Waste Rock Studies at a Diamond Mine

The growth of the diamond mining industry in the Northwest Territories has led to significant economic benefit; however, the long-term benefit to northern communities will depend on careful environmental management of mining activities. An important environmental aspect of mining activities is the management of leachate derived from the oxidation of sulfide minerals within unsaturated waste rock piles. The rate and extent of sulfide oxidation is function of a number of factors; including climate, the physical and geochemical characteristics of the waste rock, water, gas and thermal transport processes, and significantly, the interaction and coupling of these characteristics and processes. My research at the Diavik Diamond Mine has included scaling of leaching results from laboratory experiments to field scale waste-rock piles, the investigation of gas transport processes in waste rock, and coupling of thermal and gas transport processes. 

Installing bore-hole instrumentation in the waste rock at the Diavik Diamond Mine.Installing bore-hole instrumentation in the waste rock at the Diavik Diamond Mine.

Gas transport research at Diavik has involved the design, construction, and installation of an automated data logging system to directly measure air pressures, and thus the driving force for air movement, within the waste-rock stockpiles. The ability to experimentally measure air pressure gradients within unsaturated waste rock is a marked improvement over previous studies and has shown that in the Canadian north, where wind speeds often exceed 20 km/hr, wind is a major driving force for oxygen and thermal transport within these large porous structures. 

Scaling studies have involved the application of analytical and numerical models to predict the geochemistry of leachate from field-scale waste-rock piles using leaching rates from laboratory experiments. Results have indicated that reliable predictions of leach water quality can be made if the waste rock at both scales is sufficiently characterized in terms of mineralogy, particularly sulfur and carbonate content, and particle size, and environmental factors such as temperature are accounted for. These studies will enable mining companies to plan and employ more effective closure strategies for protecting the local environment.

Advanced Reactive Transport Modelling for Groundwater Contaminant Remediation

The transport and fate of contaminants in groundwater often involves complex biogeochemical reactions coupled to physical transport and attenuation processes. Furthermore, remediation schemes, particularly in-situ methods, often seek to manipulate the geochemical or physical conditions in the subsurface to enhance contaminant attenuation or bio/degradation. Due to the coupled and non-linear nature of these processes, numerical modelling techniques have been developed and employed to analyse these systems. Reactive transport models (RTM) in particular couple geochemical reactions with physical transport processes and can be used to quantify intra-aqueous chemical reactions, mineral dissolution-precipitation reactions, chemical and physical sorption processes, mass transfer between aqueous and non-aqueous phases, and other processes that effect contaminant transport and attenuation. When applied appropriately RTM are able to provide a more in-depth quantitative analysis of geochemical data sets and can often provide additional insights into the underlying mechanism of contaminant transport and attenuation than can be achieved through analytical means of data analysis. This level of understanding is essential for the design, application and improvement of efficient and cost effective stabilization and remediation strategies.

My research has involved the development of reactive transport models and application to contaminant transport and remediation problems. Particularly, this research has focused on; 1) coupling of physical and chemical processes, such as, the effects of gas bubble formation and entrapment on contaminant transport, the effects of gas production on gas phase transport in the vadose zone, and the coupled effects of gas bubble formation and secondary mineral precipitation on the reactivity and permeability of zero-valent iron permeable reactive barriers, and 2) the use of tracers for elucidating geochemical and physical processes. These include dissolved gas tracers and stable isotopes.

Experimental data and reactive transport simulation results showing Cr(VI) reduction by organic showing with two distinct fractionation factors.Experimental data and reactive transport simulation results showing Cr(VI) reduction by organic carbon with two distinct fractionation factors.

Ongoing research will involve the development and application of modeling tools to interpret mechanisms and rates of contaminant release, transport and attenuation, with a focus on metal contamination at industrial sites. This objective will be carried out through the further development of the reactive transport model MIN3P. The key areas for development will include 1) simulation of nano-particle and colloid transport and reactivity, 2) simulation of stable isotope fractionation in metals and gases, and 3) simulation of integrated thermal, hydrological and geochemical processes. The long-term objective of this research is to develop a better qualitative and quantitative understanding of coupled physical and bio/geochemical mechanisms that affect contaminant transport and attenuation mechanisms.

Coupled Physical and Biogeochemical Processes in Multiphase Systems

At an oil spill site near the town of Bemidji, MN, methanogenesis has been shown to be a significant biodegradation pathway of the petroleum contaminants in the groundwater and vadose zone. However, the rate and extent of methanogenesis is difficult to quantify given the complex interaction between physical gas transport processes and geochemical reactions. Field work conducted at the site as part of my PhD and post-doctoral research has demonstrated that naturally occurring stable gases tracers, argon and nitrogen, could be used to identify and quantify methane production, transport, and attenuation processes in both the saturated and unsaturated sediment in the vicinity of the oil spill. High resolution direct push sampling used to collect samples for dissolved and vapour phase gases, geochemical parameters, and stable carbon isotopes, along with core samples for sediment iron extractions and microbial enumerations, demonstrate that two primary processes are likely contributing to methane attenuation downgradient of the oil spill: 1) the entrapment of oxygen rich gas bubbles near the water table leading to enhanced oxygen transport into the contaminant plume and aerobic methane oxidation, and 2) anaerobic methane oxidation coupled to reduction of sediment iron. This research has been conducted in collaboration with several researchers from the United States Geological Survey, 

2-m sand tank experiment.2-m sand tank experiment.

Current research has involved the construction of a 2-m long sand tank to study contaminant transport in the presence of entrapped gas near the water table. Experiments will measure the contributions of bubble entrapment, recharge, and diffusion to gas transport processes near the water table, and quantify the effects of entrapped gas on gas transport, water flow, and contaminant transport and attenuation in the capillary fringe and groundwater in the vicinity of the water table. Stable gases will be used to distinguish between physical and geochemical processes and stable isotope techniques will be used to quantify biogeochemical reactions. The long-term objectives of this research are to contribute to the overall understanding of contaminant transport near the water table, which is often considered to be a highly reactive and important zone with respect to contaminant degradation, particularly for organic contaminants. Future work will use information obtained from laboratory studies to investigate field sites and potentially demonstrate the application of gas bubble entrapment to enhance in situ remediation.