Phisical and characterization of the water of Rio Grande Reservoir Riacho Grande, São Paulo State, Brazil

Abstract

 A limnological investigation was undertaken on the Rio Grande (or Riacho Grande) Reservoir, which was an arm of Billings Reservoir; it is located at 23°46'S and 46°38'W, altitude of 746.5 m. The climate of the region is tropical humid, the air temperature of the warmer month is 21 to 22°C and that of the coldest is between 14 and 15°C. During the last 50 years, Billings Reservoir has been received, directly or not, the domestic and industrial sewage of São Paulo City, which today has about 12 million people. In December 12, 1981 Rio Grande Reservoir was isolated from the main body of Billings Reservoir and a study for the water quality characterization has started in July of 1982 in three sampling stations along the main axis of the reservoir. This characterization was based on the parameter estimations of the following variables: temperature, Secchi disc water transparency, color, turbidity, electric conductivity, pH, alkalinity and ammonia, nitrite, nitrate, orthophosphate, soluble silica, total iron and dissolved oxygen as well as oxidability and BOD. The results confirm previous papers that showed that the reservoir is an eutrophic environment which has a polymictic behaviour where water circulation is frequent. The presence of the dam that isolated Rio Grande from Billings Reservoir induced a decrease in the polluted load, and contributed to the dilution of the autochthonous and allochthonous material accumulated in the reservoir. Besides this, the load of polluted material that still comes from the Grande River and Pires Stream is diluted along the major axis, causing a better water quality near the dam.