Influence of Soil Moisture Dynamics on DNAPL Spill Zone Architecture and Its Impact on Mass Removal Mechanisms During Soil Vapor Extraction in Heterogeneous Porous Media

Abstract

153 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2005.The spatial distribution of DNAPL saturation, water saturation, and soil permeability (i.e., spill zone architecture) determines the pore-scale processes that control soil vapor extraction (SVE). To test this hypothesis, a fully coupled multiphase flow and transport simulator, STOMP was chosen. STOMP was first used to develop a new conceptual model for the migration and distribution of DNAPL in heterogeneous porous media for different NAPL and water loading histories (i.e., soil moisture dynamics). A 2-D vertical cross-section with layered heterogeneity mimicking the Hanford Site was assumed. In cases of co-disposal of NAPL with large volumes of wastewater, the form and location of NAPL were most strongly influenced by high water recharge rates. The effect of NAPL evaporation on NAPL migration was dramatic just after the spill event when the NAPL was present near the ground surface, resulting in a high diffusive mass flux into the atmosphere. For low water infiltration rate scenarios, the distribution of water content prior to a NAPL spill event had a significant impact on NAPL migration and distribution. Mass transfer processes for slow desorption and rate-limited dissolution from trapped NAPL were incorporated into the STOMP simulator. The modified STOMP was used to determine the effects of heterogeneity, slow desorption, and rate-limited NAPL mass transfer on mass removal mechanisms during SVE. Gas flow by-passing of low permeability zones was one of the dominant factors for diminished SVE effectiveness at late time. Rate-limited mass transfer from trapped NAPL led to a longer tailing when most remaining NAPL in the low permeability layer was trapped NAPL. These simulations indicate that NAPL-water spill-driven gas advection, vapor diffusion, and NAPL vertical movement enhanced by water flow must be considered in order to predict NAPL distribution and migration in the vadose zone. In addition, if trapped NAPL is expected in heterogeneous porous media, an improved mass transfer rate can be achieved not only by delivering gas directly to zones where trapped NAPL exists, but by changing the NAPL form from trapped NAPL to free NAPL.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD