Influence of distance on transport of some pesticides by runoff
The aim of this study was to investigate how the distance covered by runoff water containing pesticides influenced the resulting quantity of pesticides liable to enter neighbouring waters courses. We carried out primary experiments on a bare soil, equipped with runoff water collection equipment placed at the following distances from the area to which the pesticide was applied: 1.5 m, 5 m, 9 m and 13 m. Each 10 m2 test surface was treated with several pesticides: atrazine, simazine, cyanazine, 2,4-D and 2,4-DB. Results show that the pesticide quantities found in the water collectors decreased exponentially with distance. Under the experimental conditions, a distance of about 4 m led to a 10-fold reduction in pesticide runoff transfer rates. Further similar experiments were carried out on soil cultivated with wheat and with maize. Two different pesticides, atrazine and lindane, exhibiting different behaviour in soil and different chemical and physical properties, were chosen. Observed results were similar and the total pesticide quantity was again found to be a decreasing exponential function of the distance from the treated plot. Reduction in the amount transferred, as a function of runoff distance, depends mainly on vertical migration, which in turn reduces the quantity of surface pesticide available. This vertical migration is favoured by a low affinity between pesticide and soil. This phenomenon is amplified with increasing soil water reserve. It is due to both pesticide diffusion and vertical displacement of water. The state of the surface due to arable farming practices and vegetation also plays a determining role in the vertical and horizontal displacement of water, and consequently affects pesticide transfer by runoff. Therefore the distance factor was less prominent in spring for soils cultivated with maize than in winter for the same soil cultivated with wheat. However, in all cases, the influence of runoff distance upon the quantities of pesticide transferred to surface waters remains crucial.