This research had as objective to analyze the quality of the groundwater in the neighborhood Parque Guajará, Icoaraci-Pa. Septic tanks were considered as potential sources of contamination, with emphasis on the detection of nitrogen compounds. The wells studied have depths ranging from 6 to 24 meters and capture water from the Barreiras aquifer. Four shallow groundwater samples were collected in 15 wells during the rainy season (january 2016 and may 2016) and less rainy (august 2016 and october 2016). The following physico-chemical and bacteriological parameters were analyzed: pH, chloride, sodium, electrical conductivity, nitrite, ammonium, nitrate, Escherichia coli and total coliforms. The analyzes were made at the Evandro Chagas Institute. The results were compared to the maximum values allowed by Ministry of Health Ordinance nº 2.914/11. The interpretation of the data was based on seasonal variations and isovalues maps. The bacterias Escherichia coli and total coliforms were detected in the rainy season in 33.33% and 40% of the waters, respectively. The pH ranged from 3.93 to 6.22 during the rainy season and from 3.95 to 6.42 in the less rainy season. Chloride and sodium presented maximum values of 68.46 mg/ L and 68.41 mg/ L, respectively. The electrical conductivity reached its maximum value of 527 μS/cm following the elevation of sodium and chloride values. Ammonium ranged from 0.00 mg/L to 6.01 mg/L, with maximum value in the rainy season. Nitrite was more expressive in the less rainy period, with values reaching 0,44 mg/L. Nitrate (N-NO3-) was present in all sample periods ranging from 0.91 mg/L to 64.03 mg/L. The nitrate contents found in the samples reached levels higher than allowed by legislation, being 80% and 86.67% in the rainy season (january 2016 and may 2016) and 66.67% and 80% in the less rainy season (august / 2016 and october / 2016). The nitrification process was identified in 13.33% of the sampled waters. In january / 2016 (rainy season) and august / 2016 (less rainy season) the waters presented sodium bicarbonated characteristics. To the ionic classification, in may / 2016 (rainy season) the predominance was sodium sulphated water and in october / 2016 (less rainy period), sodium chlorated water. In the research area, the high levels of nitrate in shallow groundwater were attributed to the presence of septic tanks, the absence of technical criteria for the construction of
wells and the circulation of domestic animals around the wells of water catchment for human consumption.
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