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Radioactive Fountains of the Deep

March 29, 2012

There's more than just liquids, such as lava and groundwater, that flow from the depths of the Earth onto the surface and the atmosphere. There is, of course, natural gas, which is on our mind because of its energy value; and also the helium that's contained in natural gas and is harvested for its obvious value.

Natural gas contains more than 5% helium, much more than the 0.00052% by volume that's found in the atmosphere, and it's extracted using the fractional distillation process known and loved by all organic chemistry undergraduates. This helium is created by the radioactive decay of thorium and uranium; for example,
238U -> 234Th + 4He
232Th -> 228Ra + 4He

Since their symbols are similar, the radium, Ra, in the decay of thorium shouldn't be confused with radon, Rn. The most common isotope of radon, 222Rn is produced in the alpha decay of 226Ra
226Ra -> 222Rn + 4He

Since radium-226 is a common trace element in many rocks, quite a bit of radon is produced and released into the environment. Since 222Rn has a half-life of just 3.8 days, that equates to a lot of natural radioactivity. It's estimated that 2,400 million curies (9 x 1013 becquerels) of radon are annually released into the environment.[1]

Humans have existed on the Earth for quite a while, so our exposure to radon can't be all that bad, or we would be extinct. Radon was essentially ignored until 1985, when radiation alarms at the entrance of a nuclear power plant were triggered by an employee. The interesting thing about this event was that the plant had not yet been fueled. An investigation revealed a radon level of 100,000 Bq/m3 in the basement of his house. A typical indoor radon reading is about a thousand times less than this.

The modern problem of radon exposure comes from our living indoors, and not in the great outdoors. The enclosed space of our homes, now nicely sealed to save energy, traps the radon that enters from cracks and gaps in the foundation where our homes interface to Earth's surface. The US Environmental Protection Agency has a web site devoted to radon mitigation in homes.[2]

Radon map of the United States (EPA)
Radon map of the United States (EPA). Zone 1 > 4 pCi/L (picocuries per liter); Zone 2 between 2 and 4 pCi/L; Zone 3 <2 pCi/L. (via epa.gov web site).

If all this scares you into pitching a tent in a forest to escape radon, a paper published last year by scientists from the International Laboratory for Air Quality and Health, the Queensland University of Technology (Brisbane, Australia), presents evidence that trees harvest radon from groundwater.[3-4] This conclusion came from a study investigating why the concentration of ions in forest areas is consistently higher than in other areas. Natural ions in the air are created by cosmic rays, and by the radiative decay of radon.[4]

Radon is water-soluble, so it's taken up by trees and plants through the groundwater they absorb, and it's released into the atmosphere by transpiration. The study estimates that a forest composed of eucalyptus trees, spaced four meters apart, would account for about 37% of atmospheric radon during the middle of the day when the rate of transpiration is highest.[3] However, when the value is annualized, the contribution drops to just four percent.[3] Said Queensland scientist, Dr Rohan Jayaratne,
"Trees act as radon pumps, bringing the gas to the surface and releasing it to the atmosphere through transpiration - a process where water absorbed by the root system is evaporated into the atmosphere from leaves. This is especially prevalent for trees with deep root systems, such as eucalypts."[4]

The ions produced by the radon could be a problem, also. In experiments at six locations around the Brisbane area, the Queensland research team found that the positive and negative ion concentrations in the air in heavily wooded areas were about twice that of open, grassy areas.[Q] About half of the particles inhaled remain in the respiratory system for a time, and charged particles are more likely to deposit in the lungs. This effect is harmful if there are pollutants in the air.[4]

Infrared photo of a maple treeA tree of a different color.

An infrared image of a maple tree.

In such images, hotter regions are white, and cooler regions are black.

(Via Wikimedia Commons).

How can I write an article about trees without a mention of the mighty larch,[5] or lumberjacks?[6]

References:

  1. J.H. Harley, Environmental Radon, in Noble Gases, Richard E. Stanley and A. Alan Moghissi, Environmental Research Center (Las Vegas, 1975), pp. 109-114 (via Google Books).
  2. Radon - Indoor Air, US Environmental Protection Agency.
  3. E.R. Jayaratne, X. Ling and L. Morawska, "Role of Vegetation in Enhancing Radon Concentration and Ion Production in the Atmosphere," Environ. Sci. Technol., vol. 45, no. 15 (August 1, 2011), pp. 6350–6355.
  4. Electricity from trees, Queensland University of Technology Press Release, March 21, 2012.
  5. Monty Python, "The Larch Sketch," YouTube video.
  6. Monty Python, "The Lumberjack Song," YouTube video; German version; and, a very thorough review on Wikipedia.