Abstract :
[en] Northern Benin is recognized as a cotton growing area that uses large quantities of insecticides. The same agricultural area has water reservoirs developed for the production of fish that are a source of protein for most of the northern population. Unfortunately, these impoundments and the fish that live there are subjected to strong pressures linked to the massive use of insecticides for cotton cultivation. The presence of residues of these insecticides in the water or fish of these reservoirs can cause health problems for consumers. In order to evaluate the levels of contamination of these insecticides in reservoirs and in fish, this work consisted in developing analytical methods for the control and monitoring of these pollutants.
The first part of this document (the introduction) presents the study environment (the North of Benin), some generalities on the insecticides likely to be or to have been used for the cotton cultivation in the North of Benin, their become in the environment, their physicochemical, eco-toxicological and toxicological characteristics. The insecticides of interest in this work are some organochlorines (endosulfan, DTT and related compounds, isomers of HCH), some pyrethroids (lambda-cyhalothrin, permethrin, cypermethrin, beta-cyfluthrin) and neonicotinoids (especially acetamiprid) and the chlorpyrifos.
The second part is the experimental section with 4 studies.
In the first study, the development of a generic pyrethroid ELISA test was investigated using polycyclonal antibodies directed against the phenoxybenzene group common to a large number of pyrethroids. The polyclonal antibodies obtained, however, did not have sufficient affinity for lambda-cyhalothrin, one of the targeted pyrethroids of this study, for insecticide determination at the expected concentration levels in the environment (and water in particular).
The second experimental study concerns the development of a gas-chromatographic mass spectrometry (GC-MS) multiresidue method for the determination of 16 compounds in water (10 organochlorines, 4 pyrethroids, chlorpyrifos and trifluralin). After validation of the method, a test for the recovery of the analytes in water samples stored under different conditions of time, temperature and container (glass or plastic) was carried out. This test made it possible to highlight that, whatever the conditions of conservation of water samples (glass or plastic bottles, 4 ° C or -20 ° C) supplemented at the rate of 1 g/L with various pyrethroids, these are no longer available for analysis, from the second day of storage, probably due to adsorption on the walls of the container. For organochlorines insecticides and chlorpyrifos, the recovery seems good if the water sample is stored at -20 ° C in glass bottles.
The third study presents the development of a UPLC-UV method for the determination of acetamiprid in water. After optimization of the extraction, detection and quantification steps, the method was submitted to an inhouse validation according to several international guidelines including the European Commission's reference document SANTE/11945/2015 (SANTE 2015). The method was then applied to water samples taken from 4 reservoirs in northern Benin, close to cotton fields. This study shows that acetamiprid residues are present in impoundments at concentrations ranging from 0.2 to 7.7 μg/L. These concentrations of acetamiprid do not, however, constitute a risk to the health of the fish or to the man who consumes the water of these reservoirs.
In the fourth study, the GC-MS multiresidue method developed for water in the first study was adapted for the determination of the same compounds in sediment and fish samples. After validation, the method was applied to samples of water, sediment and fish taken from the 4 reservoirs in northern Benin, already mentioned in the third study. In the water samples, analysis of these samples revealed the presence of trace organochlorines only (<1μg / L). It is in the sediment samples that the greatest number of residues has been recovered and in larger quantities, in particular with endosulfan levels of the order of 100 μg/kg. For pyrethroid residues, concentrations ranged from 0.2 to 14 μg/kg. Low amounts of chlorpyrifos were found (at concentrations of the order of 1 μg/kg). On the other hand, a wider range of concentrations was found for organochlorines (between 0.6 and 101 μg/kg). Finally, in fish, the concentrations of pyrethroids ranged between 1.5 and 11 μg/kg, those of chlorpyrifos between 1.8 and 3.3 μg/kg and those of organochlorines between 1 and 36 μg/kg. The risk assessment for each chemical contaminant taken alone shows that there is no risk to fish consumers in these water reservoirs.
Finally, the last part presents a general discussion of the results obtained. Organochlorine, chlorpyrifos and pyrethroid residues have been found in sediment and fish matrices, but sediments appear to be more contaminated than fish and than water.
At the level of the two fish species sampled, Clarias gariepinus, a sediment-dwelling species, appears to be more contaminated than the Oreochromis niloticus which live in the water, but a statistical comparison is not possible due to the small number of individuals sampled. Based on the results of this study concerning the contamination of fish with pesticides, and literature studies concerning the contamination of other foodstuffs with pesticides, it appears that the use of pesticides in the cotton basin results in the presence of pesticide residues in food in such a quantity as to constitute a risk for the health of the population. It is the case for chlorpyrifos and lambda-cyhalothrine, which can cause a chronic risk for children.
In conclusion, analytical methods were developed during this study for the determination of different pesticides in water, sediment and fish. The risk assessment, for the local consumer, associated with the ingestion of pesticide residues shows that there appears to be no risk, considering the consumption of fish from the reservoirs only, but considering the total exposure of the population via other commodities, the quantities of pesticides ingested exceed the allowable doses, safe for health, in the case of children. Thus, to reduce the risk to the health of the population linked to the use of pesticides, measures should be put in place in Benin to monitor the food chain, reduce the amount of pesticide used in the cotton basin and propose alternatives (such as biopesticides) for the treatment of cotton and other crops.
