Assessment of water quality degradation in the Carlos Botelho Reservoir (Broa) (Itirapina, São Paulo State, Brazil), using Sentinel 2 satellite images

ABSTRACT

This study evaluated the water quality of Carlos Botelho reservoir, São Paulo, Brazil, using the Environmental Company of the São Paulo State (Companhia Ambiental do Estado de São Paulo) database for the years 2017 to 2019. Chlorophyll concentrations at and the number of cyanobacteria cells, relating them to the image processing algorithm of the European Space Agency. Images from European Space Agency’s Sentinel 2 satellites were used, with a spatial resolution of 10 m and with Case 2 Regional Coast Color atmospheric correction. Chlorophyll concentrations at data from Companhia Ambiental do Estado de São Paulo and those estimated by the algorithm showed high correlation (R²=0.6052, Normalized Root Mean Square Error=27%, n=8) and were also statistically related to cyanobacteria cells number, which allowed estimating cyanobacteria cells in the number Sentinel 2 images, by means of chlorophyll concentrations. Based on the estimates, Carlos Botelho reservoir showed a eutrophic pattern in the year of 2020; the data suggest the presence and substantial predominance of cyanobacteria in the reservoir in 2020 (a range of 10,000–20,000 cells.mL); in your water, the presence of potentially toxic cyanotoxins has already been found; the reservoir has a public beach, for primary contact recreation; and there are condominiums with houses, clubs, and farms in the surroundings. This set of characteristics should serve as a warning to the local authorities, as there is a need to discuss the monitoring program now underway, with an indication of reducing the monitoring to a minimum of monthly and complementary evaluations, for periodic verification of the presence of cyanotoxins.

Keywords:
cyanobacteria; trophic state index; Lobo; remote sensing