Lake Água Preta is the main water reservoir, which together with Lake Bolonha, constitute the surface water sources for public supply in the city of Belém. Both reservoirs are dams fed by small drainages and water pumped from the Guamá River. These springs are vulnerable to the release of domestic and industrial effluents due to the disorderly urban occupation in their surroundings and their proximity to the Aurá sanitary landfill. In this work, the historical record of the isotopic composition of lead and the concentration of heavy metals in sediment cores from the Água Preta lake was investigated. Different analytical and spectroanalytical methods such as gravimetry, titrimetry, ion exchange chromatography, atomic absorption spectrometry and mass spectrometry were used to obtain the results. In the chemical separation of lead, specific Sr resin (Sr-Spec) was used in Teflon columns, adapting the methodology of Gale (1996). Through preliminary sampling of bottom sediments along the entire length of the Água Preta lake, the distribution of the isotopic ratios of lead and heavy metals was recognized. The lowest values of the 206Pb/207Pb ratio and the highest levels of heavy metals were identified in the lake ramifications close to the urban occupation zones. In this part of the lake, a 77.5 cm column of sediments, called profile 01, was sampled. The highest values of the 206Pb/207Pb ratio were found near the lake dam, which is constantly influenced by the water pumped from the Guamá River; where profile 02 was witnessed with approximately 110 cm of sediment. The 206Pb/207Pb isotopic ratio values were basically grouped into four segments in profile 01 and in three segments in profile 02. -1.20431 in profile 02, refer to the regional isotopic signature. The 206Pb/207Pb isotopic ratio with values varying between 1.19346-1.19602 in profile 02 correspond to the isotopic signature from the influence of suspended sediments in the water of the Guamá River pumped to the lake and whose 206Pb/207Pb ratio is 1. 19225. The isotopic ratios 206Pb/207Pb varying between 1.16223-1.17621 found only in profile 01, correspond to the anthropogenic contribution. The higher averages of heavy metal concentrations in the upper segment of profile 01 justify the interpretation that the lower values for the 206Pb/207Pb ratio are characteristics of anthropogenic contribution. The average levels of heavy metals, found at the top of profile 02, probably reflect the contribution of suspended sediments from the Guamá River to the Água Preta lake, evidenced by the 206Pb/207Pb ratio. The lower values of the average concentrations of the metals found at the base of two profiles reinforce the interpretation that the higher values of the 206Pb/207Pb ratio reflect a regional contribution, prior to anthropogenic action, basically given by the rocks in the area.
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