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

The relative diffusion coefficient (D_r) 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 (θ_br) in the relationship between θ and D_r has been demonstrated at low-θ, in which the value of n becomes approximately 4 when θ < θ_br. 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 D_r measurements require considerable time to render sufficient data at low-θ, few studies have measured D_r at θ < θ_br, and the relationship between D_r and θ at θ < θ_br remains unclear. In this study, we investigated if the indirect D_r measurement method can be applied in the low-θ region of θ < θ_br. An indirect method for measuring D_r was employed to determine the soil electrical conductivity (EC_s). Using dune sands in which the presence of θ_br was confirmed, D_r from high- to low-θ was calculated based on EC_s measurements and compared with directly measured D_r. The relationships between θ and D_r calculated based on EC_s and using the transient state method were almost the same. D_r could be calculated from EC_s even at θ < θ_br. The results confirmed that the indirect Dr measurement method can be applied for the low-θ region of θ < θ_br.