Currently, most of the plutonium found in the Earth's environment results from human activities--in particular, the testing of nuclear weapons in the atmosphere. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has estimated that early above-ground nuclear weapons tests released about three tons of Pu-239/240 into the atmosphere; nearly 80 percent of this atmospheric plutonium is in the northern hemisphere.
The practice of testing nuclear weapons above ground has been essentially stopped but there is still some plutonium in the environment, including the upper atmosphere. In addition, there are also small (but measurable) quantities of plutonium near a few nuclear facilities in the U.S. and in Russia.
Thus, contrary to popular belief, plutonium does occur naturally in the environment and is not solely a manmade material. According to Glenn Seaborg, perhaps we should rethink the number of naturally occurring elements and recognize that rather than 92, there are really 94 such elements.
Gregory A. Lyzenga, a physicist at Harvey Mudd College in Claremont, Calif., adds this additional information.
Plutonium does occur naturally, but at very low concentrations. Indeed, it is all but unobservable, except by very sensitive modern analytical techniques. The reason that plutonium (and other transuranic elements) are so rare in nature is that being radioactive, they decay with a characteristic half-life.
All of the heavy elements making up the Earth were formed in nuclear reactions during supernova explosions, occurring millions or even billions of years before the Earth formed. Any element formed at that time with a half-life much less than the Earth's age--or 4.5 billion year--has nearly all decayed into lighter elements by now.
In addition to the few atoms of plutonium that may have survived since the Earth's formation, a very small, steady inventory of such unstable elements is also maintained in the environment by naturally occurring nuclear reactions (for example, those involving cosmic rays). Even so, the levels are very small