Radon and hydrogeochemical monitoring at Popocatépetl volcano, Mexico

Measurements of radon in soil gas were carried out around Popocatepetl volcano during the period from September 1997 toApril 1999. Soil gas was sampled using a stainless steel probe, and measured in the field with an alpha scintillation counter. At onelocation concentration variations of 222 Rn in soil have been related with changes in the volcanic activity. In addition, chemicalvariations of spring waters at active volcanoes may be useful for volcano monitoring. In 1993 an increased period of volcanicactivity began at Popocatepetl volcano. Since 1995, the chemical analyses of springs have been carried out as part of the surveillanceof the volcanic activity. Conductivity, pH, and temperature were measured in the field. Concentration of the major ions:HCO 3 -, Cl-, SO 4 2 -, Mg 2 +, Na+, K+, Ca 2 + have been determined in the laboratory by standard methods. The minor species boron,sulfide and fluoride were analyzed by colorimetry and potentiometry. Concentration increases of B, SO 4 2 -, and Cl-, have beenobserved preceding some changes in volcanic activity during the studied period in some of the springs. Radon anomalies werefound to approximately coincide with some of the chemical variations. The highest 222 Rn levels at one location were measuredduring the period that preceded the eruption that occurred on December 24, 1997, which included the emission of relatively largetephra fragments.