Analytical solutions of the convection–dispersion equation applied to transport of pesticides in soil columns

Mathematical solutions are presented to describe leaching and degradation of pesticides in a specific type of column experiment, which is frequently required for the official registration of pesticides. In the column experiment in question a 2 cm layer of soil containing a known amount of the pesticide is placed on top of a soil column with a length of 28 cm. Water is added to the layer on top of the column for a certain period at a constant rate, while allowing free drainage at the bottom of the column. This induces leaching of the compound from the top layer. The mathematical solutions presented allow interpretation of the experiment in terms of the half-life and the sorption coefficient of the compound from the measured amount in the leachate and the amount remaining in the column after leaching. Example calculations are carried out for the German and US leaching scenarios, which are currently used in admission procedures for new and existing pesticides. It is concluded that, for a standard soil with an organic carbon content of 0.01 kg kg−1, the German BBA guidelines are suitable to estimate half-lives (T50) and sorption coefficients (Koc), for compounds with T50<20 days and Koc<0.06 m3 kg−1. An alternative leaching scenario, as described in the US–EPA guidelines seems to be useful to estimate T50 in the range 10–250 days and Koc in the range 0.05–0.20 m3 kg−1.