Chemical characteristics of the crater lakes of Popocatetetl, El Chichon, and Nevado de Toluca volcanoes, Mexico

Three crater lakes from Mexican volcanoes were sampled and analyzed at vari ous dates to determine their chemical characteristics. Strong differences w ere observed in the chemistry among the three lakes: Nevado de Toluca, cons idered as dormant, El Chichon at a post-eruptive stage, and Popocatepetl at a pre-eruptive stage. Not surprisingly, no influence of volcanic activity was found at the Nevado de Toluca volcano, while the other volcanoes showed a correlation between the changing level of activity and the evolution of chemical trends. Low pHs (<3.0) were measured in the water from the active volcanoes, while a pH of 5.6 was measured at the Nevado de Toluca Sun lake. Changes with time were observed at Popocatepetl and El Chichon. Concentrat ions of volcanic-gas derived species like Cl-, SO42- and F- decreased irreg ularly at El Chichon from 1983 until 1997. Major cations concentrations als o diminished at El Chichon. A 100% increase in the SO42- content was measur ed at Popocatepetl between 1985 and 1994. An increase in the Mg/Cl ratio be tween 1992 (Mg/Cl = 0.085) and 1994 (Mg/Cl = 0.177) was observed at Popocat epetl, before the disappearance of the crater lake in 1994. It is concluded that chemical analysis of crater lakes may provide a useful additional too l for active-volcano monitoring. (C) 2000 Elsevier Science B.V. All rights reserved.