Eutrofização em reservatórios tropicais semiáridos: Fontes de fósforo e subsídios à mitigação

Abstract

Eutrophication is recognized as one of the main problems that cause the deterioration of aquatic ecosystems, resulting in economic and biodiversity losses. The main implication of the eutrophication process is the occurrence of cyanobacterial blooms, which can produce toxins that have direct effects on human health and cause an increase in water treatment costs. In semi-arid regions, long periods of drought occur, favoring the concentration of nutrients and cyanobacterial blooms in water sources, and compromising the quality of water for human consumption. Within this problem, phosphorus (P) deserves to be highlighted, as it is one of the main actors in the eutrophication process. Many efforts have been made to reduce the loads of this nutrient in water sources, both from external sources, resulting from the contribution of the use and occupation of the hydrographic basin, as well as internal sources, such as the release of P from the sediment. In addition, the determination of the P forms present in the sediment is a key factor to predict the active release mechanism in the system. Thus, the general objective of this thesis was to identify and quantify the external and internal sources of phosphorus in springs in the Brazilian semiarid region, in order to subsidize the mitigation of eutrophication. In order to identify external sources, mainly diffuse sources of pollution, we evaluated the use and land cover of different sub-basins and their potential influence on the water quality of semi-arid tropical reservoirs. The hypothesis raised was that sub-basins with a higher percentage of exposed soil, urban area and sparse vegetation are more susceptible to eutrophication due to the inflow of alien nutrients, with higher values of nutrients, solids and biomass. Our results refuted the hypothesis. The Epitácio Pessoa reservoir showed low concentrations of nutrients, solids and chlorophyll despite the greatest evidence of exposed soil and agricultural activity in the Alto Paraíba (PB) sub-basin. In contrast, the Boqueirão de Parelhas reservoir showed higher concentrations of nutrients, solids and biomass, despite the sub-basin of the Seridó River (RN) having the highest percentages of dense vegetation. Therefore, the results of this study suggest that sources other than diffuse sources may be responsible for the intensification of eutrophication in Boqueirão de Parelhas. To investigate this gap, we evaluated the potential for internal phosphorus fertilization of the Boqueirão de Parelhas reservoir on an experimental scale. For this we quantify the phosphorus fractions of the sediment and the release rate. The results showed that warmer and alkaline environments, such as those in the semiarid region, are susceptible to the release of P linked to aluminum from sediments. This work also showed that the levels of P in the sediment before and after the experiment were not significantly different because the amount of P in the sediment is much greater than the released P. But the amount of P released is already high for water, emphasizing the importance of sediment as a source of nutrients that feed the eutrophication process. Therefore, as a suggestion for mitigating eutrophication, the application of the geoengineering technique, on a laboratory scale, was evaluated in order to verify the efficiency of the application of coagulants and clays, isolated and combined, in the control of the internal fertilization of P of a eutrophic reservoir in tropical semiarid region. The isolated application of coagulants has an effective action on sedimentation of cyanobacteria and total phosphorus in the water of an eutrophic environment. To inactivate phosphorus in the sediment or even minimize its flow, a material with a high phosphorus adsorption capacity, such as lanthanum modified bentonite (BML), is required. Therefore, for a complete effect in the control of eutrophication, the joint application of coagulant and clay (Floc & Lock) is necessary.