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

Abstract

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(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.