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Keywords: Água subterrânea;  Caracterização hidrogeoquímica;  Isótopos estáveis;  Contaminação;  Groundwater;  Hydrogeochemistry characterization;  Stable isotopes;  Contamination
Issue Date: 31-May-2012
Abstract: The Macacu sedimentary basin, located at the northeast of Guanabara Bay, embodies one of the three extended continental basins with sediments thickness reduced in Rio de Janeiro state. The basin is formed by aluvionary deposits, lacustrine and marine tertiaryquaternary sediments which can exceed 200 meters depth. The main clay minerals verified in these sediments consist in caulinita, illite and illite-smectite formed by weathering and erosion of the rocks in the basement composed by gnaiss and alkaline rocks of the Tanguá Alkaline Complex. The clay minerals, as well as the micas and feldspars weathered and deposited in these sediments, have a fundamental role in the physical-chemical composition of the groundwaters in this region, in which three sedimentary units hydrostratigraphic were studied: Aquifer System of Macacu, Aquifer System Alluvial-lacustrine and Fluvial-marine System. The hydrogeochemistry characterization of aquifers was based in five wells of multi-level monitoring. Between August/2009 and August/2010, it was determined the concentration of the dissolved ions Na+, K+, Mg+2, Ca+2, Cl-, SO4 -2, HCO3. Moreover, it was also determined the concentration of the metals Al, Ag, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sb, Zn, Fe, Mn and Ba, and of other elements such as As, B and F, H2S, SiO2 in addition to nutrients as N-NH4, NNO2, N-NO3 and P-PO4. The hydrochemical types for groundwaters were verified as calcium sodium bicarbonate faces integrating the Aquifer System of Macacu. For the alluviallacustrine, they belong to sodium-bicarbonate, calcium-chloride and sodium-chloride faces and for Fluvial-marine system they belongs to sodium-chloride faces. Besides hydraulic connections among these aquifer systems and the distribution of sediments in a restrict area, since Macacu sedimentary basin covers an area of 375 m2, its groundwater present distinct salinity, which reflect a diverse geochemical behavior. The source of waters salinity was analyzed through hydrodynamic data, rC1/Br ratio associated to chloride concentrations and isotopic ratios (18O e 2H). Based in the results, the highest salinity level was found in the Fluvial-marine system and reflects the influence of saltwater intrusion. On the other hand, the salinity of the Alluvial-lacustrine aquifer would be originated by salt leaching of paludial tertiary sediments, whereas the low levels of salinity in Macacu aquifer system would be directly associated to leaching of the ions from foundation rocks and overlying sediments. The results showed concentrations above the reference standard (CONAMA 420/2009) for at least one groundwater sample regarding to one of the following elements or substances: Cd, Mo, Sb, As, B, Fe, Mn, N-NO2, Pb, F, Ba and H2S. Two of these dissolved ions (Ba and F) were analyzed due to their occurrence in the geological material of the region. It was verified that Ba concentrations are higher in shallow aquifers and are subordinated to reduced environments. The variations in the watershed are responsible for sulfide oxidation in aquifer systems leading to sulphate availability in groundwater, which may associates to the absorbed barium from feldspars forming barite. This sulphate can be reduced again and precipitate as mackinawita, releasing the ion Ba2+ to groundwater. The highest fluoride concentrations were found in deeper aquifers close to Caceribu River, having weathering of clay minerals and micas deposited in the basin as its main source. The use of groundwater by the rural population of Itaboraí without the environmental diagnosis of the collected waters may be harmful to people s health, especially due to natural incidents and the availability of hydrochemical elements such as Ba, F-.
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