PLUTONIUM-237: AN IMPORTANT RESEARCH TOOL FOR STUDIES OF ENVIRONMENTAL PLUTONIUM KINETICS John R. Trabalka and L. Dean Eyman Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, Tennessee ABSTRACT The oroduction and use of the ohoton-emitting isotone olutonium-237 in investigations of the untake, retention, and distribution of monomeric olutonium (IV) both in an aquatic vertebrate, the channel catfish Ietalurus punetatus, and in a littoral aquatic micro-ecosvstem, are presented. The rationale for use of plutonium-237 in environmental studies is discussed. Chelation can either enhance or reduce the uptake of ingested plutonium relative to plutonium hydroxide (monomer) in channel catfish. The highest observed retention (whole bodv) at 63 days was 3.8% of ingested dose for olutonium-237 citrate, while retention of the fulvate was 0.6%. Reduced uptake of the fulvate complex is due either to its high molecular weight (> 10,000) or to its stability in metabolic svstems. Increased uotake of plutonium-237 citrate is attributable to instability of the comolex in metabolic svstems. Tissue distribution studies revealed that relatively little (< 10%) of intracardially injected plutonium citrate was excreted. Blood clearance rates were similar to those found in smal] mammals, with the plutonium being primarily associated with the plasma protein transferrin. The fractional bodv burdens in bone, liver, and kidney 17 days after injection were 31%, 24%, and 9% of the injection dose, respectively. High kidney burdens relativy to mammals are expnected, since the kidney functions as the major site of homopotesis in teleosts. Absence of significant excretion indicates that a short half-life component of elimination following qut clearance in gavage studies is due to plutonium labeling of the gut. A distribution coefficient of 9 x 104 was observed for sediment in a year-old aquatic microcosm spiked with plutonium-237 nitrate. A materials batance at 90 days post-spike provided the following estimates: 0.001% in water, 0.04% in biota, and over 99.9% in sediments. Concentrations 48S