Estimating the Impact of Vadose Zone Sources on Groundwater to Support Performance Assessment of Soil Vapor Extraction

Abstract

Soil vapor extraction (SVE) is a prevalent remediation remedy for volatile organic compound (VOC) contaminants in the vadose zone. To support selection of an appropriate condition at which SVE may be terminated for site closure or for transition to another remedy, an evaluation is needed to determine whether vadose zone VOC contamination has been diminished sufficiently to keep groundwater concentrations below threshold values. A conceptual model for this evaluation was developed for VOC fate and transport from a vadose zone source to groundwater when vapor‐phase diffusive transport is the dominant transport process. A numerical analysis showed that, for these conditions, the groundwater concentration is controlled by a limited set of parameters, including site‐specific dimensions, vadose zone properties, and source characteristics. On the basis of these findings, a procedure was then developed for estimating groundwater concentrations using results from the three‐dimensional multiphase transport simulations for a matrix of parameter value combinations and covering a range of potential site conditions. Interpolation and scaling processes are applied to estimate groundwater concentrations at compliance (monitoring) wells for specific site conditions of interest using the data from the simulation results. The interpolation and scaling methodology using these simulation results provides a far less computationally intensive alternative to site‐specific three‐dimensional multiphase site modeling, while still allowing for parameter sensitivity and uncertainty analyses. With iterative application, the approach can be used to consider the effect of a diminishing vadose zone source over time on future groundwater concentrations. This novel approach and related simulation results have been incorporated into a user‐friendly Microsoft® Excel®‐based spreadsheet tool entitled SVEET (Soil Vapor Extraction Endstate Tool), which has been made available to the public.

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

“…This application is illustrated using a recently developed vapor-discharge tool (Carroll et al, 2012; Oostrom et al, 2014). The tool is based on a data base relating long-term, maximum contaminant concentrations in groundwater to short-term vapor-phase mass discharges associated with vadose-zone VOC sources.…”

“…The results obtained from application of the nomograph may have a significant degree of uncertainty because the nomograph is not tuned specifically to the TIAA site. Hence, the full-scale vapor-discharge tool (Oostrom et al, 2014) was used to obtain a site-specific estimate.…”

An Investigation of Plume Response to Soil Vapor Extraction and Hypothetical Drum Failure

“…Soil vapor extraction is a well‐established method for VOC remediation in the vadose zone (Truex et al, 2013; Fischer et al, 1996; Oostrom et al, 2014). Soil vapor extraction is conducted using wells that pull vapor from the subsurface and filter it for rough particulates.…”

A Rapid Decision Support Tool for Estimating Impacts of a Vadose Zone Volatile Organic Compound Source on Groundwater and Soil Gas

“…SVE performance is predicated on several sitespecific factors including but not limited to soil moisture, permeability, sorption/partitioning, VOC vapor pressure(s), and subsurface heterogeneity. Temporal decline of VOC removal efficiency using SVE is common, especially over extended periods of operation, and this typically warrants SVE termination and the subsequent investigation of alternative remedial strategies (Oostrom et al 2014).…”

“…The SVEET version 1.0 and Vapor Intrusion Estimation Tool for Unsaturated-zone Sources (VIETUS) quantitative tools were previously developed to assist with decision-making processes regarding remedy transitions from SVE (Truex et al 2013;Oostrom et al 2014;Johnson et al 2016). However, limitations with respect to input parameter variability restrict the utility of these tools across a wide variety of VOC-impacted sites.…”