Diagnóstico de dados de concentração e de cargas de nutrientes na Bacia do Rio das Velhas

Abstract

The main topic of the research was the evaluation of environmental contamination by nutrients in the Velhas River Basin, using different databases, such as monitoring series using water quality parameters and river flows, a map of land cover and land use, inventory of sewage collection and treatment, and demographic and agricultural censuses. The topic gains relevance due to the combined application of multivariate statistical techniques and a water quality model on concentrations and loads, in different sub-basins of the study area. The work was divided into four independent and linked themes. The research began with a study of methods related to censored treatment data, which permit to estimate statistical variables in time series with values below the analytical instrumentation detection limit. This action helped in studies that aimed to spatially distribute nutrients throughout the basin, identifying critical areas and primary sources of contamination throughout the study area, using nutrient concentrations. After that, regionalization studies then made estimating average daily flows and contamination loads, at the nutrient concentration measurement points possible. Finally, a simplified water quality model was developed based on annual median loads, mass balance in sub-basin outflow, and nutrient attenuation in rivers through first-order kinetic reactions. This technique was used with permanence curves and multivariate statistical techniques to establish the specific loads in the incremental areas, the influence of contaminant permanence on the violation of legal parameters, and to compare the scenarios obtained using nutrient concentrations and loads. The use of stochastic simulations is a robust methodological approach to prepare the guideline mentioned earlier. In addition, the methodology showed that the censoring percentage and the coefficients of variation that generate the synthetic series significantly influence the estimation accuracy. Regarding the second and fourth stages, it was noticed that the environmental analysis using joint data on nutrient concentrations and loads gains robustness, as it presents distinct and complementary results. Therefore, the interconnected monitoring of water quality parameters and flow rates and the incorporation of load analysis into the legal framework for assessing the water quality, can significantly assist in water management. It was also found that the Metropolitan Region of Belo Horizonte is the most critical region of the basin. However, it was estimated that animal contamination represents 71% of the domestic contamination for nitrogen and 83% for phosphorus. In the third stage, the long-term duration curve allowed the accurate estimation of average daily flows at water quality points, except for flows with high permanence.