PNNL

Abstract

Soil vapor extraction (SVE) is typically effective for removal of volatile contaminants from higher permeability portions of the vadose zone. However, contamination in low-permeability lenses or layers can persist due to slow mass transfer processes that limit the removal effectiveness from these zones. After SVE has been operated for a period of time and the remaining contamination is primarily located in low-permeability zones, the remedy performance needs to be evaluated to determine whether the SVE system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. A methodology to quantitatively support this decision was developed based on estimating the impact of persistent vadose zone sources on the groundwater. Mathematical-model simulations are used to characterize the relationship between the short-term source vapor mass discharge rate, measurable using SVE system operational data, and predicted groundwater contaminant concentration as a function of vadose zone source size and strength. The method enables consideration of alternative conceptual site models to evaluate how uncertainty in site parameters affects the predicted impact to groundwater. The performance of SVE in addressing contamination in low-permeability zones can also be evaluated using data from cyclic operation of the SVE system. Data from a Department of Energy Hanford Site SVE system was used to illustrate the method and the effect of variations in vadose zone source size/strength, recharge, sorption, and groundwater Darcy flux on the predicted groundwater impact.