2.4 Cesium- 137 in the Surface Waters of the 

 Central Equatorial Pacific 



VLADIMIR I. MEDINETS 5 , VLADIMIR G. SOLOVEV , and BORIS V. GLEBOV* 



'State Oceanographic Institute, Odessa Branch, USSR 



* Institute of Global Climate and Ecology, State Committee for Hydrometeorology and Academy of Sciences, Moscow, USSR 



Introduction 



The study of radioactive contamination of ocean waters 

 throughout the world is one of the most important tasks facing 

 researchers concerned with the effects of human activities on 

 the ocean environment. Man-induced radioactivity of the 

 marine environment has three origins: 7. nuclear weapons 

 testing; 2. nuclear power plant operation; and 3. operation of 

 nuclear fuel treatment and reprocessing plants. 



The need for more information about the present status of 

 radioactive contaminants of World Ocean waters prompted 

 our research efforts in the course of the 47th cruise of the R/V 

 Akademik Korolev in the central portion of the central equatorial 

 and western Pacific from September to November 1988. 



The indicator of radioactive pollution selected was cesium- 

 1 37, a radionuclide with a long half-life. It is this radionuclide, 

 together with strontium-90 and plutonium-239, that presents 

 the greatest potential threat to the marine environment. 



Methods 



Seawater samples of large volume (0.8-1.1 m 1 ) were 

 drawn from various depths (0-200 m) using NIVA and Malysh 

 immersion pumps. Selective extraction of the cesium- 1 37 was 

 performed with the aid of Milton-T fibrous sorbent impregnated 

 with copper ferrocyanide (Vakulovsky, 1986). Following 

 extraction, the sorbent was reduced to ash in a flameless muffle 

 furnace at a temperature not exceeding 450°C. The ash residue 

 was then hermetically sealed in a polyethylene capsule. In 

 December 1988. the samples were measured using a gamma- 

 spectrometry setup at one of the laboratories of the State 

 Oceanographic Institute, Odessa Branch. The error of the 

 cesium- 137 determinations did not exceed 10%. The detection 

 threshold was 0.01 Bq. 



Results 



Cesium- 137 levels in surface waters were determined in 

 the central equatorial portion of the Pacific, as well as in the 

 Philippine, South China, East China, and Japan Seas. The 

 results are summarized in Table 1. Figure 1 shows the location 

 of cesium-137 level measurements in the surface (0-3 m) layer 

 of Pacific Basin waters along the expedition route. 



Analysis of the data showed that the spatial distribution 

 of cesium-137 levels was very uneven. The absolute 

 minimum and maximum concentrations were recorded in the 

 equatorial Pacific. The minimum level recorded in the course 

 of the entire expedition was situated at 8°N, 1 56°35'W between 

 Hawaii and Christmas Island. This was at a time when the 

 Akademik Korolev was traversing the northern boundary 

 of the intratropical convergence zone. Precipitation was 

 abundant, and the resulting dilution was what, in our view, 

 lowered the cesium- 1 37 concentration in the top layer down to 

 0.7 Bq/m\ 



A maximum concentration of 7.0 Bq/m 3 was recorded in 

 the vicinity of Kusaie Island in the Carolines group. 



During this period, the vessel was at its shortest distance 

 from the Marshall Islands that include the Bikini and Eniwetok 

 Atolls, sites where nuclear weapons were tested in the 1950's. 

 The elevated levels observed were attributable either to the 

 transport of the cesium-137 enriched water masses from the 

 Marshall Islands or to the presence of the local sources on the 

 Kusaie Island, lying eight miles north of the cruise track. A 

 more detailed survey of the region might allow identification of 

 sources. 



Elevated (3.4-3.9 Bq/m 1 ) cesium-137 levels in the waters 

 off Tarawa and Caroline Atolls and Christmas Island may have 

 been due to the transport of oceanic waters from the Tuamotu 

 Archipelago, where nuclear weapon testing is regularly 

 conducted by France. The considerable difference between the 

 cesium-137 levels in samples taken two miles off Caroline 

 Atoll and those measured in the waters of its lagoon was 

 especially noteworthy. 



The cesium-137 level in the lagoon water was double the 

 value obtained for oceanic water taken off the atoll. This may 

 be attributed to cesium-137 enrichment of the inner lagoon 

 waters by the flushing of radionuclides deposited on the atoll 

 surface by atmospheric fallout. Analysis of the average levels 

 of cesium-137 for each region (Table 2) showed that the 

 'cleanest' waters of all the ocean areas studied were those of the 

 equatorial Pacific. In terms of radioactive pollution, this region 

 must be ranked between the Bering Sea (Medinets et al. . 1 992) 

 and the rest of the regions of the Pacific. 



The findings of the present study agree with the results 

 obtained by Japanese researchers who worked in the central 

 Pacific in 1980-82 (Nagaya & Nakamura, 1985). 



191 



