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Mechanisms for 85Sr migration through glacial sand determined by laboratory and in situ tracer tests

Abstract Prediction of solute transport based on the laboratory measurement of radionuclide/sediment sorption parameters has been compared with field tracer tests involving the release of 85Sr and a conservative tracer 131I into a radially divergent flow field around a recharge well in a confined glacial sand aquifer. Tracer breakthrough was monitored in multi-level sampling installations. Two tests at different imposed rates of groundwater flow were conducted. At average flows up to 5 m day−1, the retardation of 85Sr at a given sampling point was lower than that observed with an average flow rate of 0.5 m day−1, suggesting a kinetic control on sorption. Sequential selective leaching was also undertaken to study the geochemical phases responsible for sorption. In a core consisting predominantly of medium sand, approximately 80% of the 85Sr was present in ion exchange sites, with 12% in the carbonate phase. The remaining 8% was recovered easily by extraction with distilled water. Kinetic data for Sr sorption on sand and silt core material suggested a two-stage process which could be fitted by two empirical sorption/desorption rate constants. These were used to simulate the effect of kinetics of sorption on the breakthrough curves for strontium. The retardation of 85Sr at the lower flow rate was in relatively good agreement with laboratory values derived from batch sorption experiments lasting several days. At the high flow rate, longitudinal dispersivity αL for 85Sr was larger than that for 131I, but this discrepancy reduced at the lower flow rate where some convergence to the 85Sr values was observed. A possible explanation is that at the high flow the opportunity for 131I diffusion into relatively static porewater was limited; whereas with solutes moving at a lower rate there was a greater opportunity for diffusion into ‘dead end’ pores.