Qualidade das águas subterrâneas rasas do aquífero Barreiras: estudo de caso em Benevides - PA

Abstract

The objective of this research was to study the groundwater quality of the free aquifer Barreiras, by means of hydrochemical characterization and classification of the chemical type of groundwater, hypothetically polluted by anthropic activities, at the municipal headquarters of Benevides - PA. The groundwater of the municipality of Benevides is an important natural resource both for domestic supply and for industrial and commercial use. The material and methods used for the development of this work consisted of: a bibliographical survey on the local and regional geology of the Amazon Basin and regional aquifers; organization and consistency of data available in the SIAGAS (Groundwater Information System) project; treatment and analysis of existing well information (definition of lithological and hydrogeological standards); registration of 43 new wells, of which 30 wells were used as monitoring wells; and systematic physico-chemical analysis of shallow groundwater during the months of March, June, August, October and December 2017. The analytical techniques adopted consist of physical-chemical measurements in situ, with the use of (pH, temperature, electrical conductivity, OD, total dissolved solids, turbidity, fluoride, total iron, manganese, aluminum, barium, lead, copper, cobalt, chromium, cadmium, nickel, zinc, NaCl , Ca+2, Na+, K+, Mg+2, Cl-, SO4 -2, CO3 -, HCO3 -, NH4 + e NO3 -), which were carried out at the Evandro Chagas Institute. For the hydrochemical characterization of the waters, maps of isoteores of the elements and graphs of seasonal variations were elaborated, besides the analysis of the diagram of Piper for classification of groundwater. For the evaluation of water for human consumption was used Ordinance Nº 2914/2011, Ministry of Health. The pH ranged from 4.33 to 5.84 during the rainy season and 4.48 to 5.96 in the least rainy period. The static level variation was 0.8 to 20.4 meters in the rainy season and 1.7 to 25.34 in the less rainy period. Aluminum and lead presented maximum values of 0.515 mg/L and 0.02 mg/L, respectively. The electrical conductivity reached its maximum value of 195.8 μS/cm, accompanying the elevation of sodium and chloride values. Ammonium values were more significant during the rainy season, with an average value of 0.175 mg/L. The highest total iron concentration index was 1.28 mg/L. The nitrate levels found in the samples reached indices higher than allowed by the legislation in half of the samples analyzed. The hydrochemical characterization evidences a character of chlorinated sodium nature. The high concentrations of nitrate are related to anthropic activities originated by the absence of local basic sanitation and characterized by the discharge of domestic sewage and the use of black cesspits, near most wells.