Experimental study on the relation between the fractal characteristics and solute transport parameters of sandy soil

Abstract

Assessing solute transport in porous media is important for better understanding groundwater contamination. Transport parameters are closely related to the soil geometry, which exhibits self-similarity both in particle size distribution and pore characteristics, which can be modeled mathematically with fractals. Therefore, studying the relation between soil fractal characteristics and solute transport parameters is crucial. In this study, hydrodynamic dispersion experiments were carried out in six sandy soil columns that represent different porous aquifers to calculate the solute transport parameters including the hydraulic conductivity (Ks) and dispersion coefficient (DL). The mass fractal dimension (Dm) was calculated on the basis of measuring the particle size distribution of the six soil samples, and the pore volume fractal dimension (Dv) of the soil was calculated by analyzing scanning electron microscope images of the soil with the software Pores (Particles) and Cracks Analysis System (PCAS). The relationships among Dm, Dv, and the solute transport characteristics (saturated permeability and hydrodynamic dispersion) in the sandy soil were analyzed. The results suggested three mass fractal scaling domains for the studied soils: 1~10 μm (I), 10~60 μm (II), and 60~420 μm (III). Better soil gradation resulted in a larger Dm in scaling domains I and III and a smaller Dm in domain II. Dv decreased slightly with the coefficient of uniformity (Cv). Better soil gradation resulted in a larger DL and a smaller Ks. Ks decreased with increasing Dm, and DL increased slightly with increasing Dm in the whole scaling domain. The Dm results in the divided scaling domains had little significance for the evaluation of DL and Ks. Ks decreased with increasing Dv, and DL increased with increasing Dv for the studied soils. In short, the studied soils showed three typical mass fractal scaling domains, and the Dm varied differently among the three scaling domains. A better soil gradation and more fine soil particles led to a higher Dm. Dv decreased with Cv. Better soil gradation resulted in a larger DL and a smaller Ks. The influence of Dv on Ks and DL is greater than that of Dm on Ks and DL. The use of one or more comprehensive indicators should be promoted, and other fractal characteristics of soils, not only the fractal mass, should be considered. More attention should be paid to the pore volume properties to more accurately characterize solute transport in soil.