Modelagem deterministica e probabilistica da qualidade das águas superficiais utilizando o QUAL-UFMG: uma ferramenta para gestão de recursos hídricos - estudo de caso em regiões de cabeceira da bacia hidrográfica do Rio Grande

Abstract

The objective of carrying out this work was to evaluate the use of a mathematical modeling methodology for the quality of surface water, deterministic and probabilistic, applicable as a water resource management tool in decision-making for the allocation of resources and definition of uses in any watersheds. For this purpose, the GD-2 hydrographic basin (Rio Grande slopes), located in the state of Minas Gerais, Brazil, was used as a case study. The discharges of sanitary sewage from the urban centers of 30 municipalities were considered, using the Qual-UFMG model, together with Monte Carlo simulations, allowing simulations of different conditions to be carried out. Using the variables biochemical oxygen demand (BOD), dissolved oxygen (DO), thermotolerant coliforms, nitrogen and phosphorus, 4 scenarios were considered: current scenario (C-01); trend for the year 2033 (C-02), considering the population growth in the municipalities; compliance with the new Sanitation Legal Framework in 2033 (C-03), with 90% of sewage collection and treatment; and compliance with the framework proposed for the basin (C-04). In C-01 and C-02, the condition of the basin was systematically outside the framework limits for the variables phosphorus and thermotolerant coliforms. As for BOD and DO, the situation is better, with the “nonattendance” stretches being restricted to the headwaters of streams where the dilution capacity is reduced, almost similar situation for ammoniacal nitrogen. In C-03, in which the Sanitation Legal Framework was applied, the situation eases, but it is still not enough to meet the framework and, consequently, the preponderant uses foreseen. The variables phosphorus and thermotolerant coliforms remained with values outside the framework limits in most of the basin, while the same was observed for BOD and DO in the headwaters. In C-04, the treatment efficiencies necessary for the proposed framework to be achieved were defined. Tertiary treatment, with disinfection and removal of phosphorus, was necessary in most of the basin, it is recommended to define medium and long-term progressive targets for the launch pattern for these variables. As for BOD and DO, the concern would remain only for the stretches where the effluent dilution ratio is low, similar case for N, with the release standard limit of 20 mg L-1 being sufficient for most cases. In general, the proposed methodology proved to be an adequate and applicable tool in the management of water resources, helping to prioritize and allocate resources. The obligations foreseen in the environmental legislation proved to be insufficient to comply with the framework at the headwaters of watercourses and often exaggerated in the lower reaches, where the dilution ratio is high. Therefore, the definition of temporary goals or the revision of some implemented ones may be necessary, as well as raising the rates of collection and treatment of sewage in the basin.