The analysis of solute breakthrough curves to predict water redistribution during unsteady flow through undisturbed structured clay soil

Abstract

Solutions of 5 or 10 m M CaCl 2 were applied at constant rates of 7–14 mm hr. −1 to the surface of large (23-cm dia. × 20–24 cm long) undisturbed cores of cracking clay soil. Cl − breakthrough was analysed assuming a log-normal distribution of solute travel times. The resultant transfer function model (TFM) fitted the data well for moist or prewetted cores (volumetric water content θ i from 0.35 to 0.49) but was inadequate for very dry cores ( θ i ⋍ 0.30 ) at the higher input rates. When extensive bypass flow occurred in the soil, the probability density function of solute travel times was not well represented by a log-normal distribution. The breakthrough data were also analysed using a mixing model which postulated convective mixing of applied solution with resident soil water to form a mobile miscible volume, and diffusion of solute between the miscible and immiscible volumes of soil water. This model was used to predict the miscible volume and its change with time during unsteady flow through each core. The time-weighted average fractional miscible volume θ' m derived from the mixing model was in close agreement with θ m, calculated as the input water flux divided by the mean pore-water velocity given by the TFM. An essential feature of any model of solute transport in structured soils is the recognition of two domains of different flow characteristics which is some cases give rise to a log-normal distribution of pore-water velocities.