The development of agricultural activities in Africa, occupies an important place for human nutrition and contributes to the national gross domestic product (GDP) of some countries. The growing demand for agricultural products, associated with yields improving involves the systematic use of chemical pesticides to control crop pests (Ahouangninou et al. 2011 ; Andres and Lebailly, 2011 ; Illyassou et al. 2015 ; Kanda et al., 2013), which consist mainly of tropical insects and parasites (Bruinsma, 2003). For African countries in the Sahelian zone that are part of the Interstate Committee for Drought Control in the Sahel (CILSS), the use of these pesticides is regulated by the Sahelian Pesticides Commitee (CSP). Several studies have shown some high concentrations of pesticides with a frequency of pyrethroids, dieldrin and organophosphate pesticides (Donald et la., 2016) in water, sediment, soil, food, air and aquatic organisms in countries concerned with the Niger River Basin including Guinea (Traoré and Haggblade, 2017), Mali (Berthe Dem et al., 2007; Le Bars et al., 2020), Burkina (Gnankiné et al., 2013; Lehmann et al., 2017, 2018; Son et al., 2017; Tarnagda et al, 2017), Benin (Okoumassoun et al., 2002; Agbohessi et al., 2011, 2012; Adechian et al., 2015; Agboyi et al., 2016, Gouda et al., 2018), Nigeria (Adeyemi et al., 2011; Ogbeide et al., 2015; Isogi et al., 2016, Ojo 2016; Gushit et al., 2017), Niger (Mamadou et al, 2005, 2008; Mamane et al.,2016; Zabeirou et al. 2018), Cameroon (Ahmadou et al., 2016 ; Norbet et al., 2017 ; Branchet et al., 2018), Côte d'Ivoire (Traoré et al., 2006 ; Doumbia and Kwadjo 2009 ; Coulibaly et al., 2012 ; Traoré A. et al., 2015) and Chad (Sougnabe et al 2010). On the Mékrou River, studies have only involved Benin part and the work has shown alarming concentrations of endosulfan (746 µg/l), DDT (100 µg/l), dieldrin (48 µg/l), and heptachlor in some of the waters of the W Park and the river, as well as endosulfan and lindane in fish and sediment (Elisabeth et al, 2006 ; Agbohessi et al. 2011, 2012, 2015 ; Ahouangninou et al. 2011 ,2012 ; Adechian et al., 2015). The W Park is a large transboundary biosphere reserve straddling Benin, Burkina, and Niger and has been listed as a Unesco World Heritage Site since 1996 and protected since 2007 by the Ramsar Convention.
In Niger, the annual population growth rate is 3.9%, one of the highest in the world, and agriculture is the country's third largest source of income, after uranium mining and livestock. Agriculture is mainly practiced on small family farms used for food crops (millet, sorghum, cowpea and cassava), rice cultivation, and some cash crops such as corn and peanuts, without recourse to mechanization (MHELD, 2005). It is mainly practiced in the southern part of the country and along the Niger River, and corresponds to about 13% of the national territory (Guengant and Banoin, 2003). Market garden crops such as tomatoes and onions are grown in the depressions created by ancient or recent rivers.
The Mékrou River is a temporary tributary of the right bank of the Niger River that originates in Benin in the plain west of the Atakora Mountains (Le Barbe et al., 1993; Robert Vernet, 1994). This river is heavily involved in cotton production, which began in 1965 and was encouraged by the Compagnie française pour le développement des textiles (CFDT). This production was greatly expanded in the 1980s with the large-scale application of plow-down cultivation (Leo J. De Haan, 1992). Benin is the leading cotton-producing country in Africa, ahead of Mali, Côte d'Ivoire and Burkina Faso, with production of 712,000 tons in 2020 (Benin Ministry of Agriculture), accounting for 80% of the country's export earnings (AIC, 2019).
The use of pesticides allows a good crop yield, but several works have highlighted environmental and health risks including reproductive function expectations, neurological disorders and cancerous pathologies (Multigner 2005; Ahouangninou et al. 2011; Mamane et al., 2015; Gouda et al., 2018; Le Bars et al., 2020). According to the WHO, there are approximately one million severe pesticide poisonings worldwide with some 200,000 deaths (WHO, 2006 and WHO, 2008).
Several studies in Niger have highlighted the use of pesticides and human and food exposures. The application of organophosphates, pyrethroids, and avermectins was evidenced in market garden crops of onion, cabbage, and tomato in Madaoua Department and 25 pesticide formulations, all of which were not registered by the Sahelian Pesticides Committee (Zabeirou et al. 2018). The harmful effects of chlorpyrifos ethyl and fenitrothion used against the desert locust on the two species of pimelia (Coleoptera, tenebrioniadae) in the Tafidet valley (Agadez region) and occupational exposure to organophosphates and carbamates in rural Niger were highlighted in the work of Mamadou et al., 2005, 2008. On the Niger River, the exposure of the diet of the city of Niamey to residues of phytopharmaceutical products during the hot and cold dry season was highlighted and showed that the risk of exposure was higher in children than in adults for all the residues detected and whatever the product (Massalatchi et al. 2018). These exposure risks also concern small farmers with an exposure level above the acceptable exposure level for all active substances and vary from 0.0013 mg / kg bw / day to 0.4125 mg / kg bw / day (Massalatchi et al. 2017). The result of surveys conducted on the risk of respiratory disorders in adults and children related to cultivation in Niger showed that people living in agricultural areas have an increased risk of respiratory symptoms in adults (wheezing, dyspnea, sudden shortness of breath, and cough without fever) and in children (cough without fever) compared to those in pastoral areas (Mamane et al.,2014, 2016).
Although the work carried out has highlighted the risks of exposure of certain producers through the use of pesticides, there is currently no work assessing the quality of water to pesticides along the Niger River and on the Mekrou River. Chemical pesticides are used by farmers to protect their crops from pests and are mostly exported from neighboring countries. The use of these mainly unregistered pesticides for agricultural purposes could be a source of environmental and health problems in the Niger River watershed in Niger. The objective of this study is to study the occurrences of polar pesticides contamination in the Niger River valley and its tributary, the Mekrou River, and to evaluate the inputs of pesticides from the river and its tributaries in the Niger portion of the watershed.
The passive sampling technique will be used to detect and quantify polar compounds in the water. It has advantages in terms of the preconcentration of compounds in water and increase the possibility of detecting or even quantifying compounds present at very low concentrations in water and improving the diagnosis of contamination (Di Carro et al., 2018). Samplers will be placed along the Niger River and downstream of tributaries to accumulate molecules on a receptor phase by diffusion (mass transfer) of compounds. This technique will be used for the first time in the assessment in pesticide contamination of the Niger River and its tributaries.