Constraints on the applicability of ‘in-situ distribution coefficient’ values

Quantification of solute transport in natural waters is a key aim, or major component, of many geochemical studies. In models which describe such transport, the interactions of solute with solid phases are usually bulked into a simple distribution coefficient (Kd) term. Recently, there has been an increasing number of papers which purport to measure in-situ Kd values, which may be further claimed to be preferable to conventional laboratory measurements. This paper demonstrates that the general application of such in-situ values to transport modelling may be incorrect - especially in low-flow environments. In particular, for applications in which the movement of contaminants such as toxic wastes is concerned, these in-situ values may be non-conservative in the sense that they underpredict releases.