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