Accelerated Transport of 90Sr Following a Release of High Ionic Strength Solution in Vadose Zone Sediments
Numerical simulation of cation exchange and mineral precipitation / dissolution reactions using the multiphase reactive geochemical transport code TOUGHREACT has provided important insight into the distribution of 90Sr between layers of geologic strata in a complex vadose zone at the U. S. Department of Energy’s Idaho National Laboratory. During a transfer operation in November 1972, 70.4 m3 of acidic, high ionic strength liquid containing 15,900 Ci of 90Sr was released over five days into alluvial gravels 137 m above the Snake River Plain Aquifer. Sampling data from perched water zones 33 m below the release contain very high levels of 90Sr as do soil samples obtained nearer the point of release. Use of traditional simulation approaches using laboratory measured constant partitioning coefficients (Kd) cannot simultaneously explain perched water and soil concentrations. To address the discrepancy, a reactive transport approach was adopted to include competitive cation exchange, dissolution / precipitation of calcite, carbon dioxide gas production and transport, and gibbsite precipitation. Simulation results using this model suggest that some of the 90Sr could have been transported very rapidly immediately after the release with the acceleration facilitated by competition with high sodium concentrations in the released liquid, by calcium dissolved from calcite, and to a lesser extent by formation of aqueous complexes with nitrate. It is known that once the leading edge of the liquid assemblage was flushed from the alluvium, the mobility of the remaining 90Sr decreased significantly in the absence of the competing cations. Calculations indicate that there should be a net increase in calcite, suggesting that 90Sr could be entrained in the mineral lattice, but insufficient field data exists for confirmation. Sensitivity studies show that the ion exchange capacity and the Na/Sr ion exchange selectivity coefficients were the most sensitive parameters. Because of the sensitivity to total ion exchange capacity, 90Sr migration was found to be very sensitive to the combination of sediment cation exchange capacity and the area wetted by the infiltrating solution. In contrast, the future mobility of 90Sr was found to be relatively insensitive to the normal range in the composition of influxing precipitation and anthropogenic waters.
- Research Organization:
- Idaho National Lab. (INL), Idaho Falls, ID (United States)
- Sponsoring Organization:
- DOE - EM
- DOE Contract Number:
- DE-AC07-99ID-13727
- OSTI ID:
- 932234
- Report Number(s):
- INL/JOU-07-12646; JCOHE6; TRN: US200812%%666
- Journal Information:
- Journal of Contaminant Hydrology, Vol. 97, Issue 3 - 4; ISSN 0169-7722
- Country of Publication:
- United States
- Language:
- English
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