Distributions of radionuclide sorption coefficients (K_d) in sub-surface sediments and the implications for transport calculations

摘要

The effect of the spatial variability of K_d on calculations of contaminant travel time in the vadose zone was determined. Depth discrete measurements of K_d were made for a suite of radionuclides (~(109)Cd, ~(57)Co, ~(60)Co, ~(85)Sr, ~(137)Cs, and ~(88)Y) utilizing a sediment core from the E-Area at the Savannah River Site. The K_d's were ordered as ~(85)Sr~(2+) < ~(137)Cs~+ < ~(109)Cd~(2+) < ~(57)Co~(2+) = ~(60)Co~(2+) ? ~(88)Y~(3+) and the values generally fell below or near the lowest quartile of values reported in the literature. Correlations were generally weak between soil properties and K_d values. Most importantly, all of the K_d distributions could be reasonably approximated as log-normal. Deterministic and stochastic calculations of contaminant travel time to the water table were made. The deterministic calculations were based on each of three conceptual models of the vadose zone: complete stratification (17 strata, each with a different K_dJ), two strata (two sections of the vadose zone, each characterized by a single, average K_d), and unstratified (a single zone with an average K_d). Stochastic calculations were based on log-normal fits to the K_d data. The two strata model generally yielded travel times 2 × greater than those in the completely stratified model. The unstratified model yielded travel times that were between 3 and 5 times greater than the completely stratified model. The stochastic mean travel times were comparable to those of the two strata model.