Speaker
Description
The behavior of pollutant sources after remediation remains a relatively underexplored topic. For sites contaminated by volatile organic compounds (VOCs), soil vapor extraction is a widely applied field technique. Following a venting phase, an increase in VOC concentrations in the soil air can be observed. This increase is partly due to the return to re-equilibration governed by mass transfer between the dissolved phase and the volatile liquid phase of the contaminant. Previous venting experiments conducted on a decimetre scale were set up to observe the consequences of changes in air extraction flow rates and phases of re-equilibration (rebound effect). The chemical species studied was perchloroethylene (PCE).
The aim of this study was to investigate mass transfer of PCE in the unsaturated zone of a model aquifer at the multi-decameter scale by conducting controlled experiments on the SCERES facility. SCERES is a watertight basin that is 25 m long, 12 m wide and 3 m deep which is covered by a fixed roof to prevent rainfall infiltration. The hydraulic gradient, flow rate, water table levels and water sampling are controlled and monitored from two pits located at the upstream and downstream ends of the basin. The system reproduces a three-layer alluvial aquifer system that includes two less-permeable blocks.
Results are presented from a large-scale vapor plume experiment involving a well characterized PCE release, including multiple campaigns of soil air extraction to explore rebound effects and to track the fate of the PCE plume up to source depletion. Following the release of 3 liters of PCE into the unsaturated zone through 38 injection points located beneath the low-permeability surface layer, PCE vapor concentrations were subsequently monitored for 6 weeks with a multi-gas analyzer, using 25 gas sampling points installed at different depths. Once the vapor plume had reached a steady state, a brief one-hour venting phase was carried out at two air extraction wells, during which roughly 1 cubic meter of soil air was extracted.
As expected, local PCE vapor concentrations measured 1.5 m upstream and downstream of the source zone dropped significantly, reaching half of their initial values. Within ten days however, vapor concentrations rose again substantially and even surpassed pre-venting levels. This rebound effect could be clearly attributed to the still highly active spill. Vapor concentration measurements at the spill showed that vapor levels in its core were at saturation vapor pressure, which, due to the resulting increase in the local concentration gradient relative to the surrounding area, promoted enhanced volatilization of the PCE phase. A second venting stage of up to 5 hours is planned, aiming to double the total extracted air volume. The PCE vapor plume will be monitored again until the contamination source is depleted.
Parallel to the experiments, numerical simulations of the PCE vapor plume originating from the PCE source zone, as well as relaxation tests, are carried out using the multiphase simulator cubicM. It should be emphasized that the water (dissolved PCE)/air mass transfer kinetics quantified in laboratory column experiments will be implemented in cubicM.
| Country | France |
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