Factors Influencing the Partitioning of Metal Residues ( Lead ) in Nokoué Lake , Benin

The impacts on housing and living species (fish, shrimp) related to the disposal of untreated sewage by municipalities and discharges of solid waste was studied by assessing the whole Organic Matter (OM) of the sediments, Chemical Oxygen Demand (COD), Suspended Materials (SM), nitrogen pollution indicators (nitrate-NO3 , nitriteNO2 , ammonium-NH4 ), phosphated (ortho-phosphate ionsPO4 3), sulphide (sulphateSO4 2, sulphite SO3 2, sulphides S 2) of water and total Pb contents of water and sediments of Lake Nokoué. The organic matter content of sediments (19600 to 17164 mg / kg) and COD (130 to 1880 mg/L) of water are very high and indicate an increase in organic pollution of the lake compared to in previous years (Youssao, 2011, Mama et al., 2011). The strong variations in pH are dependent on the contributions of marine origin (basic to neutral) and the mineralization of the organic matter (pH < 7) in the areas of human settlement resulting in a greater or less mobility of the residues of metals in making sulphur available in its reduced form (sulphide and sulphite). The assessment of pollution indicators in the central part of the lake shows that it is heterogeneous in terms of source and impact of pollution. The sediment Pb concentration (Pbsed) / concentration of Pb in water (Pbw) ratio, represented by Pbsed/Pbw ratio shows that lead is 200 to nearly 1000 tiSM more concentrated in sediments than in the water showing a predominance of the organic form of Pb in the ecosystem of Nokoué lake. Indeed, organic lead residues are massively released into this ecosystem by oil traffickers who use motorized canoes to transport their products from Nigeria. © 2019 Elixir All rights reserved. Elixir Aquaculture 126 (2019) 52438-52444 Aquaculture Available online at www.elixirpublishers. com (Elixir International Journal) Alassane Youssao Abdou Karim et al./ Elixir Aquaculture 126 (2019) 52438-52444 52439 Figure 1. Location map of sampling sites on The Lake Nokoué. all the wastes and carries out dangerous activities such as the trafficking of petroleum products from the oil-producing country of Nigeria located in the East of Benin. 2.2 Selection of sampling sites A previous study was carried out in the area indicating a high level of pollution in the surrounding areas at garbage dumps and discharges in the peripheral areas of the effluents transported via the gutters and collectors. To deepen the paradox of a higher contamination in the sediment lead core area, the present study focused a little more on the central zone where four sites were identified in more than four others in the peripheries which were already followed during from the previous study. 2.3 Sampling and conditioning technique for water samples The taking of a water sample is a delicate operation to which great care must be taken; it conditions the analytical results and the interpretation that will be given (J. Rodier, 1978). For this, the containers used for sampling are a function of the physicochemical characteristics of the element to be analyzed. As part of our study, we used glass and plastic bottles, previously washed and dried in an oven at 105 °C. Water was taken with a bottle plunged about 10 cm below the surface. The bottle is filled flush to avoid trapping air bubbles. All samples of water, sediment, fish are kept in a cooler. Cold accumulators are used to keep them cool in the cooler. Once brought back to the laboratory, the water samples are refrigerated at 4 °C while the sediment samples are placed in the freezer. 2.4 Measurement of physical parameters These are temperature, pH, conductivity, total dissolved solids (TDS) and total salinity. The pH was measured in the field with a PCS Testr 35 multimeter by immersing the probe in the water taken from a 50 mL beaker by immersing the probe about 10 cm from the surface of the water, and reading digital display of pH and temperature, as well as conductivity, TDS, and salinity successively. These parameters were measured using the PCS testr multi parameter mark 35. The multiparameter’s probe has been immersed in the water about 10 cm below the surface and the reading is done on the screen of the device. The soil pH determination method consists in weighing 20 g of fine sediment screened to 2 mm, add 50 mL of demineralized water and shake for 15 minutes. Leave the suspension at rest for 30-60 minutes, immerse the pH-meter probe in the supernatant water and read to the nearest 0.1; the pH meter is calibrated beforehand. This protocol is that of ISO 1842: 1992. 2.5 Determination of organic and inorganic pollution indicators The indicators of organic pollution measured are the total organic soil material (OM), the Chemical Oxygen Demand (COD) and the Suspended Material (SM). The OM of the soils was determined by loss on ignition (Rodier, 1978) by carrying a determined quantity of sediment in the oven at 105 °C, weighing after 24 hours and then at 550 °C and weighing again. The difference in mass allows the determination of total organic matter (OM). This parameter was measured by the gravimetric method using a filter with a diameter of less than 45 μm. The Determination of the Chemical Oxygen Demand (COD) by potassium dichromate in an acid and a hot medium. Indicators of inorganic nitrogen pollution were determined by: the ammonium molybdate method for phosphate ions. The principle is based on the fact that in acidic medium and in the presence of ammonium molybdate, the phosphates give a phosphomolybdic complex which, reduced by ascorbic acid, develops a blue colour susceptible to a colorimetric dosage. the Nessler (alkaline potassium iodo-mercurate) reactive method for ammonium ions with formation of dimercuriammonium iodide which can be assayed spectrophotometrically at the wavelength of 400 to 425 nm. the sodium salicylate colorimetric method for nitrate ions by the use of sodium salicylate in the presence of nitrates which gives sodium paranitrosalicylate colored in yellow and which is capable of a colorimetric assay at the wavelength of 420 nm. the Zambelli reagent method and the sulphanilic acid in hydrochloric acid medium for nitrite ions. The complex formed with the NO2 ions is colored yellow and is capable of a colorimetric assay at the wavelength of 435 nm. Sulphur and orthophosphate pollution indicators were determined by kit-based colorimetry, except sulphate ions, which were determined gravimetrically using barium chloride. The lead residues were determined by the colorimetric method exposed by Youssao et al. (2018b). 3. Results and discussion 3.1 Evolution of the physical parameters The conductivity is 7.94 at the Middle Lake site1 (Mil1) at 392 mS/cm at the mouth of the river with an average of 105.27 mS/cm. These values are well above those of Dongui et al. (2017) in fresh water (48.2 to 98.6μS/cm) in the Aghien lagoon and Bhange et al. (2018) whose values are in the range of 149 to 348 μS/cm; but close to the results of Dimon et al. Alassane Youssao Abdou Karim et al./ Elixir Aquaculture 126 (2019) 52438-52444 52440 Table 1. Physico-chemical characterization of the environment. Sites pH Temperature (°C) Cond (mS/cm) TDS (ppm) Salt (ppm) O2 (ppm) Cal 7.7 28.6 8.22 5820 4690 8.0 Gan_m 6.6 28.1 9.65 685 486 5.0 Gan_c 6.4 28.3 292.00 208 145 1.5 Sô 6.4 28.4 392.00 277 194 3.8 Mil1 7.7 30.0 7.94 5530 4480 9.0 Mil2 8.0 29.5 10.20 8 6 7.5 Mil3 8.4 28.7 16.90 >LD >LD 12.0 Jes 8.0 29.0 >LD >LD >LD 8.0 Min 6.4 28.1 7.94 8 6 1.5 Max 8.4 30.0 392.00 5820 4690 12.0 Moy 7.4 28.8 105.27 2088 1667 6.9