2,4-D mobility in clay soils: Impact of macrofauna abundance on soil porosity

Pesticides in agriculture are commonly used to meet the growing food demand; however they compromise the quality of water and soil. The pesticide 2,4-D is one of the most widely used herbicides in the world, it belongs to the group of synthetic herbicides that control broadleaf weeds. In this paper the risk of groundwater contamination by 24-D and its major metabolite, 2,4-DCP, is studied in a context of high density of soil worms. We compared the adsorption, desorption, degradation and displacement of 2,4-D in soils from Tabasco Region having different properties: clay, organic matter, iron and aluminum oxides contents. In addition to the classical soil physicochemical characterizations, a 3D analysis of the soil structure and porosity was performed by analyzing images acquired by Computed Tomography. The objective was to evaluate the effect of soil properties and macroporosity produced by the macrofauna activity on solute movement.All the four soils studied sorbed the herbicides, more importantly 2,4-DCP, the metabolite, than 2,4-D itself. The distribution coefficients for 2,4-D sorption were linear and varied between 1 and 4 while those for 2,4-DCP were above 10. The contents of iron and aluminum have an important role in the adsorption of these two compounds. In aerobic conditions, the herbicides half-life was about 2days. Water movement occurred in physical equilibrium in three of the four soils; soil dispersivity ranged from 1.2 to 7cm, clay content being the main factor. After 20 to 60days depending on the soil, no 2,4-D leaching was observed through the soil columns, except for one soil were there was preferential flow. Earthworms burrows were exhibited and quantified in the soils samples through the analysis of Computer Tomography (CT) images, they appeared as small, snail-shaped, rounded volume of 3 to 7mm radius with a higher density with respect to the surrounding soil. They were extracted from the original data using a combination of image processing and mathematical morphology operators. Based on the results obtained, it can be concluded that preferential flow caused by both high clay content and the presence of macrofauna pores significantly reduces the buffering capacity of the soil, increasing the risk of contamination by herbicides of the underlying aquifer.