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ORIGINAL RESEARCH

Experimental Determination of Effective Diffusion Parameters in the Matrix of Fractured Till

^a Versar, Inc., 6850 Versar Center, Springfield VA 22151
^b Dep. of Geological and Atmospheric Sciences, Iowa State Univ., Ames IA 50011
^c Agronomy Dep., Iowa State Univ., Ames, IA 50011
^* Corresponding author (mhelmke{at}versar.com)

Received 13 June 2003.

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 (D_e) and effective diffusive porosity (_De) 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 (_T) 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 _De from the model were 28.3 (Br^–), 26.5 (PFBA), and 21.6% (PIPES) and there were significant differences in _De among the three tracers (p = 0.05). Mean values of D_e were 5.6 x 10^–10 m² s^–1 (Br^–), 2.9 x 10^–10 m² s^–1 (PFBA), and 1.3 x 10^–10 m² s^–1 (PIPES). Values of D_e 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 () were 8.4 x 10^–7 (Br^–), 4.1 x 10^–7 (PFBA), and 1.6 x 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.

Abbreviations: BTC, breakthrough curve • DML, Des Moines Lobe • IES, Iowa Erosion Surface • MCL, maximum contaminant level • MSEA, Management Systems Evaluation Area • SIDP, Southern Iowa Drift Plain

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