Superfund Research Program
New Approaches for the Physical Characterization of Wetland Deposits with Emphasis on Wells G and H Site
Project Leader: John Germaine
Grant Number: P42ES004675
Funding Period: 1995 - 2000
Project investigators are measuring the physical properties affecting subsurface contaminant transport in the wetland deposits. A laboratory testing program was undertaken to measure the physical properties (mechanical and hydraulic) of the peat and organic clay deposits typical of the wetland (Bailon, 1995). A fixed piston sampling technique was developed to obtain high quality, undisturbed samples for the laboratory-testing program. Also, bulk peat is resedimented in the lab. Since resedimentation eliminates some of the variability in our specimens, it is easier to define how some of the physical characteristics of this peat are related to its hydrogeologic properties. Both the undisturbed and resedimented specimens will be tested in a modified triaxial permeameter. This equipment was developed to enable measurement of hydraulic conductivity, as well as the measurement of breakthrough curves resulting from pulses of a conservative tracer solution, while controlling the flow rate and effective stress acting on a soil specimen. The breakthrough curves (Concentration vs. time data) can then be analyzed with a curve fitting package to obtain values for transport parameters such as effective porosity and the mass transfer coefficient governing the transport of contaminants between the mobile and immobile regions in the soil (Ramsey, 1996).
A new approach to define flow in wetland deposits is being developed. Magnetic Resonance Imaging, MRI, has been successful in defining the pore space in re-sedimented wetland specimens. With MRI techniques, the portion of pore space that takes place in advective flow can be calculated when heavy water is used as a tracer. Testing equipment for large samples (> 1" in diameter) needs to be developed.
An experimental testing program is underway that aims to establish the mechanisms controlling airflow through saturated porous media. Various soil and system parameters are known to affect the performance of an air sparging system, namely medium grain size, homogeneity, gas injection pressure, and injection rate. It is unclear how these variables interact to dictate airflow pathways through the soil. In order to isolate the relative importance of these parameters on air transport through soils, experiments are being conducted in a geotechnical centrifuge under a wide range of g-levels. Centrifuge testing allows for investigation of the effects of these variables on identical samples under increasing injection pressures, while realistically replicating in situ soil stresses and fluid pressures.
Airflow characteristics are obtained from analysis of digitized video images collected during experiments. A transparent porous medium was developed by submerging crushed borosilicate glass in a fluid of matching index of refraction. Airflow through this see-through medium is easily monitored, and image analysis provides continuous information on the resulting radius of influence of the treatment. Grain size distribution of the glass can be controlled, and experiments have been conducted on media with two different particle sizes. Pore fluid can also be changed as long as the value of the refractive index is maintained constant. Two fluids with drastically different values of viscosity were used, in order to study the influence of pore fluid properties on patterns of airflow.