Posted 14 December 2010
Mine tailings can contain toxic substances that can leach into the environment over long periods of time. Image source: Jim Hendry's web site.
by Dan Zlotnikov
A lot can happen in ten thousand years. This is longer than the lifespan of any human civilization to date, and longer than all but the sturdiest human-built structures can remain standing. Yet predicting what will happen over this time frame - and building to withstand anything from earthquakes and floods to beaver infestations - is a challenge uranium mining operators must overcome.
The millennia-long time frame is one mandated by Canada's nuclear regulators, explains Jim Hendry, a professor of hydrogeochemistry and Cameco Research Chair at the University of Saskatchewan. The regulators place the same restrictions on uranium mine tailings as those placed on nuclear waste, says Hendry.
"What we have to do is take what we know about the tailings and ask, 'What could happen in 10,000 years? What could change?' And some of these changes could be arsenic, selenium or molybdenum leaching into a solution." Coming up with progressively more accurate answers to this question is what Hendry and his team have been working on for almost 20 years.
The main danger in the tailings is not radiation, but a mix of arsenic, selenium and molybdenum. All three are highly toxic and can pose a significant danger if they happen to leach into the groundwater. Radiation is a lesser concern; the tailings contain a small amount of radium-226. But Hendry points out that radium is fairly short-lived, the element's half-life being roughly 1,600 years - the term "short-lived" having a very different meaning for geologists.
To construct a reliable model for what arsenic, for example, might do over a few thousand years, Hendry first had to find out where it was now. This was not as straightforward a question as one might first think because in the overall mass of earth and clay, the dangerous minerals comprised a very small percentage and could be attached to any number of otherwise harmless particles, in any number of ways.
"It's only because of synchrotron radiation, the high-energy beam lines, that we've been able to answer this," says Hendry, looking back on the last decade's worth of efforts. "Now we know fairly well where these materials sit, what solids they're on, what they're associated with, what the binding angles are, and so on."
Most recently, Hendry has been using the information to run experiments, attempting to further refine and validate the models for what factors might influence the tailings ponds in the very distant future.
Despite the extremely long outlook, there are some short-term implications for the uranium industry's work. Hendry's team has been working in partnership with Cameco and, more recently, AREVA Canada. Today, Cameco is looking at modifying their extraction process based on Hendry's results.
Hendry explains: "Now that we understand the geochemical controls on present-day tailings, the question is, can we go back into the mill process and modify that process to improve on the minerals going into the tailings? So, if there is a potential that one of the minerals may form and not be very stable at certain pH levels, we'll say 'we're not going to precipitate that mineral. We're going to consider precipitating this other mineral by changing the mill process, and the new mineral may have a lower solubility and, thus, fewer chances of contaminating the groundwater.'"
Hendry's work is not limited to the uranium industry - most mining operations create tailings ponds and must ensure the long-term safety and stability of those. Radium is the only aspect that is unique to the uranium industry, he says; most of his lab's work can be applied elsewhere in the mining world. Illustrating this is a second set of projects Hendry has been working on, looking at the long-term movement of salts from tailings generated by the potash industry.
"When potash producers take potassium chloride salts out from approximately 1.2 kilometres under the ground, what you're left with are tailings ponds that are very high in salt content; it's a high-salt brine," Hendry says. The salt concentrations are so high that Hendry cannot recall having ever seen the ponds freeze, even when the Prairie winter temperatures drop to minus 40 degrees Celsius.
The salt brine is not without its dangers. There have been cases of brine ponds migrating under the surface, and in a few cases, this migration was blamed for affecting the quality of residential water wells.
The challenge when it comes to salt transport, explains Hendry, is in making sure your tailings pond is built on an impermeable geologic medium. Here too, what would be a straightforward engineering task is made difficult by the time spans under consideration - a potash tailings pond may remain undamaged for a few hundred years. The geological deposits across most of Canada go a good way to ensuring that, as the ponds, in many cases, are located on glacial till.
The till, says Hendry, is a uniform mixture of silt, clay and sand: a highly impermeable layer, through which brine moves at very slow rates. Hendry's experiments peg the hydrotransport rate as low as one metre per thousand years. "That is, if the till layer isn't fractured," he adds. "And it's fractured quite often. If the till is fractured, all bets are off. You can move salt rapidly over great distances, because the porosity increases considerably."
The question of how the salt moves through fractured and unfractured till has kept Hendry's team busy for a number of years now. He and his team have had to do a lot of development of testing techniques because most of the commonly used approaches were designed to work with fresh, or slightly salty (brackish), water and are not usable with the extreme salt concentrations of brine ponds.
Despite the fractured and unfractured tills - and below them an even thicker and less permeable layer of shale - comprising the majority of Western Canada's geological structures, Hendry says most researchers focus on the aquifers. The main reason for this, he feels, is the difficulty in getting the less permeable deposits to yield scientific data.
The till and shale are "aquitards," a name stemming from their inability to pass water. "To get data from these materials, it's not like an aquifer, where you can put a well in and collect a sample right away," explains Hendry. "You have to put a well in, design it so you don't contaminate it, and just wait and wait and wait; you get a very small volume of water because it's so tight - it yields water very slowly."
Getting sufficient data to publish scientific results can take 10 to 15 years, Hendry adds, and funding for such a long time frame is hard to secure. But despite the challenges - or possibly because of them - Hendry is planning on going even deeper. Next on the list is the shale itself, where water movement is measured on a scale of millions, rather than thousands, of years.
"When we do our transport work on the tills, we need to define a lower boundary," he explains. We often use the upper shale contact layer for that. But understanding what happens below that gives us a more complete base to build on for this information. Very few people have looked at this material in the context of salt transport."
Source: Published in CIM Magazine May 2010, Volume 5 No. 3
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