Gamma Radionuclide Deposition at SMO During Recent French Nuclear Weapons Testing on South Pacific Atolls

The Environmental Measurements Laboratory (EML) has maintained a global network of deposition sampling sites for nearly 40 years. Through CMDL support, American Samoa (SMO) and Mauna Loa (MLO) have been a part of this network for many years. This network was initiated to investigate the transport and fate of radioactivity produced from atmospheric testing of nuclear weapons. Strontium-90 was the radionuclide of primary interest due to the relatively high quantity released and its physical and chemical properties that made it a concern to human health. The global distribution and inventories of ⁹⁰Sr through 1990 have been determined in this program [Monetti, 1996]. Now that the period of atmospheric weapons testing appears to be past, EML has modified this program to meet new objectives. The most significant program change is that gamma spectrometric techniques are being used to measure the activity of several gamma-emitting radionuclides instead of the radiochemical procedure previously used to make a single ⁹⁰Sr measurement. Gamma radionuclides of particular interest include both anthropogenic (¹³⁷Cs, ⁹⁵Zr, and ¹⁴⁴Ce) and natural isotopes (⁷Be and ²¹⁰Pb). This technique will allow EML to continue to monitor for atmospheric releases of fission products. In addition, the development of a database on ⁷Be and ²¹⁰Pb deposition is valued by colleagues who can use this data to verify global circulation models. EML no longer has the resources to make these measurements in all of the samples collected from the 78 stations in the global network. As a result, samples from certain locations are analyzed and others are being archived for future interests. Whenever there is an indication of activities that can potentially release fission products into the atmosphere, EML will use the network samples to identify if a release has occurred and determine the extent and magnitude if a release is confirmed. Following announcements of France's intentions to perform a series of underground tests at the Mururoa and Fangataufa Atolls, EML conducted a special study to monitor the atmosphere in the South Pacific Ocean for radioactivity that may be released during the testing. This study involved the use of the SMO sampling station as described below.

The first sample from SMO was collected in a 23-L polyethylene bucket that was exposed from September 1 through September 20, 1995. This sample was eluted through an ion-exchange column at EML. All other samples were weekly collections of bulk precipitation from September 20, 1995 until February 14, 1996, using EML's ion-exchange fallout collector [EML, 1996]. Any sample collected within 2 weeks following a French detonation was processed for analysis individually. Samples collected during intermediate periods were composited by annual quarter and then processed. Processing of the samples involved homogenizing the surface paper pulp and ion-exchange resin and sealing the sample in a 90-cm³ aluminum can. The samples were then counted on a gamma spectrometer equipped with an n-type coaxial high-purity germanium detector, and the activities of the radionuclides of interest were determined by computer analysis of the spectral data. These spectra were also visually checked for the presence of any other anthropogenic gamma radionuclides. In all, twenty samples from SMO were analyzed.

Radionuclide deposition data from SMO obtained in this study is presented in Table 1. The results are shown in units of becquerels per square meter (Bq m^-2). A counting error is reported whenever a radionuclide was detected. The lower limit of detection (equivalent to three times the statistical variation of the background activity for each radionuclide) was used to calculate maximal deposition values for ²¹⁰Pb, ¹³⁷Cs, ¹⁴⁴Ce and ⁹⁵Zr when their activity was not detected. In addition, precipitation data in centimeters is provided for most sampling intervals.

TABLE 1. Gamma Radionuclide Deposition at SMO from September 1995 Through February 1996

	Precipitation	7Be	Error	210Pb	Error	210Pb	Error	144Ce	95Zr
Collection Date	(cm)	(Bq m-2)	(%)	(Bq m-2)	(%)	(Bq m-2)	(%)	(Bq m-2)	(Bq m-2)
3rd Quarter 1995	66.0	271.8	6	8.8	25	< 0.6		< 0.7	< 1.8
Sept. 1-20, 1995	18.1	90.8	9	< 1.5		< 1.0		< 1.1	< 2.7
Oct. 4-11, 1995	4.3	88.5	8	< 1.5		< 1.0		< 1.1	< 2.9
Oct. 11-18, 1995	0.8	14.9	16	< 1.3		< 0.8		< 1.0	< 2.2
Oct. 18-25, 1995	8.4	ND	0	< 1.7		< 1.0		< 1.2	< 2.9
Oct. 25-Nov. 3, 1995	1.0	21.4	10	5.9	40	< 0.7		< 0.9	< 2.0
Nov. 3- 8, 1995	8.0	22.4	14	< 2.0		< 1.6		< 2.0	< 4.8
Nov. 8-15, 1995	13.1	27.7	9	< 1.0		0.9	14	< 0.8	< 1.6
Nov. 15-22, 1995	3.1	21.4	11	< 0.7		< 0.4		< 0.5	< 1.2
Nov. 22-29, 1995	13.2	45.1	9	< 1.1		0.7	18	< 0.9	< 1.9
Nov. 29-Dec. 6, 1995	2.2	33.7	13	< 1.3		< 0.8		< 0.9	< 2.0
Dec. 6-15, 1995	1.1	20.1	14	3.4	36	< 0.9		< 1.0	< 2.4
Dec. 15-20, 1995	0.0	ND		2.8	37	< 0.8		< 0.9	< 2.1
Dec. 20-29, 1995	2.7	54.6	8	< 1.6		< 1.2		< 1.5	< 3.4
Dec. 29-Jan. 3, 1996	2.7	30.0	14	8.5	38	< 1.2		< 1.4	< 3.0
Jan. 3-10, 1996	8.1	46.5	7	4.7	45	< 0.6		< 0.8	< 1.8
Jan. 10-24, 1996	18.6	48.7	7	3.6	38	< 1.0		< 1.3	< 2.9
Jan. 24-31, 1996	0.9	10.1	22	< 1.5		< 1.1		< 1.4	< 3.3
Jan. 31-Feb. 7, 1996	NA	24.1	16	< 1.2		< 0.8		< 0.9	< 2.0
Feb. 7-14, 1996	NA	27.2	13	< 1.2		< 0.7		< 0.8	< 2.1

ND - Not detected

NA - Data not available

The results presented in Table 1 indicate that the primary source of gamma activity in deposition from September of 1995 through February of 1996 was from natural radionuclides. In all but two of the samples the anthropogenic component of gamma activity was below the detection limit. The ¹³⁷Cs activity detected in two samples is only slightly higher than the detection limit. Cesium-137 has a long half-life (30 years) and it is ubiquitous because of past nuclear weapons testing. Since no other anthropogenic radionuclide was detected in the samples, it is unlikely that there was a "fresh" release of anthropogenic radioactivity from the underground French test or any other source. Deposition data from other sampling sites used for this study revealed similar observations (unpublished data). Therefore, the presence of ¹³⁷Cs in these two samples was probably due to resuspension of previously deposited material. Of the natural radio-nuclides, ⁷Be was detected in most of the samples, but ²¹⁰Pb was only detected in seven of the twenty samples. Beryllium-7 is a cosmogenic radionuclide produced in the earth's upper atmosphere, while ²¹⁰Pb is a daughter in the primordial 238U decay series and its atmospheric presence follows the release of ²²²Rn from the earth's surface. The deposition of ⁷Be and ²¹⁰Pb ranged from undetectable to 271.8 and <0.7 to 8.8 Bq m^-2 respectively. The ⁷Be deposition appears to be related to both the amount of precipitation and duration of the collection period. The ²¹⁰Pb data is subject to large counting errors (from 25 to 45%) as a result of the low deposition rate and high detector background counts at low energies. The concentration of ²¹⁰Pb in the surface atmosphere at SMO has been shown to be relatively low [Larsen et al., 1995] presumably as a result of the oceanic influence. It is more difficult to identify if a similar correlation exists between the ²¹⁰Pb deposition and precipitation because of the large number of nondetects. Real ²¹⁰Pb deposition values occurred during periods of lesser precipitation which may suggest that there is a higher ²²²Rn flux during these periods. Further measurements and comparisons with atmospheric concentrations of ²¹⁰Pb may reveal if this interpretation is valid. In the future, EML will analyze deposition samples collected at SMO and MLO to address this observation.

Acknowledgment. The special assistance provided by Mark Winey at SMO was invaluable to this study. The continued sampling efforts by the CMDL staff at SMO and MLO are greatly appreciated by EML.

EML, EML Procedures Manual, 28th Edition, HASL-300, Vol. 1, pp 2.3-1 to 2.3-6, U.S. Department of Energy, Environmental Measurements Laboratory, New York, 1996.

Larsen , R.J., C.G. Sanderson and J. Kada, EML Surface Air Sampling Program, 1990-1993 Data, EML-572, 247 pp., U.S. Department of Energy, Environmental Measurements Laboratory, New York, 1995.

Monetti, M.A., Worldwide Deposition of Strontium-90 through 1990, EML-579, 56 pp., Environmental Measurements Laboratory, New York, 1996.
 

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