- 3h basalts. Some of the radioisotopes are leached from the rocks by weathering and carried to sea. Their fate in the sea is described in the following statements by Arnold and Martell, ibid. Thorium washed into the ocean rapidly precipitates as insoluble compounds. tion, Potassium remains in solu- but some of its ions become attached to part- ieles of clay which fall to the bottom. precipitates more slowly than thorium. product, thorium-230, decays into radium, into Uranium Its decay precipitates rapidly, but some of which dissolves back the ocean. The lesser amount of radioactivity in water than in rock and also the lesser exposure of organisms in water to cosmic rays result in a smaller environmental radiation dose to fish than to man. The radiation dose in terms of millirads per year has been calculated to be 207 for man living over granite rock at 10,000 feet elevation, man Over granite rock at sea level, mentary rock at sea level, 142 for 75 for man over sedi- 64 for a large fish living near the surfact of the sea and 30 for a large fish living at a depth of 100 meters (Folsom and Harley 1957).