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DNAPL Source Depletion: 1. Predicting Rates and Timeframes

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Abstract

Given the relatively rapid rate of dense nonaqueous-phase liquid (DNAPL) ganglia depletion, source zones are generally dominated by horizontal layers of DNAPL after a release to the saturated zone. Estimating the time required to attain specific source strength reduction targets resulting from partial DNAPL source depletion is challenging due to a lack of available screening models, and because little has been done to synthesize available empirical data. Analytical and semi-analytical models are used to study general DNAPL pool dissolution dynamics. The half-life for the decline in DNAPL source strength (i.e., aqueous mass discharge) is demonstrated as proportional to the square root of the pool length, the thickness of the pool, and the solubility for single component DNAPLs. The through-pool discharge is shown to be potentially significant for thin pools or in upper regions of thicker pools. An empirical analysis is used to evaluate average concentration decline rates for 13 in situ chemical oxidation (ISCO) and 16 enhanced in situ bioremediation (EISB) sites. Mean apparent decline rates, based on the time required to achieve the observed source strength reduction, are calculated for the ISCO and EISB sites (half-lives of 0.39 year and 0.29 year, respectively). The empirical study sites are shown to have faster decline rates than for a large, complex study site where ISCO was implemented (half-life of 2.5 years), and for a conceptual pool-dominated trichloroethene source zone where EISB was simulated (half-life of 2.5 years). Guidance is provided on using these findings in estimating timeframes for partial DNAPL depletion goals. © 2014 Wiley Periodicals, Inc.

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