Formatted Title
How to Estimate Sub-Slab Constituent Concentrations Using Riser Monitoring Data
Background/Objectives
The San Francisco Bay Area Regional Water Quality Control Board (RWQCB) released an update to the Vapor Intrusion Mitigation Guidance in June of 2022, which includes new requirements for both vent riser and sub-slab sampling. Older vapor intrusion mitigation systems (VIMS) installed prior to this requirement have, in many cases, been monitored only via riser sampling ports. In cases where sub-slab sampling has not been performed (e.g., due to absence of sub-slab sampling ports) or is not feasible (e.g., very shallow groundwater table), Langan has determined that a data-derived mathematical relationship between sub-slab and riser concentrations exists. Using this relationship, sub-slab concentrations can be estimated for sites at which only riser data has been collected.
Approach/Activities
To date, Langan has analyzed sub-slab and riser sampling data from seven passive VIMS sites in the San Francisco Bay Area. These sites were selected based on the availability of both sub-slab and riser data for each sampling event. The average concentration of each site’s sub-slab data was graphed against the average concentration of each site’s riser data to evaluate the relationship between the two. Once graphed, the linear regression equation and R2 value of each graph was determined. Several parameters were considered to compare the data for each site, including test port location (i.e., on roof level versus ground level), and compound type (chlorinated and petroleum hydrocarbons). As a final comparison, the percent drop between the average sub-slab and riser concentrations was calculated.
Results/Lessons Learned
Comparisons of riser to sub-slab data showed that there is a correlation between the two, although care should be taken when using riser data to estimate sub-slab constituent concentrations. The percent drop in concentration from sub-slab to riser generally ranged between -40% and 80%, indicating that, on average, the constituent concentrations in the riser samples provide a reasonable estimate of sub-slab concentrations, regardless of whether the constituents are associated with petroleum hydrocarbons or chlorinated solvents. In some cases, the riser(s) may be located closer to localized hotspots than any of the sub-slab sampling ports, but in other cases the opposite may be true. Therefore, if it is reasonable to assume, based on riser locations and site conditions, that the risers are located away from any potential hotspots, the data analysis indicates that the constituent concentrations in riser samples are approximately 50% of the sub-slab concentrations; otherwise, the riser concentrations may be providing an accurate approximation of sub-slab concentrations using the regression equation derived from the data set, regardless of whether the riser samples are collected at the ground level or the roof level. As sub-slab and riser data are available for additional passive VIMS sites within the Bay Area, they will be added to the data set to confirm, adjust, or update these findings.