Usos múltiplos e qualidade das águas da Bacia do Baixo Itajaí-Açú-SC: elementos para um gerenciamento integrado.

Abstract

The Itajaí-Açu river watershed is characterized by moderate agricultural use and rapid urban development. The risks on freshwater resources are notorious, but there is a lack of systematic technical data to qualify the level of degradation and to support decisions and forecasting. The main purpose of the present study was to diagnose the water quality of the lower Itajaí-Açu river and determine its relationship with land use, generating background information to integrated water resources management activities. Three procedures were executed: (a) the generation of land use charts; (b) the monitoring of chemical, biological and ecotoxicological variables in water and, (c) the preliminary characterization of sediments and pollution sources. The land use showed the predominance of forests (59%), followed by pasture/agriculture (27%) and urban areas (7%). The general water quality was regular, with 90% of legal framing for pH, BOD5, heavy metals, inorganic dissolved nutrients, surfactants and faecal coliforms; and about 70% of legal framing to dissolved oxygen and turbidity. Nutrients and other pollution indicators were directly related to rainfall and river discharge before reaching large urban areas and inversely related after these areas. In the estuarine area, physical-chemical processes induced by marine salinity caused the P-PO4 3- and suspended material to diminish in the water column. Regarding pollution sources; chemical industries, domestic wastewater and urban streams generated the highest toxicity levels. An ilustrative approach regarding the budget of nutrients and other materials and the river self purification capability showed four distinct situations or trends: (1) the case of the benchmark station, subjected to only few impacts presenting self purification processes of little importance; (2) the case of urban stations, subjected to many impacts from its contributing area, having a limited capacity to locally neutralize it; (3) the case of rural stations, which receive impacts from the upper river and specific impacts from its contribution area, but show the occurrence of self purification processes and; (4) the case of the estuarine and urbanized stations which receive many impacts, but present considerable conditions of self purification due to the influence of marine processes.