Modeling of multiphase multicontaminant transport in the subsurface

Abstract

This review addresses modeling of subsurface hydrological systems in which contaminants are transported in more than one fluid phase. The primary focus is on problems involving nonaqueous-phase liquids (NAPLs), a dynamic gas phase, or both, so that the unsaturated and saturated zones are both discussed. Basic research in simpler settings is included if it develops concepts that could be applied in modeling of such problems.Recent developments in practical models are reviewed, along with experimental work and theoretical issues related to the formulation of mathematical models. It will be seen that the extent of validity of the local equilibrium assump tion (LEA) for component partitioning among phases is a major question influencing the direction of these formula tions. Also emphasized are upscaling to macroscopic and grid lengths, and the choice and coupling of primary vari ables in multicomponent systems. Relations to and possible use of developments in petroleum reservoir simulation are discussed.The significance of the type of modeling reviewed here is growing, as the last several years have seen increased aware ness of the complexity and difficulty of groundwater contam ination problems that involve multiphase flow. For example, emplacement of NAPLs, such as gasoline or trichloroethylene, in the subsurface typically entails downward migra tion through the vadose zone, leaving some (or all) NAPL mass trapped behind by capillary forces in globules of widely varying shape and size. Depending on the NAPL density and other factors, the remaining mass will float at the wa ter table or continue down into the saturated zone. Some contamination can occur via transport of the NAPL phase itself, but even when this phase reaches immobile residual saturation, its components can dissolve in groundwater in concentrations well above environmental thresholds. These components can then be transported in the water phase to contaminate water far from the NAPL source. Many such components, known as volatile organic compounds (VOCs), will also vaporize into the air phase in the unsaturated zone and can then be transported in that phase.Conventional pump-and-treat techniques will not read ily remove immobile NAPL from the subsurface, where it l A]»o at U.S. Geological Survey, Paper number 95RG00289. 8755-1209/95/95RG-00289$15.00 may persist for decades as a source of contamination. Other mechanisms, such as interphase mass transfer, must be ex ploited. One example of such a process is vapor extraction, pumping air through the contaminated zone and relying on volatility to partition some mass of the offending compo nents into the air, which is then extracted from the sub surface. This technique may be enhanced by injection of steam, if the chemicals are such that a higher temperature will yield a more favorable partitioning. Partitioning can be quite sensitive to the multicontaminant compositions of the phases present. This discussion is merely illustrative, not exhaustive; for the purpose of this review, it suffices to realize tha...

Simulation and optimization technologies for petroleum waste management and remediation process control

“…Since 1960s, a significant number of numerical simulators were developed for modeling NAPL flow and transport in multiphase subsurface systems (Helmig, 1997;Huber and Helmig, 1999;Zheng and Bennett, 2002). Excellent reviews of these simulators were given by Abriola (1989), Mercer and Cohen (1990), Russell (1995), and Miller et al (1998).…”

Numerical solution of two-phase flow for the advection-dominated and non-linear case

“…Traditionally, the solution of the system of equations can be grouped in one of the methods discussed by Aziz and Settari [3]: SS (Simultaneous Solution), IMPES (IMplicit Pressure, Explicit Saturation) or SEQ (SEQuential solution). Dorgarten [14], Helmig [21], Miller et al [36] and Russell [41] presented overviews about possible approximations and 0309-1708/$ -see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.advwatres.2004.08.017 applications.…”

Adaptive moving mesh modeling for two dimensional groundwater flow and transport

“…multiphase flow and transport in groundwater environment; e.g. see [21,24] and references therein. Here we consider a specific case where nonaqueous phase liquids (NAPLs) are dissolved into the aqueous phase [22].…”