Processos geoquímicos que controlam a salinização dos reservatórios Carira e Coité, bacia do rio Vaza-Barris, estado de Sergipe

Abstract

The salinization of reservoir waters, in semi-arid regions, has motivated a search for the mechanisms that control their chemical composition, aiming to identify the geochemical processes at work, the possible sources and to anticipate future changes in the composition of these waters. The primary processes that trigger these changes, which are naturally responsible for salinization, are atmospheric precipitation, weathering, and the evaporationcrystallization process. Depending on factors such as climate, land use, and occupation, among others, one or more of these processes will be relevant. In this context, the geochemical processes that control the ionic content of the waters of the Carira and Coité reservoirs, which make up the watershed of the Vaza-Barris River, in the State of Sergipe, were investigated. For this study, data from campaigns carried out in the years 1996, 2013, 2014, 2017, and 2018 were used, in which the values of pH, temperature, electrical conductivity (EC), total dissolved solids (STD) and ions were obtained, Cl- , Ca2+, Mg2+, Na+ , K + , SO4 2- and HCO3- . Geochemical assessments were conducted by classifying the waters for anionic and cationic dominance by the Piper's Diagram, the prior definition of the geochemical process acting by the Gibbs Diagram, analysis of ionic ratios and simulations of the evaporative process using the PHREEQC software. The program was fed with the mineralogical phases that made up the rocks and soils of the drainage basins of these reservoirs, together with the hydrochemical parameters of a less saline sample, which were tested through the removal of water mols from the solution, until reaching a concentration in agreement with another more saline sample. The ionic ratios and the simulations were able to reproduce the compositional variations observed in the field and confirmed the results obtained with the Gibbs and Piper diagrams, in which evaporation-crystallization was the main geochemical process that promoted and dominated the salinization of the waters of the Carira and Coité reservoirs. In both, the modeling indicated an enrichment of the waters in Cl- , Na+ , Mg2+ and Ca2+, as a consequence of successive evaporations. The ionic reasons and the simulations also indicated that the reverse ion exchange promoted the removal of sodium and increased the concentrations of Ca and Mg. With the excess of these nutrients, supersaturation and precipitation of the carbonate mineral phases, calcite and dolomite, and dissolution of gypsum, halite and sylvite occurred. To minimize the effects of evaporation in the reservoirs, it is recommended to adopt residence times of less than five years and to recover and protect the drainage basin from the weathering of rocks and soils, maximized by the anthropic action.