Radiostrontium Migration from a 1953-54 Liquid Release to a Sand Aquifer

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Abstract
Aqueous ammonium nitrate solutions containing a variety of fission products were released to an infiltration pit overlying a sand aquifer in the early 1950’s. Almost all of the undecayed 137Cs and 154Eu are still retained within the 5 m thick unsaturated zone beneath the pit. Radiostrontium, however, has moved up to 350 m through the hydrogeologically simple and fast-moving groundwater flow system; the inventory of the aquifer plume’s 90Sr content is in good agreement with the estimated initial release. Data from 6 determinations of plume front position yield a linear increase in distribution ratios for 90Sr at the leading edge, from 0.8 to 4.8 mL·g−1 between 1955 and 1983. Mean Kd’s between 3 and 9 mL·g−1 have been determined from short-term laboratory and field experiments, and the rate of advance of the plume front is expected to reach a steady state within this range. Analyses of 340 contaminated sands and associated pore waters from various locations in the plume provided in situ distribution ratios with a log mean of 12.9 mL·g−1, and with no significant variations between sediments that have been in contact with 90Sr for from 10 to 28 years. Differences between plume front distribution coefficients and the in situ values observed in older portions of the plume are explained primarily by kinetically slow chemisorption onto secondary oxide coatings on the sand surfaces. This reaction apparently has half-reaction times for both sorption and desorption of less than 2 years, and hence can be treated as an equilibrium process in long-term studies. A substantial conflict remaining in the data, however, is the rate of advance of the centre of mass of the plume. Most of the radiostrontium in the aquifer la apparently moving at a much slower rate than the log mean in situ distribution ratio would predict - one explanation for this discrepancy is that 90Sr movement is greatly retarded by volumetrically small sandy silt strata that have much higher sorption capacities than the fine to medium sands that dominate the overall aquifer sampling. A simple, conservative, model that assumes equilibrium sorption of radiostrontium predicts acceptable concentrations of radiostrontium in surface waters at the CRNL property boundary.

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