VOLCANIC IMPACTS ON LAKE CAVIAHUE, ARGENTINA

Lake Caviahue is a large (0.47 km³; 10km²) glacial lake that receives hydrothermal inputs from the active volcano Cophaue (37.8 S, -72W). It also has meltwater inputs and an outflow in its northern arm. Eruptions occurred in 2000 and 2012 and chemical data for the lake and rivers are available from 1997 till 2013. The pH in Lake Caviahue (2.2 to 3) was the lowest during the eruptions. Mean concentrations of F, SO₄ and Cl (volcanogenic elements-VE) are resp. 10, 350 and 80 ppm, whereas the major cations (rock forming elements-RFE) range from a few ppm to a high of 30 ppm for Al. The variations in lake composition depend on the element fluxes into the lake, which were measured in the volcanic river every year. Non-steady state modeling allows the reconstruction of mean flux values for all elements, which can be compared with the annual, instantaneous flux measurements. All element fluxes except for Mg diminished strongly after the 2000 eruption and only reached again pre-eruptive values over the last two years. Initial data show that the Cl flux has decreased again by >30% in 2013. After the 2000 eruption, the hydrothermal reservoir became saturated in alunite-jarosite, reducing permeability and overall elements fluxes through the hot springs, with significant flux reductions in K, Al and Fe. The mean apparent residence time (MART) for the VE, based on the 1997-2009 record, is 41 months, similar to Varekamp (2009). The MART for the RFE is much more variable, with very low values for K (13 months) and very long MART for Ca (>200 months) suggesting non-conservative behavior in the lake system. The total lake element contents was compared to the local volcanic rock composition, showing that Lake Caviahue contains the equivalent of 364000 tons of dissolved rock (150,000 m³), with strong depletions for Si, Al, Fe and K. The lake was close to Schwertmannite saturation in 2009, but the new acid pulse associated with the 2012 eruption prevented the lake from reaching saturation. The outflow river (pH ~ 3.2) has been saturated with schwertmannite since 2003. Today’s lower lake pH values cause re-dissolution of the mineral in the riverbed, which is significant, because with the Schwertmannite dissolution, the associated arsenic (1000 ppm), phosphorus (5700 ppm) and vanadium (1000 ppm) are also released into the environment (Kading, 2011).