Groundwater contaminant flux reduction resulting from nonaqueous phase liquid mass reduction

Abstract

[1] This work presents a stream tube-based analytical approach to evaluate reduction in groundwater contaminant flux resulting from partial mass reduction in a nonaqueous phase liquid (NAPL) source zone. The reduction in contaminant flux, R j , discharged from the source zone is a remediation performance metric that has a direct effect on the fundamental drivers of remediation: protection of human health and the environment. Closed form expressions are provided for analyzing remediation performance under conditions of joint spatial variability of both groundwater flow and NAPL content. The performance measures derived here are expressed in terms of measurable parameters. Spatial variability is described within a Lagrangian framework where aquifer hydrodynamic heterogeneities are characterized using nonreactive travel time distributions, while NAPL spatial distribution heterogeneity can be similarly described using reactive travel time distributions. The combined statistics of these distributions are used to evaluate the relationship between reduction in contaminant mass, R m , and R j . A portion of the contaminant mass in the source zone is assumed to be removed via in situ flushing remediation, with the initial and final conditions defined as steady state natural gradient groundwater flow through the contaminant source zone. The combined effects of aquifer and NAPL heterogeneities are shown to be captured in a single parameter, reactive travel time variability, which was determined to be the most important factor controlling the relationship between R m and R j . It is shown that as heterogeneity in aquifer properties and NAPL spatial distribution increases, less mass reduction is required to achieve a given flux reduction, although the overall source longevity increases. When rate-limited dissolution is important, the efficiency of remediation, in terms of both mass and flux reduction, is reduced. However, at many field sites the combined effects of field-scale heterogeneities and site aging will result in favorable relationships between mass reduction and flux reduction.

Relationship between mass-flux reduction and source-zone mass removal: Analysis of field data

“…An early effort to quantify the relationship between contaminant mass flux reduction and mass removal, and the resultant reduction in risk, was presented by Freeze and McWhorter (1997). The specific relationship between mass flux reduction and mass removal has since been examined and discussed in a number of studies , Rao et al, 2002, Rao and Jawitz, 2003Stroo et al, 2003;Brooks et al, 2004; Jayanti and Pope, 2004;Lemke et al, 2004;Parker and Park, 2004;Phelan et al, 2004;Soga et al, 2004;Jawitz et al, 2005; NRC, 2005;Fure et al, 2006;Lemke and Abriola, 2006;Brusseau et al, 2007Brusseau et al, , 2008.…”

“…Several approaches, based on "source-depletion" models or simple "mass-removal" functions, have recently been proposed for estimating the relationship between mass flux reduction and mass removal (e.g., Enfield et al, 2002;Rao et al, 2002;Parker and Park, 2004;Zhu and Sykes, 2004;Falta et al, 2005a;Jawitz et al, 2005). One simple approach is based on treating changes in mass flux as a direct function of the change in contaminant mass:…”

“…An early effort to quantify the relationship between contaminant mass flux reduction and mass removal, and the resultant reduction in risk, was presented by Freeze and McWhorter (1997). The specific relationship between mass flux reduction and mass removal has since been examined and discussed in a number of studies , Rao et al, 2002, Rao and Jawitz, 2003Stroo et al, 2003;Brooks et al, 2004; Jayanti and Pope, 2004;Lemke et al, 2004;Parker and Park, 2004;Phelan et al, 2004;Soga et al, 2004;Jawitz et al, 2005; NRC, 2005;Fure et al, 2006;Lemke and Abriola, 2006;Brusseau et al, 2007Brusseau et al, , 2008.Three simplified, prototypical relationships between mass flux reduction and mass removal, representative of systems for which the source zone is undergoing continuous water flushing, which are useful for comparative discussion are presented in Figure 1a. These relationships can be readily developed by employing a simple limiting-case analysis of the temporal contaminant-elution/mass-removal function for immiscible-liquid systems (as shown in Figure 1b), from which the mass-flux-reduction/mass-removal relationship can be Figure 1a represents the relationship for a system for which the flushing process (mass-transfer and displacement) is relatively ideal, wherein immiscible-liquid dissolution and other mass-transfer processes are under equilibrium conditions and all contaminant mass is accessible to flowing groundwater.…”

“…The fundamental concept of contaminant mass flux, its relationship to mass-removal processes and source-zone properties, and its impact on risk has long been established (e.g., Fried et al, 1979;Pfannkuch, 1984). The impact of subsurface heterogeneity, immiscibleliquid distribution, and mass-transfer dynamics on mass-removal behavior and aqueous concentration profiles (mass flux) has been examined for some time through laboratory, modeling, and field studies (e.g., Schwille, 1988;Dorgarten, 1989;Guiguer, 1991;Anderson et al, 1992; Brusseau, 1992;Guarnaccia and Pinder, 1992;Mayer and Miller, 1996;Berglund, 1997;Nelson and Brusseau, 1997;Powers et al, 1998;Unger et al, 1998;Broholm et al, 1999;Brusseau et al, 1999a;Frind et al, 1999;Zhang and Brusseau, 1999;Nambi and Powers, 2000;Zhu and Sykes, 2000;Brusseau et al, 2000Brusseau et al, , 2002 Saba and Illangasekare, 2000;Sale and McWhorter, 2001;Rivett et al, 2001;Enfield et al, 2002;Rao et al, 2002;Rao and Jawitz, 2003; Jayanti and Pope, 2004;Lemke et al, 2004;Parker and Park, 2004;Phelan et al, 2004;Soga et al, 2004; Falta et al, 2005a, b ;Jawitz et al, 2005; Feenstra, 2005, Fure et al, 2006;Lemke and Abriola, 2006;Suchomel and Pennell, 2006;Brusseau et al, 2007Brusseau et al, , 2008). An early effort to quantify the relationship between contaminant mass flux reduction and mass removal, and the resultant reduction in risk, was presented by Freeze and McWhorter (1...…”

Lab Testing and Field Implementation of Soil Flushing

“…In the latter case, the site remediation goal is to remove sufficient DNAPL mass from the source zone to achieve the desired reduction in source strength, which results in either stabilization or shrinkage of the dissolved plume . The relationship between the amount of DNAPL mass depleted in the source and the resulting reduction in source strength is receiving increasing attention as a performance metric (Falta et al, 2005a;ITRC, 2004;Jawitz et al, 2005;Lemke et al, 2005;Rao et al, 2001;Rao and Jawitz, 2003;Sale and McWhorter, 2001). It is generally recognized that this relationship is a function of several parameters, including the aquifer hydrodynamic heterogeneity, the non-uniformity of DNAPL distribution, the correlation between hydraulic conductivity and DNAPL content, the DNAPL architecture, and the age of the site (the time since contamination event) (Jawitz et al, 2005;Rao et al, 2001;Rao and Jawitz, 2003).…”