Diel biogeochemistry of the Rio Agrio, Argentina

64. PARKER S.R., GAMMONS C.H. Y PEDROZO F.L

Cologne, Alemania

Conferencia; 17th Annual Goldschmidt Conference; 2007

Geochemical Society and the European Association of Geochemistry.

Rio Agrio in Argentina is a geogenically acidic stream that derives its low-pH waters from springs of the active Copahue Volcano. These springs are fed partially by off-gassing (SO2, HCl, HF) of the magma-chamber. This study reports the results of three diel (24-h) water samplings in three different pH regimes (3.2, 4.4, 6.3) along the river. Changes in the concentration and speciation of Fe dominated the diel chemical changes at all sites. At the two lower pH sites, total dissolved Fe and dissolved Fe(III) concentrations decreased during the day and increased at night, while dissolved Fe(II) showed inverse pattern.  These cycles are explained by Fe(III) photoreduction, as well as a diel, temperature-dependent, changes in the rate of precipitation of hydrous ferric oxide (HFO).  A correlation was observed between Fe(III) and As at the pH 3.2 site, most likely due to co-precipitation of As with HFO.  At the downstream (pH 6.3) location, Fe(II) concentrations increased at night, as did concentrations of rare earth elements.  Photoreduction does not appear to be an important process in this reach, although it may be indirectly responsible for the observed diel cycle of Fe(II) due to advection of photochemically produced Fe(II) from acidic upstream waters.  These results are very similar to diel trends recently obtained from mining-impacted streams receiving acid rock drainage.  The results help form a link between geochemistry and microbiology in acidic riverine ecosystems.   For example, Fe(III) photoreduction produces chemical potential energy (in the form of metastable Fe2+) that helps support the bacterial community in this unique extreme environment.