The de la Plata River drainage basin: a water source for the Guaraní aquifer?.

DEPETRIS, P.J., PASQUINI, A.I., GRACIA, M.G., LECOMTE, K.

Foz Iguazu

Congreso; Meeting of Americas; 2010

American Geophysical Union

The de la Plata River basin, (3.1 x 106 Km2) is one of the largest drainage regions in South America, covering in excess the underlying Guaraní Aquifer, which occupies about 1/3 of the mentioned surface area and is one of the largest groundwater reservoirs in the world. Its estimated water volume is 40,000 Km3, outcropping in some areas and reaching significant depth (1800 m) and thickness (800 m) in others. Geology is dominated by Triassic and Jurassic fluvial an eolian sandstones, 90% of which is overlaid by Cretaceous basalt. Although with low intrinsic  permeability the basalt is fractured, a characteristic that possibly favors aquifer recharging (Araújo et al., 1999). One of the de la Plata River peculiarities is its connection with ENSO events in the Pacific. Recent research has shown that its main tributaries, Paraná, Paraguay, and Uruguay rivers, have been rising their respective mean discharges since ~1970 (e.g., Pasquini and Depetris, 2007) and, also, that the Paraguay flow is increasing at a higher rate than the Paraná River does,producing a discernible signal of such feature in the middle Paraná River d18O signature (Pasquini and Depetris, 2010). This runoff increase may have a significant impact on the Guaraní Aquifer. Another interesting characteristic of the de la Plata River drainage basin, which may have an effect on the Guaraní is the high relative abundance of Fe-rich rocks, higher than the mean UCC  concentration. The tropical soils mantling a large portion of the area exhibit extreme weathering (CIA, 80-99.5) with very high losses to solution of alkaline and alkaline-earth elements (97 to 99.8%).The system´s riverine iron annual supply to the SW Atlantic Ocean is indeed significant, estimating its solid transport rate in 9.0 x 1012 Kg. Due to oxidizing conditions, only a small  fraction of iron becomes dissolved at the earth´s surface. This is obvious through high saturation indexes (oxides) in surface waters and solution/solid distribution coefficients of the order of 10-3. But when environmental conditions become reducing, as it may happen in groundwater, iron and manganese dissolve and reach high concentrations, turning water often unsuitable for human  consumption. It has been shown, however, that through seepage, Fe-rich groundwater may reach the South Atlantic, supplying micronutrients and fostering productivity in the coastal waters in  southern Brazil