236U, 237Np and 239,240Pu as complementary fingerprints of radioactiveeffluents in the western Mediterranean Sea and in the Canada Basin (Arctic Ocean).

The aim of this study was to assess the potential of combining the conservatively behaving anthropogenic radionuclides 236U and 237Np to gain information on the origin of water masses tagged with liquid effluents from Nuclear Reprocessing Plants. This work includes samples collected from three full-depth water columns in two areas: i) the Arctic Ocean, where Atlantic waters carry the signal of Sellafield (United Kingdom) and La Hague (France) nuclear reprocessing facilities; and ii) the western Mediterranean Sea, directly impacted by Marcoule reprocessing plant (France). This work is complemented by the study of the particle-reactive Pu isotopes as an additional fingerprint of the source region. In the Canada Basin, Atlantic waters showed the highest concentrations and 237Np/236U ratios in agreement with the estimated values for North Atlantic waters entering the Arctic Ocean and tagged with the signal of European Nuclear Reprocessing Plants. These results may reflect the impact of the documented releases for the 1990s. In the Mediterranean Sea, an excess of 236U presumably caused by Marcoule is reflected in the lower 237Np/236U ratios compared to the Global Fallout signal in all the studied samples. On the contrary, the 239,240Pu profiles were mainly governed by the Global Fallout. The impact of Marcoule as a local source is further corroborated when comparing the temporal evolution of these ratios between 2001 and 2013. The lowest 237Np/236U ratios observed in 2001 at the surface reflect a previous local input that is no longer observed in 2013 as it had been homogenized through the whole water column. This work presents the use of 237Np as a new ocean tracer. A more accurate characterization of the main sources is still needed to optimize the use of 236U-237Np as a new tool to understand transient oceanographic processes.

Microbial mineral weathering for nutrient acquisition releases arsenic.

Tens of millions of people in Southeast Asia drink groundwater contaminated with naturally occurring arsenic. How arsenic is released from the sediment into the water remains poorly understood. Here, we show in laboratory experiments that phosphate-limited cells of Burkholderia fungorum mobilize ancillary arsenic from apatite. We hypothesize that arsenic mobilization is a by-product of mineral weathering for nutrient acquisition. The released arsenic does not undergo a redox transformation but appears to be solubilized from the apatite mineral lattice during weathering. Analysis of apatite from the source area in the Himalayan basin indicates the presence of elevated levels of arsenic, with an average concentration of 210 mg/kg. The rate of arsenic release is independent of the initial dissolved arsenic concentration and occurs at phosphate levels observed in Bangladesh aquifers. We also demonstrate the presence of the microbial phenotype that releases arsenic from apatite in Bangladesh aquifer sediments and groundwater. These results suggest that microbial mineral weathering for nutrient acquisition could be an important mechanism for arsenic mobilization.

Using sequential extraction techniques to assess the partitioning of plutonium and neptunium-237 from multiple sources in sediments from the Ob River (Siberia).

Sequential extraction techniques have been developed to assess partitioning of anthropogenic radionuclides ((240)Pu, (239)Pu, and (237)Np), originating from a variety of sources, as identified by using bulk sediment isotopic composition. Sediments were leached sequentially with a series of six chemical treatments designed to approximate different environmental processes that may occur or to selectively extract trace metals contained in different solid phases of the sediments (i.e., exchangeable, reducible, carbonate, organic, acid leachable, and refractory). Results indicate the majority of Pu and Np is similarly distributed within many of the extracted fractions, with the largest percentage (66-97%) of both elements being observed in sediments treated with buffered citrate dithionite (CDB), which targets easily reduced constituents such as Mn and Fe hydrous oxides. While these results do indicate an association of Pu and Np with redox sensitive elements, the environmental implications are unclear given that the CDB treatment is more extreme than naturally occurring conditions. Minor amounts of Np partition differently from Pu in sediments. The NH(4)-acetate treatment, which is designed to liberate trace metals that are loosely adsorbed onto the surfaces of sedimentary materials such as hydrated iron oxides and humic substances, or present at exchangeable sites in clay minerals, mobilized approximately 12% of the total Np while Pu levels were below detection. The H(2)O(2) treatment, which is designed to liberate trace metals bound to organic matter, mobilized approximately 8 and approximately 1% of Np and Pu, respectively. These results indicate that a minor portion of the total Np may be affected by environmental conditions that have little or no effect on Pu. Between 7 and 24% of the Pu was observed in treatments designed to liberate Pu and Np that are tightly bound to lithogenic phases or refractory silicates. The (240)Pu/(239)Pu observed in accessible and refractory fractions ranged between 0.11 and 0.18. With the exception of one sample, the (240)Pu/(239)Pu isotope ratios measured in the refractory fractions are essentially identical having a mean ratio value of 0.123+/-0.001 (1sigma). The (240)Pu/(239)Pu ratios provide isotopic evidence that suggests a portion of non-fallout contamination has a refractory nature. The presence of similarly low (240)Pu/(239)Pu ratios in refractory fractions of sediments from the Ob and Irtysh Rivers suggests the existence of a source of refractory Pu which is consistent with refractory "hot particles" derived from surface tests at the Semipalitinsk test site.