Hidroquímica e contaminação em aquíferos sedimentares e rochosos fraturados da bacia do Alto Rio Negro – Amazonas/Brasil

Abstract

The Amazon region has the largest hydrographic network on the planet. However, cities and communities along rivers obtain water from tube wells without studying the chemical quality and, consequently, the potability of their water. In specific geological contexts, there is a significant probability of natural contamination by arsenic in groundwater in the region, already attested in previous works in the gutters of the Solimões, Madeira and Amazonas rivers, but not yet witnessed in the Rio Negro basin. As the Negro River is the second largest river in the Amazon Basin, it has become necessary to study the chemical quality of its groundwater, relating it to the geology of the region. In this work, water samples were collected from 21 wells and 4 sources of water for human consumption, following the protocols proposed by the US - EPA. In the field, the physicochemical parameters of the in-situ water were measured, and alkalinity tests were carried out on the samples. The cations were analyzed by the INPA and the anions by the Laboratory of Mineral Analysis (LAMIN), of the Geological Survey of Brazil (SGB). Of the 16 points analyzed, 12 showed contamination, with As, Mn, Al, Pb and Fe found in concentrations above the maximum allowed value (MPV). The presence of values below the permitted level, but above what is considered normal, of NO3- in 66% of the contaminated points, associated with the shallow depth of most of the wells, installed in areas of recent sedimentary deposits and that do not show any or few signs of care demonstrate that the contamination in these localities is of anthropic origin. Two samples (16% of the contaminated samples) are from deep wells installed in fractured aquifers, which in this case indicate contamination of geogenic origin. Arsenic was found in association with iron and aluminum in a well installed in alluvial reservoirs, which indicates that more studies are needed for this specific point to know the quality of this water. For other locations and cities, short, medium and long-term measures are needed to remove contaminants from circulation and prevent further outbreaks of contamination.