The overall goal of this project was to demonstrate that the multi-stage vapor-phase contaminant mass discharge (MS-CMD) test and vapor-phase tomography (VPT) can effectively characterize persistent VOC sources in the vadose zone and measure their associated mass discharge. It is anticipated that these technologies will improve evaluation of vadose zone source impacts on groundwater and vapor intrusion.
The specific performance objectives for this demonstration were as follows:
1. Produce quantitative measurements of contaminant mass discharge.
2. Produce contaminant mass discharge values with uncertainty bounds.
3. Assessment of mass-transfer conditions.
4. Produce three-dimensional map of contaminant source distribution.
5. Improved analysis of risk.
All 5 performance objectives were met.
Multi-stage Vapor-phase Contaminant Mass Discharge Test
The vapor-phase cyclic or multi-stage contaminant mass discharge (MS-CMD) test was recently developed to measure mass discharge for vadose zone sources under both forced-gradient and pseudo natural-gradient conditions (Brusseau et al., 2010). The vapor-phase MS-CMD test consists of three stages: an extended initial extraction stage (which is identical to a standard CMD test), a rebound stage, and a second extraction stage. In brief, Stage 1 consists of an initial extraction wherein concentrations of contaminant in the effluent gas are monitored. The extraction continues until quasi steady state is attained with respect to effluent concentrations. The purpose of the initial extraction stage is to sweep vapor-phase contaminant from the advective domains. At this point, the extraction is stopped, and the system is monitored to characterize potential rebound (Stage 2). This rebound represents transfer of contaminant from poorly accessible domains to the accessible domain. A second extraction stage is then implemented (Stage 3). Contaminant mass removed during the second extraction stage is tabulated to determine the mass that transferred from the poorly accessible domains to the advective domain during the rebound stage.
This method is based on conducting a short-term vapor extraction test at a vertically-discrete point, while collecting vapor-phase contaminant concentration data at multiple vertically-discrete sampling points surrounding the extraction point. For the test, air is extracted from the vadose zone at a specific point (depth and location). The vapor-phase contaminant concentration (and the gas-flow rate if desired) is measured simultaneously at several other locations. The test is of sufficient length to remove the initial resident volume of contaminant in the local area, allowing interrogation of the source-associated mass flux. In essence, the data collected from the test is analogous to a 3-D “snapshot” of the source(s) of the vapor-phase contamination.
Vapor concentrations collected during the MS-CMD test was plotted as a function of elapsed time. The resultant contaminant-elution curve was examined to evaluate mass transfer conditions. The appearance of specific landmarks was evaluated, such as length of the steady-state stage, occurrence of an asymptote, and occurrence of a rebound. This information was used to qualitatively assess the conditions influencing vapor-phase mass transfer and mass removal.
The VPT test produced 3D maps of VOC concentrations and mass flux in which spatial differences were observed, indicating the presence of a vapor source located in the S–SW quadrant of the test area, between the water table and the monitoring-well screened interval. These results were consistent with those of the sediment data. In comparison, the standard soil gas survey (SGS) test was unable to identify spatial variability of vapor concentrations or delineate a potential source. Thus, the VPT test provided a more robust source characterization compared to the SGS method. These results illustrate the utility of the VPT test as a higher-resolution method for characterizing VOC source distribution in the vadose zone. The analyses of mass-transfer conditions illustrate the ability to use the results of the MS-CMD and VPT tests to evaluate mass-transfer conditions and delineate the presence of persistent contamination.
Overall, the MS-CMD test has a relatively low implementation cost (modest infrastructure and sampling requirements) and relatively simple data-analysis requirements. Therefore, it is expected that this test would be of beneficial use under many conditions, and applicable for a wide array of sites. In many situations, the cost of implementing the MS-CMD test will be lower than the cost of implementing a standard SGS test. The application of a full-scale VPT test (with multiple local extractions) is anticipated to be reserved generally for two scenarios. First, for large or complex sites that have active SVE operations. Second, application of the tomography methods for sites that do not have substantial infrastructure present may be warranted for sites that have a greater degree of complexity or other special circumstances that warrants the additional costs. For other scenarios, a more limited VPT test set could be implemented, thus reducing overall costs.