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Yucca Mountain – hydrogeology
Although I’m a radiation safety professional I studied geology as an undergrad and for my MS (my doctorate is in Environmental Science). This included several classes in hydrogeology – these were some interesting classes, and I added to the experience by doing some work as a field hydrogeologist for a project at a Department of Energy facility in the early 1990s. Although I’m not a professional hydrogeologist, between my classes and field experience I’ve at least got some familiarity with the basic concepts of field geology, with groundwater modeling, and with some of the practicalities of work in the field. But before applying this to Yucca Mountain, I’d like to go over some of the fundamentals of hydrogeology (you can find much more in a wonderful publication by the United States Geological Survey).
The biggest of these is sort of obvious – water flows downhill. Or, to be precise, water flows down-gradient; from an area of high pressure to an area of lower pressure. This means that groundwater can flow uphill, just as water can flow through pipes from the ground floor to higher floors in a building, if the water pressure is high enough.
Although water normally doesn’t exist in the form of underground rivers or streams it does flow through sediments. But water in a river or stream will flow hundreds or thousands of kilometers in a year; water flowing through an aquifer might flow only a few meters in a year. And unlike surface water where you can pump seemingly unlimited quantities of water (up to the capacity of the river or stream of course), most aquifers are more limited in capacity.
There are a number of factors that affect the speed at which water flows through an aquifer and the rate at which it can be pumped from the ground. One factor is the porosity of the aquifer – the percentage of the rock or sediments that is pore space. An aquifer with higher porosity (all else being equal) will hold more water. But there’s more to an aquifer than how much water it will hold – if the pores are interconnected the water can flow easily from one pore to the next, if they are isolated then water remains locked in the pore space. So permeability is another important factor – clay can actually have a high degree of porosity but, since the pores are blocked off from each other, it is highly impermeable; so much so that clay is frequently used to seal landfills to isolate whatever they hold. Groundwater flows slowly through low-permeability rock and sediments – flow velocities can be as low as centimeters per year. One other thing – permeability depends on more than just the interconnection between pores – cracks and fractures can pipe water through the rock at high velocities, adding to the permeability.
Something else to keep in mind is that water can be corrosive and it’s a solvent, so if we’re trying to isolate radioactive waste – to keep it safe for millennia – then it behooves us to try to keep water away from it. This is why designers do their best to keep water out of all hazardous materials landfills and disposal sites – not just radioactive waste facilities. And since high-level waste can remain fairly highly radioactive for centuries or longer (and since it can also contain things like plutonium and americium), we need to do our best to try to keep water away from the waste containers for the same length of time.
One last thing to consider is that virtually every place on Earth has a water table. Water falls on the surface as rain or snow, the water flows downhill to collect in lakes, rivers, and streams, and water can leak out of the bottoms and sides of the rivers, lakes, and streams into the soil. This water will percolate downwards until it can’t descend any further – usually because it fetches up against an impermeable layer of rock or soil (clay, for example). The water will collect there and will begin flowing downgradient. Sometimes the water table is only a few feet underground, in other places it’s thousands of feet down.
So let’s put this together to try to figure out what sort of hydrological conditions will be good for a high-level waste repository. And remember that the whole idea is to keep water away from the radioactive waste as long as possible and, when contact with water is inevitable, to minimize its duration. So what conditions are we looking for?
The first condition should be to have the waste as far from water as possible. Satisfying this means in part putting the waste as far from the water table as possible. In the case of Yucca Mountain the proposed waste location is both several hundred feet above the local water table and several hundred feet below the surface upon which rain and snow will fall. The depth to the water table means that there are no worries about water rising to flood the site from below – and that any leaking waste will take a long time to descend to the water table; the depth below ground means that precipitation has quite a distance to flow before reaching the waste.
So it’s nice that the waste will be stored several hundred feet belowground, but a more fundamental question is whether or not surface water can reach it – after all, if the rock is heavily fractured and permeable then water can be piped directly down from the surface, possibly reaching the waste in hours or days. On the other hand, if the rock is impermeable then the water will take so long to reach the waste that it’ll be absorbed by the intervening rock before it ever penetrates to the waste repository. Porous, fractured rock speeds the flow of water from the surface to the waste; low-permeability rock impedes it.
Something else can slow the flow of water as well – geometry and chemical reactions. The parched desert rock that overlies the Yucca Mountain repository soaks up water in two ways – first, water percolating through the rock will coat and fill the pore spaces it’s flowing through and until the pores are filled with water it won’t flow further. So even permeable rock – if it’s bone dry – will absorb quite a bit of water. As the water coats the pores it also reacts chemically with the minerals that comprise the rock. Feldspar, for example, is a dry mineral that hydrates to form clay minerals. So groundwater in contact with feldspar will be absorbed by the feldspar, forming clay. There are other hydration reactions as well – most dry minerals will weather to form hydrated versions. Water flowing slowly through rock will undergo these hydration reactions until all of the available minerals are converted – only then will water start flowing through the rock, if the new minerals haven’t expanded to block the passages, which sometimes happens.
So think of the rock overlying the waste repository as a sponge – if you trickle water slowly onto a sponge, the sponge will slowly expand as it absorbs the water. The sponge swells and swells until it’s saturated – only then will water start to drip from the bottom. And – in the absence of large fractures – surface precipitation will not reach the waste repository until the rock overlying the repository site is saturated and converted to hydrated minerals. Cracks and fractures are a different story – but even here, if the flow path is sufficiently circuitous or if the fractures are sufficiently small, the water will react with dry minerals before it can reach the waste.
So we know that the Yucca Mountain repository site is situated several hundred feet from both the water table and the surface precipitation, but what about the rock? Well, it’s reasonably porous and it has some fractures – not the ideal rock to form an impermeable barrier. Geologists have found fractures and faults in the overlying rock, as well as evidence that hydrous minerals have formed in the past. This means that not only will the rock hold a lot of water, but there are some flowpaths to get that water to the waste location. On the other hand, the rock pretty dry, so the water has to first fill up the pore space and then hydrate the minerals – something that can take hundreds of thousands of years in a climate as dry as Nevada’s. So from this standpoint, the hydrogeology of Yucca Mountain might not be perfect, but it’s pretty good.
The kicker is that, while we know what today’s climate is like, we don’t know what it will be like in a thousand years. If it stays dry then the waste will be in good shape. But if the climate changes and puts Nevada in the monsoon belt then all bets are off. This is not disqualifying, by the way – but it would call for relying on engineered barriers as well as natural ones to help keep the waste dry. The engineering is something we’ll look at in a future posting.
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