Hydrogeochemical and Flow Modeling of Aquitard Percolation in the Cariri Valley-Northeast Brazil

Abstract

Flow modeling and hydrogeochemical inversion calculations were performed on the aquifer systems of the Crato-Juazeiro Graben in the Araripe Sedimentary Basin of the Cariri Valley, Northeast Brazil, in order to assess aquifer interconnections through an aquitard of the Araripe Plateau. Steady state hydraulic head numeric calculations with the finite difference method used Modflow software with hydrologically based boundary conditions and a piezometric surface as initial condition. Hydrogeochemical inversion with PHREEQC was performed for two water samples representing typical initial and final waters along the flow path. Flow simulation shows that approximately 20% of all recharge water from the superior aquifer system passes through the Santana aquitard reaching the middle aquifer system. Quantification of mineral dissolution/precipitation processes, ion exchange and microbiological redox reactions taking place in the aquifer systems also

Salinization mechanisms of a small alluvial aquifer in the semiarid region of northeast Brazil

“…The quantified mole transfers are consistent with the ones from other studies (Uliana & Sharp, 2001;Mahlknecht et al, 2006;Machado et al, 2007;Sharif et al, 2008), which For all the other wells, the simulations were performed for the same hypothesis and in all of them the results show mole transfers similar to the ones found for the P10 well, with volumetric contributions of water from the fissure aquifer ranging from 1.53 to 27.99% to the enrichment of salts in the water of the alluvial aquifer.…”

“…In Table 7, where the positive values describe dissolutions and negative values indicate precipitations, the simulation results show that the evolution in salt concentration is due to: dissolution of CO 2 , which favors the increase of the ion bicarbonate in the waters; dissolution of anorthite and K-feldspar; precipitation of albite and K-mica; a small release of methane gas; and ionic exchanges of the cations Ca 2+ , Mg 2+ and Na + , through interaction processes with the metallic ions adsorbed to the clays (Machado et al, 2007) present in the sandy clay soils of the alluvial terrain, favoring the absorption of Ca 2+ and the release of Na + (Mahlknecht et al, 2006).…”

“…The results of the multivariate analysis for the process of mixture do not discard, however, the possibility of salt increment in the alluvial aquifer by the contribution of the crystalline basement, since the complexity of the hydrogeological dynamics allows a hydrochemical evolution through combined processes by a variety of factors (Tóth, 1999;Pereira et al, 2006), such as mixtures of groundwaters, ionic exchanges and salt dissolution in the flow pathways of alluvial aquifers (Machado et al, 2007;Cloutier et al, 2008).…”

“…This ion was used in this study as a tracer of the water from crystalline crevices, which is rich in chloride. Cruz & Melo (1968) and Costa et al (2006) show that groundwaters can acquire their composition through the progressive concentration of dissolved ions by evaporation, and studies in nearby regions (Pereira et al, 2006;Machado et al, 2007) pointed out the importance of the contact between water and rock as the origin of the salts dissolved in groundwaters.…”

Calibration of spatial aquitard distribution using hydraulic head changes and regularisation

“…Inverse mass balance models and an axisymmetric radial groundwater flow model are determined using FEFLOW (Diersch, 1998), furthermore hydrochemical changes were accounted for with PHREEQC (Parkhurst and Apello, 1999). Machado et al (2007) studied an aquifer-aquitard system in order to understand whether rainfall waters could percolate an upper limestone aquitard and contribute to groundwater storage in the underlying aquifer. In this study they used the groundwater model MODFLOW (Harbaugh et al, 2000) for regional flow modeling and PHREEQC for hydrogeochemical inversion for identifying the contribution of groundwater flow through the Santana aquitard (of unknown hydraulic conductivity) to Cariri Valley aquifers.…”