Interpolation of solid/liquid partition coefficients, Kd, for iodine in soils

The measurement of Kd is difficult for most radionuclides: a different value is expected for every different soil. This study explored a modification of the constituent-Kd approach used to estimate Kd in geological materials. Here we selected five soils of very different compositions, four were field soils and one was an artificial potting soil. The soils were blended together in ratios of 1:1, 1:3 and 1:1:2 for all possible (60) 2- and 3-soil combinations. The Kd was measured for each soil and each of the combined soils using additions of stable iodine. Our hypothesis was that the weighted average of the Kds of the original, unblended soils, weighted by the blending ratios, would be a reasonable estimate of the measured Kds of the blended soils. The ratios of expected/measured Kd values did not deviate significantly from unity (a geometric mean of 0.91) for the four field soils. This result suggests that Kd in the combined field soils could be estimated by the weighted average Kd for the constituent soils. The resulting variation is consistent with other estimation methods. The practical implication of this finding is that, with Kd data for a few benchmark soils in a region, one could estimate Kd for other soils. The potting soil did not conform, and there are several possible explanations for this.