Predicting Attainable Goals and Depletion Timeframes for DNAPL Source Zones

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Carey, Grant
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University of Guelph

The primary goal of this research was to develop and validate new statistical and process-based, screening-level modeling tools for estimating attainable goals and depletion timeframes for DNAPL source zones under a wide range of soil texture and other conditions. A process-based screening model (NAPL Depletion Model, or NDM) was developed to simulate the relative influence of surface discharge and through-discharge for one or more DNAPL sub-zones in an overall source zone. Empirical assessment of depletion rates at field sites are compared to model simulated results for pool-dominated source zones to provide insights on the influence of site complexity. Statistical analysis of empirical data for a similar site dataset defined the mean confidence interval for technology-specific, attainable mass discharge reduction. The lower end of this confidence interval is demonstrated to be applicable to complex sites. Empirical regression equations are derived for estimating the tortuosity coefficient based on either hydraulic conductivity or effective porosity, and for estimating total porosity and effective porosity based on hydraulic conductivity. An empirical regression equation is also developed for estimating transverse vertical dispersivity based on hydraulic conductivity, demonstrating that this relationship is inverse to what has been concluded in previous studies. An alternative model for simulating the decline in through-discharge for layers of residual DNAPL is presented, and is demonstrated to be more applicable to a broad range of conditions relative to previously developed regression equations. NDM was used to simulate complex multicomponent DNAPL dissolution trends based on a previously published study (Emplaced Source experiment in Borden, Ontario). Model results demonstrate the influence of re-equilibration and transient effective solubility in a multicomponent DNAPL source zone, on the overall through-discharge decline rate that was observed during the physical experiment. An empirical regression equation is developed for estimating the through-discharge decline rate for residual DNAPL layers over a broad range of conditions. This regression equation allows estimation of the decline rate a priori i.e. without fitting to site-specific data.

DNAPL, dissolution, depletion, mass discharge, pool, tortuosity, transverse dispersivty, dilution