Pesticides residues in Lake Kivu, Rwanda

Abstract

Rwanda has advanced toward a modern agriculture; the use of pesticides and agrochemicals are crucial in modern agriculture and contributed to an increase in Rwandan agriculture productivity. However other problems could results from the use of pesticides in agriculture. Contamination of water resources via runoff, leaching and drift of pesticides are the main issues that today’s agriculture is facing. In a country like Rwanda where the surface water is used as the source for drinking water and the water treatment process does not include the micro pollutant removal, the surface water needs a more stringent protection from the contamination and needs good management policies. Regular monitoring is an important tool that helps both policy makers as well as scientists in water management. However, there is currently no systematic monitoring of water resources in Rwanda with regard to pesticides residues due to the lack of analytical facilities. The present study was conducted to investigate the level of pesticides residues in Lake Kivu and to compare the level of pollution with other small Lakes in Rwanda namely Muhazi and Mugesera. From the obtained results, risk associated with the use of Lakes water as drinking water was investigated. In a preliminary stage, a survey was conducted to be aware on which types of pesticides are used in the neighboring area of Lakes. The results of the survey showed that 34% of the farmers use mancozeb, 22% use malathion, 18% use copper based pesticides, 14% use cypermethrin and 12% use carbendazim and sulfur based pesticides. Pesticides like acetamiprid, metalaxil and profenofos are used by 6% of the farmers. Only 4% of the responded farmers use chlorpyrfos and 2% reported to use cyhalothrin pesticide. To evaluate extend of pesticides contamination to water bodies, water samples were analyzed for 34 pesticides residues. Water samples were collected from Lake Kivu, the Sebeya River and from two small lakes; Lake Mugesera and Lake Muhazi. Lake Kivu water samples were collected from different depths by using a NISKIN bottle on which an RBR 620 CTD (Conductivity, Temperature and Depth) probe was attached to record the depth and physical properties of the water. Solid Phase Extraction (SPE) cartridges were used for extraction of pesticides from water samples. Samples were loaded at the field and the cartridges were xi transported to the laboratory. Pesticides in water samples were analyzed by using Ultra High Pressure Liquid Chromatography Mass Spectrometer (UPLC-MS/MS). The results indicate the presence of traces of metalaxyl, malathion and amitrol in water samples. Two pesticides were detected in Lake Kivu; malathion (0.22 μg/l) and metalaxyl (162.5μg/l). Malathion was also detected in the River flowing to Lake Kivu (19.26 μg/l). Amitrol is the only pesticide detected in Mugesera Lake (10.23 μg/l). The detected concentrations of pesticides residues in Lakes are greater than 0.1μg/l, a maximum value recommended by the World Health Organization (WHO) and European Economic Community (Directive 98/83/EC). The risk of pesticides to aquatic organisms was evaluated by using Risk quotient method which is a ratio of measured environmental concentration (MEC) to the predicted no effect concentration (PNEC). The health risks were determined based on USEPA Guidelines by calculating the Risk index which is the life exposure dose or Estimated Daily Intake (EDI) divided by the Acceptable Daily Intake (ADI) for chronic risk or acute reference (ARfD) dose for acute risk. The results show that the detected concentrations of pesticides in water samples represent a risk to people and to aquatic organisms. More vulnerable are infants and children. The presence of the traces of pesticides in Rwandan lakes is attributed to the use of pesticides in agriculture and the environmental factors such as runoff, erosion, flooding and land sliding. Human health and aquatic risks associated to pesticides in Rwanda is expected to increase in future as the agriculture use of pesticides is expected to increase. This will be accelerated by the topography and the degradation of wetlands which are important in water purification. Wetlands in Rwanda are facing enormous pressures from agriculture and this pressure will continue to increase considering the current high rate of increase in population. A further research is needed to investigate the quality of groundwater, drinking water and rivers. It is assumed that the concentration might be high in rivers as the dilution power is very small for rivers as compared to lakes. A continuous monitoring is recommended for a better protection of water resources in Rwanda and to ensure a good agricultural practice that provides economical benefits without harming the environment.