Size-fractionated plutonium isotopes in a coastal environment

We have examined the distribution of individual Pu isotopes (Pu-239, Pu-240 , and Pu-241) in seawater from the Gulf of Maine (GOM). Samples were size-f ractionated with a 1 kD crossflow ultrafiltration (CFF) membrane. Subfracti oned samples were radiochemically purified and Pu isotopes were analyzed us ing a three-stage thermal ionization mass spectrometer (TIMS). To our knowl edge, this is the first time that both size class and Pu isotopic data have been obtained for seawater samples. Within measurement uncertainties a sin gle Pu-240/Pu-239 atom ratio of 0.18 was found for all sample collection de pths and sample size fractions. This signifies a current, single Pu source in GOM waters, namely global fallout, and suggests that no measurable isoto pic fractionation occurred during CFF processing. The majority of Pu was fo und in the low molecular weight fraction (< 1 kD). Colloidal Pu varied from 8% of the total in surface waters to <1% in the deepest (250 m) seawater s ample. Evidence suggests that the vertical distribution of Pu in GOM is pri marily controlled by conservative mixing processes. The high Pu fraction fo und in the low molecular size fraction implies that most of the Pu is in th e non-particle-reactive oxidized fraction, and is consistent with the conse rvative Pu behavior. The activity levels are in agreement with other studie s which show a slow decrease in Pu with time due to continued mixing and re latively slow particle removal. (C) 2000 Elsevier Science B.V.. All rights reserved.