Herbicide Residues in Soil of Former Phosphate Sludge Basins Converted to Agricultural Land in Mboro, Niayes Area, Senegal

Herbicides have been widely used in agriculture for many years to control weeds. Triazines are one group of herbicides. Their degradation products are mobile and soluble in water, and can be strongly adsorbed to soil. They are highly toxic and a source of concern in terms of environmental safety. Open-pit mining of the Taïba phosphate mines by Chemical Industries of Senegal (ICS) leaves deep basins on the mined sites. Downstream, these basins serve as storage areas for phosphate washing sludge. This highly fertile farmland is used by over 2,000 local farmers of all kinds. In such a context, it is essential to study the evolution of the behavior and fate of herbicides in the soil of the ponds. In this study, residue levels of three herbicides (atrazine, Terbutryne and Cybutryne) were determined in the soil of four of these ponds (B9, B10, B11 and B17). Extraction was performed using the QuEChERS method. Analyses were carried out by gas chromatography coupled with tandem mass spectrophotometry (GC-MS/MS). Statistical analysis of the results shows an abundance of active substances in the following descending order: Terbutryne ˃ Atrazine ˃ Cybutryne. It also shows that: Terbutryne residue levels range from 2.351 to 83.457 µg.kg-1, with a mean of 32.172±5.777 µg.kg-1 and a median value of 31.681±1.810 µg.kg-1. Atrazine residue levels range from 0.059 to 0.444 µg. kg-1, with a mean value of 0.312±0.082 µg.kg-1 and a median value of 0.318±0.004 µg.kg-1. Cybutryne residue levels range from 0.300 to 0.402 µg.kg-1, with mean and median values of 0.309±0.006 and 0.304±0.002 µg.kg-1 respectively. The presence of these three pesticides in sometimes significant quantities is partly due to the clayey nature of the soil studied. Their behavior depends on agricultural practices in the basins: the timing of herbicide application, soil turning, crop rotation and soil dilution by watering and rain. This study shows soil contamination by unregistered pesticides. The search for these and other more toxic and banned phytosanitary substances in the soils of the entire Niayes area and the market garden produce grown there is therefore essential to protect the environment and ensure the health and food safety of the population.