Abstract

Diffusion is often the dominant mode of solute transport in soils when advection is minimal. This paper describes the application of a radial diffusion cell method to estimate the effective diffusion coefficient (De) and effective diffusive porosity (θDe) for use in solute transport models for fractured-porous media. Twenty-four experiments were conducted for 28 d using three conservative solutes (Br, PFBA, and PIPES) on eight late Wisconsinan and Pre-Illinoian till samples from Iowa. The mean value of the total porosity (θT) of the till samples was 30.0%. Concentrations of the three tracers in the reservoir decreased with time and eventually approached equilibrium concentrations. A model simulated the observed concentration data and the modified goodness-of-fit (d1) values ranged from 0.878 to 0.950. Mean values of θDe from the model were 28.3 (Br), 26.5 (PFBA), and 21.6% (PIPES) and there were significant differences in θDe among the three tracers (p = 0.05). Mean values of De were 5.6 × 10−10 m2 s−1 (Br), 2.9 × 10−10 m2 s−1 (PFBA), and 1.3 × 10−10 m2 s−1 (PIPES). Values of De differed significantly by compound and were significantly different (p = 0.05) from the aqueous diffusion coefficient (D0). Calculated mean values of the first-order mass exchange coefficient (α) were 8.4 × 10−7 (Br), 4.1 × 10−7 (PFBA), and 1.6 × 10−7 s−1 (PIPES); they differed by compound (p = 0.05) and generally decreased with increasing molecular weight of the tracer. This study confirmed that the radial diffusion cell method is an efficient method to estimate effective diffusion parameters necessary to accurately model solute transport in fractured till and soil.

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