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.
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