Mathematical modeling of oxadixyl transport in onion crop soil

Pesticides used in crop production are the most important source of diffuse pollution to groundwater, and their discharge into surface water may be a contributing factor towards the decline of living resources and the deterioration of ecosystems. In this work, we studied the movement of oxadixyl through soil columns (30 cm length and 14 cm diameter) in laboratory conditions using onion soil from Lake Tota (Boyacá, Colombia). A solution of 0.01 M CaCl2, containing a tracer (bromide) and oxadixyl was sprayed onto the surface of the soil column, and then simulated rainfall was applied at an intensity of 0.034 cm h-1. After 30 days, and 2.13 relative pore volumes, oxadixyl percentages recovered at the bottom of the column were 92.1%. The oxadixyl experimental elution curve was analyzed using the Stanmod program (inverse problem) to obtain transport parameters. The non-equilibrium chemical model described the experimental elution curve well. The tail of the elution curve was particularly well captured. The retardation factor calculated for the fungicide was 3.94 and the partition coefficient, kd, was close to 1 kg L-1, indicating low adsorption in this soil. Under the experimental conditions, it could be concluded that oxadixyl is mobile in this soil, and therefore presents a risk of potential groundwater contamination.