The vapor-phase multi-stage CMD test for characterizing contaminant mass discharge associated with VOC sources in the vadose zone: Application to three sites in different lifecycle stages of SVE operations

Abstract

Vapor-phase multi-stage contaminant mass discharge (CMD) tests were conducted at three field sites to measure mass discharge associated with contaminant sources located in the vadose zone. The three sites represent the three primary stages along the soil vapor extraction (SVE) operations lifecycle- pre/initial-SVE, mid-lifecycle, and near-closure. A CMD of 32 g/d was obtained for a site at which soil vapor SVE has been in operation for approximately 6 years, and for which mass removal is currently in the asymptotic stage. The contaminant removal behavior exhibited for the vapor extractions conducted at this site suggests that there is unlikely to be a significant mass of non-vapor-phase contaminant (e.g., DNAPL, sorbed phase) remaining in the advective domains, and that most remaining mass is likely located in poorly accessible domains. Given the conditions for this site, this remaining mass is hypothesized to be associated with the low-permeability (and higher water saturation) region in the vicinity of the saturated zone and capillary fringe. A CMD of 25 g/d was obtained for a site wherein SVE has been in operation for several years but concentrations and mass-removal rates are still relatively high. A CMD of 270 g/d was obtained for a site for which there were no prior SVE operations. The behavior exhibited for the vapor extractions conducted at this site suggest that non-vapor-phase contaminant mass (e.g., DNAPL) may be present in the advective domains. Hence, the asymptotic conditions observed for this site most likely derive from a combination of rate-limited mass transfer from DNAPL (and sorbed) phases present in the advective domain as well as mass residing in lower-permeability (“non-advective”) regions. The CMD values obtained from the tests were used in conjunction with a recently developed vapor-discharge tool to evaluate the impact of the measured CMDs on groundwater quality.

Remediation of a Petroleum Hydrocarbon-Contaminated Site by Soil Vapor Extraction: A Full-Scale Case Study

“…In such cases, the removal can often be improved by modifying the extraction strategy (e.g., operating different or new wells, transitioning to a lower extraction rate, pulsing SVE operation). In particular, the rebound test provides valuable data for assessing the time of operation to achieve performance objectives as well as a benchmark for assessing progress toward cleanup during subsequent operations [36]. Consequently, soils with low air permeability are more difficult to remediate and exhibit reduced removal efficiency [35].…”

Assessing the Economic and Societal Benefits of SRP-Funded Research

“…The approach can provide a more accurate measure of vapor-phase contaminant mass discharge, characterize mass-transfer conditions, and provide a higher-resolution characterization of the groundwater source distribution ( Brusseau et al. 2010 , 2013 , 2015 ; Mainhagu et al. 2015 ).…”

“…2010 ). Follow-up testing and demonstration of the methods were conducted at a Tucson, Arizona, site on the National Priorities List (NPL), which documents hazardous waste sites eligible for long-term remedial action ( Brusseau et al. 2015 ; Mainhagu et al.…”

PFOS Mass Flux Reduction/Mass Removal: Impacts of a Lower-Permeability Sand Lens within Otherwise Homogeneous Systems

“…Regression between mass removal (MR) and reduction in contaminant mass flux (MFR) has been developed as a method for characterizing contamination sources and remediation of contaminants in heterogeneous porous media. , Some authors use a different formulation by correlating relative concentration with relative mass . These assessments have focused on the remediation of nonaqueous phase liquid (NAPL) contaminant source zones, ,,− with some examination of volatile source vapor extraction, − and also extension to groundwater-contaminant plume extraction and treatment. , DiFilippo et al found that a single exponential function correlating MFR to MR was not appropriate when multiple NAPL sources existed, and they observed a nonsingular or multi-step, nonlinear MFR/MR trend for the heterogeneous source zone systems. Thus, a non-singular or multi-step regression was needed for the characterization of the heterogeneous contaminant distribution.…”