Title: Western Region Hazardous Substance Research Center Project 2-SU-04
Novel Methods for Laboratory Measurement of Transverse Dispersion in Porous Media

Investigators: Peter K. Kitanidis and Craig Criddle, Stanford University

Institution: Oregon State University

Research Category: Groundwater, transport

Project Period: 2004-2007

 

Goal: (1) Develop, refine, and critically evaluate novel methods for the laboratory measurement of transverse dispersion in homogeneous isotropic unconsolidated porous media; (2) develop experimental protocols and methods of data analysis; (3) independently verify the accuracy of the new methods; (4) perform extensive experiments to determine relations of transverse dispersivity with conductivity, longitudinal dispersivity, mean grain size, degree of non-uniformity, etc.

Rationale: Transverse dispersion in porous media measures the rate of spreading of a solute in the direction perpendicular to flow. Pore-scale transverse dispersion is widely accepted as playing a dominant role in determining the actual rate of dilution of solutes and mixing of reactants in porous media. For example, consider a long plume of contaminants emanating from a constant source. The rate of intrinsic remediation is determined by the rate of transverse mixing of contaminants in the plume with reactants from the surrounding groundwater. The rate may be primarily determined by the value of the transverse dispersion coefficient. Better understanding of transverse dispersion would ultimately improve our understanding of diffusion-limited processes, such as intrinsic remediation. Despite its importance, transverse dispersion remains insufficiently understood.

Approach: Part of the difficulty has been the lack of accurate and efficient methods for laboratory measurements. In most existing methods for the determination of transverse dispersion, the measured quantity is proportional to the dispersion coefficient, and thus small and swamped by experimental error. However, we developed new methods for the measurement of local transverse dispersion in isotropic porous media based on a helical and a cochlea-like device. The idea was to perform an experiment similar to the tracer test through a laboratory column packed with a porous medium and to measure the breakthrough curve; however, the objective was not to determine the column-scale longitudinal dispersion but the transverse dispersion. The principle was to induce shear flow inside the device that creates strong longitudinal dispersion in the observed breakthrough curve; transverse mixing tended to negate the effects of shear flow and thus reduced the observed column-scale longitudinal dispersion. Then, from the spreading of the observed breakthrough curve, we could estimate the unknown, the pore-scale transverse dispersion. The measured quantity varies inversely with transverse dispersion coefficient.

Summary of Findings: The project was completed and a PhD dissertation has been submitted that has served as final report. Highlights of the dissertation: We discuss instrumentation and tracers that we used to obtain experimental concentration breakthrough curves. We describe the numerical simulation and parameter estimation methods used to analyze the experimental data. We discuss the results and describe the relative advantages of each device, instrument, and methodology that we have used to estimate transverse dispersivity. Perhaps the most noteworthy conclusions of this research are that the results from the two devices, helix and cochlea, are in agreement and that the ratio of transverse dispersivity to longitudinal dispersivity that we estimate agrees with the higher ratios reported in the literature.

 

 Publications:

Journal Articles

Benekos, I. D., O. A. Cirpka, and P. K. Kitanidis (2006). Experimental determination of transverse dispersivity in a helix and a cochlea. Water Resources Research, 42, W07406, 10.1029/2005WR004712.

 

Conference Proceedings and Presentations

Benekos, I., P.K. Kitanidis, M.A. Rahman, and O.A. Cirpka (2001). Experimental and Mathematical Studies of Pore-Scale Transverse Dispersion in a Helical Soil Column. AGU Fall Meeting, December 10-14, San Francisco, CA.

Benekos, I., and P.K. Kitanidis (2004). Experimental Determination of Transverse Dispersivity in a Cochlear Device. Western Pacific Geophysics Meeting, August 15-21, Honolulu, HI.

Benekos, I., and P.K. Kitanidis (2005). An Optimization Approach Using Tracer Concentration Breakthrough Curves for Determining the Transverse Dispersivity in a Cochlear Device. EPA-HSRC Workshop on Risk Assessment and Monitoring Research, November 4-5, Las Vegas, NV.

Benekos, I., and P. Kitanidis (2005). On the Determination of Transverse Dispersivity: Experiments and Simulations in a Helix and a Cochlea. AGU Fall Annual Meeting.

 

Theses

Benekos, Ioannis D. (2005). On the Determination of Transverse Dispersivity: Experiments and simulations in a helix and a cochlea. Ph.D, Stanford University.

Supplemental Keywords: characterization; groundwater; chemical wastes.