Cost Comparison of Soil Vapor Extraction and Subslab Depressurization for Vapor Intrusion Mitigation.

Soil vapor extraction (SVE) can be applied for remediation, and also as an alternative to sub-slab depressurization (SSD) for vapor intrusion (VI) mitigation. This study compares capital, operation, and treatment costs of SVE and SSD systems using data collected during a multi-year demonstration project conducted at eight buildings in an urban setting. The capital cost of the SVE system is substantially less than the estimated total capital cost of individual SSD systems. The SVE operating costs are higher, especially in the early operating years when it is being operated for mass removal and treatment. As a result, the cumulative SVE system cost rises above that of the SSD systems in the sixth year of operation. A significant portion of the operations and maintenance cost advantage of the SSD systems comes from the assumption that off-gas treatment is not required. Alternative cases show SVE costs are likely to be lower in scenarios where numerous small buildings requiring independent SSD systems overlie the SVE zone of influence. Conversely, SSD systems are less costly for cases with few small buildings overlying the SVE zone of influence. An additional benefit of SVE is continued mass removal. In a situation where an existing SVE can be repurposed for VI protection from residual volatile organic carbon (VOC) mass, the SVE cumulative costs over 30years can remain lower than the cost of installing and operating SSD systems in multiple buildings.

Field Study of Soil Vapor Extraction for Reducing Off-Site Vapor Intrusion.

Soil vapor extraction (SVE) is effective for removing volatile organic compound (VOC) mass from the vadose zone and reducing the potential for vapor intrusion (VI) into overlying and surrounding buildings. However, the relationship between residual mass in the subsurface and VI is complex. Through a series of alternating extraction (SVE on) and rebound (SVE off) periods, this field study explored the relationship and aspects of SVE applicable to VI mitigation in a commercial/light-industrial setting. The primary objective was to determine if SVE could provide VI mitigation over a wide area encompassing multiple buildings, city streets, and subsurface utilities and eliminate the need for individual subslab depressurization systems. We determined that SVE effectively mitigates offsite VI by intercepting or diluting contaminant vapors that would otherwise enter buildings through foundation slabs. Data indicate a measurable (5 Pa) influence of SVE on subslab/indoor pressure differential may occur but is not essential for effective VI mitigation. Indoor air quality improvements were evident in buildings 100 to 200 feet away from SVE including those without a measurable reversal of differential pressure across the slab or substantial reductions in subslab VOC concentration. These cases also demonstrated mitigation effects across a four-lane avenue with subsurface utilities. These findings suggest that SVE affects distant VI entry points with little observable impact on differential pressures and without relying on subslab VOC concentration reductions.

Upscaled modeling of complex DNAPL dissolution.

A straightforward, upscaled DNAPL mass dissolution model is developed using relatively simple input consisting of characteristic dimensions and saturations of a DNAPL accumulation. Multiple accumulations are aggregated into a single source zone volume. Physically, the dissolution process is a combination of flow through the mass (advective component) and flow around the mass (dispersive component). The contribution of each component is based on initial characteristic length scales and the average initial saturation. Changes over time with the depletion of mass are captured with a changing relative permeability and a power law relationship for the fraction of initial mass remaining. The utility of the upscaled process model is demonstrated with data from three studies: numerical simulation of multiple pools, two-dimensional test cell experiments with mixed architecture and with heterogeneous soil, and a controlled field study of multicomponent DNAPL release and depletion. Use of the model successfully reproduced the observed multistage mass discharge in each study and illuminated the governing processes. The power law exponent was relatively constant for the various conditions and relative permeability changes were integral to the success. The numerical and experimental studies were run to complete mass depletion which the upscaled model matched. The input parameters are minimal and are found in typical DNAPL source zone characterization data.

In-situ determination of field-scale NAPL mass transfer coefficients: Performance, simulation and analysis.

Better estimates of non-aqueous phase liquid (NAPL) mass, its persistence into the future, and the potential impact of source reduction are critical needs for determining the optimal path to clean up sites impacted by NAPLs. One impediment to constraining time estimates of source depletion is the uncertainty in the rate of mass transfer between NAPLs and groundwater. In this study, an innovative field test is demonstrated for the purpose of quantifying field-scale NAPL mass transfer coefficients (kl(N)) within a source zone of a fuel-contaminated site. Initial evaluation of the test concept using a numerical model revealed that the aqueous phase concentration response to the injection of clean groundwater within a source zone was a function of NAPL mass transfer. Under rate limited conditions, NAPL dissolution together with the injection flow rate and the radial distance to monitoring points directly controlled time of travel. Concentration responses observed in the field test were consistent with the hypothetical model results allowing field-scale NAPL mass transfer coefficients to be quantified. Site models for groundwater flow and solute transport were systematically calibrated and utilized for data analysis. Results show kl(N) for benzene varied from 0.022 to 0.60d(-1). Variability in results was attributed to a highly heterogeneous horizon consisting of layered media of varying physical properties.

Corrosion of unexploded ordnance in soil-field results.

Unexploded ordnance (UXO) are found on 400 000 Ha of land across 1400 different sites in the United States. In many cases, UXO contain high explosives posing a risk to groundwater quality. This paper provides results from a field survey of 14 inactive Army ranges distributed throughout the mainland United States. Metal samples and adjacent soil were collected from 161 UXO. Soil samples were analyzed for a variety of chemical and physical properties. Metal samples were analyzed for pitting corrosion. Climate data were also compiled. Of these analyses, only rainfall correlated with extent of corrosion. Samples had been buried from the Civil War era through the 1990s. Photographs taken in the field illustrate that corrosion occurs more rapidly on the underside of a UXO. Field observations also revealed that low-order detonations and UXO are difficult to distinguish in the field. The deepest individual pit measured on any of the 161 metal samples was 2400 microm. Low rainfall environments had shallower average pit depths (590+/-130 microm)than moderate and high rainfall environments, which produced deeper average pit depths (940+/-90 microm). The UXO studied in this effort were not a significant source of explosives to the unsaturated soil environment.