84 The difference in the specific activity (pCi/g) of the fine-grained debris collected at stations B-21 and B-20 can be explained by one or combinations of two processes. One is a dilution in the concentration of radioactive particles deposted at Station 8-71 by lower specific activity material arising from biological activity or erosion of the reef. Secondly, the difference in the specific activity of the sediments at stations B-21 and B-20 could arise via a physical or chemical fractionation of the radioactivity in, or from, debris which is transported in suspension. A physical fractionation could arise by differences in the specific activity of different sized particles. The specific activity of relatively larger sized particles deposited at Station B-2], for instance, may be lower than that of small particles deposited further downstream. Chemical fractionation of radionuclides may be a function of the length of time radioactive particles remain suspended. Particles which were deposited at Station B-21 may thus have lost a higher proportion of their surface-associated radioactivity than those deposited at stations B-2 or B-20. The relatively high specific activity of the sediments collected at stations B-22 and B-24 would be consistent with deposition of the high specific activity material fractionated--whether by physical, chemical or biological processes-from sediments suspended upstream. Figure 20 is a photomicrograph of surface sediments from Station B-20, adhering to scotch tape, at 20x magnification. are about 200 p in diameter. The largest particles observed However, many of these particles are agglomerates characteristic of dried sediments. Figure 2] is a photomicrograph of Station B-20 surface sediments dispersed in water (pH = 10) and magnified at 80x. Nearly all of the particles observed in these samples are smaller than 50y in diameter and a large number are smaller than 10y in diameter. The very sma?)