Influence of suspended matters on iron and manganese presence in the Okpara Water Dam (Benin, West Africa)

Iron and manganese, which their concentrations are seasonally high in the Okpara Water Dam, have a high mobility links to suspended matters (SM). The presence of the water plants constitutes SM retention support within the water column. The anthropoid disturbances caused by the fishermen activities constitute a factor of sediments renewing in the suspension. Solid (>0.45 µm)-liquid (< 0.45 µm) fractionation has been carried out and enabled us to observe that SM are responsible for iron content for more than 10 to 98% and for Manganese content about 23 to 93% filtration removes from 10 to 98% of the colloidal iron which is related to filtrated SM. The phosphorus constitutes a combined factor of the iron mobility in the dam. We have noticed in the top water layers a high proportion (low repartition coefficient) of dissolved (and colloidals) iron respectively. The iron retained by SM is more concentrated in the middle of the water column than anywhere. The west side of the dam is identified as a manganese enrichment source. The manganese ion concentration influences positively on pH (r = 0.57), conductivity (r = 0.78), color (r = 0.66), and SM (r = 0.66) after decantation. Furthermore, the iron concentration is negatively influenced by the pH (r = -0.52) and positively by the TDS (r = 0.51) after filtration. A pre-filtration or a pre-decantation could reduce the quantity of chemicals used during water treatment.   Key words: Iron, manganese, suspended matters (SM), surface water, fractionation, mobility.