, moisture profiles, and other climate factors (Sakaki et al., 2013). In general, the temporal changes in the vapor emission rates from groundwater have yet to be studied in great detail and further study is needed to more intelligently design sampling plans.

Because the costs and complexity of vapor intrusion assessment have been increasing without a commensurate increase in the mechanistic understanding of the exposure pathway, the resulting response actions reflect a conservative management approach.

MONITORING

Monitoring of groundwater is conducted over the entire life cycle of a complex site and can represent a significant percentage of life-cycle costs if residual contamination remains after active remediation has been completed, especially when monitoring extends over multiple decades. Traditionally, the monitoring of temporal changes in groundwater contamination



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