Plant Talk!

Why it’s a bad idea to burn down a radioactive forest…

Although anyone who has ever read a comic book could probably tell you this already, as scientists it’s nice to have numbers that will back us up. Nice, solid, repeatable numbers. And that’s what three Russian scientists have gotten from the Baikal region in Siberia.

What’s happened in Baikal is something of an offshoot of biomonitoring. All that term means is that we can assess the ecology and environment of a certain place by studying the organisms that live in it. Focussing on plant life, as we do in this blog, one of the common plant biomonitoring schemes occurs with radioactivity and heavy metals. Trees, lichens, mosses, and other plants suck in these things from their environment, and testing the levels found in the plant can tell you how much (and what sort) of pollution is in that environment.

For example, if there is a factory in Leadville pumping out lots of lead-laden smoke, elevated levels of lead will turn up in the local lichen. Lichen and mosses are particularly good indicators of air pollution, as they have no root systems, and therefore all their nutrients are sucked out of the air in one atmospheric snapshot.

This can also happen with radioactivity. What’s more, we can actively use plants that suck up radioactive particles as part of clean-up measures for contaminated sites. Willow trees are a popular option, especially on sites that are contaminated via radioactive ground water. If, for example, contaminated water from a nuclear reactor is accidentally leaked into the surrounding countryside, willow trees can help mop it up by sucking up the contaminated water through their root systems. The trees can later be harvested and treated as waste, or left in the ground, “locking” the radioactivity up within themselves. It’s due to a process called phytoremediation – using plants to fix the soil. If, for some ungodly reason, you’ve spilled chemicals or radioactive material in your back garden, you can always help nature suck it up by planting correctly.

Unless, of course…

Unless you live in a region, like Siberia, where you can get around thirty thousand forest fires a year. There’s a lot of forest in Siberia – don’t be fooled into thinking that it’s all icy waste. And unfortunately, that forest, through no fault of its own (it has never gone poking around radioactive spiders, hoping to get bitten) has gotten more than its fair share of radionuclides.

Chernobyl, you see, is just down the road – relatively speaking. The radioactive fallout area includes some of this Russian forest, and the trees have gone sucking up radionuclides and locking them within themselves like the generally eco-friendly beasts that they are. But when those unlucky trees burn down, those particles go up in smoke. And it’s not just trees either – mosses, lichen and dry litter contribute around 11t/ha in the Siberian forests, and forest fire emission compares with volcanic eruption in terms of dust and aerosol contribution to the atmosphere. Clearly, this is a question of potentially high importance for polluted, forested regions.

And that smoke travels to new and unaffected areas, areas that escaped being in the fallout zone the first time around. The same is true for heavy metal pollution.

In an orgy of assonance, three scientists called Shcherbov, Strahkovenko and Sukhorukov went out to find just how much of these nasty toxins would go up in smoke at any given opportunity.

They started in burnt pine forests, in the middle mountains of the Ust-Ordynsky Okrug, Aginsky Okrug, and Chita regions. Both ground and crown fire regions were tested. In the worst cases, mosses, lichens, and ground litter burned away entirely, while in the lighter fires only the upper parts of the mosses and litter were singed. Samples from these areas were taken, as were samples from unburned areas under the smoke plumes of the fires.

Comparison between the burned and unburned by smoke-affected regions showed that up to 40% of 137Cs and 90Sr can spread from polluted areas to previously unpolluted areas after a forest fire.

Just looking at 137Cs we can see the increase in content (Bq/kg) under the plumed area. Increases are typically found in those plants most open to atmospheric influence – lichens increased in 137Cs by a factor of 3.2, and mosses by 2.6. There was also significant increases in forest litter (2.7) and conifer needles (2.5).

Similarly, some heavy metals are also able to travel via smoke plumes and pollute previously unaffected soils. Cadmium, lead, and mercury increased in soil samples under smoke plume by factors of 2.2, 1.33, and 1.75 respectively. These same heavy metals were also shown to increase their concentrations in vegetation under the plume (specifically, lichens) by factors of 1.4, 1.27, and 2.1 respectively. Those heavy metals that do not travel well – such as Ni, Co, Cr, and V – are metals that have a high heat tolerance and are not easily removed from the upper layers of the soil.

The severity of the fallout, however, was dependent on several factors, such as atmospheric conditions, fire type, soil composition, and others. Windless days saw much of the burned material fall back into the burned areas, while precipitation at the time of the fire increased fallout from the atmosphere by a factor of 9.

This has implications for people living in or near contaminated areas. Being bitten by a radioactive spider may give you super powers, but breathing in recycled atmospheric fallout in the form of radionuclides or heavy metals is unlikely to do any such thing. If ever there was good reason to get out of the way of a smoking forest fire, this is it. We have to remember that just as plants can function as a bioindicator, and even a repository, of unfavourable ecological elements, they are not a permanent safe-box.

Reference:

Shcherbov, B.L. and V.D. Strahkovenko, F.V. Sukhorukov. The ecogeochemical role of forest fires in the Baikal region. Geography and Natural Resources. 29: 150-155, 2008.

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