Composition Dependent Evolution in Mass Flux from Binary Trichloroethene/Tetrachloroethene-DNAPL Source Zones
Abstract
In order to accurately predict the efficacy of subsurface remediation for sites contaminated with multicomponent dense non-aqueous phase liquids (DNAPLs), it is necessary to link changes in aqueous phase contaminant discharge with source composition and distribution. Dissolution from a binary 1:1 (molar) mixture of trichloroethene- (TCE) and tetrachloroethene- (PCE) DNAPL was measured in three separate 2-dimensional aquifer cells (100 x 48 x 1.4 cm) that were packed with different background media (1:1 mixture 20:30 and 40:50 mesh; 20:30 mesh and 40:50 mesh Accusand) and low permeability zones. Initial DNAPL source zone architectures were varied to yield ganglia to pool (GTP) ratios of 0.44, 1.56, and 1.72. Down-gradient plume evolution and DNAPL spatial distribution were measured every 5 pore volumes (PV) from side port samples and a light transmission system that allowed non-invasive measurement of volumetric DNAPL saturation and source descriptive metrics at a resolution of 0.03 to 0.08 mm2. Flux-averaged PCE and TCE effluent concentrations were measured every 0.7 PVs from a fully screened effluent chamber. To accelerate changes in source zone architecture and overall mass removal, two surfactant floods (4% w/w Tween 80) were completed after mass discharge from the source zone reached a steady state. Mass flux reductions for a given amount of DNAPL mass removed were found to correspond strongly to the molar composition of DNAPL in the source zone and the initial DNAPL saturation distribution metric (e.g., GTP). Percent reductions in mass flux from the aquifer cells with ganglia dominated architectures were 98 and 72% for TCE and PCE respectively, with a final overall NAPL source zone molar ratio of 0.49:0.51 TCE: PCE ; and 97 and 79% for TCE and PCE with molar ratios of 0.19:0.81 TCE:PCE. Reductions in mass flux from the pool dominated source zone were 90 and 53% for TCE and PCE with a final overall DNAPL source zone mole fraction of 0.26:0.74 TCE:PCE. These results indicate that observed mass flux reductions have to be considered with respect to the composition and distribution of DNAPL source zones in heterogeneous subsurface environments.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H53F1133W
- Keywords:
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- 1831 HYDROLOGY / Groundwater quality;
- 1832 HYDROLOGY / Groundwater transport