Effect of sediment and groundwater flow heterogeneity on accurately modelling radionuclide transport at UK nuclear sites

Strontium-90 (90Sr) is a major contaminant at nuclear legacy sites and is of interest due to being a strong beta emitter, t1/2 of 28 years, radiotoxic and is relatively mobile in groundwater. This thesis investigates the reactive transport processes affecting the behaviour of 90Sr in groundwater through heterogeneous unconsolidated glacial deposits. The gravel fraction (> 2 mm) has largely been ignored when measuring sorption and determining Kd values by confining batch sorption studies to < 2 mm fraction. Sr2+ adsorption was investigated in heterogeneous unconsolidated aquifer sediments, representative of the glacial deposits underlying the Sellafield site. The heterogenous gravel sediment was shown to have a higher Kd value than the outwash sand, due to the higher fines fraction and internal porosity present in the sandstone clasts. Results showed that Sr2+ sorption to heterogeneous sediment units could be estimated from Kd < 2 mm using appropriate gravel corrections. Additionally, pH sorption edge data and ionic strength behaviour could be effectively modelled by cation exchange and surface complexation processes. Intermediate scale adsorption-desorption column experiments showed that the sand packed column was predicted within error by the Advective Dispersion Equation, whilst the response of the gravel packed column showed non Fickian early breakthrough and tailing. Non equilibrium transport modelling approaches were investigated to better simulate transport through heterogeneous sediment. Results indicate the importance of tortuous pathways through heterogeneous sediment when modelling reactive transport of Sr2+. Assessment of fines washing remediation using heterogeneous and aged sediments showed that recently contaminated sediment was best suited to treatment and that gravel sediments had lower Sr recovery due to co precipitation of Sr into Ca mineral phases.