Abstract
Dispersion characteristics are important factors affecting groundwater solute transport in porous media. In marine environments, solute dispersion leads to the formation of freshwater aquifers under islands. In this study, a series of model tests were designed to explore the relationship between the dispersion characteristics of solute in calcareous sands and the particle size, degree of compactness, and gradation of porous media, with a discussion of the types of dispersion mechanisms in coral sands. It was found that the particle size of coral sands was an important parameter affecting the dispersion coefficient, with the dispersion coefficient increasing with particle size. Gradation was also an important factor affecting the dispersion coefficient of coral sands, with the dispersion coefficient increasing with increasing d10. The dispersion coefficient of coral sands decreased approximately linearly with increasing compactness. The rate of decrease was −0.7244 for single-grained coral sands of particle size 0.25–0.5 mm. When the solute concentrations and particle sizes increased, the limiting concentration gradients at equilibrium decreased. In this study, based on the relative weights of molecular diffusion versus mechanical dispersion under different flow velocity conditions, the dispersion mechanisms were classified into five types, and for each type, a corresponding flow velocity limit was derived.
Highlights
Coral sands are a kind of biogenic soil in marine environments, originating from the fracturing and sedimentation of coral skeletons by wind and waves [1]
Given that the dispersion coefficient of coral sands is a key factor affecting the conservation of fresh groundwater, uncovering the dispersion pattern of groundwater solute in coral sands will provide the basic parameters and theoretical basis for the numerical simulation of the formation and evolution of fresh groundwater aquifers in the South China Sea islands and reefs, as well as the conservation and utilization of these aquifers
In order to uncover the effect of particle size on the one-dimensional dispersion coefficient, dispersion tests were performed on a total of six groups of single-grained coral sand samples, with the sample groups having a particle sizes of < 0.1 mm, 0.1–0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm, and 2–5 mm; the groups had particle size ranges within the categories of silt, fine sand, medium sand, coarse sand, gravelly sand, and crushed stone, respectively
Summary
Coral sands are a kind of biogenic soil in marine environments, originating from the fracturing and sedimentation of coral skeletons by wind and waves [1]. Klotzd et al explored the relationship between the longitudinal dispersion coefficient and average pore flow velocity, the fluid viscosity coefficient, and the characteristic parameters of soil media by conducting a large number of field and laboratory experiments [6]. According to the results of our study, the empirical value of dispersion coefficients can be provided for calcareous soils with common gradation and compactness. In this way, a more stratigraphic collocation design can be considered in the numerical simulation, so as to find the optimal stratigraphic design scheme that can promote the formation of underground fresh water.
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