2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 11
Presentation Time: 1:30 PM-5:30 PM


LEAL, Luiz Rogério Bastos1, DUTTON, Alan2, SILVA, Hailton Mello1 and BARBOSA, Johildo3, (1)Department of Geology and Geophysics, Institute of Geosciences, Federal University of Bahia, Rua Barão de Geremoabo, S/N, Campus Universitário de Ondina, Federação, Salvador, 40210-190, Brazil, (2)Department of Geological Sciences, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX, San Antonio, TX 78249, (3)Departamento de Geoquímica, Universidade Federal da Bahia/Instituto de Geociências, Rua Barão de Geremoabo, s/n Campus Universitário de Ondina, Salvador, Bahia, 41210-190, Brazil, lrogerio@ufba.br

Karst aquifers provide important supplies of potable water in Northeast Brazil and occupy approximately 7% of the land surface of Brazil. The fractured-karst limestone aquifer of the Irecê region is one of the most important agricultural areas in the semiarid part of Bahia State, and groundwater represents an important source of water for agriculture, domestic and industry uses.

The Irecê fractured-karst area is divided into three principal structural domains:

(1) North and central regions of the aquifer are characterized by subvertical beds and isoclinal folds with E-W fold axes, and fractures with strikes of N, N40-50oE and N40-50oW. The northern part of the aquifer has many fewer karst features evident at ground surface and it is assumed to behave more like a fractured aquifer than a typical karst aquifer. The central part of the aquifer has active juvenile-stage karstification with (a) a high density of narrow (5 to 10m wide) but elongate sinkholes oriented in E-W and (b) vertical shafts. The elongate sinkholes and shafts focus recharge. They pose a serious geohazard for agricultural work and urbanization while putting the aquifer at risk of contamination.

(2) The second structural domain lies at the contact between the Neoproterozoic carbonates and the older metasediments of the Chapada Diamantina Group around the boundary of the karst basin. The carbonates unconformably overlie the metasediments and have a subhorizontal dip. The carbonates are crossed by faults and fractures that strike N30-40oW. In this domain, karstification has formed from cross-formational groundwater flow from the subadjacent metasediments. Sinkhole lineaments commonly follow the structural trend.

(3) The southern part of the aquifer has large folds, subhorizontal beds, large fractures with principal N-oriented strike and secondary fractures with orientations of ~N15oW and N15oE. Karstification is more developmented than other parts of the aquifer. There is a high density of generally large (100 to 200m diameter) dolines, which typically follow N-striking lineaments.

Considering the structural domains, variations in water chemical composition, and differences in land use across the Irecê basin, four groundwater management zones can be defined for which alternate strategies might be considered to protect groundwater quality.