Does radioactive waste present a danger to our drinking water? Whether posed as a question or presented as a headline: this is an understandable concern as water is our most precious resource and key to survival. That is why protecting it is so important. The deep geological repository for Switzerland’s radioactive waste is not being constructed despite the groundwater – quite the opposite: emplacing the waste at depth will protect the water in the long term.
Admittedly: at first glance, the idea that a repository can protect our lifeblood seems contradictory because construction at such depths takes us closer to the groundwater.
Water at different levels
The example of a multi-storey house is a helpful comparison to explain this. The subsurface is made up of different layers of rock that lie on top of each other like storeys. Some of these storeys contain groundwater.
We ourselves are up on the roof of the house, looking down to the ground floor 1,000 metres below, or around 300 million years back in time. The uppermost and therefore youngest storey consists of glacial deposits from the last 2.5 million years. Like a sponge, the permeable mixture of gravel and sand holds the groundwater that we use as drinking water. This storey must therefore be particularly well protected during the construction of the deep geological repository.
In general, the following holds true: the more permeable a storey or rock is, the faster the water moves within it. Usable groundwater is sometimes only a few years old because fresh rain, river or lake water seeps in.
The situation is completely different for water located in lower storeys: some of this deep groundwater has been there for thousands of years, sometimes even for several hundreds of thousands of years. Its connection pathways to the earth’s surface are so long that the water is replaced very slowly. The considerable age of the water also demonstrates how well the individual storeys are separated from each other.
Where time practically stands still
The Opalinus Clay, an inconspicuous grey rock, is so tight that it is next to impossible for water to pass through. This is the main reason why the deep geological repository for radioactive waste is to be constructed in it: as there is almost no movement in the Opalinus Clay, time practically comes to a standstill there. In this “time capsule”, the radioactive elements can decay until they are harmless to groundwater and thus also to humans and the environment.
The clay rock is a good 100 metres thick and lies at a depth of around 800 metres in the Nördlich Lägern siting region –- i.e. near the bottom of the “house” and therefore far away from usable groundwater. Nagra wants to construct the deep geological repository in the northern part of Canton Zürich known as the Zürcher Unterland. The very tight rock formed around 175 million years ago on the seabed, when minute particles were deposited there layer by layer. To this day, water from earlier geological periods is confined within the microscopic voids of this clay. This porewater has barely changed in hundreds of thousands or even millions of years.
Incidentally, unlike the usable groundwater in the uppermost storey, some deep groundwater would not be potable because the longer water remains underground, the more minerals dissolve from the rock into the water. As a result, such water is often very salty or can contain naturally occurring pollutants – potentially including radioactive elements, making this water often unsuitable for drinking or irrigation.
Tiny particles on the move
In all three potential siting regions that Nagra has investigated in depth in recent years, the Opalinus Clay can enclose the radioactive waste so effectively that the long-term protection of the groundwater can be ensured. Moreover, the Nördlich Lägern region, where Nagra wants to construct the deep geological repository, has an additional safety reserve in this respect.
Investigations have shown that the Opalinus Clay porewater in this region is older than in the other two regions. What is the reason for this age difference? While the storeys are well separated from each other, practically bringing the water exchange between the rock layers to a standstill, particles can still migrate through the rock over very long periods of time.
This physical process – known as diffusion – causes differences in the concentration of substances, such as water, to equalise over time. This means that radioactive particles from the waste can also start to move, albeit at an extremely slow rate and in minute quantities.
Extra safety in Nördlich Lägern
In over- and underlying rock formations of the Opalinus Clay, there are storeys containing deep groundwater. In Nördlich Lägern, the distance between the Opalinus Clay and the nearest layer in which such water flows is substantial. Moreover, the water within this water-bearing layer moves very slowly. For this reason, the Opalinus Clay porewater in this region is the least diluted and therefore the oldest, demonstrating that the radioactive waste can be particularly well enclosed in Nördlich Lägern.
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