EXPERIMENTAL DETERMINATION OF HOW SEDIMENT SIZES AND GRADATION CONTROL FICKIAN TO NON-FICKIAN DISPERSION WITH FLOW RATE – A LABORATORY APPROACH

To improve predictive capabilities of evaluating groundwater contamination and nutrient transport, we determine the relationship between the scale-dependent Fickian dispersion and dispersivity as a function of sediment sizes and gradation through conducting non-reactive tracer experiments simulated within sand column of different length-scales and at different flow rates. Numerous studies have examined the effect of sediment particle size and distribution on dispersity, but few have focused explicitly on how grain-size heterogeneity related to sediment gradation affects Fickian to non-Fickian dispersion. Moreover, how the transition from non-Fickian to Fickian transport regime is affected by different flow rates or Péclet number remains unclear. In this study, breakthrough curve (BTC) data from several column tests at different length-scales are used to investigate the effect of sediment grain size and gradation under different flow rates. Grain size distribution and grading indexes such as Uniformity Coefficient (Cu), Coefficient of Curvature (Cc), and sorting coefficient (So) are used to quantify the magnitude of heterogeneity in sediment sizes and gradation. Hydrodynamic dispersion is quantified as a function of geologic heterogeneity, i.e., Cu, Cc, and So and Peclet number. Attempts are made to determine unique power-law exponent describing the relation between heterogeneity and the dispersion/dispersivity coefficients.

Keywords: Dispersivity. Peclet number. Sediment gradation. Fickian Dispersion. Non-Fickian Dispersion. Breakthrough Curve.