eget ht re” Too yk SES a 738 Noshkin et al. Table 7. Radionuclides in surface sediments at station B-3 compared to levels in most recent coral sections, C-5 cm sediment core section at B-3 fine coarse* 24lam:55py 80¢5:155 py 207py 155m, Recent coral section 0.6240.03 0.7740.03 0.6240.03 0.3840.02 4.4 +1.2 0.8340.47 0.10+0.01 0.1340.01 1.0 40.4 *Sedimentary components greater than 0.5 mm. ume exchanges with the open ocean every 39 days. During the summer the average exchange is about half. the winter rate. These rates imply about seven changes per year between the lagoon and open ocean. The coral data show that the rate of natural displacement of any radionuclide from the lagoon water may not be directly assessed from physical circulation estimates alone. The radiological data stored in the yearly growth increments yield longerresidence times for this initially contaminated lagoon, or for the region of the lagoon aroundstation B-3, than those predicted by Von Arx (1954). Any chronological assessment of the availability of pollutants to - Marine organisms, in an aquatic environ- ment where the flow characteristics are similar to those in an atoll, should be treated with these findings in mind. Table 6 shows that the rate constants for supply of a specific radionuclide from diagenetic processes are smallest for **°Pu, 240Pu, 741Pu, and ‘Eu. Surprisingly the value of ky for *°*Pu is about twice that of 239Py and equivalent to the values for ®°Co and *°7Bi. *88Pu appears to be released to the lagoon faster than 72°Pu and one must conclude that the processes acting on the reservoirs regulating the amount of **8Pu released to the lagoon are different from those regulating *°°Pu. The average concentration of 7°9.2!0Py in Bikini Lagoon water during the fall of 1972 was 49 fCi liter-+ which converts to a Jagoon water inventory of 1.4 Ci. Assuming the *#Pu: 7%°Pu ratio in the water was equivalent to that in the recent coral sec- tions there were 0.8 Ci of 7°°Pu and 0.6 of *40Py in the lagoon. Substituting these values into Eq. 5 with t= 13.5 years and the respective values of k,; and k» we can com- pute the initial size of the reservoir supplying *°Pu and *#Pu to the lagoon. The source contributing 79°749Pu must have contained, in 1958, at least 97 total Curies of the radionuclides; by 1972, of this amount, 32 Ci have been lost to the lagoon water and advectively transported to the open ocean. A number of sediment samples from Bikini have been analyzed for 74tAm and a few for *9240Py. The total amount of “41Am in the surface 2.5 cm of sediment (see Fig. I) is about 200 Ci and represents on the average only 27% of the total activity in the sediment column (unpublished data). The mean **#1Am : 79°749Py ratio in samples of specific sedimentary components from several lagoon locations was 0.74 + 0.17. Assuming that this ratio is constant over the entire lagoon basin, we estimate there are at least 250 Ci of 75°-749Pu in the surface 2.5 cm of lagoon sediment. This source alone is more than sufficient to account for the size of the 7°°?4°Pu reservoir predicted by the coral data. Since the estimated size of the reservoir is 97 Ci, or 65 Ci by 1972, substantially less than the amountpresently contained in the surface 2.5 em of surface sediment alone, we can at present only ask whether new or different diagenetic processes will act on this larger reservoir in the future (equiva- lent to a variable rather than constant ky ) to increase lagoon concentrations, or whether a quantity of 7°%?40Pu in the atoll will forever remain unavailable to the water and the pelagic organismsof the lagoon. Concentration factors—On the basis of the average activity from the four most recent growth sections and the average water concentrations of °*3%74°Pu and Sr given earlier in this report, the concentration factors for these radionuclides in Bikini coral are, respectively, 2.7 x 103 and 1.1 x 10°. Stable strontium in the coral sections averaged 8.94 + 0.35 mg g?. The specific activity of °Sr in the coral is 0.072 pCi mg";