Caracterização e evolução geoquímica das águas subterrâneas da mina de Candiota (RS), Brasil

Abstract

In order to evaluate the mining activities influences of Candiota Mine, the largest Brazilian coal reserve, over the groundwater quality, geochemical studies were carried out on water and paralic sandstones of the Rio Bonito Formation, which constitutes a granular aquifers system. Geochemical modeling indicates that for most cases, groundwater is subsatured in relation to the balance water-carbonates and supersatured in relation to the balance water-goethite and -kaolinite. The phisico-chemical results of the water samples showed little evidence of significant acidification or sulphatisation of groundwater by acid mine drainage of mining activities. A natural but limited pyrite oxidation by the restricted entrance of oxygen in the aquifer system seems to be responsible for the positive correlations between sulphate and iron. However, the iron contents is lower than the expected value from pyrite dissolution which can be explained by the precipitation of Fe-oxi-hydroxides (e.g. goethite). The dissolution of carbonates (calcite, siderite and ankerite) is an important process of the neutralization of acidity, controlling the chemical composition of the groundwater by consuming protons and releasing Ca2+ and Mg2+. Relatively to the superficial waters, calcium and magnesium are more significantly depleted than sodium and potassium in the groundwater, probably related to the selective adsorption. For the calcium-bicarbonate correlation, atmospheric and soil CO2 as well as organic matter oxidation are considered to contribute to the increase in the total content of bicarbonate