The study evaluates the urban impacts in the groundwater quality of Barreiras aquifer, caused mainly by nitrogen compounds (nitrate and ammonium) from domestic sewage and aromatical composites (benzene, tolueno, ethylbenzeno and xylenes - BTEX), from underground fuel storage tanks of gasoline and diesel. The study was carried out on an area of 15 Km2, in the quarters of the Reduto, Nazaré and Umarizal in city of Belém/PA. The used methods incluided a cadastre of wells, analyses and stratigraphic well log correlations, execution of 9 periods of sampling, chemical analysis of water from and interpretation of results. The unsaturated zone, in the study area, has thickness varying of I to 9m, being conditional to the seasonals variability. Mineralogical studies of sediments collected in drill rig wells had defined a domain of quartz and, of a more subordinated form, caulinite. The grain sized analysis show a predominance of the fractions silte sand, silte arenaceous and silte clay. Test of infiltration indicated value corresponding to a material silte to silte arenaceous. Sediments with these textures, associates to the varied infiltration capacity, mineralogic composition, water levei flat and raised load of contaminants, of mainly septic and casspools, can lead to the contamination of the aquifers of the area. The aquifers ones identified are correlated to the aquifers of the Barreiras Group, having been defined two main ones. First, the most superficial one, was classified as unconfined-leaky, and as the second aquifer one was characterized as confined-leaky. The water level, by diverse reasons, had not been measured during the execution of this work. In this case, the presented measures correspond to those done ones during the time of drilling of the wells, in which had varied of less of lm, in the areas of lower topographical quotas, until 11m in the wells located in the more raised paris. Vulnerability data had established mainly that, to a large extent of the area, the Barreiras Aquifer presents contamination possibility, considering that the studied area is urban and, consequently, concentrate some potentially polluting activities. The interpretation of the directions of groundwater flows, of the studied area, shows that the main lines of preferential flow are guided for the center of the area, containning the quarter of the Reduto and part of the Nazaré. The global behavior of the variables shows that the hydrogeochemical characteristics of studied waters are differentiated, when compared with the regional standard. The main parameters that had marked this differentiation had been the electric conductivity, chloride, sodium, pH, ammonium and nitrate. It is standed out that the grades of ammonium and nitrate are above or very Glose to the limit to potability standards, and that such parameters are the main contamination indicators of waters by effluent domestic sewage. The global analysis showed for nitrate a positive correlations with electric conductivity and a weak correlation with chloride, sodium, calcium, sulphate and hardness. For ammonium one observes a positive correlation with chloride and negative correlation with calcium, sulphate, pH and hardness. This study it allowed to indicate the conductivity, chloride and calcium as good pointers of the contaminations. The effected factorial analyses had come to confirm the main comments gotten in the study of correlations. The statistical study by period of sampling, they indicate that the regional seasonality is influencing only in secondary way in the chemical behavior the studied waters. The slight enrichment, of some components, in the rainy period can be explained because, in this period, the groundwater level is raised until next to the ground , being next to the contamination sources. In the study of correlation by period of sampling, the results can be considered, without exception, similars to the gotten ones in the analysis of correlation made with the global data. The statistical analysis for a series of wells had indicated that the wells, when analyzed individually, they had shown a relatively homogeneous behavior along the periods, but when compared between itself, it was possible to distinguish wells with values always raised from those with the raised values less. In the study of the behaviors for nitrate and ammonium, it was observed that the wells, when analyzed individually, they had homogeneous behaviors. On the other hand, the joint study of the wells it allowed to the classification of 4 main styles of behavior for nitrate and ammonium. Style 1 would correspond the wells with high values of nitrate and low values of ammonium; style 2 would be characterized by high values to nitrate and ammonium; style 3 would correspond the wells with low to intermediate concentrations the nitrate and high the relatively high ammonium grades and, finally, style 4 would be translated by lower grades for ammonium, followed for intermediate nitrate grades. These styles can mean greater or minor distance of active sources of contamination, that would be, therefore, the preponderant factors in the hydrogeochemical characterization of the wells in an urban area as the studied one. In two wells a more detailed water sampling was effected. These samplings had evidenced, one more time, to little influence of the seasonality in the hydrochemical characterization of the studied wells. To identify the main types of waters, representative samples of the registered in cadastre wells had been located in the diagrams of Piper. The results had shown a trend, so that, the samples, independent of the period of sampling, to be concentrated in calcium chloride sulfate, cl sodium chloride and sodium nitrate cloride hydrochemical facies. These behaviors reflect that they are being strong affected for antropogenic factors, in detriment of natural processes. The analysis of the space distribution, of the main parameters, showed that, for nitrate, the more raised grades are located in the central and south portion of the area. The ammonium presented an inverse relation to the one of nitrate, with more raised grade the northwest and northeast of the area, diminishing for the center of the area. The map of electric conductivity coincides, in good part, with the one of nitrate, while that the chloride iso-grade map is, in a general, similar way to the one it ammonium. The iso-grade map of pH did not show any trend of correlation with nitrate. These correspondences had coincided with the comments of the studies of correlations. In the study of water contamination by gasoline/diesel, three critical areas were defined. The study finally sugests that, in regard to water samples with qualitative identification of diesel and or gasoline, the main biodegradation process, bisede the aerobic one, seems to be of nitrate one.
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