NUMERICAL INVESTIGATION ON THE ABILITY TO PREDICT NON-FICKIAN DISPERSION FROM AQUIFER PROPERTIES

In contrast to the assumptions of Fickian, or Gaussian-based, dispersion, field and laboratory-scale dispersion in ground water systems demonstrates scale dependency. Scale dependent, or non- Fickian, dispersion results in a heavy-tailed solute distribution that is poorly predicted using the Gaussian-based Advection Dispersion Equation. Solute plumes are believed to disperse in a non-Fickian manner due to small scale heterogeneity and preferential pathways. While non-Fickian based solute transport models are able to mimic the scale dependent nature of dispersion, there is no clear understanding on how to predict dispersion parameters from the underlying heterogeneity. This link is needed in order to forecast the nature of dispersion based on key features of the aquifer prior to a contamination event.

This project tracks how heterogeneity characteristics such as correlation length, variability, anisotropy, and preferential pathways play a role in our ability to predict non-Fickian dispersion based on aquifer properties. In this study, we take a numerical approach to simulate solute transport through a wide range of heterogeneous fields and compare the nature of solute dispersion with our ability to predict that dispersion from the underlying heterogeneity. Heterogeneous fields are first based on sequential Gaussian simulation and move towards using more realistic representation of natural geologic materials. While several methods exist to estimate the nature of dispersion directly from heterogeneity, their use has not proven robust over various hydrologic settings (Herrick et al., 2002). This study is designed to test the limitations of existing estimation techniques and provide insight on how to improve estimation.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Herrick, M.G., D.A. Benson, M.M. Meerschaert, and K.R. McCall, Hydraulic conductivity, velocity, and the order of the fractional dispersion derivative in a highly heterogeneous system, Water Resources Research, 38(11), 1227, 2002.