Avaliação do transporte de nutrientes e sedimentos em suspensão na bacia hidrográfica do Alto Rio Pardo, sul de Minas Gerais

Abstract

The river waters chemical composition is influenced by the physical, chemical and biological factors that occurs in the hydrographic basin, as well by the anthropic factors interferences. The river forms the basin main drainage, therefore, integrates all the phenomena occurring the mountain slopes, and the consequences of natural and anthropic processes at the nutrients and sediments that can be reflected in downstream processing, in the decrease of runoff sections and silting reservoirs. In this context, the present paper work aimed to evaluate the nutrients and sediments suspension transport in Alto Rio Pardo hydrographic basin (BHARP) upstream Caconde Reservoir, Caconde-SP. The river waters were monitored once a month during May 2018 to April 2019. BHARP was physiographically and morphometrically characterized from the Rio Pardo source to the sampling point, located 44 km to the Caconde Reservoir, in 1,627.05 km2 total area. The physiographic characterization results demonstrated predominance of monzonite (21.33%) and orthogneiss (19.88%) rocks, Red-Yellow Argisol (43.46%) and Cambisol Háplico (37.12%) soils, agricultural cultures (39.40%), followed by native and planted forest (18.81%) of predominance areas. BHARP average altitude is 1,188m, with undulating relief predominance (57.41%). The morphometric characterization results demonstrated 3,229 km of water courses study area, well drainage capacity (Dd = 1.98 km/km2 ) and intense dissection. The annual flow average was 33.97 m3 /s and the specific flow 20.88 l/s/km2 . The rainfall average according methods arithmetic was 1,740.94 mm and Thiessen 1,763.47 mm, was higher than the historical pluviometric averaged for the 1942 to 2018 period (1,457.78 mm). The dry season occured from May/18 to September/18 with deficits in the water system, and the rainy season from October/18 to April/19, with a water surplus. The water balance demonstrated that area studied is very representative to regional aquifers recharging. Temperature, pH, turbidity, OD, nitrate and sulfate values were within limits by CONAMA Resolution No. 357/2005 for class 2 rivers. The trophic state index for the nutrient phosphorus (IET-PT) indicated that Rio Pardo dry period was classified mesotrophic, and in the rainy period it was eutrophic. In relation to annual transport, Caconde Reservoir total affluent of 259,453.64 tons, distributed in 3,002.22 t nitrate (NO3 - ), 428.88 t phosphate (PO4 3- ), 5,682.78 t sulfate (SO4 2- ), 41,923.95 t sediments suspension, 24,017.81 t dissolved solids and 184,398.00 t bottom sediments was observed during the study period, showing the Caconde Reservoir possible impacts, such as the silting acceleration. The physiographic and morphometric characterization combined with the hydrological evaluations, water quality and nutrients and sediments transport can to provide water resources management subsidies in this important basin located in the southern region of Minas Gerais state.