Water is one of the essential natural resources for life on Earth, being fundamental for the development
of biochemical and physiological processes. The general objective of the research is to evaluate the
current conditions of the water quality of the São João stream in the municipality of Porto Nacional -
TO, through the determination of physical, chemical and biological parameters. Four water samples
were collected, one during the dry period (July 2021), the second at the end of the dry season (October
2021), the third collection in the wet period (December 2021), and the fourth in end of the wet period
(May 2022), in the region of Porto Nacional. Physical-chemical parameters (temperature, conductivity,
turbidity, dissolved oxygen, phosphorus concentration, ammonia concentration and pH) and biological
parameters (chlorophyll-α concentration and heterotrophic bacteria count) were evaluated. Data from
each collection were compared using ANOVA and the Tukey test (α = 0.05) in the Sisvar® statistical
program. The results of the parameters analyzed in the study, turbidity showed an average variation of
567.50 UNT, at point 7 in collection III. Resolution 357 of 2005 CONAMA, establishes the permissible
limit of 100 UNT for freshwater class II rivers. Dissolved oxygen showed values below those allowed
by CONAMA resolution. The pH in the third collection values indicated acidity (below 6) and alkalinity
(above 7), CONAMA in resolution 357/2005 assigned limits from 6 to 9 for class II waters. As well as
the concentration of chlorophyll-α, values lower than the permissible limit per CONAMA resolution
were obtained, being up to 30 μg/L. The Principal Component Analysis (PCA) of the four collections
showed that collection III, which marks the beginning of the wet period in the region, remained
independent of the others. Conclusions: The turbidity variable (collection III) has values above that
allowed by CONAMA resolution 357/2005, this high turbidity value is possibly related to seasonality,
as it occurred in the rainy season, when the presence of suspended sediments in the water is very
common. The other variables of the study are within the limits allowed by current legislation. Thus,
monitoring the water from the supply sources, which is the case of the São João stream, the main source
that supplies the city, is of paramount importance.
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