Estudo hidrogeológico da área localizada entre o depósito de lixo metropolitano de Belém (Áura) e o Lago Água Preta

Abstract

Belem, the biggest city of the Amazonia, with an estimated population of 1.280.614 habitants, directs all types of residues produced in the urban area to the Metropolitan Waste Deposit, which was inaugurated in 1991 and is located in Santana do Aura. Currently, approximately 1000 tons of solid residues are deposited daily, turning it a local permanent source of pollution. The fact that the Agua Preta (Black Water) Lake – main source of water supply for the population of the Metropolitan Region of Belem – is located at 1.4 km west of the Residues Deposit of Aura, has raised speculation on the possible of contamination as a result of the proximity between those two locations. The main goal of the present study is the hydrogeologic characterization of the area located between the Metropolitan Waste Deposit of Belem (Aura) and the Agua Preta Lake, by correlating geophysical, hydrogeologic, and geochemical data, in order to determine the influence of the deriving contaminants of the deposit of residues on the underground water resources of the studied area, and also evaluate the possibility of their influence in the water sources of Belem. Electromagnetic geophysical measurements (EM) carried through, showed the presence of clayed layers and of the shallow water table. The results achieved through this indirect method allowed the identification of anomalous values (high conductivity) in some locations that were later checked through the installation of monitoring wells and geochemical analysis of water samples. In the hydrogeologic analysis of the area, a hydraulic gradient of approximately 0.011, effective porosity values 28%, hydraulic conductivity between 4,01 x 10-3 and 3,16 x 10-4 cm/s were obtained showing that the underground flows run at approximately 20.03 m/year, in the east-west and east-south directions. The predominant aquifers in the area are of confined type, protected by layers of clayed material with thickness ranging from 4 to 17 meters and are associated to Barreiras Group and to the Pos-Barreiras unity. The chemical analysis obtained for leachate tracers (pH, electric conductivity, alkalinity, ammonia, nitrate, chloride, sodium, calcium, magnesium and potassium) carried out in groundwater samples collected from monitoring wells have shown environmental characteristics of the water resources in the area, not evidencing contamination due to leachate. The analysis of the geochemical parameters together with the underground flows indicates that in spite of the flow be towards the Agua Preta lake, we cannot affirm that this actually happens because besides the analytical results do not show evidence of water resources pollution, other wells and mainly the narrow Santo Antonio river (located between the deposit and the lake) act as a draining system for the superficial and underground flows in the area. The narrow river would act as a natural and protective barrier, diminishing the possibility of the aquifers located more west in the studied area be reached by superficial and underground flows of leachate generated in the waste deposit.