The Guajará Bay is the main water body that receives sewage from Belém's official network, receiving a daily volume of 18,294 m3/day, which is discharged in natura from the Una channel. This work aims to evaluate the influence of the release of this sewage on the quality of the sediments on the edge of Guajará Bay. For the purpose of comparison, two areas were chosen: one more impacted, located between the Una channel and the district of Icoaraci (Area 1) and another, less affected by the release of sewage, located near the island of Cumbu (Area 2). Sediments were collected at 18 points, 12 in Area 1 and 6 in Area 2. Mineralogical compositions were determined by X-ray diffraction and the concentration of metals in the total sample and in the soluble fraction was studied. The following metals: Cu, Cr, Ni, Pb, Zn, Mn and Fe. Concentrations were determined by atomic absorption spectrometry. The results show that the studied sediments are clayey and constituted mainly by kaolinite, illite and smectite. In Area 1, they present an average of 2.6% by weight of organic matter, with average concentrations of carbon and organic nitrogen of 1.51% and 0.08%, respectively. The concentration of total phosphorus shows an average of 216 mg/kg and as the sampled points move away from the Val de Cans channel, it shows a strong decrease (308 mg/kg to 132 mg/kg). The metals in the total fraction showed the following concentrations: Cu (16 mg/kg), Cr (66 mg/kg), Ni (24 mg/kg), Pb (59 mg/kg), Zn (70 mg/kg) , Mn (531 mg/kg) and Fe (37,181 mg/kg). Regarding the remobilizable fraction, the percentages observed were as follows: Cu (50%), Cr (15%), Ni (17%), Pb (51%), Zn (71%), Mn (83%), and Fe (47%). In Area 2, the average concentration of organic carbon was 0.43% by weight and that of organic nitrogen was 0.1% by weight. Organic matter reached a value of 0.74% by weight and organic phosphorus 495 mg/kg. Regarding metals, the average concentration observed was: Cu (13 mg/kg), Cr (36 mg/kg), Ni (35 mg/kg), Pb (55 mg/kg), Zn (33 mg/kg), Mn (214 mg/kg) and Fe (17,029 mg/kg). In the rewoundable fraction, the metals presented the following percentage in relation to the total content: Cu (41%), Cr (12%), Ni (32%), Pb (32%), Zn (73%), Mn (67 %) and Fe (70%). The concentrations of metals found in Area 2 were lower than those observed in Area 1, with the exception of phosphorus. The results confirm the strong anthropogenic influence on the quality of sediments in the Guajará estuary.
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