Uranium is often considered to be a mobile radioelement in the natural
environment owing to its tendency to form stable complexes with a num
ber of aqueous anions, particularly in oxidising milieu. A series of i
nfiltration experiments were devised to investigate this migration beh
aviour under rigidly controlled laboratory conditions. Intact cores of
Permo-Triassic Clashach Sandstone were pre-equilibrated with syntheti
c groundwater solutions and continuous flow-through of uranium monitor
ed together with Ph and concentrations of other ions. Prior to perform
ing each experiment a simulation was carried out using a one-dimension
al coupled chemical transport code, encompassing a thermodynamic descr
iption of the electrical double layer. These calculations together wit
h electron microscopy indicated the potential role played by iron oxyh
ydroxide grain coatings in retarding the uranium plume. Thus, a second
series of experiments was initiated on pre-acidified cores from which
all surface exposed iron had been removed, allowing an assessment of
the retention capacity of non-ferric components. Taken together, the d
ata clearly illustrate the strong affinity of aqueous uranium species
for natural surfaces even under strongly oxidising conditions. The suc
cess of the model in predicting a priori the dominant trends in uraniu
m migration behaviour is encouraging and may aid in prioritising analy
tical requirements for investigations in more complex geochemical situ
ations than those studied here.