Plutonium in groundwater at the 100K-Area of the U.S. DOE Hanford Site

Abstract

We examined the concentration, size distribution, redox state and isotopic composition of plutonium (Pu) in groundwater at the 100K-Area at the US Department of Energy's (DOE) Hanford Site. Total concentrations of Pu isotopes were extremely low (10 -4 to 10 -6 pCi/kg, ≈ 10 4 to 10 6 atoms/kg), but measurable for the first time in the 100K-Area wells using mass spectrometric analyses that are much more sensitive than alpha spectroscopy methods used previously. Size fractionation data from two wells suggests that 7-29% of the Pu is associated with colloids, operationally defined here as particles between 1 kDa -0.2 µm in size. These colloids were collected using a 1 kDa cross-flow ultrafiltration system developed specifically for groundwater actinide studies to include careful controls both in the field and during processing to ensure in-situ geochemical conditions are maintained and size separations can be well characterized. Pu in this colloidal fraction was exclusively in the more reduced Pu(III/IV) form, consistent with the higher affinity of Pu for particle surfaces in the lower oxidation states. While the overall concentrations of Pu were low, the Pu isotopic composition suggests at least two local sources of groundwater Pu, namely local Hanford reactor operations at the 100K-Area, and spent nuclear fuel from the N reactor, which was stored in concrete pools at this site.Differences between this site and the Savannah River Site (SRS) are noted, since groundwater Pu at the F-Area seepage basin at SRS has been found using these same 2 methods, to be characterized by much lower colloidal abundances and higher oxidation states. This difference is not directly attributable to groundwater redox potential or geochemical conditions, but rather the physical-chemical difference in Pu sources, which at SRS appear to be dominated downstream from the seepage basins by decay of 244 Cm, resulting in more oxidized forms of 240 Pu. There is no clear evidence for colloid facilitated transport of Pu in groundwater at this site, since downstream wells have both an order of magnitude lower concentrations of Pu, but also a lower fractional colloidal distribution.

Interactions of Plutonium with Pseudomonas sp. Strain EPS-1W and Its Extracellular Polymeric Substances

“…strain EPS-1W, a common environmental bacterium previously isolated from the Pu-contaminated groundwater at the NNSS. Groundwater conditions at the NNSS are oxic, ϳpH 8, and of low ionic strength (45), similar to other contaminated sites, such as the Hanford Site in Washington and the Rocky Flats Environmental Technology Site in Colorado (46,47). While our results are not directly linked to the Pu transport detected at the NNSS, our observations of Pu interaction with a common soil bacterium, Pseudomonas sp., suggest the potential importance of microbes and the specific role of EPS in facilitating redox transformation and sorption of Pu.…”

Review on subsurface colloids and colloid-associated contaminant transport in saturated porous media

“…Also a greater number of field studies did not detect any significant colloid-facilitated transport [3,192,201].…”

Transuranic Contamination in Sediment and Groundwater at the U.S. DOE Hanford Site

“…A study of the chemistry of plutonium in groundwater has been conducted at the 100-K Area (Buesseler et al 2003;Dai et al 2005). The concentration, size distribution, reduction-oxidation (redox) state, and isotopic composition of plutonium in groundwater from the 100-K Area were examined.…”