A. Introduction

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. 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/244Pu and 242Pu spikes Radiochemical purification (WHOI) careful attention to blanks & yields prior to TIMS Thermal Ionization Mass Spectrometry (PNNL) subfemtogram detection limits (10-15 gm or 106 atoms); use 240Pu/239Pu to determine Pu sources B. Groundwater sampling and processing Well water In-line multiprobe (O2, pH, turbidity etc.) Unfiltered 0.2 mm prefiltration Permeate (<1 kD) Retentate (> 1 kD) Pu Isotopic composition with TIMS On-site Cross-flow ultrafiltration On-site oxidation state separations E. Hanford Site N2 purged and sealed Oxidized and reduced forms D. Savannah River Site Pu is primarily in reduced form near source, with a trend towards more oxidized forms further from source a.b. a) 240Pu/239Pu atom ratio in unfiltered well samples collected in August 1997. b) size fractionated 240Pu/239Pu data from samples collected in May 1998. Note two of the well sites are the same, and that for well 108D (b), 240Pu/239Pu ratio is only available for an unfiltered sample at this time. • The likely source of high 241Pu/239Pu 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 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 240Pu/239Pu ratio can only point to 240Pu 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. 240Pu (3a) and 239Pu (3b) atom abundances in filtrate (<0.2µm), Permeate (<1kD) and colloidal (1kD-0.2 µm) size fractions. Percent oxidized 240Pu in filtrate (<0.2µm), Permeate (<1kD) and colloidal (1kD-0.2 µm) size fractions. We attribute the highest 240Pu/239Pu ratios to samples that have been impacted by the decay of 244Cm (t½ = 18.1 yrs.) to 240Pu in the seepage basin wastes. A low 240Pu/239Pu 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 240Pu, we find that >75-95% of the Pu is in the oxidized form in all of the size classes. 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. 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 .