Relationship Between Solute Diffusion Coefficient and Electrical Conductivity Under Extremely Low Soil Water Conditions

Abstract

The relative diffusion coefficient (Dr) of solutes in soils is directly proportional to the “nth” power of the volumetric soil water content (θ), where n is an empirical parameter, which normally ranges between 1 and 2. The existence of a breakpoint (θbr) in the relationship between θ and Dr has been demonstrated at low-θ, in which the value of n becomes approximately 4 when θ < θbr. The change in n can be attributed to various mechanisms, including drastic changes in the geometrical distribution of pore liquid water caused by dehydration. However, as direct Dr measurements require considerable time to render sufficient data at low-θ, few studies have measured Dr at θ < θbr, and the relationship between Dr and θ at θ < θbr remains unclear. In this study, we investigated if the indirect Dr measurement method can be applied in the low-θ region of θ < θbr. An indirect method for measuring Dr was employed to determine the soil electrical conductivity (ECs). Using dune sands in which the presence of θbr was confirmed, Dr from high- to low-θ was calculated based on ECs measurements and compared with directly measured Dr. The relationships between θ and Dr calculated based on ECs and using the transient state method were almost the same. Dr could be calculated from ECs even at θ < θbr. The results confirmed that the indirect Dr measurement method can be applied for the low-θ region of θ < θbr.