The residence time of thorium in surface sea water and its implications regarding the rate of reactive pollutants

The activity ratio of ²²⁸Th/ ²²⁸Ra in the open surface ocean averages 0.21. This suggests that thorium is removed from surface water in about 0.7 yr. As plant matter is cycled within the surface sea on a similar time scale, the suggestion is made that highly reactive compounds are inadvertently removed by plants in their quest for the critical nutrients nitrate and phosphate. Combined with the coefficient for horizontal eddy diffusivity obtained from the distribution of ²²⁸Ra in surface sea water this result provides a basis for the prediction of the distribution of "reactive" pollutants released to the surface sea from coastal areas.