Determining depleted uranium （DU） contamination in soils and salt marsh sediments in the vicinity of DU munitions testing sites in SW Scotland and NW England, UK

In its natural state uranium has three isotopes; ^238U （99.27 atom % abundance）, ^235U （0.72%） and ^234U （0.0055%）, with the ^235U ： ^238U activity ratio having a constant value of 0.045 and the ^234U：^238U activity ratio commonly ranging 0.8-1.2 for soils and sediments （e.g. Fisenne 1996）. Uranium enrichment, the process by which the fissile isotope ^235U is concentrated for the production of nuclear fuel, generates a waste product that is depleted in both ^235U and ^234U. Depleted uranium （DU） therefore has a distinct isotopic signature that can be used to monitor contamination arising from the use of munitions containing it （i.e., DU weapons and armour）. The UK Ministry of Defense has tested such munitions at coastal locations in SW Scotland and NW England, resulting in possible contamination of the surrounding terrestrial and marine environments. We examined the U isotope distribution in surface soil （top 10 cm） and salt marsh sediment samples from these locations using alpha spectrometry to determine the extent of any DU contamination from the munitions testing programme. Despite 〉30 t of DU shells having been fired into the sea in this region there was no evidence of deviation from the natural ^234U ： ^238U activity ratio range in salt marsh samples. The ^235U ： ^238U activity ratio could not be used to assess these salt marsh samples because ^235U activities were at the limit of detection for alpha spectrometry.     

Adsorption and stabilization of organic carbon by mineral surfaces in soils

Soils are the largest carbon reservoir in the terrestrial system. Soils contain about three times more carbon than vegetation and twice as much as that present in the atmosphere. Soil organic matter （SOM） is very complex in composition and structure, formed of heterogeneous substances and generally associated with minerals in soils. SOM is classified as labile and stable fractions on the basis of residence time, determined not only by the chemical composition of SOM, but also by types of protection or bonds within soils. The stable carbon fraction is protected either physically or chemically. To understand the process of SOM stabilization, physicochemical properties of organic-mineral complexes were determined by Fourier transformed infrared （FTIR） with attenuated total reflectance （ATR） and diffuse reflectance （DRIFT）, atomic force microscopy （AFM）, and nuclear magnetic spectroscopy （NMR）. Humic acids and carboxylic acids with relatively short carbon chains were used as sorbates, and goethite, kaolinite, and montmorillonite as adsorbents. Humic acid was fractionated during adsorption on the minerals, which was highly influenced by the characteristics of minerals. For instance, long-chain aliphatic carbon was likely to be adsorbed onto the surface of kaolinite and montmorillonite, while goethite surface attracted carboxylic functional groups of humic acid.