Colloid Benchmark Exercise: An Interlaboratory Study of Sampling and Characterisation Techniques for Natural Colloids in Oxic Groundwater

Summary

Natural colloids have been identified as potentially important in the transport of radionuclides in groundwater. This paper presents the results of a comparative study of two methods of colloid sampling; pulsed, cross-flow ultrafiltration and tangential diafiltration. In this study a comparison was made of colloid populations, composition and size distributions in colloid concentrate samples taken in the field, of a poorly mineralised groundwater abstracted from fractured Permo-Triassic sandstone in the English East Midlands. The actinide isotopic activities associated with the colloids were also determined, using isotope dilution/alpha spectrometry, at Harwell. In order to study the effects of storage on natural colloids, additional groundwater samples were stored for approximately two months before fractionation using the cross-flow filtration and diafiltration methods previously applied to field samples. Preliminary results from this intercomparison exercise indicate that; (i) no artefacts are produced by either colloid concentration method, although some aggregation of colloidal particles may occur on storage of aqueous colloid concentrates, (ii) good agreement on colloid populations and size distributions is obtained by different laboratories applying scanning electron microscopic analysis to cross-flow ultrafilters and (iii) limited fractionation of solutes is caused by diafiltration of low salinity water, in particular uptake of sodium.