Hardy, E.P., Krey, P.W. and Volchok, H.L. (February 16, 1973). Global inventory and distribution of fallout plutonium. Nature. 241. pg. 444-445.

The following letter is one of the most important ever published in the British journal Nature, providing baseline data about the dispersal of weapons testing-derived fallout plutonium as well as plutonium isotopes derived from the 1964 satellite accident.  Hardy, et. al. used the reporting unit of mCi/km².  This can be converted directly to the more understandable (for the layperson) reporting unit of pCi/m².  Few areas in the northern hemisphere contain less than 1000 pCi/ m2 of fallout ²³⁹Pu, 1/2 T 24,240 years.  Even though this fallout is stratospheric rather than tropospheric, the higher values in soils are correlated to some extent with locations having the greatest annual precipitation, as well as mid-latitude locations.  One to four thousand pCi/m² of fallout ²³⁹Pu is the minimum baseline level of plutonium contamination in the northern hemisphere.  More recent research identifies numerous areas with much higher levels of plutonium in soils, see especially the data collected pertaining to the Rocky Flats facility in Colorado.

Below is a scan of page 444 followed by a more readable enlargement of the table.  See RAD 8:5 Anthropogenic radioactivity: Baseline data: Plutonium and Americium for more comments on this article and other information on plutonium fallout.  For more information on this satellite accident, consult RAD 11:9 Anthropogenic radioactivity: Major plume source points: Nuclear Powered Satellite Accidents.