Abstract
Study regionThe Guarani Aquifer System (GAS), São Paulo State, Brazil, an important freshwater resource regionally and part of a giant, transboundary system. Study focusGroundwaters have been sampled along a transect. Based on environmental tracers (REEs, Br, B, δ11B, Sr, 87Sr/86Sr) aquifer vulnerability and sustainability issues are identified. New hydrological insights for the regionFor sites near to aquifer outcrop, REE and Sr signatures (and relatively light δ13C) trace possible vertical recharge from flood basalts directly overlying the GAS. This highlights aquifer vulnerability where confined by fewer basalts and/or having cross-cutting fractures. 14C activities for these waters, however, suggest the impact of this recharge is significantly delayed in reaching the GAS. Anthropogenic sources for boron are not currently encountered; δ11B highlights feldspar dissolution, isotopically lighter signatures in the deepest sampled GAS waters resulting from pH/hydrochemical speciation changes downgradient. Br/Cl signatures (and heavier δ18O, δ2H) in these latter samples appear to reflect a past climatic signature emplaced under more arid conditions; carbon isotope systematics (δ13C, 14C) support that these represent fossil waters, and care must be taken in their water resources management in regard to sustainability of abstraction. δ18O, δ34S (sulfate) signatures confirm hydrological arguments that waters leak out of the base of the GAS aquifer in this locality rather than underlying formations with poorer water quality potentially mixing in, although other deep GAS waters in the region do show this inter-aquifer mixing.
Highlights
Introduction and aquifer contextThe Guarani Aquifer System (GAS) is an important freshwater resource regionally across São Paulo (SP) State and forms part of a giant, transboundary aquifer system, areally some 1.2 million km2, within the Paraná sedimentary basin comprising southern Brazil, eastern Paraguay, NW Uruguay and the NE extreme corner of Argentina
Rainfall samples published by the International Atomic Energy Agency (IAEA) for Rio Claro city (IAEA GNIP data, World Meteorological Office (WMO) station code 8374701; Lat. 22°23′57′′S, Long. 47°32′37′′W) during 2013–14 (Fig. 3) establishes a Local Meteoric Water Line (LMWL) relevant to the GAS recharge zone
Bonotto (2006; his Fig. 9) previously has noted an elevated piezometric level in the GAS formation at Bernardino de Campos indicating potential recharge contribution here. It is noted from the borehole lithology that Bernardino de Campos has the thickest layer of weathered mantle (0–21 m; Table 1) which might suggest some hydraulic effect focussing recharge in this locality
Summary
The Guarani Aquifer System (GAS) is an important freshwater resource regionally across São Paulo (SP) State and forms part of a giant, transboundary aquifer system, areally some 1.2 million km, within the Paraná sedimentary basin comprising southern Brazil, eastern Paraguay, NW Uruguay and the NE extreme corner of Argentina. The total freshwater availability overall in the GAS based on a depth of abstraction limit of 400 m is estimated as being 2014 ± 270 km (Organization of American States, 2009, their Table 7). Some areas (near the borders of Paraná and Santa Catarina States and in parts of Rio Grande do Sul in Brazil) are already below this critical 400m depth threshold
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