Oxygen, BOD and oxidability of the Salto Grande Stream Micro-basin: 22º09`S and 48º19`W, Dourado, SP, Brazil

Abstract

ln the central region of state of São Paulo, Brazil, the Salto Grande Stream is part of the hydrographic basin of Jacaré Pepira River. The water was found to be well oxygenated (median values 7.67 mg/L O₂ and 94 oxygen saturation). Anoxic conditions were absent and the spring water saturation was higher than 30. Oxygen increased along the stream mainly after the crossover of waterfalls and decreased when swamps were crossed or by receiving organic effluents. During the cold and dry period (April to September), the water showed little higher oxygen content than during the warm and rainy season (October to March). The organic matter as reflected by BOD and oxidability, was low (medians 1.57 and 2.82 mg/L O₂, respectively) with high sporadic values (maximum of 21.7 and 14.5 mg/L O₂, respectively). The BOD variation was more sensitive to the reception of domestic sewage while oxidability was more sensitive to the cattle-range run-off. The presence of swamps decreased oxidability. During the warm and rainy season, a small increase of the organic matter and decrease of oxygen was observed. During the three years of study the atypical climatic conditions with a more severe dry season induced the fall of almost all the leaves of the woods of the basin. The organic matter from the decaying leaves probably masked the punctual effect of the discharge. This decomposition was also reflected in the direct reIation between the increase in the drainage area (as the stream descends) and the concentration of organic matter in the water.