Systematic radon survey over active volcanoes

In-soil radon-222 monitoring has been conducted on active volcanoes, in par ticular on the Arenal, Irazu and Peas volcanoes in Costa Rica and on the Pi ton de la Fournaise volcano, La Reunion island. Automatic electronic probes buried in soil at one meter depth were used to study the short- and long-t erm fluctuations of radon related to the external parameters and/or related with the volcanic activity. Three radon stations are in operation on each Costs Rican volcano and a network of 24 stations (with 3 teletransmitted) i s operated on the Piton de la Fournaise. Data obtained since 1993 on Costa Rica volcanoes are presented and radon an omalies recorded before the December 8, 1994 eruption of the Irazu volcano are discussed. The Piton de la Fournaise volcano is inactive since mid 1992 . We could intensively study the influence of the external parameters on th e radon behavior and individuate the type of perturbations induced on short -term measurements. One seismic crisis occurred on November 27, 1996. Radon anomalies appear on most of 50% of the stations 36 hours before the occurr ence of the cl isis.