Experiments are performed at the lab scale with soil columns allowing a good control of operating conditions and following the methods of frontal chromatography. Saturated flow experiments are conducted in a conventional bench-scale column apparatus, including a pump, a glass column, on-line detectors (pH and conductivity) and data acquisition. A specific soil column system is used to carry out experiments with an unsaturated steady-state flow. This set-up is composed of two stainless steel columns (outside diameter: 10 cm; length: 20 cm), equiped with two tensiometers, with a sprinkler unit and a stainless steel porous plate at the bottom end. Suction can be applied thanks to a vacuum pump and a multichannel peristaltic pump allows the feeding of the columns and the depletion of the outflow liquid. Experiments are monitored thanks to a specific software. In both cases, samples are collected at the column outlet for further analysis (PAH, ion analysis, TOC).
Different porous media were selected: a sand, a sand coated with kaolinite, a sandy-silty soil sample, a contaminated soil sample (PAH, heavy metals) from a former coking plant. For each sample, the soil-moisture characteristic curve (representing matrix suction versus water content) is experimentally determined in order to find the best operating conditions for our experiments with unsaturated flow. Numerical codes allowing us to model reactive solute transport under variably saturated water flow conditions are also used to model the results.
The results are currently in acquisition and will be presented.
This study at a bench scale aims at developing at first a methodology under unsaturated conditions and better understanding the main mechanisms controlling reactive solute transport in natural soils. This is a necessary preliminary step before upscaling at the lysimeter and field scales. Modelling of the observed processes will enable us to predict long term fate of contaminant like PAH in soils.
Keywords: solute transport, column experiments, vadose zone, bioavailability, organic contaminants