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AGU: Water Resources Research

 

Keywords

  • fractional derivative
  • numerical method
  • radial dispersion
  • subordination

Index Terms

  • Hydrology: Groundwater transport
  • Mathematical Geophysics: Numerical solutions
  • Mathematical Geophysics: Fractals and multifractals
  • Hydrology: Stochastic processes

Abstract

WATER RESOURCES RESEARCH, VOL. 40, W12416, 9 PP., 2004
doi:10.1029/2004WR003314

Radial fractional-order dispersion through fractured rock

David A. Benson

Desert Research Institute, Reno, Nevada, USA

Charles Tadjeran

Department of Mathematics and Statistics, University of Nevada, Reno, Nevada, USA

Mark M. Meerschaert

Department of Physics, University of Nevada, Reno, Nevada, USA

Irene Farnham

Stoller Navarro Joint Venture, Las Vegas, Nevada, USA

Greg Pohll

Desert Research Institute, Reno, Nevada, USA

A solute transport equation with a fractional-order dispersion term is a model of solute movement in aquifers with very wide distributions of velocity. The equation is typically formulated in Cartesian coordinates with constant coefficients. In situations where wells may act as either sources or sinks in these models, a radial coordinate system provides a more natural framework for deriving the resulting differential equations and the associated initial and boundary conditions. We provide the fractional radial flow advection-dispersion equation with nonconstant coefficients and develop a stable numerical solution using finite differences. The hallmark of a spatially fractional-order dispersion term is the rapid transport of the leading edge of a plume compared to the classical Fickian model. The numerical solution of the fractional radial transport equation is able to reproduce the early breakthrough of bromide observed in a radial tracer test conducted in a fractured granite aquifer. The early breakthrough of bromide is underpredicted by the classical radial transport model. Another conservative, yet nonnaturally occurring solute (pentaflourobenzoate), also shows early breakthrough but does not conclusively support the bromide model due to poor detection at very low concentrations. The solution method includes, through a procedure called subordination, the effects of solute partitioning on immobile water.

Received 29 April 2004; accepted 25 October 2004; published 28 December 2004.

Citation: Benson, D. A., C. Tadjeran, M. M. Meerschaert, I. Farnham, and G. Pohll (2004), Radial fractional-order dispersion through fractured rock, Water Resour. Res., 40, W12416, doi:10.1029/2004WR003314.

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