Drainage-imbibition tests and pumping of heavy chlorinated solvents in saturated porous media: measurements and modeling of the effects of thermal and chemical enhancement

International audienceRecovery of chlorinated solvents (CSs) as a free product is mainly based on pumping and pumping/skimming (Perez et al., 2014). However, this technique is time consuming and does not allow significant recovery of CSs in the form of free product and associated dissolved emissions (McGuire et al., 2006). Our study focuses on the beneficial effects of thermal and chemical enhancements for recovering free product composed of heavy chlorinated compounds (Hexachlorobutadiene, Hexachloroethane, Perchloroethylene, Pentachlorobenzene, Trichloroethylene…).This study is part of the SILPHES project, financially supported by ADEME (French Environment and Energy Agency) in the framework of the Investing for the Future (“Investissements d'Avenir”).1. Materials and methodsThe rheological parameters of the DNAPL were characterized: density, dynamic viscosity, interfacial tension (water/DNAPL) and contact angles (water/DNAPL). First, the imbibition-drainage tests were conducted in small columns to determine the capillary pressure-saturation functions. Then, the tests were elaborated in a one-dimensional large column, in order to validate the numerical model. Finally, pumping tests at different flow rates were carried out in a 2D tank, in order to improve the processes. These tests were carried out using glass beads of different diameters (0.1 to 1 mm) at different temperatures (10 °C and 45 °C) and with application of different surfactants (SDBS, Triton X-100, Aerosol MA-80, Tween 80). DNAPL flow was monitored using Time Domain Reflectometry (TDR) probes, geophysical probes (induced polarization) and also imaging technique (Light Reflected Method-LRM).Numerical models have been developed using COMSOL Multiphysics® for 1D and 2D cases and the experimental results were compared with simulations.2. Results and DiscussionThe results show that increasing the temperature from 10 to 45 °C decreases the dynamic viscosity of DNAPLs (respectively from 5.5 (±0.2) to 2.8 (±0.08) mPa.s) and has a limited influence on the interfacial tension (12 mN/m) and the contact angles (70 °). However, surfactants reduce the interfacial tensions (from 12 mN/m to 1 mN/m, depending on surfactant nature and concentration).Retention curves of the two-phase system (capillary pressure as a function of water saturation) obtained using small column are compared to various analytical models (Chen et al., 2006). The best correlations are obtained with the van Genuchten - Mualem model and the Brutsaert - Burdine model.These retention curves show that: (i) The residual saturation of DNAPL decreases by 30% with increasing the temperature from 10 to 45 °C; (ii) Adding the surfactants globally decreases the irreducible saturation of water by 25% (i.e., the surfactants can, at constant pressure, increase the mobility of DNAPLs) and; (iii) Aerosol MA-80 decreases the residual saturation by 24%.The pumping experiments using 2D tanks are under way to confirm these data and to estimate the radius of influence and the optimal pumping rate.Finally, the developed two-phase pressure-pressure numerical model accurately reproduces the experiments, in particular, the movements of the water-DNAPL interface.Perez, J., Lang, C., Roure, J. and Baccelli, A.: 2014, Taux d’utilisation et coûts des différentes techniques et filières de traitement des sols et des eaux souterraines pollués en France - synthèse des données 2012, Technical report, Agence de l’Environnement et de la Maîtrise de l’Energie (ADEME) / Ernst & Young, Angers, France. 148 p.McGuire, T., McDade, J. and Newell, C.: 2006, Performance of DNAPL source depletion technologies at 59 chlorinated solvent-impacted sites, Ground Water Monitoring and Remediation 26(1), 73–84.Chen, Z., Huan, G. and Ma, Y.: 2006, Computational methods for multiphase flows in porous media, number 978-0-898716-06-1, Society for industrial and applied mathematics edn, Philadelphia, PA , USA. 531 p