Validation of a General 3-D Numerical Model for Simulating Organic Pollutants Migration and Application to Site Remediation

Abstract

Abstract This paper presents a general numerical model able to simulate both organic pollutants migration (3-phase compositional flows, mass transfer, transport) in soils and aquifers and decontamination techniques such as pumping, skimming, venting, hot venting, steam injection, surfactant injection and biodegradation. To validate the simulator, a 3-D experiment in a large pilot (25 m × 12 m × 4 m) was carried out. A total of 0.475 m3 of diesel oil was injected into the pilot, and numerous in-situ measurements were performed to determine pollutants location and concentrations within the vadose and saturated zones. Prior to the pilot test, a predictive simulation computed the extent of the contaminated zone and the oil saturations. Numerical results showed good agreement between experiment and simulation. To demonstrate the simulator abilities to improve remediation operations, a soil vapor extraction (venting) of weathered gasoline in the vadose zone under a service station was simulated. Fourteen wells were drilled on the site and extraction took nine months. The simulation closely matches the field data. Further simulations show the possibility of venting optimization for this site. Introduction Groundwater contamination by organic compounds such as hydrocarbons or chlorinated solvents has become a major environmental concern. To help in understanding of the complex phenomena associated with the migration of non-aqueous-phase liquids (NAPL) and the design of remediation operations. specific numerical tools have been introduced. Understanding the evolution of NAPL in the subsurface requires knowledge of the migration of the contaminants (advection and dispersion) in both the vadose and saturated zone, transfer of components from NAPL to water and gas phases. Prediction of long-term evolution of the contamination may also require knowledge of interactions between organic pollutants and the porous matrix and biodegradation of both dissolved pollutants and residual NAPL trapped in the pores. Good comprehension of the above mechanisms will allow numerical simulators to be efficient tools for risk assessment purposes. Remediation processes aimed at removing NAPL from the subsurface. Different technologies are available or under development:–direct mobilization of NAPL, e.g. pumping, surfactant injection;–partitioning of contaminants from either the aqueous phase or oil phase into the gas phase, e.g. in situ stripping, vacuum extraction (venting), steam injection;–biodegradation of dissolved pollutants and of residual NAPL. While these various methods of remediation have been studied on the laboratory scale and have been used in field conditions, the design of an operation is often based on empirical guidelines or previous experience. Use of a comprehensive numerical tool may significantly increase our capability of optimizing such remediation operations. Numerous numerical models have been applied to oil-spill problems and decontamination processes as reviewed by Pinder and Abriola (1986), Corapcioglu and Panday (1991) and Panday et al. (1995). Compositional models developed to examine groundwater contamination scenarios include those by Abriola and Pinder, Corapcioglu and Panday, Forsyth and Shao, Sleep and Sykes, Falta et al., Le Thiez and Ducreux, and Unger et al. The goal of this work is to present two simulations cases performed with the SIMUSCOPP numerical model and the comparison between computed results and field observations. P. 723