| joon ean on of Bikini Lagoon id sources of radiogoon. 1) in the annual ial to the concen- nvironment, On il detected radioave decreased in ‘orders of magni- radionuclide contime is not con- > was most rapid some years (e.g. d 1965), the connuclides changed but real changes Ne tried to correrith the peak in Northern Hemid amount deposperiod wasinsigobserved change. lerived from only suggest that unie atoll may lead concentrations of the lagoon envi1965 the concenidionuclides have ter correction for it recycling from ological activity, ee ee ee eee c {North equatorial current) Transuranics in Bikini Lagoon and surface runoff, or some combination of these or other biogeochemical processes, are responsible for replenishing activity levels of some radionuclides in the lagoon at rates that compensate for the rates of removal. No model can adequately account for these unanticipated changes in concentration unless the specific mechanisms responsible for the changes are understood. However, even though a degree of uncertainty exists, we can use a simplified model of the lagoon environment based on the diagram in Fig. 6 and the coral data to describe the rate at which the radionuclides are exchanged between the lagoon and open ocean and the rate at which specific radionuclides are recycled from theatoll. The statement of the mass balance in terms of the change in the amountof a radionuclide, no, in the lagoon water with time is dn = kyno— Ane + kong + Kym, dt (1) where k, is a universal rate constant in yr! and is independent of the particular radioactive species considered. k, is the mean residence time of the lagoon water. A is the radiological decay constant in yrand k, is the rate constant in yr? defining the supply of a particular dissolved species from all diagenetic processes. If m, the quantity of species ns supplied to the lagoon from the ocean reservoir, is small compared to the amounts contributed by the atoll, as it is for all radionuclides de- tected in the coral except ®Sr and 1°"Cs, Eq. 1 reduces to SM = na ky + A) + Kans. (2) The change in ns, the quantity of species m2 supplied to the lagoon from diagenetic processes, with timeis d 7 =—g(A + ke). (3) Solving Eq. 3 and substituting the solu- tion in 2, the solution for m2 as a function of timeis 737 Table 6. ks and ky values computed from Eq. 3 for specific radionuclides. Radionuclide k, 9 - mk 30 239py 0.07 0.12 240py 0.07 0.091 241 py 0.06 3.90 238py 0,13 0,014 135eu 0.06 1.25 20754 0.13 0.39 80cq 0.12 1.52 Ne —_ = Noe -At “'e-™ p-kit + ke k, -k, noe! (e-Fat — eit) x (4) We propose that the radionuclide concentrations in each annual growth section are proportional to the amount of species nm. in the surrounding water environment during the respective year of growth. The last nuclear test series at Bikini was held in 1958. Taking 1958 as t, there was an amount M39 of species nz in the atoll reser- voirs, We assume that the rate at which the lagoon is flushed with uncontaminated ocean water is rapid enough so that after 5 years e** can be taken to be zero. Equation 4 then reads ny (after 1962-1963) = kye-™"#, (5) where ko = (Kontago) /( ki — ke). Using the data retained in the coral sec- tions from 1962 to 1972 we can compute a best-fit unique value of ky and ke from Eq. 5. These values for each radionuclide detected are listed in Table 6. Substituting the values of ky and ky into Eq. 4, and now using the 1958 and post-test year coral data, we get an average value for k, of 1.98 + 0.14 yr. The lagoon volume along with any dissolved species is exchanged 1.98 times a year with the open ocean. The residence time of the lagoon is 127 to 198 days. From calculated flows into and out of Bikini Lagoon, Von Arx (1954) estimated that during the tradewind season one lagoon vol-