Geochemical characterization in karst basin tributaries of The San Franciscan Depression: Corrente River, West Bahia, Brazil

Abstract

Karst aquifers are important freshwater resources for thegrowing population in Brazil. The sandstones of Urucuia plateau and thelimestone of Bambui Group constitute important aquifer systems in thewestern part of Bahia state. The Corrente River provides ~30 % of thetotal water flow of the São Francisco River and crosses karstifiedstructures. Surface and groundwater samples were collected during the dryperiod, the beginning of the wet season, and the wet season. The mainobjective was to define sources and distribution of dissolved elementsand to describe the geochemical processes that govern their mobilitywithin the system. Water samples are classified into three groups,depending on the dominant weathering process. When carbonate dissolutiongoverns, waters are bicarbonate-calcium-type; whereas when theatmospheric precipitation signal is present, the samples in siliciclasticterrain are more Cl- - Na+. Groundwaters reflect bicarbonate-mixed-type,with the highest dissolved concentrations. In contrast to the majorelements, trace elements, including Rare Earth Elements (REEs), showseasonal behavior: their concentrations increase with the beginning ofthe wet season, due to re-mobilization and release into the solution ofadsorbed elements from the system and the atmospheric dust. The totaldissolved REE concentration (800 - 7500 ng L-1) is one order of magnitudemore concentrated in karsts than in siliciclastic rocks. Principalcomponent analysis was performed, explaining > 77% of the variance. Firstfactor extracted (REE, Y, Th, Al, Fe) explain the washout and enhancementof atmospheric dust weathering throughout the beginning of the wetseasons. The second component comprises variables related to karstslithology, representing calcite and dolomite dissolution.