1. Isotopic Composition, Speciation and Mobility of actinides in the Groundwater at DOE Savannah River and Hanford Sites K.O. Buesseler (1), M.H. Dai (1), J.M. Kelley (2), S. Pike (1), J. Andrews (1), T.C. Maiti (2), and J.F. Wacker (2) (1) Woods Hole Oceanographic Institution, MS 25, Woods Hole, MA 02543; (2) Pacific Northwest National Lab, PO Box 999, Richland, WA 99352 A. Introduction Prior studies have revealed that speciation and/or complexation chemistry have dramatic effect on the mobility of many elements in aquifers because sorption onto any solid phase will depend upon the speciation in solution (Choppin, 1988; Choppin, 1992; Moulin and Ouzounian, 1992; Marley et al., 1993; Cooper et al., 1995). Less well understood, is how interactions between source characteristics (i.e. Pu from fallout, underground test debris, liquid vs. vitrified waste) and ambient geochemistry and hydrology interact to change speciation leading to enhanced or retarded actinide migration in groundwater. Total radionuclide activity data are not particularly revealing since speciation, not total activity levels, are needed to predict mobility. Extreme care must be taken not to alter the ambient geochemistry and colloid abundances during sampling. Finally, isotopic information is needed to identify Pu sources which can be linked to the physical-chemical form of Pu and hence its mobility. a) 240 Pu/ 239 Pu atom ratio in unfiltered well samples collected in August 1997. b) size fractionated 240 Pu/ 239 Pu data from samples collected in May 1998. Note two of the well sites are the same, and that for well 108D (b), 240 Pu/ 239 Pu ratio is only available for an unfiltered sample at this time. We attribute the highest 240 Pu/ 239 Pu ratios to samples that have been impacted by the decay of 244 Cm (t½ = 18.1 yrs.) to 240 Pu in the seepage basin wastes. A low 240 Pu/ 239 Pu ratio in background well (108D) signifies a source other than global fallout and is most likely due to weapons grade Pu waste of some local origin. A low colloidal abundance is consistent with the higher oxidation states, and with overall low natural colloidal organic carbon levels at this site. For 240 Pu, we find that >75-95% of the Pu is in the oxidized form in all of the size classes. 240 Pu (3a) and 239 Pu (3b) atom abundances in filtrate (<0.2µm), Permeate (<1kD) and colloidal (1kD-0.2 µm) size fractions. G. Summary Overall, our data suggest that redox speciation can vary considerably at both SRS and HS. Most of the Pu is in the form <1 kD in size at both sites. The highly varied 240 Pu/ 239 Pu ratio can only point to 240 Pu being enhanced downstream due to its production from the more mobile Cm precursor and its own enhanced mobility in the oxidized form at SRS, and we note that the forms of Pu found at the SRS would be significantly more mobile than standard models predict. Pu from Hanford operations is observable from 100K area although the concentration is extremely low. I. Acknowledgments: This work was supported by the Department of Energy under Grant #DOE DE-FG07-96ER14733. We are grateful for the kind assistance from James Haffener and Keith Johnson from the Savannah River Site and Shannon Goodwin, Evan Dresel, Scott Conley, Teresa Wilson, Debi Morgan, Mike Thomson, Stuart Luttrel, Loni Peurrung, Dennis Brooks from the Hanford site. a.b. Percent oxidized 240 Pu in filtrate (<0.2µm), Permeate (<1kD) and colloidal (1kD-0.2 µm) size fractions. Well water On-site Cross-flow ultrafiltratio n N 2 purged and sealed Unfiltered Permeate (<1 kD) Retentate (> 1 kD) On-site oxidation state separations Pu Isotopic composition with TIMS Oxidized and reduced forms 0.2  m prefiltration In-line multiprobe (O 2, pH, turbidity etc.) D. Savannah River Site C. Issues important for accurate Pu speciation studies Groundwater sampling maintain and measure in-situ geochemistry; use low flow sampling to reduce colloid formation Cross-flow filtration for separation of colloids maintain redox state & keep trace metal clean; demonstrate low sorptive losses and negligible blanks; calibrate CFF with colloid standards Redox speciation studies performed immediately in field under nitrogen gas; lanthanium fluoride ppt w/ 244 Pu and 242 Pu spikes Radiochemical purification (WHOI) careful attention to blanks & yields prior to TIMS Thermal Ionization Mass Spectrometry (PNNL) subfemtogram detection limits (10 -15 gm or 10 6 atoms); use 240 Pu/ 239 Pu to determine Pu sources E. Hanford Site  The likely source of high 241 Pu/ 239 Pu in wells K-109A and K-27 is N-reactor waste (the K-East reactor basin is currently being used to store irradiated fuel from the N- reactor).  The isotopic ratio in the other wells reflects the K-reactor signal, possibly mixed with fallout  Pu from Hanford operations found in all groundwater samples from 100K area- low levels (fg/l, 10 -4 to 10 -6 pCi/l)  Colloidal Pu is minor fraction of total Pu in groundwater-<5-15% colloidal Pu is primarily in reduced form near source, with a trend towards more oxidized forms further from source B. Groundwater sampling and processing H. Significance to DOE EMSP Migration of Plutonium in the Environment is a major problem at several DOE sites, and fundamental data concerning the interactions between various chemical forms of plutonium with compounds in the environment are essential to predicting environmental behavior. The results of our research program would: i) provide the basis for accurate modeling and prediction of actinide transport; ii) allow for remediation strategies to be planned that might use in-situ manipulations of geochemical variables to enhance (for extraction) or retard (for immobilization) Pu mobility in the vadose/groundwater zone, and iii) identify specific Pu sources and the extent of far field, or long-term migration of actinides in groundwaters. This new knowledge is essential to ensure continued public and worker safety at the DOE sites and the efficient management of cleanup and containment strategies.