Hidrogeoquimica e a contaminação por nitrato em agua subterranea no bairro Piranema, Seropedica - RJ / Geochemistry and the nitrate contamination of groundwater in Piranema neighborhood, Seropedica-RJ, Brazil

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

Nitrate is found in waters, vegetable and animal tissues. Due to man activities, the concentration of nitrate in waters has increased in the last decades and a maximum allowed value for drinkable water was established by regulatory agencies. A common source of aquifers contamination by the nitrate is the use of in situ sanitation systems, like as cesspits and ditches. This work has been developed in Piranema neighborhood, between Itaguaí and Seropédica municipalities , in the state of Rio de Janeiro, where also the Piranema aquifer is situated. The main purposes of the study were to determine the spatial and temporal variations of nitrate concentration in the Piranema aquifer and the consequent change in the groundwater s composition and quality. Fourteen shallow wells (untill5 meters deep) of houses in the neighborhood were monitored for one year, with the monthly measurement of physical-chemical parameters (pH, Eh, temperature, conductivity) and the chemical analysis of water samples in three distinct samplings.. Water samples from five wells of the first sampling had nitrate concentrations over 45mg/L, the Maximum Allowed Value (VMP), while in the two other samplings, on1y two wells exceeded this limit. In samples of some wells, nitrite and ammonium were also detected. It was possible to associate the nitrate concentrations with the seasonal potentiometric leveI and the direction of the groundwater flow , being concentrated in the discharge region of the local aquiferflow. Some wells systematically presented higher and/or lower values of pH, Eh, conductivity and temperature parameters, while others did not appear to have any kind of tendency to lower and/or higher values, but no relation between those parameters and the hydrochemistry could be established. The sediments, constituted by quartz, feldspars, and layers of c1ay and sand and the Piranema aquifer dynamics have very little influence in the sodiumchloride composition of the groundwater, which mainly comes from oceanic rain water. The distribution of species was calculated by geochemical modeling, which confirmed that the main cations (Na+, Ca2+, Mg2+ e K+) are present in their free form. On1y a small part reacts with HCO3 - and sol- to form complexes. The anions are present as free ions (Cr) and in the respective oxidized forms (HCO3-, NO3 - e SO42-) The presence of NH4+ + in some wells indicates that the contamination might be recent, not having passed enough time for its oxidation

ASSUNTO(S)

aguas subterraneas nitrato

ACESSO AO ARTIGO

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