Solid-liquid partition coefficients, K_dS: What's the value and when does it matter?

ECOMatters, 24 Aberdeen Avenue, P.O. Box 430, Pinawa, Manitoba ROE 1L0, Canada

Abstract

Environmental risk assessments hinge on our ability to predict the fate and mobility of radionuclides and metals in terrestrial soils and aquatic sediments. Solid-solution partitioning (the Kd approach), despite its shortcomings, has been used extensively. Much attention has been devoted to grooming the existing key compendia for values applicable for each nuclear risk assessment carried out worldwide. This appears to be an important task. For example, soil Kd values for a single nuclide can vary over several orders of magnitude, yet the soil Kd value is the most important parameter in the soil leaching model. Similarly, plant uptake depends primarily on the nuclide present in solution phase. Despite this apparent sensitivity, our experience has shown that risk assessors dwell too much on the precision of the Kd value for all nuclides. This paper discusses the effect of the Kd value on the resulting soil concentration during leaching and identifies those radionuclides and assessment conditions where a precise value is required. Only those radionuclides that typically have a soil Kd value of 10 L/kg or less (Tc, Cl, I, As) need be accurately described for timeframes of up to 10,000 years for desired soil model prediction outcomes (soil concentrations within two-fold).