Seasonal Herbicide Monitoring in Soil, Runoff Waters and Sediments in an Olive Orchard.

Several herbicide pollution episodes in surface and ground waters have occurred in areas where olive crops are established, and hence it is necessary to know the evolution of herbicides in olive trees plantation depending on their seasonal application. A monitoring study of two herbicides (terbuthylazine and oxyfluorfen) in the first 2 cm of soil, runoff water and sediment yield, was carried out after several rainfall events. Terbuthylazine was monitored in spring and winter seasons, whereas oxyfluorfen was monitored only in winter season. At the end of the experiment, no terbuthylazine soil residues were recovered from the winter application (107 days after application) whereas 15% of terbuthylazine applied in spring still remained (85 days after application) in the soil. Oxyfluorfen was more persistent than terbuthylazine and 48% of this applied herbicide remaining in the soil at the end of the winter (107 days after application). Very low percentage of the applied terbuthylazine was recovered in runoff water, although it was slightly higher in winter (0.53%) than in spring (0.16%). No differences in terbuthylazine sediment yields between both seasons were observed. Due to the low water solubility of oxyfluorfen, very low amounts of this herbicide was recovered in runoff waters, whereas in sediment yields 39.5% of the total applied herbicide was recovered. These data show that the dissipation of terbuthylazine due to runoff is low and it could be related to other soil processes like leaching and degradation, whereas oxyfluorfen dissipation is more affected by runoff processes, being mainly either co-transported  in sediment yields or  tagged to soil aggregates. These results show that the fate of terbuthylazine and oxyfluorfen depends partially on seasonal and environmental conditions, mainly on the rainfall events, but also, and even at larger extension, on the physicochemical character of the herbicide molecules.