Estruturas geoelétrica e hidroquímica do sistema aquífero cristalino da bacia do alto rio Curaçá, semi-árido da Bahia

Abstract

ABSTRACT. In this work we discuss a geophysical-hydrogeological strategy proposed to explore rationally the groundwater stored in the semi-arid terrains, such as those of the upper Curaçá river basin, Bahia. Three main subjects were considered and solved in this study. The first one was to develop a conceptual geohydrologic model to represent the water storage and transmission within the aquifer zones of the area. Based on surface and aero-geophysical information and on geological data obtained in deep cored mining-wells we propose the model to consist of: (i) a thick layer of alluvial deposits and weathered rocks (saprolites) having up to 30 m thickness, covering almost continuously the Curaçá river plain and those of their tributaries; and (ii) large and wide belts of densely fractured rocks, laterally and vertically alternating with zones of impervious crystalline basement. These belts of open, subvertical fracturing may reach more than 500 m in width and depths larger than 300 m. The second was to optimize a geoelectrical procedure capable of imaging, with good spatial resolution, these complexes underground reservoirs. Thus, a two-dimensional scheme of IP-resistivity imaging was performed with a closely spaced association of deep vertical electrical soundings. The third was to search and explain the mechanisms and processes responsible for the high water salinization in the wells drilled in this system. Based on a simplified model for a steady-state, two-dimensional flow of water in a water-table homogeneous and isotropic aquifer, we suggest this salt concentration process is mainly due to the intense and almost continuous evapo-transpiration, that characterizes the semi-arid lands. This process acts over a hydraulic circulating system controlled by the terrain topography. The circulation cells have different sizes and depth of propagation, the salt accumulation being concentrated in the smaller cells.Keywords: crystalline aquifers, geoelectrical structure, saltiness. RESUMO.Neste trabalho se analisa uma estratégia geofísico-hidrogeológica proposta para explorar racionalmente aágua subterrânea armazenada nos terrenos cristalinos semi-áridos, como aqueles da bacia do alto rio Curaçá, Bahia. Num estudo integrado, três principais problemas foram considerados e resolvidos. O primeiro envolveu a proposição de um modelo hidrogeológico conceitual para representar o armazenamento e a transmissão deágua no sistema aquífero daárea. Usando informações geofísicas aéreas e terrestres com dados geológicos de testemunhos de sondagens, propõe-se um modelo consistindo de: (i) uma cobertura aluvionar e de saprólitos, com cerca de 20 a 30 m de espessura e que se estende, continuamente, ao longo da planície do rio Curaçá e de seus tributários; e (ii) zonas largas e extensas de densos e profundos fraturamentos subverticais, geralmente abertos e que formam faixas com mais de 500 m de largura e profundidades maiores que 300 m, separadas entre si por ombreiras de rocha cristalina mais fechada. O segundo consistiu em aperfeiçoar um...

Relationships of stable isotopes, water-rock interaction and salinization in fractured aquifers, Petrolina region, Pernambuco State, Brazil

“…The high salinity of groundwater is given by the joint action of factors such as: climate, soil, high rate of evaporation, plane relief, low connected fractures, restricted circulation of water in the aquifer, and high waterrock time contact. Several authors have presented explanations for the causes of the increasing ion content in the water which are focused on different processes: dry and warm climate conditions, salt enrichment in the soils and water-rock interaction (Silva Junior et al, 1999;Santiago et al, 2000;Silveira & Silva Junior, 2002;Costa et al, 2006;Lima, 2010).…”

Different Approaches on the Investigation of Ground Water

“…Information about the flow, depth and different lithologies was obtained [13]. At Bacia do Alto Rio Curaçá, BA, a hydrogeological model of water storage and transmission was proposed in order to explain salinization mechanisms [14]. In Minas Gerais, a study of the water flow in the recharge area of an aquifer was performed.…”

Estimativa da velocidade de fluxo subterrâneo por imageamento elétrico

“…A Geofísica tem um papel importante nos estudos desenvolvidos para água subterrânea. A principal aplicação da metodologia geofísica na prospecção de água subterrânea é voltada para a procura de rochas, estruturas ou ambientes geológicos que possam permitir a extração de água (LIMA, 1990;MEDEIROS e LIMA, 1990;HARARI, 1996;SOUZA FILHO et al, 2006;LIMA, 2010;OLIVEIRA, 2011;SOUSA e LUIZ, 2012;MENDES et al, 2014). Os métodos geofísicos podem, porém, ainda ser usados para: a) Estimar características físicas dos aquíferos, como porosidade, condutividade hidráulica, per-meabilidade e transmissividade (GRIFFITHS, 1976;KELLY, 1977;PONZINI et al, 1984;LIMA e NIWAS, 2000;NIWAS e LIMA, 2003;LU e SATO, 2007;SOUPIOS et al, 2007); (b) Indicar alguns parâmetros de potabilidade da água, como, por exemplo, seu grau de salinidade (HAGREY e MÜLLER, 2000;BENKABBOUR et al, 2004) ou contaminações por poluentes químicos e orgânicos (COSTA et al, 1995;BENSON et al, 1997;SAUCK et al, 1998;NUNES e LUIZ, 2006;BAESSA et al, 2010;BAHIA et al, 2011;CUNHA e SHIRAIWA, 2011).…”