Text Box: Text Box: Page # Text Box: Volume 10, issue 2 Text Box: investigations. The worst-case building, however, is not always readily apparent due to the large number of factors that contribute to vapor intrusion migration, including COC concentrations in groundwater or soil gas, depth to groundwater, soil types, building construction and ventilation, and groundwater flow direction”. “...it may be appropriate to collect additional groundwater and/or soil gas data to narrow down the area with the highest concentrations before selecting individual properties or buildings for site-specific evaluation…” On Page 38… “At some sites, a statistical approach has been adopted for selecting properties or buildings to begin investigations. Buildings might be selected based on the magnitude of underlying concentrations, with bias toward grid cells overlying higher groundwater concentrations (stratified, systematic sampling approach). This method is most appropriate when concentrations are fairly similar over a broad area and there is little to distinguish the most susceptible building or area”. “Groundwater concentrations should be measured or reasonably estimated using samples collected at the top of the water column from wells screened at or across the top of the water Table”. On Page 40... ‘Preferably, groundwater monitoring data should Text Box: reflect the concentration at the groundwatervadose interface (top of the water table) as that is the point where partitioning from the groundwater to soil gas occurs. In the event that sampling shows no contamination at the groundwater-vadose interface, the project manager may correctly interpret this as indicating no vapor intrusion risk to overlying buildings unless other sources of the contaminant are known in the vadose zone sufficiently near the building of potential concern. Surficial recharge that occurs along the predominantly horizontal flow path of a groundwater plume will tend to establish a downward vertical hydraulic gradient and cause the affected groundwater plume to also migrate downward in proportion to the amount of recharge and in relation to total flow in the hydraulic unit. This effect is often referred to as a “diving” or “sinking” plume. While it is most apparent in situations where a groundwater plume has migrated over a significant lateral distance, in some situations plumes may start to dive relatively close to their source area. The value of techniques that sample from the upper horizon (with the zone of seasonal water table fluctuations) of the groundwater cannot be minimized. Where rainfall is abundant, sinking plumes can occur even with dissolved contaminants that are traditionally thought of as “floaters.” Targeting collection of groundwater samples from the top of the water table can provide significant data that will drastically reduce the extent of area over which one should consider vapor intrusion to be an issue, regardless of climate (wet, arid, etc.). For additional Text Box: information, refer to NJDEP 2005b, Chapters 2 and 6. In practice, many groundwater monitoring wells are screened in portions of the aquifer other than the interface. In addition to not revealing the conditions at the groundwater-vadose interface, purging and sampling these wells results in average contaminant concentrations that may under/overestimate soil gas concentrations and subsequently the vapor intrusion risk. Where possible, groundwater data from a relatively narrow interval across the water table should be used in the vapor intrusion assessment. The California Department of Toxic Substances Control states that a screen length of 10 feet or less will yield representative groundwater data suitable for vapor intrusion assessment (Cal DTSC 2004)’. “The investigator may be asked to determine whether vapor intrusion may pose a risk beyond the known plume boundaries for groundwater plumes that have not reached steady-state conditions. Information regarding the groundwater flow direction and gradient is often useful in determining the potential for contaminant migration. Groundwater characterization can entail groundwater flow direction; vertical and horizontal gradients, including as appropriate, seasonal variation, tidal influences, and the effects of groundwater withdrawal; the rate of groundwater flow movement;

ITRC Vapor intrusion intro - continued

CONTINUED ON P. 21

PAERAB Home Next Page Previous Page