Hydrochemistry of the acidic Rio Agrio / Lago Caviahue catchment and it's response to the year 2000 eruption of Copahue Volcano / Argentina

KRINGEL, R.; WOELFL, S.; SCHIMMELE, M.; BAFFICO, G.; MAGES, M.; TEMPORETTI, P.; GELLER, W.; PEDROZO, F

Caviahue, Argentina

Workshop; International Workshop Geogenically Acidic Water Systems: Volcanic Waters, Mining Lakes and Rivers.; 2004

CRUB - UNComahue

The upper Rio Agrio/Lago Caviahue is an alpine catchment in the Eastern Andes at Latitude 37° 45 S, Longitude 71°10.2' W, acidified by hydrothermal activity of Copahue, an active andesitic stratovolcano (Argentina/Chile). The hydrochemistry of the catchment has been investigated to and hydrochemical constraints for biological and microbiological activity in the naturally acidic stream and the glacial lake forming a large reservoir during two field campaigns in the year 2000 and 2001. In winter 2001 Copahue volcano went into eruption and caused changes to the drainage by a. evaporating the glacier{fed crater lake and b. by depositing moderate amounts of reactive ash and lapilli. Before the eruption Rio Agrio showed distinct diurnal cycles in accord with meltwater production. 2 month after the eruption hydrothermally controlled element ux to the lake was back to a preeruption level and the anion ratio Cl-/SO4 was unchanged. In march 2001 the diurnal cycle was not found, as the ash was insulating the snow cover. The ash and it's redistribution caused major changes to the bogs supplying fresh water in the catchment and the conditions in the river and the lake. Suspended glass{shards with up to 6500 mg L-1 abraded the river bottom. Fines transported into the lake basin changed the light conditions in the lake from 9 m to only 2 m. The changes with eruption were visible in the mineralization of Lago Caviahue, it's mineral acidity reduced by 0.9 meq L-1 to 10.0 meq L-1 and the cation ratios changed. The history was visible in the depth profile of electrical conductivity changed by +200 µS cm-1 to 2400 µS cm-1 over the whole 85 m water column. Ash deposited into the environment was found to be highly reactive with up to 385 meq kg-1 of mineral acidity and 38000 mg kg-1 of soluble salts. Detrimental changes to the plant cover (mainly bogs) were already visible. S°{derived acidity was visible in originally neutral streams, draining into the Lago Caviahue. The point source of Copahue (when in quiet hydrothermal mode) had added a diffuse source of acidity to the catchment with the eruption.