2h water as it does at greater depths. This greater concentra- tion in the surface layer, where phytoplankton is most abundant» coupled with the facts that plankton has a higher spe- cific activity than water and that a few samples of microplank- ton taken during Operation Troll showed an even higher specific activity than the macroplankton suggests that the microplankton may be the principal source of the radioactive particulate matter. The aotivity in the particulate matter was measured from surface samples taken every half-hour (approximately every 8 miles) while approaching and leaving station 62 off the western end of Bikini. Water for these samples was taken from the out- let of the tank for continuous monitoring. The results (Fig. 6) indicate that some radioactive materials are being eluted from Bikini Atoll to the northwest. However, these materials probab- ly add only an insignificant amount to the total activity in the area between Bikini and Eniwetok. The discrepancy between the activity in the sample taken from the tank and that taken from the water bottle at the station off Bikini is probably largely due to 4 lag in passage of water through the tank. Continuous Monitoring The scintillation probe used is sucoessful for the determination of levels of activity in contaminated sea water during the first few weeks following the detonation of a nuclear device; however, on the basis of our limited experifence, the probe is not of sufficient sensitivity for the measurement of low levels of Y activity in sea water as late as two months fol] owing as * a of -