Influência do uso e ocupação do solo na qualidade da água do reservatório da barragem Santa Bárbara, Pelotas/RS

Abstract

The indiscriminate and non-rationalized use of water resources has been causing degradation of quality, threatening availability, hindering treatment and increasing risks to human health due to consumption. Therefore, it is essential to ensure the promotion of adequate conditions, especially for use as a source of public supply. Therefore, the objective of this study was to evaluate the influence of land use and occupation on the water quality of the Santa Bárbara dam reservoir, Pelotas/RS, and to investigate the occurrence of Ceratium furcoides blooms using orbital remote sensing. To this end, the following steps were performed: characterization of the use and occupation of the drainage area containing the reservoir through orbital remote sensing; evaluation of physical-chemical parameters of water quality in the reservoir and its main tributaries (data from 2016 to 2022); analysis of data on Chlorophyll-a concentration in the reservoir (available from 2010 to 2022) associated with local records of rainfall, level and phytoplankton count from 2016 to 2022, with an emphasis on identifying facilitators of the occurrence of Ceratium furcoides blooms; perform classical decomposition of Time Series of Chlorophyll-a concentrations and the Normalized Difference Chlorophyll-a Index (NDCI) from 2016 to 2024 and, after temporal pairing, generation of a predictive equation for Chlorophyll-a from the NDCI. The areas drained by the Sanga da Barbuda and Santa Terezinha tributaries were identified as the most anthropized regions (agricultural practices and urbanization), whose water quality analysis revealed them as having the highest nutrient contents, highlighting them as priorities in the management of the sanitary conditions of the contributions to the reservoir. Despite this, the Passo do Cunha tributary was revealed as the most important predictor of the reservoir's Total Phosphorus concentration, showing the need to preserve its drainage area to maintain the existing good dilution conditions. The occurrence of Ceratium furcoides blooms was associated, in addition to the biotic and abiotic factors favorable to phytoplankton development in general, with disturbances caused by operational interventions necessary in periods of intense drought. The analysis of the time series revealed similarity in the temporal behavior of Chlorophyll-a concentration and NDCI, even in the presence of Ceratium furcoides, demonstrating that NDCI can be used in the remote detection of Chlorophyll-a in environments subject to this organism in the absence of a specific hyperspectral index for it. This similarity supported the development of a predictive equation for Chlorophyll-a from the NDCI to aid in the hydrobiological monitoring of the reservoir, which had the best fit (R2=0.64) through second-order polynomial regression. Although Ceratium furcoides is not generally associated with toxicity, the changes it causes in the dynamics of aquatic ecosystems suggest that it should be evaluated with more robust techniques in future research.