Contaminação no lago Xolotlán causada pelos despejos vertidos pela área metropolitana da cidade de Manágua, Nicarágua

Abstract

Lake Xolotlán, the smaller of the two large lakes that occupy the Nicaraguan Depression, has been polluted by the metropolitan area of ​​the city of Managuá since 1928, when it began to be used as a receiving body for the city's sewage. It currently also receives wastewater from the main industries in Nicaragua (located in Managua); sediments, garbage, high content of nutrients and organic pollutants from the drainage basin and the water that percolates from the main dump in the city of Managua (Lixão Acahualinca). To determine the levels of contamination by chromium, copper, arsenic, cadmium, mercury and lead on the southern coast of Lake Xolotlán, two sampling campaigns were carried out in six profiles with a north direction, starting from the discharge points of three sanitary sewage collectors, an industrial wastewater effluent, a rainwater drainage channel and in front of a "sanitary" landfill. In the first campaign, sampling was carried out up to a distance of 1000 meters from the lake shore and in the second, up to 500 meters. Water and sediment samples were collected for the analysis of trace elements and water samples for the determination of physical and physical-chemical variables. A third sampling campaign was carried out on two profiles to determine the concentrations of ƩDDT and toxaphene in sediments from Lake Xolotlán. For the determination of trace elements, the atomic absorption spectrometry method was used. ƩDDT and toxaphene were determined by the gas chromatography method. In the analysis of physical and physical-chemical variables, descriptive methods by APHA (1992) were used. The concentrations of chromium, copper, arsenic and cadmium in water were lower than the limits established by the National Council for the Environment (CONAMA) of Brazil in 1987 for brackish water intended for primary contact recreation, the protection of aquatic communities and the creation of species. intended for human consumption. Lead was exactly at the limit established by CONAMA for this pollutant (10ppb) at the point of discharge of the influent sampled from industrial wastewater in the first sampling. Mercury concentrations in water were high throughout the wastewater effluent profile, ranging from 0.6 to 0.1 ppb in both samples. In sediments from the coast of Lake Xolotlán, chromium was found at an average concentration of 15.1 µg/g and its distribution in most profiles indicates contamination by point sources, similarly to what occurs with copper (57 - 28 µg/g) and arsenic (1.3 - 11.2 µg/g). Cadmium contamination (0.7 - 4.7 µg/g) is a product of volcanic activity. Sediments sampled at the point of discharge of the sampled wastewater show a strong mercury contamination (57.6 µg/g), which decreases with distance. In turn, lead contamination (average concentration of 36.8 µg/g) is widespread throughout the study area and, with the exception of one profile, its spatial distribution indicates contamination by non-point sources. Toxaphene was only detected at the industrial wastewater discharge point, but the concentration was high (10 ppm). The distribution of ƩDDT suggests contamination by an unidentified point source. The results of the analysis of the physicochemical variables demonstrate that the volcanic activity is contributing with high values ​​of Ph, solids, boron, fluorides and higher ion concentrations. The significant decrease in the concentration of nutrients and the chemical and biochemical oxygen demands, with the distance from the point contamination sources, reveal that Lake Xolotlán has a high stabilization capacity.