The isotopic composition, size distribution, and redox speciation of plutonium (Pu) in the groundwater in the vicinity of the F-area seepage basins at the U.S. Department of Energy Savannah River Site (SRS) were examined. A low 240Pu/239Pu ratio in the upstream control well signifies a Pu source otherthan global fallout and indicates reactor-produced Pu. Elevated 240Pu/239Pu atom ratios downstream from the seepage basins are due to the decay of transplutonium isotopes, mainly 244Cm to 240Pu, which were generated at the SRS. Evidence suggests that the migration of basin-released Pu isotopes is minor. Rather, it is the transplutonium isotopes that migrate preferentially downstream and in the process decay to yield progeny Pu isotopes. Size fractionation studies with cross-flow ultrafiltration show that <4% of the 239Pu or 240Pu is found in the colloidal fraction, a finding that is consistent with the higher Pu oxidation states observed in the SRS groundwater. The observation of a low abundance of colloid-associated Pu in SRS groundwater cannot be extrapolated to all sites, but is in contrast to the conclusions of prior groundwater Pu studies at the SRS and elsewhere. This work is unique in its application of a novel combination of sampling and processing protocols as well as its use of thermal ionization mass spectrometry for the detection of Pu isotopes. This allows quantification of the Pu source terms and better determination of the ambient Pu size and redox speciation representative of in situ conditions.