Thursday, May 24, 2012

The Accretionary Wedge 46 - Deep Geologic Repositories

The 46th Accretionary Wedge is being hosted by Cat at Knowledge Flocs. She has chosen an absolutely fantastic topic: Geology, Life and Civilization. What Cat means by geology, life and civilization is what impact does geology have on us and what impact do we have on geology? This is a topic that I think a lot of geobloggers are really going to enjoy so please head over to her blog and read all of the posts in the carnival.

I have always felt that the impact of geology in our lives is under appreciated. Indeed, the more I learn about geology the more I realize its uses and applications are endless. As long as geoscientists continue to think freely and creatively there is almost no limit to the applications that the resources of the Earth can be used to help society...hopefully without causing damage to the Earth itself.

One of the uses that I allude to is the storage of nuclear waste in geological formations. Ever since the dawn of nuclear energy we have always had the problem of what to do with the waste? Nuclear waste is obviously dangerous due to the radiation it emits, and it stays that way for a long time. This means that any place we put it has to be able to hold for at least a million years. However, there is always the possibility that as technology and science advance we will either be able to use it again or dispose of it completely without having to wait the for the radioisotopes to decay on their own. Therefore it also needs to be recoverable. Taking all of this into account means finding a location and designing storage is a pretty tall order since it needs to be both impermeable and accessible over million year time scales.

Many ideas have been kicked around over the years as to what to do with rad. waste such as sending it into space, or burying it in ocean trenches, but the best solution that seems to meet all necessary criteria is geologic disposal. Geologic disposal is a technical term for burying waste in a large, engineered repository in a very secure and stable rock. One proposed site for a deep geologic repository (DGR) is in the Bruce Penninusla, Ontario, Canada. Myself and many others have been involved in characterising the rocks, the hydrogeology of the site, the geochemistry of porewater and umpteen other investigations that are still ongoing.

This post is going to introduce you to the geology of the Bruce DGR and why it is a good place to store low and intermediate level radioactive waste (L/IRW).

Source: Wikipedia
Where is the Bruce DGR?

The Bruce Peninsula is located in south-western Ontario and juts out into Georgian Bay. It is known, in Ontario, at least as a beautiful place to visit and is home to both the Bruce Peninsula National Park and Fathom Five National Marine Park as well as several other provincial parks and conservation areas. The Bruce, as it is affectionately known, is also home to the Bruce Power Nuclear Generating Station.

The proposed DGR site is located next to the generating station and you can see what it is expected to look like in the conceptual model below. For reference the DGR is located at 680m below ground (DGSM) within very low permeability limestone and is expected to hold 200,000 cubic metres of waste.

A conceptual model of the Bruce DGR. Source: NWMO's Descriptive Geosphere Site Model
What is the local geology?

The general term that could be used to describe the geology of the Bruce Peninsula is sedimentary, and lots of it! Obviously the geology is more complex than that, although not as much as you might think.

Source: NWMO's Descriptive Geosphere Site Model
The geology for the region, as you can see from the map above, is mostly Devonian and Silurian limestones that get older as you move from west to east. The pink in the top right is the Canadian Shield, which is mostly metasedimentary, metavolcanic or volcanic intrusive rocks. The overall geologic picture is that this is the eastern flank of the Michigan Basin, which was a large sedimentary basin that existed from the Cambrian to the Jurassic and extends from Chicago to Toronto, with Michigan at its centre. The stratigraphy of the basin at the DGR site is pictured below. 
Source: NWMO's Descriptive Geosphere Site Model
As you can see from the figure above the repository horizon will be within the middle Ordovician Cobourg limestone (named after my hometown of Cobourg, Ontario). The Cobourg formation is a clay rich, lime mudstone that has very, very low permeability. The Cobourg is overlain conformably by the Collingwood shale and the upper Ordovician shales that are about 200 metres thick. The shales are then overlain by Silurian and Devonian dolostones and evaporites. 

Is it a good place to store waste? How do we know?

I'll lead with the punchline on this one. The Bruce DGR site is a good place to store low and intermediate level radioactive waste. But, how do we know? The evidence for this comes from many, many research projects that have been conducted across Canada and Europe over the past several years. The overall results outlined in the Descriptive Geosphere Site Model have condensed the work of hundreds of researchers. Overall, this has been a herculean task, but one that has taught us a lot about storing nuclear waste.

Obviously, I can't summarize the thousands of pages of results in one little blog post, but I can give some of the highlights for why we know that the Bruce site will be secure.

- The hydrogeological investigation has found that the hydraulic conductivity of the the Cobourg Formation is 1 x 10^-14 m/s and the overlying 200 metres of shale is 3 x 10^-14 m/s. The underlying Sherman Fall is 9 x 10^-15 m/s. These are some of the lowest hydraulic conductivities ever recorded and make these rocks nearly impenetrable. To put these numbers in perspective it would take water 1 year to move 10 nanometres, which is about the width of a single protein molecule. In a million years water would move about 1 metre (Agosta

-The rock quality itself is extremely high and there are almost no fractures or faults in the rock whatsoever that could encourage water movement.

- The geochemical evidence from porewater is another way we can tell how secure the rocks at the Bruce DGR are. Porewater was extracted from samples of rock core using a process called vacuum distillation. The ususal recovery was between 0 and 4 mL of water, which is more than enough to analyze for isotopes. The core was then leached in water, which was then analyzed for other elements of interest. Some cores were also analyzed for helium and methane as well. The amount of data that these analyses produced is staggering. The conclusions drawn from these different data sets all agree well and show that the Cobourg limestone is a very secure geologic unit that is bounded on both top and bottom by other very impermeable and stable rock units. The overall conclusion is that the porewater trapped in the rocks is hundreds of millions of years old and has not moved since it was emplaced when the rocks formed in the Ordovician period, 450 million years ago.

So to sum up, the Bruce DGR is a necessary development. There is no question that as time passes and nuclear energy, while it has taken a recent setback, will continue to grow in importance. This can only mean the production of more radioactive waste which only serves to highlight the urgent need to find a safe, long-term storage solution. The Bruce DGR is such a solution due the the highly impermeable nature of the rock units and the physical and geochemical evidence that shows this is a stable and unchanging environment. When looking at the big picture it is easy to see that the geology around us dictates our land usages while also providing us with opportunities to exploit its unique properties serving as a reminder that we are inextricably linked to the geology around us.

Also, please feel free to post any questions that you may have and I'll do my best to answer them.



NWMO's DGR Website:

NWMO's Descriptive Geosphere Site Model:

Agosta, S., Scharf, V., & Herod, M. (2011). Conference Report - The Second Canadian Symposium on Aquitard Hydrogeology. Geoscience Canada, 38(4), 151-153.

All of the information in the post can be obtained freely at the NWMO's website on the Bruce DGR. The opinions expressed in this post are my own and do not represent those of the NWMO, OPG, uOttawa or my direct supervisors.

Monday, May 14, 2012


Pinned Image

What can I say...this is hilarious on so many levels and I had to share it. Stay tuned for more serious posts to come.