Development of natural colloid sampling and characterisation techniques

A sampling technique based on diafiltration has been developed for the in situ collection of colloids (1 nm-1 μm diameter) from groundwater systems. A large-scale field rig is used to obtain the colloid sample, and a laboratory-scale rig is used to fractionate the colloid samples into convenient size ranges. The equipment has been tested in the field by using it to collect colloid samples both from a public water supply borehole at Markham Clinton, Notts., and from several boreholes at the uranium deposit in the Alligator Rivers region, Australia. These samples have been characterised in terms of their elemental composition, actinide isotopic activities and particle size. For each site about 1-2% of the uranium activity and 10-70% of the thorium activity in the sub-1 μm size fraction were found to be associated with the colloid fraction. There is some evidence that particle aggregation can occur during storage and possibly during concentration. (author)