Relation between sedimentary framework and hydrogeology in the Guarani Aquifer System in São Paulo state, Brazil

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• 作者：Ricardo Hirata^a ; ^{rhirata@usp.br"" rel=""nofollow} ; Ana Gesicki^b ; ^{gesicki@usp.br"" rel=""nofollow} ; Ondra Sracek^c ; ^d ; ^{srondra@yahoo.com"" rel=""nofollow} ; Reginaldo Bertolo^a ; ^{bertolo@usp.br"" rel=""nofollow} ; Paulo Cé ; sar Giannini^e ; ^{pcgianni@usp.br"" rel=""nofollow} ; Ram&#xf3 ; n Aravena^f ; ^{roaraven@sciborg.uwaterloo.ca"" rel=""nofollow}
• 关键词：Guarani Aquifer System ; Transboundary aquifer ; Sedimentary framework ; Flow pattern ; Cation exchange ; Environmental isotopes
• 刊名：Journal of South American Earth Sciences
• 出版年：2011
• 出版时间：April 2011
• 年：2011
• 卷：31
• 期：4
• 页码：444-456
• 全文大小：1888 K

文摘

This paper presents the results of a new investigation of the Guarani Aquifer System (SAG) in São Paulo state. New data were acquired about sedimentary framework, flow pattern, and hydrogeochemistry. The flow direction in the north of the state is towards the southwest and not towards the west as expected previously. This is linked to the absence of SAG outcrop in the northeast of São Paulo state. Both the underlying Piramb&#xf3;ia Formation and the overlying Botucatu Formation possess high porosity (18.9 % and 19.5 % , respectively), which was not modified significantly by diagenetic changes. Investigation of sediments confirmed a zone of chalcedony cement close to the SAG outcrop and a zone of calcite cement in the deep confined zone. The main events in the SAG post-sedimentary history were: (1) adhesion of ferrugineous coatings on grains, (2) infiltration of clays in eodiagenetic stage, (3) regeneration of coatings with formation of smectites, (4) authigenic overgrowth of quartz and K-feldspar in advanced eodiagenetic stage, (5) bitumen cementation of Piramb&#xf3;ia Formation in mesodiagenetic stage, (6) cementation by calcite in mesodiagenetic and telodiagenetic stages in Piramb&#xf3;ia Formation, (7) formation of secondary porosity by dissolution of unstable minerals after appearance of hydraulic gradient and penetration of the meteoric water caused by the uplift of the Serra do Mar coastal range in the Late Cretaceous, (8) authigenesis of kaolinite and amorphous silica in unconfined zone of the SAG and cation exchange coupled with the dissolution of calcite at the transition between unconfined and confined zone, and (9) authigenesis of analcime in the confined SAG zone. The last two processes are still under operation. The deep zone of the SAG comprises an alkaline pH, Na–HCO₃ groundwater type with old water and enriched δ¹³C values (<−3.9), which evolved from a neutral pH, Ca–HCO₃ groundwater type with young water and depleted δ¹³C values (>−18.8) close to the SAG outcrop. This is consistent with a conceptual geochemical model of the SAG, suggesting dissolution of calcite driven by cation exchange, which occurs at a relatively narrow front recently moving downgradient at much slower rate compared to groundwater flow. More depleted values of δ¹⁸O in the deep confined zone close to the Paraná River compared to values of relative recent recharged water indicate recharge occur during a period of cold climate. The SAG is a “storage-dominated” type of aquifer which has to be managed properly to avoid its overexploitation.