RIVER WATER QUALITY MANAGEMENT USING MATHEMATICAL MODELLING J.M.P. VIEIRA (*) , J.L.S. PINHO (*) , A.A.L.S. DUARTE (*) (*)Department of Civil Engineering, University of Minho, 4719 Braga Codex, Portugal. ABSTRACT: A mathematical model has been calibrated and run for a range of River Cavado flow conditions to predict river water quality changes after pollutant loads. Biochemical oxygen demand (BOD), dissolved oxygen (DO) and faecal coliforms (FC) bacteria were used as water quality control parameters to assess critical situations near the proposed site for the construction of a new water treatment plant for the portuguese metropolitan area of Oporto. 1. INTRODUCTION Frequently, urban areas management authorities have to deal with high water demands for different uses, as well as with the progressive deterioration of available water resources quality, mainly due to intensive urbanisation and industrialisation policies. A wide range of mathematical models has been developed and applied to predict water quality changes in receiving waters, and it appears to be a useful tool for water quality management. However, the selection of the right model for a given management problem represents a hard task for decision makers: the more accurate and feasible the model, the more expensive monitoring programmes needed. The river Cavado basin, located in the north-western region of Portugal, has a very intensive use for water supply, irrigation and hydropower generation. In the last years pollution of water has increased due to industrial and domestic wastewater discharges, and, simultaneously, new water treatment plants (WTP) (Braga, Esposende), and wastewater treatment plants (WWTP) (Vila Verde, Amares, Braga, Esposende) have been installed. In the near future, a very important water supply project to the Oporto metropolitan area (that will serve seven municipalities - Esposende, Barcelos, Póvoa de Varzim, V.N. Famalicão, Vila do Conde, S.to Tirso, Maia, with a population of 0.9 million inhabitants and a design flow of 2.7 m 3 /s) will introduce new challenges in the river water quality management. Previous technical studies (PGIRHN, 1992) were conducted in order to determine the best site for WTP construction. Since the River Cavado flow regime is artificially controlled by reservoirs located upstream in its the mountain region, a flow discharge policy is needed. Moreover, the consideration of wastewater loads in the basin (WWTP effluent discharges, untreated industrial wastewater discharges and agricultural diffuse pollution) must be considered to guarantee adequate river water quality for supply purposes. In this paper a brief description of objectives, assumptions and structure of a slightly complex mathematical model (DUFLOW) is given. Its applicability is worked out in the simulation of water quantity and quality of the river Cavado basin, considering various planning scenarios concerning with the new WTP performance. 2. CHARACTERISATION OF THE STUDY AREA The river Cavado basin is located in the north-western region of Portugal, oriented from WSW to ENE, and includes the territory of eight municipalities (see Fig.1).