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The Gorleben glacial sand/silt aquifer system has been studied using both field-based and laboratory techniques to determine the elemental composition and natural series actinide loading on organo-metallic (humic) colloids. Colloid concentrations of 10 17 particles 1~1 have been found whose size and concentration may be directly related to the chemical nature of DOC (dissolved organic carbon) and its concentration. There is unequivocal evidence that the bulk of the actinide and lanthanide concentration in the groundwater is associated with the humic colloids. The activity ratio for 234 U/ 238 U indicates that uranium in the colloid fraction is not in chemical equilibrium with that in the solution. Most of the colloid population occurs in the smallest size range of 15 -1.5 nm which provides the largest surface for potential interaction with radionuclides. The amount of DOC not filterable with a 10,000 molecular weight (MW) cut-off filter (approximately 1.5 nm) consists mainly of fulvic acid which may undergo complexation with metal ions in the groundwater to produce organic complexes. These may behave similarly to organic colloidal particles but are less than 1.5 nm in size.

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