A novel mechanistic anion exclusion model to investigate partially water‐saturated transport in soils and shales: A case study of nitrate solution flow

Abstract

AbstractAnion exclusion is an important phenomenon in agricultural, environmental and engineering fields which still needs suitable quantitative models. In this work, a novel model is proposed which considers the porous media as parallel capillaries with negatively charged walls that cause total anion exclusion at a distance from the walls. By defining the ‘anion velocity coefficient’, , as the ratio of anion velocity to pore fluid velocity, equations to estimate the value of at the injection inlet have been derived by three different methods, namely from the profiles' data, , from the concentration ratio at the inlet, , and from the electrical double layer thickness model, . Finally, a simple sensitivity analysis for the third method has been presented. The estimates of presented in this paper were examined versus 22 data points for nitrate solution flow in initially dry soil columns for various soil and salt types and solution concentrations. The coefficient of determination between and for the whole dataset was 0.7755. Also, the estimate could predict the value estimated from the breakthrough anion concentration curve for available data of an advective injection test in a clayey rock. As a result, when anion exclusion dominates, the anion flux decreases but the anion velocity increases. In the case of using nitrate fertilisers, this point may be used to regulate the amount of optimal irrigation and also to explain the extent of contamination of a distant groundwater body.Highlights Modelling the anion exclusion phenomena in soils and shales. A mechanistic capillary model is developed to extract the anion velocity coefficient from data. Anion exclusion explains the anion concentration peaks in the unsaturated soil column test results. The derived estimates for anion velocity coefficient are shown to be reliable and promising.