Exploring the Potential of Nonlinear and Non-Equilibrium Alcohol Partitioning for Assessment of DNAPL Source Zone Architecture

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Recent research highlights the importance of quantifying the relationship between dense nonaqueous phase liquid (DNAPL) spatial distribution, source strength and source longevity. Since acquiring knowledge of the precise distribution of DNAPL saturations (also known as the DNAPL architecture) may be impracticable (technically or economically), attention is being placed on understanding the key features of the DNAPL architecture that control contaminant mass discharge. Examples of metrics describing DNAPL architecture include reactive travel times, spatial moments of the DNAPL mass, and ratios of DNAPL mass present in ganglia to that present in pools. While other metrics may yet be advanced, the utility of any source zone architecture metric as a site management tool requires that the metric be readily quantifiable. Partitioning tracers are one tool that may permit interrogation of DNAPL architecture. Traditional use of partitioning interwell tracer tests, however, provides an estimate of DNAPL saturation that is integrated over the flow path of the tracer. While helpful in estimating overall saturations or total mass of DNAPL, few interwell tracer tests are designed to provide the spatial resolution required to assess DNAPL architecture. Here we explore the use of partitioning tracers under the conditions of nonlinear and non-equilibrium transport, with the longer-term goal of estimating DNAPL architecture in localized portions of a source zone. Initial efforts were directed toward elucidating liquid-liquid equilibria of 2-octanol, a common partitioning alcohol, in a system comprising water and TCE-NAPL. Results illustrate the importance of assessing tracer partitioning over a wide range of concentrations. Column experiments were conducted in a sandy medium to explore limitations to tracer transport with both the aqueous phase and a uniformly entrapped nonaqueous phase. These preliminary results suggest nonlinear and non-equilibrium partitioning may be exploited to assess DNAPL source zone architecture.