Water chemistry of Lake Quilotoa (Ecuador) and assessment of natural hazards

A geochemical survey carried out in November 1993 revealed that Lake Quilot oa was composed by a thin (similar to 14 m) oxic epilimnion overlying a sim ilar to 200 m-thick anoxic hypolimnion. Dissolved CO2 concentrations reache d 1000 mg/kg in the lower stratum. Loss of CO2 from epilimnetic waters, fol lowed by calcite precipitation and a consequent lowering in density, was th e apparent cause of the stratification. The Cl, SO4 and HCO3 contents of Lake Quilotoa are intermediate between tho se of acid-SO4-Cl Crater lakes and those of neutral-HCO3 Crater lakes, indi cating that Lake Quilotoa has a 'memory' of the inflow and absorption of HC l- and S-bearing volcanic (magmatic) gases. The Mg/Ca ratios of the lake wa ters are governed by dissolution of local volcanic rocks or magmas, but K/N a ratios were likely modified by precipitation of alunite, a typical minera l in acid-SO4-Cl Crater lakes. The constant concentrations of several conservative chemical species from l ake surface to lake bottom suggest that physical, chemical and biological p rocesses did not have enough time, after the last overturn, to cause signif icant changes in the contents of these chemical species. This lapse of time might be relatively large, but it cannot be established on the basis of av ailable data. Besides, the lake may not be close to steady state. Mixing of Lake Quilotoa waters could presently be triggered by either cooling epilim netic waters by similar to 4 degrees C or providing heat to hypolimnetic wa ters or by seismic activity. Although Quilotoa lake contains a huge amount of dissolved CO2 (similar to 3 x 10(11) g), at present the risk of a dangerous limnic eruption seems to be nil even though some gas exsolution might occur if deep lake waters were brought to the surface. Carbon dioxide could build up to higher levels in deep waters than at present without any volcanic re-awakening, due to eithe r a large inflow of relatively cool CO2-rich gases, or possibly a long inte rval between overturns. Periodical geochemical surveys of Lake Quilotoa are , therefore, recommended. (C) 2000 Elsevier Science B.V. All rights reserve d.