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

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 (Cd-109, Co-57, Co-60, Sr-85, Cs-137, and Y-88) utilizing a sediment core from the E-Area at the Savannah River Site. The K-d's were ordered as Sr-85(2+) < Cs-137(+) < Cd-109(2+) < Co-57(2+) = Co-60(2+) << Y-88(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-d), 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 2x 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. (C) 2010 Elsevier Ltd. All rights reserved.