LAKE CAVIAHUE (ARGENTINA) AS A SOURCE-SINK FOR VOLCANIC ARSENIC AND PHOSPHORUS

Lake Caviahue is a large (~0.5 km3, 100 m deep) seasonally stratified, glacial lake in N-Patagonia (37.5 oS, 71oW; Argentina). It is acidified to pH 2-3 through volcanic inputs from the active volcano Copahue. We studied the chemically similar elements Arsenic and Phosphorus, a toxin and a nutrient, in this lake system. The volcanic input river (pH~1-2) had 40 – 640 ppb Arsenic and 0.1 – 5.6 ppm P during the last decade; the volcanic source fluids (hot springs) had 3-11 ppm As and 3-67 ppm P. The lake waters are a mixture of glacial meltwater and volcanic fluids, and had 30-40 ppb As and 70-910 ppb P between 1997 and 2009. Downstream from the lake overflow outlet, [As] and [P] fall to very low levels, but declining As/Cl values indicate that dilution is not the only process responsible. A zone in the outlet river with pH~3.2 has extensive deposits of a poorly crystalline ferric-sulfate mineral, most likely Schwertmannite. This mineral strongly adsorbs As and P, as indicated by their high concentrations on the yellow, partially floating scum (P= 5760 ppm; As=1040 ppm). Variations in [P] and [As] in the lake waters are modulated by the input flux, non-steady state effects, local productivity, and adsorption processes. Snow melting during the Austral spring sends a pulse of dilute riverwater into the lake, which at that time is fully mixed. During the summer, the input river becomes more concentrated and the lake is stratified, creating a more concentrated epilimnion. Bottom water samples taken in 2009 contain suspended yellow-red mineral flocs, and [As] and [P] are low, suggesting that in-lake Schwertmannite formation with associated ion adsorption has taken place. Schwertmannite with its toxic element load accumulates on the lake bottom, which builds a potential mobile reservoir of As and P. Once the lake becomes re-acidified during a volcanically active period as occurred prior to and during the 2000 Copahue eruption, the Schwertmannite may re-dissolve, releasing the adsorbed ions. We use a non-steady state, dynamic two-box lake model to assess the magnitude of the As and P fluxes into the lake sediment, and predict the magnitude of the As “pulse” if volcanic re-acidification of the lake were to occur.