Formatted Title
Methods to Quantify the Potential for Vapor Intrusion from Contaminated Groundwater
Background/Objectives
Health risks from vapor intrusion (VI) are generated by transport of contaminant mass through the vadose zone where subsurface sources exist (e.g., contaminated groundwater). A prevalent method for assessing contaminant mass flux for VI is based on mass removal measured during operation of conventional mitigation systems. However, climate and building construction and usage introduce highly transient behavior in concentration data and consequent difficulties in assessing the mass flux. These shallow data also provide little insight on source location and strength. A second method evaluates upward vapor migration from persistent sources using overly simplistic analytical models. The flux calculation generally assumes a single, unmeasured diffusion coefficient for the full depth of the vadose zone although this coefficient can vary by orders of magnitude depending upon soil type, heterogeneity and moisture distribution. For contaminated groundwater, major assumptions are also required to relate water concentrations to overlying vapor concentrations. This presentation describes the utility of creating transient conditions with soil gas extraction, measuring concentration profiles, and quantifying model parameters to provide a more representative assessment of the potential for vapor intrusion.
Approach/Activities
An analytical model of vapor transport through a layered vadose zone with flux-type boundary conditions was used to interpret vertical vapor concentration profiles measured in the subsurface following a substantial period of flushing with ambient air at three sites. The model includes a mass transfer limited boundary at both the groundwater interface, i.e., the capillary fringe, and at the vapor intrusion interface, i.e., the subslab. Simplistic models assume equilibrium between a measured groundwater concentration and the soil gas immediately above; however, the capillary fringe is known to provide a barrier and is quantified from the measured vapor concentration data. Soil layering is also included; small changes in soil type and moisture content can significantly alter diffusion coefficients as demonstrated in this work. Assessing parameters at shallow depths impacted by human interventions and climate is shown to be very difficult as compared to the relative consistency in deeper native soils.
Results/Lessons Learned
Detailed results are presented from a field study over a three-year period from an industrial site with sources of PCE and TCE in the deep vadose zone and groundwater. The vadose zone geology was modestly stratified and impacts to vapor transport were observed. Methods to quantify mass transfer rates from contaminated groundwater are illustrated and compared with an estimated rate through the layers of the vadose zone. The transport rate just below a subslab vapor intrusion boundary was also estimated but data were clearly impacted by variable atmospheric conditions. Key findings from two additional sites with groundwater sources are briefly described to illustrate differing scenarios of an intervening clay interval acting as a vapor cap, seasonal perched water, and a fluctuating water table. Takeaway points include: (1) the utility of soil gas extraction to create transient conditions that provide more reliable parameter estimation; (2) reduced variability in vapor concentration data measured in native soils; and (3) the importance of incorporating soil layering in modeling of vadose zone vapor transport for VI assessments.