Variações sazonais dos componentes nitrogenados, em aquífero livre na zona urbana de Santa Izabel do Pará, nordeste do estado do Pará

Abstract

The work was carried out in the city of Santa Izabel do Pará, whose population is estimated at 45,000 inhabitants, supplied mainly by shallow hollowed and tubular wells, with depths between 1.50 and 20 m. It is estimated that about 15% of the total urban area is covered by sewage system. In the area, there are confined aquifers of the Pirabas Formation and free aquifers of the Post-Barreiras and Recent Sediments. The main objective of the study was to evaluate the variations of nitrogen components (NH4+ and NO3-) in the water of the Post-Barreiras free aquifer, considering the sanitary conditions of the area, as well as the effects of seasonality and local hydrogeological characteristics. The presence of fecal and total coliforms was also evaluated. Initially, pre-existing physical data and physical-chemical analysis, obtained by CPRM in 1994, in 33 registered wells (PIH Project) were used. The treatment of this information allowed the elaboration of maps: of natural vulnerability of free aquifers; of nitrate isotopes and potentiometric surface. Of these 33 wells, two sets were selected for this study: one located in the old area and the other in the expansion area of ​​the city. Free aquifer water was collected in the dry (November 1999) and rainy (April 2000) periods. The analyzes for ammonium and nitrate, as well as bacteriological analyzes were carried out in the laboratory of the Instituto Evandro Chagas. The sediments of the unsaturated zone, classified as medium and coarse sand, have hydraulic conductivity between 10-3 and 10-4 cm/s, with flow velocity varying between 0.80 (old zone) and 5 m/day (old zone) expansion), indicating greater soil compaction in the old zone. Based on these velocities and on the depth of the water table close to the infiltration test sites, in the old 9.02 m and 3.70 m expansion zones, it appears that in the latter, conservative contaminants (such as nitrate) from pits and /or the ground surface would take about eighteen hours to reach the water table. On the other hand, in the old part of the city, the time required for these contaminants to reach the aquifer would be on the order of eleven days. Post-Barreiras free aquifers have hydraulic conductivity K = 8.30 x 10-4 cm/s, total porosity between 0.18 and 0.21 and flow velocity of 0.03 m/day. Considering the efficient transit times in the degradation of pathogens (210 days), the protection radius of the wells that capture the free aquifer must be at least 6.30 m. However, in the case of contaminants such as nitrate, dilution and denitrification factors must be analyzed in order to protect public health. With the exception of one well, it is observed that in the expansion zone, which is relatively new and with a lower population concentration, the wells in both samples (1994 and 1999) show significantly lower nitrate values, ranging from 4.5 to 13.5 mg /L, recorded in the rainy season. On the other hand, in the old zone, the levels are between 12.8 and 29.9 mg/L, referring to the same period, suggesting a tendency to increase the levels of nitrate in the waters over time. Ammonium levels were higher in the rainy season, with the highest levels reaching 0.710 and 1.520 mg/L obtained in the old zone. In some wells, the increase in nitrate during the dry period was accompanied by a reduction in the ammonium content, suggesting oxidation reactions of NH4+, giving rise to NO3- (nitrification). Based on the correction coefficient between the electrical conductivity parameters and the nitrate content, there was an increase in the area contaminated by nitrate between 1994 and 2001, mainly in the old zone. Most groundwater records the presence of coliforms with maximum and total fecal coliform values ​​of 1,040 and 22,600 MPN/100 mL, respectively. It is observed that there is an increase in ammonium from the dry to the rainy season, at the same time that there is an increase in fecal and total coliforms. These data suggest that, during the rainy season, the transport of ammonium, fecal and total coliforms to groundwater is more efficient. Free aquifer vulnerability indices range from moderate to extreme. The presence of ammonium in water at values ​​above 0.06 mg/L shows a strong indication of contamination by sewage, garbage, or other waste rich in organic matter. In some wells, nitrate concentrations are close to or above that established by potability standards, which is 45 mg/L. The correlation between electrical conductivity (E.C.) and nitrate levels in the water table suggests that about 45% of the city of Santa Izabel do Pará has free aquifer water that is unsuitable for human consumption. The Pirabas Formation aquifers are the best alternative for drinking water.