Movement of peak solute concentration position by leaching in a non-sorbing soil

Abstract

A simple mathematical model based on mass conservation is derived and tested which allows prediction of the displacement of the solute peak concentration position in soil from a knowledge of water inputs and losses. Comparison of the model with two different sets of field leaching data for chloride and nitrate indicates that its predictions are accurate within c. 10% provided two major assumptions in the theory are satisfied. These are: (i) that there is no significant soil-solute interaction or chemical transformation; (ii) that drainage through the soil profile to field capacity takes place. Making these assumptions and also neglecting dispersion, an alternative derivation is given to that in the literature showing that the velocity of the position (depth á) of the concentration peak is equal to the pore water velocity at á. This equality is integrated, showing that the finite displacement Äá in a is equal to the equivalent ponded depth of water which has been displaced beyond the previous position of a divided by the volumetric water content at field capacity. The effect of dispersion on prediction of peak concentration position was investigated analytically. Neglect of dispersion in the model leads to an overestimation of peak displacement, this error being c. 10% in an illustrative example, decreasing in magnitude as dispersivity increases. It is concluded that the simple theory presented may be of special utility in guiding nitrate management in situations of high potential leaching loss. In situations where predictions of the model are inaccurate because assumption (i) is not satisfied, consequent management decisions are conservative rather than dangerous in the sense of inducing an irretrievable situation.