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Passive Remediation of Sulfide Wastes: A Study of Oxygen Diffusion Through Composite Barrier Covers
Primary Issue Addressed: Source Control
Secondary Issue Addressed: None
Project Site: Montana Tech of the University of Montana
Collaborating Entities: Montana Tech
Cost Share: None
Project Description
The main objective of this project is to determine oxygen flux through one component of a typical barrier cap at a variety of moisture conditions. Oxygen flux through topsoil will be determined through a combination of percent oxygen measurements and the use of theoretical equations. The goal is to develop a best fit curve to describe oxygen diffusion under differing moisture conditions.
The theory behind oxygen movement is that oxygen will move through material such as waste rock by processes called diffusion and/or convection. Based on this process, several researchers have presented methods of modeling to represent this movement. A recent method for determining oxygen movement through a geosynthetic clay liner (GCL) system was developed in 1999 by a group of Canadian researchers. In a 1999 paper entitled Evaluation of Diffusive Gas Flux through Covers with a GCL (Aubertin), the researchers specifically set out to determine the rate of oxygen movement through GCLs. Earlier methods of modeling were only able to estimate the influence of a GCL on oxygen diffusion. In this study, the GCL was specifically examined to estimate the diffusion. This method does not produce a depth of oxygen diffusion; rather, it produces a rate at which oxygen diffuses per year and an acceptable range for the prevention of acid rock drainage (ARD) production. A critical parameter in determining acid generation is the flux of oxygen, which determines the rate of acid production through the oxidation of sulfide materials contained in the waste materials. Controlling this rate over time will control the rate of acid production. Determination of the diffusion coefficient is a key parameter in determining oxygen flux. Materials at or near saturation exhibit reduced oxygen fluxes owing to the relatively low solubility of oxygen in water and the low diffusion coefficient for oxygen in water relative to that in air.
Status
Five diffusion cells were constructed of 8-inch PVC, the cells were filled with soil and compacted to 90%, which mimics field conditions. The upper and lower reservoirs of each cell were connected to oxygen meters to record oxygen concentrations. The meter connected to the upper reservoirs also has the capability to measure carbon dioxide concentrations; thus, those measurements have also been recorded. Measurements of oxygen and carbon dioxide began in April 2005 and have been ongoing since that time. Thus far, oxygen diffusion has been studied at moisture contents of 6%, 10%, and 13%. The cells are under preparation for measuring oxygen concentrations at a moisture content of 16%. Two cells began having problems retaining moisture at 13% moisture content, and water retention became troublesome in all cells when attempting to increase soil moisture to 16%.
As expected, oxygen diffusion decreased as moisture content increased. Measured oxygen concentrations in each of the five cells were very similar at 6% and 10% moisture content. However, at 13% moisture content, one of the cells that had water retention problems had an oxygen concentration curve that differed notably from the other four cells. One unexpected finding in this project was the apparent consumption of oxygen in the cells, and a corresponding production of carbon dioxide. In an attempt to determine the origin of the oxygen consumption, the soil was analyzed for metals and sulfur content. These analyses indicated that there were neither sufficient metals nor sulfur in the soil to cause oxygen consumption. Total organic carbon (TOC) was also quantified, and a TOC content of 5 mg TOC/gsoil was determined. This concentration of TOC is sufficient to cause oxygen consumption. Data collected throughout this project will be used to determine an oxygen diffusion coefficient. An in-depth evaluation of the data, along with the determined diffusion coefficient will be provided in the final report, which is expected to be completed in fiscal 2006.