[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.

[1] The use of interwell partitioning tracers to quantify the amount of nonaqueous phase liquid (NAPL) in porous media has been demonstrated in several laboratory and field tests. The primary emphasis of work to date has been on the use of first temporal moments of tracer breakthrough curve (BTC) data to estimate the average NAPL saturation. Here we extend the data analysis to the use of tracer BTC second and third temporal moments to estimate the statistical parameters characterizing the NAPL spatial distribution. In particular, we examine the fraction f of the streamlines that contain NAPL and the mean and standard deviation of the distribution of streamline trajectoryaverage NAPL saturations. Two models are presented based on discretizing tracer swept volumes into contaminated and uncontaminated zones. The models are applied to data from three-dimensional numerical simulations, two-dimensional flow laboratory experiments, and field tests at two sites (Hill Air Force Base, Utah, and a dry cleaner in Jacksonville, Florida). For all cases considered here, good agreement was found between expected (measured) and estimated values of f, the fraction of the tracer swept zone that contained NAPL. The effects of nonlinear and nonequilibrium partitioning as well as correlations between NAPL saturation and saturated hydraulic conductivity are also considered.

Abstract. The effect of solute injection mode is examined in the stochastic-advective' framework. For three-dimensional heterogeneous aquifers, uniform resident injection of solute results in nonlinear propagation of mass arrival time mean and variance with distance. Injection in flux results in a linear propagation of mean mass arrival time and mass arrival time variance that is both lower and appears to reach a linear regime more rapidly than uniform resident injection. Implementation of instantaneous injections for both modes as boundary conditions for mathematical and numerical models is discussed.

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