Avaliação espaço-temporal da qualidade das águas superficiais da microbacia do córrego Alegria após acidente ferroviário de junho de 2003 em Uberaba/MG e sua relação com os meios físico e antrópico

Abstract

On June 10, 2003, a train crash occurred in Uberaba/MG involving the derailment of eighteen wagons spilling dangerous chemicals products (methanol, octanol, isobutanol and potassium chloride) directly on the ground, affecting water of Alegria Stream, an important tributary of the Uberaba River. This paper describes the implementation of an intensive monitoring program and the results from a space-time evaluation of surface water quality in the Alegria stream's micro-basin, using environmental and anthropic variables. The study was based on the evaluation of analytical results from 292 monitoring campaigns in 9 sampling stations with a fortnightly average frequency during two periods: June 16 to August 8, 2003; and April 10, 2005 to July 4, 2012. The main parameters analyzed were: electrical conductivity, pH, chloride, soluble potassium and soluble iron. Concentrations of chloride, total iron and total potassium were also taken from the analyses of 153 samples in 51 points distributed over the area of the accident in three distinct horizons (surface, intermediate and deep), plus 10 samples (surface) collected near both banks of the Alegria stream in an area less anthropized (agriculture). Soil results from samples collected in the area of the accident show a low variability in depth for chloride, while the results from soil samples collected along the Alegria stream banks suggests a relative enrichment over the years through the use of fertilizers. An increase in concentration with depth for total potassium and total iron reflects the effect caused by the proximity with a basaltic bedrock. Variations of chloride and soluble Potassium in surface water presented a strong correspondence with rainfall periods, with concentration peaks often associated during the first rains and input of sediments to the river bed. The lithology composition seems to affect the availability of soluble potassium in surface water and, in a lesser extent, it is also relates to the adoption of potash fertilizers used in the Alegria and Uberaba basins. Comparisons between chloride and soluble potassium concentrations from water samples collected downstream and upstream from the point of the accident suggest that it did not compromise the water quality of the Alegria stream. Concentrations of soluble iron in water samples seem to be related not only as a consequence of morphometric differences between the two basins, but also as a response from different levels of environmental degradation and characteristics for use and occupation. The turbidity in the Uberaba river tends to remain high for a period greater than in the Alegria stream. Close intervals between medians were checked during rainy seasons in both rivers, while during dry seasons lower medians were observed in Uberaba river, tending to be higher than the Alegria stream. According to the selected methodology and results acquired it is correct to believe that the variations in the water quality of Alegria stream, in time and space, are associated to the following main factors: 1) rainfall (rain rate and total rain); 2) watershed form (morphometric parameters); 3) lithologic constitution; 4) vegetation cover; and 5) uses and occupation of the area (basin).