Avaliação de impactos das atividades antrópicas sobre os recursos hídricos da Bacia Hidrográfica do Rio Salitre - Bahia e classificação das fontes hídricas.

Abstract

The main objective of this work was to evaluate the impacts of the antropic activities on the water quality of the hydrographic basin of the river Salitre, identifying the most significant variables that affect the quality and grouping the sources of water in classes, according to their quality. The hydrographic basin of the Salitre was divided into five sub-basins, based on the predominant human activities, to evaluate the influence of irrigation and of other human activities on the water quality. The several variables that indicate some aspect of water quality were quantified for various surface and ground water sources in the periods of rainfall (PC) and summer (PV) for the year of 2001, and the monitoring of the water quality along the final stretch of the Salitre river was carried out. Further, the soils in the irrigated areas and the accumulated sediments in the storage reservoirs were also analyzed. From the data obtained in the sub-basins, it was observed that the influence of the irrigation on the quality of the water was noticeable only in sources close to those areas, in which significant increases in the electrical conductivity of the soils and amounts of heavy metals in the sediments were registered. On the other hand, these increases were not observed in sources located far from irrigated areas. Increases in these variables indicate a low efficiency of the agricultural production systems, associated with the high salinity of water, considering that more than 35% and 82.5%, respectively, of surface and ground water sources were classified as C3 and C4, according to US Salinity Laboratory standards. For the water sources analyzed, approximately 78% are located under calcareous formations, whose influence on the water quality can be noticed by the high values in both surface water and groundwater sources, respectively, of: total dissolved solids (TDS = 5,370 & 5,060 mg L-1), sodium adsorption ratio (SAR = 10.9 & 6.7 mmol L-1/2), total hardness (TH = 2,718 & 2,940 mg L-1), chloride (Cl = 74.9 & 68.4 mmolc L-1), and bicarbonates (HCO3 = 9.9 & 8.0 mmolc L-1), among others, thus limiting their use. On the average, more than 80.0% and 63.43%, respectively, of the surface and underground water sources are bicarbonated. The factorial analysis allowed the grouping of the variables in three main factors at the global level of the basin of the Salitre river. For the ground water sources, the main factors during the periods of 1984, the rainy and summer periods of 2001, explained 86.9%, 72.1% and 68.3%, respectively, of the total accumulated variance. In the case of the stored water in reservoirs for the period of 2001, the main factors explained 83.50% and 74.98% of the total variance; for the rivers, it was 86.92% and 71.25% of total variance during the periods of rainfall and summer of 2001. The variables within Factor 1, independent of the source and the period, are related directly to salinity. For all sources and periods analyzed, the bicarbonate part was the Factor 2, except for the river waters in the summer period. The Factor 3 contemplated the remaining variables. These results point out the need for specific actions of prevention and conservation of the water resources and of the soils of the hydrographic basin of the Salitre. The water sources were grouped into three classes, so that C1 represents the sources of best water quality. For the underground water sources, C1 grouped 25.0%; 74.3% and 61.7% of the water sources, respectively, during 1984, rainy and summer period of 2001, with average values of TDS varying from 199.00 mg L-1, in 1984, to 724.86 and 650.34 mg L-1, in 2001. Most of the sources identified as C1, in 1984, maintained the same classification in 2001. For the surface sources, 61.5% and 76.9% of the reservoirs and 38.5% and 15.4% of the rivers were classified as C1, in 2001, presenting low average values for total dissolved solids (TDS = 138.3 & 209.4 mg L-1 for reservoirs and 105.2 & 129.5 mg L-1 for the rivers during rainy season and dry period, respectively). Changes on quality as indicated by variations of classes of water among the surface sources were not significant between the two periods.