Grantee Research Project Results

1999 Progress Report: Effect of Heterogeneities on Instability and Fingering of DNAPLS Below the Water Table

EPA Grant Number: R826157
Title: Effect of Heterogeneities on Instability and Fingering of DNAPLS Below the Water Table
Investigators: Neuman, Shlomo P. , Smith, James E.
Institution: University of Arizona
Current Institution: University of Arizona , McMaster University
EPA Project Officer: Aja, Hayley
Project Period: March 1, 1998 through February 28, 2001
Project Period Covered by this Report: March 1, 1998 through February 28, 1999
Project Amount: $330,000
RFA: Exploratory Research - Physics (1997) RFA Text |  Recipients Lists
Research Category: Water , Land and Waste Management , Air , Safer Chemicals

Objective:

To investigate analytically, experimentally, and computationally the effect of soil heterogeneities on instability and fingering of dense non-aqueous phase liquid (DNAPL) contaminants below the water table in aquifers. In particular:

1. To investigate the conditions that may lead to instability when fronts of DNAPL propagate through water-saturated, randomly heterogeneous soils in multidimensional flow fields below the water table of aquifers.

2. To investigate the manner in which multidimensional fingers evolve at finite time intervals following the onset of DNAPL front instability.

3. To consider the effect of systematic trends in permeability, and high-permeability preferential flow paths, on DNAPL front instability and finger development.

Progress Summary:

A theoretical study of wetting front instability in randomly stratified unsaturated soils by Chen and Neuman (1996) demonstrates that heterogeneities may either reinforce or counteract instability, depending on the spatial trend, variance and spatial correlation scale of medium permeabilities. Chen and Neuman considered random fluctuations of permeability about their mean trend to be statistically homogeneous. During the first year of the project, we developed a similar stochastic theory for DNAPLs that propagate downward through a stratified, saturated aquifer. In particular, we derived probabilistic criteria for the onset of instability in terms of parameters that describe medium heterogeneity, such as functional forms and coefficients of permeability trend with depth, its variance and spatial covariance. Both theoretical and computational analyses of this first phase of our investigation have been completed. During the second year of the project, we have formulated a stochastic theory of stable or unstable front propagation in randomly heterogeneous media in terms of first and second conditional moments. We have solved the problem in one-dimension and evaluated it computationally. Our theoretical efforts during the third year will be directed toward an extension of these results to two and three spatial dimensions. We also intend to incorporate in our theory flow in multiscale permeability fields of the kind studied by Di Federico and Neuman (1997, 1998a-b), Neuman and Di Federico (1998), and Di Federico, et al. (1999). This extension should allow us to consider the effect of random fractal permeability distributions on the development of preferential flow paths and their impact on the instability and fingering of DNAPL fronts below the water table.

The experimental component of our project is designed to accurately and precisely measure conditions for the onset of instability, the development of DNAPL fingers, their morphology, number, spatial distribution, size, and rate of advance in heterogeneous saturated porous media. Year one was devoted to the completion of our laboratory setup, and year two to the conduct of experiments on the penetration of DNAPL into water-saturated glass beads in a two-dimensional transparent chamber. The displacement process was recorded using a video camera directly connected to, and controlled by, a computer.

In addition to the standard fingering mechanisms of tip-splitting, coalescing and shielding, we also observed tip-growing of DNAPL fingers. During the latter process, the tip of a finger (in our case about 1-2 cm) was active and growing while other parts remained essentially unchanged. Finger lengths and fractal dimensions were measured using a box counting method with a rectangular grid superimposed onto the area of measurement. The fractal dimension did not change substantially when the mean particle size was doubled. The interfacial tension coefficient tends to increase when the mean particle size decreases. The reduction of the interfacial tension upon introduction of a surfactant caused a decrease in the fractal coefficient but did not have much effect on fractal dimension, and thus led to a smaller interfacial tension coefficient.

Using an approach modified after Chang, et. al. (1994) to determine effective interfacial tension, and applying the linear stability theory of Chuoke, et al. (1959), we were able to predict wavelengths of fingering close to those we had observed experimentally in homogeneous porous media.

The primary focus of our experimental work during year three of the project will be to investigate the effects of layered heterogeneity on DNAPL finger spacing, morphology and development. Experiments in stratified media are currently underway and progressing well.

Future Activities:

During the third year of the project, we will extend our stochastic theory of DNAPL front propagation in randomly heterogeneous media to two and three spatial dimensions. We will consider the effect of random fractal permeability distributions on the development of preferential flow paths and their impact on the instability and fingering of DNAPL fronts below the water table.

The primary focus of our experimental work during year three will be to investigate the effects of layered heterogeneity on DNAPL finger spacing, morphology and development. Experiments in stratified media are currently underway and progressing well.

Journal Articles on this Report : 1 Displayed | Download in RIS Format

Publications Views

Other project views:	All 10 publications	3 publications in selected types	All 3 journal articles

Publications

Type	Citation	Project	Document Sources
Journal Article	Smith JE, Zhang ZF. Determining effective interfacial tension and predicting finger spacing for DNAPL penetration into water-saturated porous media. Journal of Contaminant Hydrology 2001;48(1-2):167-183.	R826157 (1999) R826157 (Final)	not available

Supplemental Keywords:

groundwater, solvents, DNAPL, immiscible fluids, interfacial tension, porous media, heterogeneity, preferential flowpaths, fingering, fractals., Scientific Discipline, Water, Waste, Environmental Chemistry, Physics, Remediation, Engineering, Chemistry, & Physics, Groundwater remediation, environmental monitoring, DNAPL, instability, soil screening, aquifer remediation design, chemical kinetics, soil contaminants, multiscale permeability fields, groundwater contamination, hydrologic site characteristics, stochiometry, time domain reflectrometry, contaminated aquifers

Relevant Websites:

http://www.science.mcmaster.ca/geo/faculty/smith/index.html

Progress and Final Reports:

Original Abstract

1998

Final Report