Transuranics and other radionuclides in Bikini Lagoon

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					Transuranics         and other radionuclides in Bikini Lagoon:
Concentration         data retrieved from aged coral sections1

V. E. Noshkin, K. M. Wang, R. J. Eagle, and C. Gatrousis
Bio-Medical    Division,   Lawrcncc     Livcrmore     Laboratory,     University     of   California,      Livernlore        94550

          X-radiography     and autoradiography     of thin vertical sections wcrc used to cstimatc the
       growth rate of a specimen of Fuuites sirens from Bikini Lagoon.                           Discrctc   bands of
       radioactivity    wcrc identifiable     with   specific      nuclear     test series.     The coral growth
       rate of 8.0 mm year-’ determined by autoradiography                 is in good agreement with the rate
       of 8.1 + 2.2 mm year? derived from the “seasonal” alternating                     light and dark bands on
       X-radiographs.      With these bands as growth rate indicators,               the coral was sectioned into
       yearly increments and analyzed by low-level,             nondestructive        gamma spcctrometry,       radio-
       chemical techniques,     and mass spcctromctry         to reconstruct       the variations    in the concen-
       tration of transuranics    and other radionuclides        in the marine environment            at Bikini since
       1954. From the concentration        data retained in this indicator species, the exchange rate of
       radionuclides    between the lagoon and the open ocean is computed to be longer than ex-
       change rates based on physical circulation         data. There is no constant ratio of plutonium
       isotopes in the coral growth sections, suggesting that the redistributions                of the several plu-
       tonium isotopes in the environment          may be governed by diffcrcnt                biogeochemical     pro-
       ccsscs. Increased levels of 210Po (“‘Pb)      were found in test-year growth sections, contradict-
       ing previous arguments that no “‘“Pb has resulted from weapons testing.

    Knutson et al. ( 1972), Knutson and Bud-                    rcviewcd (Buddemeier ct al. 1974; Dodge
demeier ( 1973)) and Buddemcier et al.                          and Thomson 1974).
 ( 1974) demonstrated that massive coral                            Our studies began in an attempt to re-
colonies from Enewetak Atoll contain ra-                        construct changes in transuranium concen-
dioactive growth inclusions, detectable by                      trations in the marine environment of the
autoradiography,     that can be correlated                     Pacific test sites through observations of
with the annual density-banding       evident                   the yearly response of the coral to environ-
in X-rayed sample sections. Results from                        mental changes. Although several radio-
these and other samples (Buddemcier         et                  logical surveys have been made at the Pa-
al. 1974) show periodicity in growth, intcr-                    cific test sites, no chronological information
preted as density variations in the CaC03                       is available on the behavior of specific
skeleton, that can be used to estimate                          radionuclides in these environments.        The
growth rates of massive coral colonies.                         radiological records retained in the closed
Other recent applications,      using natural                   aragonite structure of coral could circum-
                                                                vent the need for years of real-time data
 occurring radionuclides (Dodge and Thom-
 son 1974; Moore and Krishnaswami 1972),                        and provide insight into the behavior of
                                                                these radioelements in the lagoon. Knut-
have also used radiometric techniques to
                                                                son and Buddemeier (1973) found gOSr in
 determine coral growth. Radiometric stud-
                                                                coral from Enewctak and concluded, on the
ies (Dodge and Thomson 1974) have sub-
                                                                basis of the observed differential changes
 stantiated the relationship between annual
                                                                in concentration, that the lagoon commu-
 growth-band composition and the changing
                                                                nity acts as a long term source of “OSr, prob-
 environment in which the coral grows. Prc-                     ably by exchange with and solution of car-
 vious investigations of the growth rates of                    bonatc materials deposited during the
fossil and reef coral have been adequately                      period of nuclear testing. It is not clear
                                                                 that a single mechanism can explain all
   1 Work was pcrformcd  under the auspices of
the U.S. Energy Research and Development   Ad-                  environmental radiological information re-
ministration.                                                    taincd by coral colonies; we riced infor-
LIMNOLOGY      AND    OCEANOGRAPIIY                         729                                SEPTEMBER       1975,    V.   20(5)
730                                              Noshkin    et al.

                                                            Bikini Atoll. From these data we explain
                                                            the relative behavior of some specific ra-
                                                            dionuclides in Bikini Atoll.
                                                               This work was part of a cooperative pro-
                                                            gram to investigate the biogeochemical be-
                                                            havior of the transuranium elements at Bi-
                                                            kini Atoll.     Samples were collected in
                                                            November 1972 from the RV Palumho,
                                                            operated by the Puerto Rico Nuclear Cen-
                                                            ter (PRNC).      We acknowledge the help
                                                            provided by the crew of the RV Palumbo
                                                            and specifically thank F. Lowman and W.
                                                            Schell, cooperative participating    investi-
   Fig.  1. Bikini Atoll-location        of sampling sta-
tion  and Bravo Crater.      Isoactivity    lines of “IAm
(mCi km-‘) in the surface (2.5 cm) sediment of              Sectioning methods and growth results
Bikini Lagoon,                                                 The coral used in this study was a live
                                                            specimen of F. vixens dredged in Novem-
                                                            ber 1972 from a depth of 28 m at station
mation on other possible mechanisms con-
                                                            B-3, Bikini Atoll ( Fig. 1). In the periods
trolling    the redistribution       of specific
                                                            of June-July    1946, February-May       1954,
radionuclides, principally the transuranium
                                                            May-July 1956, and May-July 1958, Bikini
                                                            Atoll was the site of a series of nuclear de-
   We prepared autoradiographs           and X-
                                                            vice tests. Many of the events in the last
radiographs from vertical slices cut from
                                                            three series were conducted along the in-
the center of a sample of Favites virens col-
                                                            ner northern and southern perimeter of the
lected from Bikini Atoll. Autoradiography
                                                            lagoon. The largest test on the reef (1954:
revealed three discrete, relatively intense,
                                                            15 megatons) ( Telegadas 1961) produced
radioactive bands within the skeletal struc-
                                                            Bravo Crater, also shown in Fig. 1.
ture identifiable       with the nuclear test
series of 1954, 1956, and 1958. Fourteen
density bands between the outermost ra-              Table 1. Concentration          of   “41Am, 16’Eu, 6oCo,
                                                   and ‘07Bi in the sediment cokcmn        from location B-3;
dioactive band (1958) and the coral surface        water depth, 28.3 m; collection         date, 8 November
 (1972) confirmed the thesis that these are 1972; core diameter, 36.3 cm’.
correlated with annual growth.           Sections
from each annual growth band were ana-              Section     pci/g    dry 2 (error     in % 0-f value)*
lyzed by low-level gamma spectrometry              Thickness
                                                                              155E*        6oC0
and radiochemical         techniques to recon-
struct the chronological history of specific                            Coarse fraction      > 0.5 mm
radionuclides     incorporated     in the coral                   7.1023      9.2222      3.48?4    1.16210
                                                                  5.17?3      6.13+2      4. 0224   2.6426
skeleton. Much, if not all, of the observed          10-15        5.9323      7.01+1      x.89*6    3.6724
artificial radioactivity    in the coral at Ene-     15-20        2.4~5       2.66&4      1.7~16    2.78+4
wetak (Knutson and Buddemeier 1973)                  20-25        0.89?10     1.19+4      o.67+9    o.g6+5
                                                     25-30        < 0.1       0.15224     o.2gs8    o. 39216
was due to “OSr and its daughter 9oY, but            30-35        < 0.1       0.13?30     0.20525   0.3896
we identified      other radionuclides     in all
                                                                         Fine fraction     < 0.5 mm
 growth sections of this Bikini coral (e.g.
                             207Bi wo,                            8.37&2     13.4 fl      5'.06f2   1.38i8
 241Am O”Sr 15aEu                           216P0,      g-:0    11.1 f2      14.8 +2      6.83+3    2.3116
 238~23g~a40~241Pu),We’believe that the record       10-15      20.3 f2      22.4 +l      7.4523  11.8212
                                                     15-20      15.7 +2 17.3 f2           4.78+5  12.4123
recovered in these yearly growth incre-              20-25        8.2623      8.73?2      2.00+6    3.62,5
 ments is one of the most complete histories         25-30        1.73~6      2.09+4      0.8929    1.14,8
                                                     30-35        1.5856      1.89+4      0.60~12   0.68,lO
 of changes in concentration of specific ra-
 dionuclides in the marine environment of
                              Transuranics   in Bikini   Lagoon                            731

   The distributions      of specific radionu-    Levels of radioactivity   are relatively high
elides in the lagoon are extremely hetero-        throughout the column. The coral from
geneous (Noshkin et al. 1974). For exam-          this location should reflect the changes in
ple, the 1972 activity lcvcls and isoactivity     activity during its growth cycle in this con-
lines of 241Am in the surface 2.5 cm of           taminated region of the lagoon.
sediment deposit (Fig. 1) show that the              The coral surface was washed and the
region around station B-3 was heavily con-        specimen was dried at 110°C for 1 week.
taminated    with     artificial  radionuclides   The sample was first cut in half vertically
from the test series. Table 1 gives the lev-      from the surface through the base. (We al-
els of several radionuclides to a depth of        ways cut from presumably low-radioactiv-
35 cm in a sediment core from station H-3.        ity sections toward higher level sections to
                                              Transuranics         in Bikini   Lagoon                              733

  Table   2.   Coral   section      clata.                            growth band; at least 1 mm of material
                                                                      was lost at each cut, so that the coral rcc-
 Growth        Estimated     Amm                 At in years          ord between each two sections was lost.
section         year of     growth/              from Nov 72
  from          growth       year            (collection   date)      Each section was then ground and homoge-
outer edge                                                            nized in a ball mill and a known weight was
   (Inm)                                                              transferred to a vial for radionuclide analy-
  o-8          1971-1972          4                 0.9               sis by gamma spectromctry.         A low-back-
  8-15            19-70           7                2.4
 15-21            1969            6                3.4                ground, Compton-suppressed,        Ge ( Li) dc-
 21-29            1968            8                4.4                tector system was used for some of the
 29-38            1967
 38-46            1966            iii              2.::               gamma-emitting      radionuclides.   The sam-
 46-56            1965           10                7:4                ples were then processed for Pu, “%r, and
 56-65            1964            9                8.4
 65-75            1963           10                9.4                210Po by chemical separation followed by
 75-83            1962            0               10.4                radioassay with low-background         beta de-
 83-96            1961           13               11.4
 96-108           1960           12               12.4                tcctors and alpha spectrometry.        Selected
108-116           1959            8               13.4                samples were also analyzed by mass spec-
116-123           1958                            14.4
123-129           1957             ii             15.4                trometry to determine 240Pu, 230Pu, and
129-137           19%              8              16.4                241P~~.   The analytical techniques were es-
137-147        1954-1955                          17.9                sentially those described by Wong ( 1971)
147-156           1953             z              18.4
                                                                      and Noshkin and Gatrousis ( 1974). Stable
                                                                      strontium was determined by atomic ab-
                                                                      sorption. Our discussion here will concen-
the Bikini coral are either associated with                           trate primarily    on the transuranium        ele-
the low-density bands of the X-ray negative                           ments* however,        Tables 3-5 give the
or close to the transition zone between light                         rcsulti for all the radionuclides analyzed,
and dark bands. By correlation             of the                     expressed as pCi g-l ( dry wt ) and cor-
bands shown by both the autoradiograph                                rcctcd for decay back to the estimated year
and X-ray exposures, the years 1956                                   of growth.
through 1958 were identified.         The 1954                            Two sources contribute to the 2ZnAm lev-
growth, however, could not be resolved                                els found in the coral. Part is from the ra-
from the 1955 growth on the X-ray nega-                                dioactive decay of the parent radionuclide
tive. From the 1958 band to the coral sur-                             241P~~,while the remainder is unsupported
face ( 1972 growth), 14 alternating light                             241Am. Based on the quantities of 241Am
and dark areas were identified         ( Fig. 3).                      and 2zL1P~~,the time between separation,
Table 2 gives the thickness of each annual                             and the age of the coral section, levels of
section along with the estimated year of                               both supported and unsupported            ““‘Am
growth.                                                                are computed and given in Table 5.
    The varying dimensions of the yearly in-
                                                                       Radionuclide     results
crements show that growth rates vary from
year to year and also that the dimensions                                 Genera&The        coral bands identified
follow no predictable trend with time. The                             with the 1954, 1956, and 1958 test series,
mean annual growth from the X-radiograph                               as would be expected from the autoradio-
is 8.1 + 2.2 mm yr-r, a value in agreement                             graph, contain the highcs t concentrations
with the autoradiography      results. No ac-                          of radionuclides.   A spectrum from 12 g of
tivity from the 1946 test was detected be-                             the 1954 growth section is shown in Fig, 4
cause it predated the earliest growth of                               with each prominent gamma ray identified.
this particular coral.                                                 Several gamma-emitting radionuclides with
    A wedge, containing the arca with the                              half-lives less than l-2 years have been
more nearly linear growth record, was re-                              identified   in earlier surveys (Welander
moved from the center of the slice (see                                1969); these were not detected in any coral
wedge outline, Fig. 3). A bandsaw was                                  sections with our spectrometer system.
used to remove each defined                annual                      Other radionuclides requiring radiochemi-
734                                                     Noshkin           et al.

  Table    3.       Transuranium      concentrations     and plutonium            activity     ratios.

 Estimated year OP                       pCi/g dry weight - decay corrected                     to year   of growth         + $ error
  growth section                   23g+240                        241&
                                           PUJA     238pu*                                           240Pu: 23gpu*             241PU : 239p,*

       1971-1972                     0.1357            0.005U8               0. og+51                 0.799+0.9                   21.Ok5.5
                                                                                                     (0.216)5                     (0.0128)~
           1970                      O.lOC5            0.005~11                                       0.754                       28.7
           1969                      O.lOG             0.004+22              0.09?37                  0.740                       31.3
           1968                      0.10+2            0.003+19              0.18f41.                 0.769                       28.6
           196-i’                    0.1055            0.004+11              0.14?39                  0.775                       32.8
           1966                      0.09+5            o. 006+14             0.14-c30                 0.770                       39.6
           1965                      0.11+4            0.003+21              0.14f52                  0.753+0.9                   30.1f6.1
           1964                      0.4824            0.023+6               0.62+13                 'Z'                          36.2
           1963                      0.1423            0.007s4               o.18+29                   0: 779                     42.7
           1962                      0.20+2            0.011+18              0.27+47                   0.754kl.3                   -
           1961                     0.6053             0 * 022+9             0.50+41                   0.821                      46.5
           1960                     1.25+3             0.058+8               0.50253                   0.822                      46.9
           1959                     0.82+3             0.049"5               0.68&33                   0.687kO.8                  37.5k3.8
                                                                                                     (0.186)                      (0.0229)
           1958                      4.50?3            0.266+2               3.29&l-(                  o.8Wo.5                    36.6k2.2
           1957                     1.2023             0.14023               1.35+28                                              35.2
           19%                     13.19+3             0. goo+g              9.00+7                                               36.120.8
                                                                                                     (0.227)                      (0.0222)
      1954-1955                    38.9753             2.18 +2              17.55f13                  o.806+0.1
*Determined by alpha spectrometry.
*Determined by gamma spectrometry.
*Determined by mass spectrometry.
@Atom ration  shown within parentheses.   240Pu:239Pu activity                               ratio       divided   by 3.69;
 21r1Pu:239Pu activity  ratio divided by 1638.

  Table     4.      Other   gamma-emitting       radionuclides        detected,     decay corrected            to year of coral growth.

 Estimated year of                                                pCi/g    dry weight         + % error
  growth section                                        207,i                                    137cs              lo2%h          12YSb
                                       155Eu                                 6oC0

                                      0.11+25          0.06?28                *
          1969                        0.14+28          0.09214             0.15+24
          1968                        0.18+27          0.09+14             0.13&75
          196-f'                      0.18+24          0.10~15             0.11?28
          1966                        0.18f26          0.09+15             0.22+14
          1965                        0.23222          0.09+17             0.15+80
          1964                        1.22f9           0.23+6              0.37+12
          1963                        0.45+80          0.14+37             0.54243
          1962                        0.90267          0.42f28             o. 68+39
          1961                        1.8Oi53          0.50229             1.08+34
          1960                        4.05541          o. 86+29            1.33+22
          1959                        5.63+47          0.90~47             2.50+38
          1958                        31.5+17          5.13+10             9. go?30              0.40+40
          1957                        5.85260          1.49233             7.20+32               0.4lC49
          19%                        117.025           13.424              30.6+1-o              1.04518
       1954-1955                     288.0?6           37.6+1              88.2+2                2.16~~1             1.92220        126+47

*A blank     space indicates         the datawerenot       collated       or the concentration               was below detection
                                        Transuranics   in Bikini              Lagoon                                                                                           735

                                                             i             ,oJ23[]



                                                                                               ",,         //"   *,-../-.,-                     /   ---.----

                                             .: .i
    8     --PC._        -._-*_     2.          .*I
s                                       .‘.!fPil
                                                 . I
                                                                                     l-                                                                                             -l

                                                             ,”                                        Period             of       nuclear            test
                                                                 ;5                                    series             at       Bikini            Atoll

                                                                  @-0.01                  ~‘ll’l~‘lll~~~~l~ll~~ll~~~l
                                                                                                           cg $               N     s        ul        g       CJ   CU   x     u>
                                                                                           cnscnol                            ST    m        k?        m       2    &    Cal   k
                                                                                           7         7     ?       7          r-    -        7         P       F-   P    P     7

                                                                                          Estimated               year         of       coral           growth      analyzed

                                                               Fig. 5. Concentration   of =‘Pu and the plu-
                                                           tonium isotope activity ratios in each annual sec-
                                                           tion analyzed.

                                                           cal separation from the coral (such as 55Fe)
                                                           and known to be present in atoll samples
                                                            (Welander     1969) were not determined.
                                                           The concentration of 239Pu and the plu-
                                                           tonium isotope activity ratios in each an-
                                                           nual band, decay-corrected       to year of
                                                           coral growth, are shown in Fig. 5.
                                                               In surface water samples collected from
                                                           the north equatorial current or east of Bi-
                                                           kini, the 23g+240Puconcentration averaged
                                                           0.4 ECi liter-l (Noshkin et al. 1974); the
                                                           average in 10 filtered      lagoon samples
                                                            (ranging from 79-4 fCi liter-l)     was 40
                                                            ( Noshkin et al. 1974). The 90Sr concentra-
                                                           tion in the lagoon averaged 570 fCi liter-l
                                                           compared to 71 in the surface waters out-

                                                              Fig. 4. Gamma-ray spectrum of the 1954-1955
                   23                                      growth section. Unidentified  photopeaks are from
        13NNVH3    Ud    SlNfl03                           background   and naturally occurring radionuclides.
736                                                  Noshkin   et al.

  Table 5. “Sr and alOPo (“‘“Pb) concentrations,                                             Pacific         (North   equa-
decay corrected to year of growth.  Unsupported                                               Ocean          torial   current)
and supported z4’Am concentrations.                                                            "1

                                                                                               I kl
Estimated                pCi/e dry weight    + % error
 year of         24&*
 growth       Glsup.        sup.    g0Sr+         210pb*
1971-1972    0.09+51        0.00    lost           lost                             Bikini          Lagoon
   1970          -                 0.45+12        0.1gi5
   1969      0.09+37        0.00   0.78~12        0.15+6
   1968      0.17+41        0.00   0.71+4
   1967      0.13539        0.01   o.6gi6         0.19+5
   1966      0.12230        0.02   0.73+6         0.2025                                       n2
   1065      0.12+52        0.02   o.8924         0.22+12
   1964      0.52513        0.10   1.15+5         0.1g+5
   1963      0. o-7229      0.11   1.03-t5        0.215)
   1962                            1.21+4         0.25225
   1961      0.30+41        0.20   1.75+3         0.1g+10
   1960      0.05?53        0.45   2.38+3         0.2429
   1959      0.40233        0.28   8.68+3         0.29222
             1.85%7                               0.4824                                       )Ocean
   1958                     1.44   16.0+2
   1957      0.93128        0.42   33.39          0.25+-i'
   1956      4.4 27         4.6    32.2-tl        1.04+11        Fig. 6. Box model diagram of Bikini Lagoon
1954-1955    1.4 %3         16.1   39.321         1.35:'9      describing the general flow and sources of radio-
                                                               nuclides into and out of the lagoon.
*Unsup:     unsup8gxted 24lAm.      sup:     see 24l~m
+~~~~~~~~~Cib~y be:: ~~~%&          g"Y - daughter of          the concentration ( pCi g-l) in the annual
 9OSr on low-level     proportional    counters.
*210Pb activity    determined by counting 210~0                growth band is proportional to the concen-
 using alpha spectrometry.                                     tration in the adjacent environment.      On
                                                               this basis, we find that all detected radio-
                                                               nuclides ( except 210Po) have decreased in
side and east of the lagoon. Radioelements                     the lagoon by two to three orders of magni-
oOco a07Bi, 241Am, and laoEu were not de-                      tude since the test years.
tected by gamma spectrometry in any wa-                            The rate at which the radionuclide con-
ter samples collected outside the atoll but                    centration changes with time is not con-
were prominent in biota and sediment sam-                      stant. The rate-of change was most rapid
ples (Fig. 1, Table 1) from the lagoon. We                     after the test series. In some years (e.g.
conclude that the atoll is the principal                       1964 compared to 1963 and 1965), the con-
source of radionuclides to the lagoon en-                      centra tions of some radionuclides changed
vironmen t .                                                   abruptly: for these small but real changes
   Our investigation differs from others at-                   we have no explanation. We tried to corre-
tempting to use corals to determine the                        late the 1964 increase with the peak in
trace element composition of adjacent wa-                      fallout deposition in the Northern Hemi-
ters. We are not comparing concentrations                      sphere, but the computed amount dcpos-
from different species or concentrations in                    ited in the lagoon in this period was insig-
whole specimens from different areas. We                       nificant compared to the observed change.
expect coral samples from different lagoon                     Although these data arc derived from only
locations to have significantly different ab-                  one coral collection, they suggest that un-
solute radionuclide      concentrations,   an d                anticipated processes in the atoll may lead
preliminary data have verified this assump-                    to abrupt changes in the concentrations of
tion. IIaving only a single sample from one                    specific radionuclides in the lagoon envi-
location, however, we need only assume for                     ronment. However since 1965 the concen-
each element or radionuclide         that each                 trations 0E many specific radionuclides have
yearly increment of coral growth concen-                       decreased only slowly ( after correction for
trates the same available fraction from                        decay); this indicates that recycling from
seawater ner unit weight of coral: then,                       sedimentary processes, biological activity,
                              Transuranics         in Bikini       Lagoon                                                      737

and surface runoff, or some combination of                Table 6. k, and kO values                  computed           from    Eq.
these or o thcr biogeochemical     processes,          5 for specific ratlionuclides.
are responsible for replenishing      activity
levels of some radionuclides in the lagoon                                                                   ko =      k2      n30
                                                       Radionuclide                   k2                            kl-k2
at rates that compensate for the rates of re-
                                                          239Pu                  0.07                               0.12
   No model can adequately account for
                                                          2hOpu                  0.07                               0.091
these unanticipated    changes in conccntra-
                                                          241pu                  0.06                               3.90
tion unless the specific mechanisms respon-
                                                          23%l                   0.13                               0.014
sible for the changes are understood. IIow-
ever, even though a degree of uncertainty                  155Eu                 0.06                               1.25
exists, we can use a simplified model of the              207Bi                  0.13                               0.39

lagoon environment based on the diagram                   6Oco                   0.12                               1.52
in Fig. 6 and the coral data to describe the
rate at which the radionuclides       are ex-
changed between the lagoon and open
ocean and the rate at which specific radio-                         n2 = n20e-xte-1clt -k - k2                      x
                                                                                                     k, - 7C2
nuclides are recycled from the atoll.
   The statement of the mass balance in                                  n30e
                                                                                -At    ( e-kzt   -   e-kit    ) ,
terms of the change in the amount of a ra-                We propose that the radionuclide     con-
dionuclidc, n2, in the lagoon water with               centrations in each annual growth section
time is                                                are proportional to the amount of species
                                                       n2 in the surrounding water environment
      $=k      m - Ana + k2n3 + kInI,        (1)       during the respective year of growth. The
                                                       last nuclear test series at Bikini was held
where kl is a universal rate constant in yr-’          in 1958. Taking 1958 as to, there was an
and is independent of the particular radio-            amount nno of species n3 in the atoll reser-
active species considered. 7cl is the mean             voirs. We assume that the rate at which
residence time of the lagoon water. x is               the lagoon is flushed with uncontaminated
the radiological decay constant in yrl and             ocean water is rapid enough so that after
k2 is the rate constant in yr-l defining the           5 years e-“lt can be taken to be zero. Equa-
supply of a particular      dissolved species          tion 4 then reads
from all diagenetic processes. If nl, the
quantity of species n2 supplied to the la-                                                    t,
                                                               n2 (after 1962-1963) = kOe-(k2FX) (5)
goon from the ocean reservoir, is small                           =
                                                       where 7co (7&,)/(      kl - kz).
compared to the amounts contributed by                    Using the data retained in the coral sec-
the atoll, as it is for all radionuclides de-          tions from 1962 to 1972 we can compute a
tected in the coral except “OSr and 237Cs,             best-fit unique value of k. and k2 from Eq.
Eq. 1 reduces to                                       5. These values for each radionuclide de-
         dnZ                                           tected are listed in Table 6. Substituting
         dt=    -n2( kl + A) + k2n3.         (2)       the values of ko and kg into Eq. 4, and now
                                                       using the 1958 and post-test year coral
The change in n3, the quantity of species              data, we get an average value for kl of
n2 supplied to the lagoon from diagenetic              1.98 + 0.14 yr-I.
processes, with time is                                   The lagoon volume along with any dis-
                                                       solved species is exchanged 1.98 times a
            -- _ -n&+h).                     (3)       year with the open ocean. The residence
                                                       time of the lagoon is 127 to 198 days. From
   Solving Eq. 3 and substituting the solu-            calculated flows into and out of Bikini La-
tion in 2, the solution for n2 as a function           goon, Von Arx (1954) estimated that dur-
of time is                                             ing the tradcwind season one lagoon vol-
738                                           Noshkin   et al.

   Table 7. Radionuclides   in surface sediments at     tions there were 0.8 Ci of 23”Pu and 0.6 of
station B-3 compared to levels in most recent coral     240Pu in the lagoon. Substituting these val-
                                                        ues into Eq. 5 with t = 13.5 years and the
                  O-5 cm sediment core      Recent      respective values of kl and k2 we can com-
                      section at B-3        coral       pute the initial size of the reservoir supply-
                    fine       coarse*     section      ing 2”“Pu and 2‘ioPu to the lagoon. The
                                                        source contributing       230*240Pu must have
                 0.62~0.03    0.77kO.03    0.83ko.47    contained, in 1958, at least 97 total Curies
 6oCo:155Eu      0.6220.03    0.38~0.02    4.4 +1.2     of the radionuclides;       by 1972, of this
 207Bi:155Eu     0.10+0.01    0.13+0.01    1.0 20.4     amount, 32 Ci have been lost to the lagoon
                                                        water and advectively transported to the
 *Sedimentary   components greater   than 0.5 nun.      open ocean.
                                                            A number of sediment samples from Bi-
                                                        kini have been analyzed for 2Z11Amand a
ume exchanges with the open ocean every                 few for 230~240Pu, The total amount of
39 days. During the summer the average                  241Am in the surface 2.5 cm of sediment
exchange is about half the winter rate.                  (see Fig. 1) is about 200 Ci and represents
These rates imply about seven changes per               on the average only 27% of the total activity
year between the lagoon and open ocean.                 in the sediment column (unpublished data).
The coral data show that the rate of nat-               The mean 241Am : 23g1240Pu      ratio in samples
ural displacement of any radionuclide from              of specific sedimentary components from
the lagoon water may not bc directly as-                several lagoon locations was 0.74 + 0.17.
sessed from physical circulation estimates              Assuming that this ratio is constant over
a1 one. The radiological data stored in the             the entire lagoon basin, we estimate there
yearly growth increments yield longer resi-                                                in
                                                        are at least 250 Ci of 23g~240Pu the surface
dencc times for this initially contaminated             2.5 cm of lagoon sediment. This source
lagoon, or for the region of the lagoon                 alone is more than sufficient to account
around station B-3, than those predicted by             for the size of the 23gv   2”oPu reservoir pre-
Von Arx ( 1954). Any chronological assess-              dicted by the coral data.
ment of the availability      of pollutants to               Since the estimated size of the reservoir
marine organisms, in an aquatic environ-                is 97 Ci, or 65 Ci by 1972, substantially less
ment where the flow characteristics          are        than the amount presently contained in the
similar to those in an atoll, should be                 surface 2.5 cm of surface sediment alone,
treated with these findings in mind.                    we can at present only ask whether new or
    Table 6 shows that the rate constants for           different diagenetic processes will act on
supply of a specific radionuclide from dia-             this larger reservoir in the future (equiva-
genetic processes are smallest for 239Pu,               lent to a variable rather than constant k2)
240Pu, 241Pu, and la5Eu. Surprisingly        the        to increase lagoon concentrations,             or
value of k2 for 23sPu is about twice that of                                                   in
                                                        whether a quantity of 23gy240Pu the atoll
23”Pu and equivalent to the values for G°Co             will forever remain unavailable to the wa-
and 207Bi. 23sPu appears to be released to               ter and the pelagic organisms of the lagoon.
the lagoon faster than 230Pu and one must                    Concentration factors-on        the basis of
conclude that the processes acting on the                the average activity from the four most re-
reservoirs regulating the amount of 23sPu                cent growth sections and the average water
released to the lagoon are different from                concentrations of 23g,240Puand “OSr given
those regulating 23gPu.
                                                         earlier in this report, the concentration fac-
    The average concentration      of 23gv 240Pu
                                                         tors for these radionuclides in Bikini coral
in Bikini Lagoon water during the fall of
 1972 was 49 fCi liter-l which converts to a             are, respectively, 2.7 X lo3 and 1.1 X 103.
 lagoon water inventory of 1.4 Ci. Assuming              Stable strontium in the coral sections aver-
the 240Pu : 230Pu ratio in the water was                 aged 8.94 10.35 mg g-l. The specific ac-
equivalent to that in the recent coral sec-              tivity of g”Sr in the coral is 0.072 pCi mg-l;
                                Transuranics    in Bikini   Lagoon                                739

its specific activity in the Bikini Lagoon wa- only slowly change the lagoon concentra-
ter, assuming an average of 8 mg liter-l of tion with time.
strontium in seawater (Goldberg               et al.     Specific radionuclides in the coral MC-
1971)) is 0.071 pCi mg-l, The “OSr there-            tio*Precise      measurements of 230Pu, 240PU,
fore is incorporated         by the living coral     and 241Pu were made in nearly all sections
polyps in direct proportion to its conccn-           by mass spectrometry. Small, but neverthe-
tration in the water and there is no discrim-        less significant, changes are noted in the
ination between 9OSr and stable strontium.           240Pu : 239Pu activity ratios (Table 3, Fig.
The concentration factor for g”Sr is in good 5) in the coral sections. The average 240Pu :
agreement with the average specific activ-           23gPu activity ratio was 0.77 2 0.07, the
ity of 37 * 10 pCi g-l strontium in recent           range from 0.57 to 0.88. In each post-test
 coral surface sections from Enewetak re- series year, 1957 and 1959, the ratio was
ported by Knutson and Buddemeier (1973). reduced to an average of 72% of its test
 If the stable strontium in coral is 8.9 mg year value. In 1960 it increased by 20%
 g-l, the g”Sr concentration is 0.33 pCi g-l         over the 1959 ratio and has since changed
 coral. Based on our average 1972 concen-            by no more than -t-10% of the 1960 value.
tration of “OSr in water ( 0.33 pCi liter-‘)             Assuming that the coral does not dis-
 at Enewetak (Noshkin et al. 1974), the criminatc                  between the same chemical
 concentration factor in Enewetak coral is forms of 241Pu and 23gPu, we found more
 1.0 x 103.                                           variation in the activity ratio, as a function
      Our computed 28g,240Pu concentration            of time, than was anticipated. If both iso-
 factor agrees well with the results of Imai          topes are released to the environment at
  and Sakanoue (1973) who reported a con- the same rates, the ratio change in the coral
  centration factor of 1 to 2 X lo3 for fallout       sections should follow a 14.3-year decay
  239,240Pu   coral collected from Yoran Island       curve (Fig. 5). An inspection of Fig. 5 and
   (27”04’N, 128’25’E).        The similar values     Table 3 will show that the 241Pu : 23gPu de-
  in corals from different environments with          cay-corrected     ratios in the test year
  different levels of contamination         indicate  growths, 1954, 1956, and 1958, are lower
  that coral species take up gOSrand 23g*240Pu than any extrapolated post-test year ratio
  in proportion to the concentration in the would predict. These differences in test
  water; therefore they serve as excellent in-        and post-test year ratio can only be ex-
  dicators for environmental       levels of these plained if the plutonium isotopes had dif-
  radionuclides.                                      fercnt ratios, in unique chemical or physical
      Values of 370 and 9,400 fCi liter-l for forms, after production.             A smaller amount
   “OSr and 137Cs were reported from a single          of soluble 241Pu relative to 23gPu was de-
  filtered midlagoon bottom water sample               posited in the lagoon water during the
  collected at Bikini in August 1964 (We-              years of the test series while relatively more
  lander et al. 1967). In our November 1972
                                                       241Pu ended up in the atoll reserviors that
  filtered midlagoon bottom water sample
    (Noshkin et ,al. 1974) we detected 315 and now supply both 241P~~and 23gPu to the la-
  340 fCi liter-l gOSr and 137Cs. The simi-            goon.
   larity in the “OSr values after decay correc-          Even more significant are the variations
  tion shows little change in the lagoon con- with time in the 23*Pu : 23gPu values. In 17
   centration at a specific location over the Bikini water samples collected during 1973
   8-year period. Our coral record over the            (Noshkin et al. 1974), the average 238Pu :
   same period confirms this observation.              230,2’oPu ratio was 0.018 -t- 0.006 (range
   Again we must conclude that the mecha-              0.011-0.026).    In the three most recent
   nisms now releasing O”Sr and, as the coral          coral growth sections the average ratio is
   record indicates, plutonium        nuclides and 0.040 * 0.005. The plutonium              concentra-
   lanthanides as well, are supplying these ra- tion factor based on 239Pu is higher than
    dionuclidcs to the lagoon at a rate that will      the computed value using 23g+240Pu       concen-
                                            Noshkin    et al.

                                                            stood. The failure of the data to conform
                                                            to predictable patterns is a most important
                                                           feature of this study and complicates the
                                                           interpretation of the behavior of plutonium
                                                            radionuclides in this environment.      It does
                                                            not appear possible, with these data, to pre-
                                                            dict unequivocally    the behavior of all plu-
                                                           tonium isotopes in the environment from an
                                                           assessment of a single isotope of the ele-
                                                           ment; a great deal of research is still re-
                                                           quircd to unravel the biogeochemistry         of
                                                           this element.
                                                               The 23gPu activity in the coral sections
                                                           seems to correlate better with ls5Eu (Fig.
                                                           7) than with 238Pu. The 100Eu : 23nPu ratio
                                                           in the coral sections decreases in value with
                                                           a best-fit half-l& of 5.1 years which is the
                                                           radiological decay half-life of lSBEu. Simi-
                                                           lar geochemical processes appear to govern
                                                           the fate and behavior of this lanthanide
                                                           and of 23gPu in the lagoon, and the way in
                                                           which the change in ratio correlates with
                                                           time supports the age assessment of each
                                                              We pointed out earlier that the 2”1Am in
                                                           the coral growth sections originates from
                                                           the decay of 241Pu in the coral and also di-
                  Year of coral  growth section
                                                           rectly from the environment.       If the envi-
                                                           ronment‘al source of 243Am is from 24LPu
     Fig. 7. ls5Eu : B”P~~ and unsupported         24’Am : decay only, and the coral does not discrimi-
“‘Pu ratios in the yearly coral growth sections.
                                                           nate between these two transuranics, the
                                                           excess z41Am to 241Pu ratio in the coral
trations. At this time we can offer no ex-
                                                           should follow a predictable growth curve
planation of this difference
                                                           with time. Plotted in Fig. 7 are the ratios
     Assessing the possible influence of 238Pu
                                                           of unsupported 241Am to 241Puin each coral
from fallout and from SNAP-9A on lagoon
                                                           growth section and a calculated growth
concentrations, we found this input to have
                                                           curve of 23IAm from 241Pu (to = 1954). Al-
little or no effect on the concentration levels            though the errors are large we find that the
during any one year. Consider, for exam- ratios in the post-test years, with the ex-
plc, that the cumulative deposition of 238Pu ception of the 1960 value, are changing
up to 1971 has been only 0.009 mCi km-2 with time in a manner consistent with the
between 10” and 20” N latitude (Hardy                      predicted curve. However, all the values
et al. 1973). Comparison of this value with                are displaced above the 1954 curve and
the 1972 average standing level in Bikini
                                                           would be further removed from a growth
Lagoon of 0.033 mCi km-” (mean depth of
                                                           curve originating     in 1956 or 1958. The
the lagoon is 46 m) shows that the present
lagoon level alone exceeds the maximum                     21LAm in the 1954-1955 growth section falls
possible concentration          derived from fall-         on the predicted curve, showing there was
out. The mechanisms regulating the dis- essentially no 241Am directly produced dur-
crimination oE these plutonium isotopes are ing the 1954 test series. If we assume this
mcshed with           biogeochemical           processes to be true for the 1956 and 1958 test series,
within the lagoon and arc as yet not under-                the coral must tither have been preferen-
                               Transuranics   in Bikini     Lagoon                                       741

tially enriching 2Zi  ‘Am over 211Pu, or the      sections identified with the test years, how-
concentration     factor of 241Am must bc         ever, there are small but definite increases
greater than plutonium for this marine or-        in 21 OPo ( 2‘OPb) concentrations, which cor-
ganism.                                           relate in time with the increases noted for
    Concentrations of several radionuclides       the artificial radionuclides.   Several investi-
in the sediment from station B-3 are shown        gators (citecl in Beasley 1969) have dis-
in Table 1. The ratios of Z43Am, OOCo,and         counted the possibility       that 210Pb was
207Bi to looEu in the surface 5 cm of the         produced from weapons testing, but the
fine and coarse (>0.5 mm) fractions are           elevated levels recorded in the coral during
compared to the ratios in the most recent         nuclear test series are at least circumstan-
coral section in Table 7. Both the 241Am :        tial cvidcnce that elevated levels of 21oP~
 ‘saEu and the B°Co : 207.Biratios in the coral    ( 210Pb) were present in the Bikini environ-
are similar to those in the scdimentaly           ment during those periods. It appears,
phases. However,        the “OCo : looEu and      from the later years’ growth, that “l”Po
207Bi  : lli6Eu values are greater in the rc-      ( 210Pb) levels in coral from remote cnvi-
cent coral than in the sedimentary cnviron-       ronments may be used as another means to
ment. If the sediments are a principal            date modern coral growth, confirming the
source now supplying radionuclides to the         work by Dodge and Thomson (1974) and
lagoon, by dissolution or exchange or other       Moore and Krishnaswami ( 1972).
processes, then we find that relative to the
sediment the coral does not discriminate          Refewnces
between lsoEu and 2’L1Am, but G°Co and            BEASLEY, T. M.     1969. Lead-210          production by
207Bi are greatly enriched relative to I “sEu.        nuclear devices:   1940-1958.         Nature (Lond.)
This implies that processes governing the                 224: 573.
                                                  BWH~EMEIER, R. W., J. E. MARAGOS, AND D. W#
fate of lcsEu in the lagoon are similar to             KNUTSON. 1974. Radiographic                studies    of
those for 241Am. From the above compari-               reef coral exoskclctons 1. Rates and patterns
sons, and the concentration factor argu-               of coral growth.        J, Exp. Mar. Biol. Ecol.
ments, we may discount the possibility that            141: 179-200.
                                                  DOUGE, R. E., AND J. THOMSON. 1974. The nat-
the source of the radionuclides in this par-           ural radiochemical        and growth        records in
ticular coral that we analyzed is in trapped,          contemporary       hematypic    corals from the At-
previously resuspended, sedimentary mate-              lantic   and Caribbean.         Earth     Planet.   Sci.
rial. The coral is functioning rather as an            Lctt. 23: 313-322.
indicator of the aquatic environment.             GOLDBERG, E. D., W. S. BROECKEH, M. G. GROSS,
                                                       AND K. K. TIJREKIAN.          1971. Marine chem-
    In an attempt to see whether a naturally           istry, p. 137-146.       In. A. II. Seymour [cd.],
occurring radionuclide     could be used to            Radioactivity       in the marine        cnvironmcnt.
confirm the age of the coral sections, we              Natl. Acad. Sci.
separated from the coral and measured the         IIARDY, E:. P., P. w. KICEY, AND )-I. L. VOLCIIOK.
                                                        1973. Global inventory         and distribution      of
2LoPo, the daughter of 2roPb ( tli2 = 22 yr),          fallout    plutonium.      Nature     ( Lond. ) 241:
Provided that the environmental levels of                 444-445.
“l”Pb are constant, the amounts taken up          LMAI, T., AND M. SAKANOUE. 1973. Contents of
by the coral reflect the age of any section            plutonium,   thorium and protactinium        in sea
relative to the youngest section, because the          water and rcccnt coral in the North Pacific.
                                                       J. Oceanogr. Sot. Jap. 29(2):       76-82.
concentration within the coral changes due        KNUTSON, D. W., AND R. W. BUDDEMEIER. 1973.
to radioactive decay. When the activity                Distribution   of radionuclidcs   in reef coral:
levels are corrected to the date of coral              Opportunity    for data retrieval   and study of
growth (as shown in Table 5)) the concen-              effects, p. 735-746.     In Radioactive    contam-
                                                       ination of the marinc environment.        IAEA,
tration-time relationship should be invari-       --       , -,      AND S. V. SMITII.      1972. Coral
ant. The data from 1966 to 1971 fit this               chronomctcrs : Seasonal growth          bands in
model very well. The decay corrected                   reef coral.   Science 177: 270-272.
                                                  Moonq       W. S., AND S. KRISHNASWAMI.             1972.
2LoPo ( 210Pb) concentrations during these             Coral growth      rates using PnsRa and 21”Pb.
years averaged 0.20 * 0.03 pCi g-l. In the             Earth Planet. Sci. Lctt. 15: 187-192.
742                                               Noshkin    et al.

NOSIIKJN, V. E., AND C. GAT~OUSIS. 1974. Fall-               WELANDER, A. D. 1969. Distribution            of radio-
     out 240Pu and =‘Pu in Atlantic marine organ-                nuclides in the environment      of Eniwetok    and
     isms. Earth Planet. Sci. Lett. 22: 111-117.                 Bikini   Atolls, Aug. 1964, p. 346-354.           In
-,        K. M. WONG, R. EAGLE, AND C. GATROU-                   Proceedings of the second symposium on ra-
     SIS. 1974. Transuranics        at Pacific atolls 1.         dioccology.     CONF-670503.       NTIS,    Spring-
     Concentrations    in the waters at Enewetak                 field, Va.
     and Bikini.     Univ.   Calif.    Livermore     Rep.    -,         AND OTIXERS. 1967. Bikini         Eniwetok
     UCRL-51612.      29 p.                                      studies, 1964. Part 2. USAEC Rep. UWFL-
TELEGADAS, K. 1961. Announced               nuclear   det-       93. NTIS, Springfield,      Va.
                                                             WONG, K. M.        1971. Radiochemical      determina-
    onations.     USAEC Health Safety Lab. Fall-
                                                                 tion of plutonium    in seawater, sediments and
     out Program Quart. Summ. Rep. HASL-111,                     marine organisms.       Anal. Chim. Acta 56:
    p. 169-185.                                                  355-364.
VON Anx, W. S. 1954. Circulation              systems of
     Bikini   and Rongelap      lagoons.     US.    Geol.                         Submitted:       31 July 1974
     Surv. Prof. Pap. 260-B, p. 265-273.                                          Accepted:       14 April 1975

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