Geochemical monitoring of the thermal waters of the Phlegraean Fields

Thermal waters from the Phlegraean Fields (PF) caldera area have been periodically sampled and analysed, during the period 1990–1999, for major chemical constituents and chemico-physical parameters, to define the most useful variables for geochemical surveillance. The variations observed in waters from different locations were compared with ground movements and microseismic events recorded at the time of observation. The discontinuous monitoring of the hydrothermal system of the PF has given indications about the geochemical variations brought about by changes in the chemical and physical characteristics of the deep aquifers. Because of the complexity of the PF geothermal system in each sampling location, different variations have been observed and then evaluated on the basis of the different kinds and degrees of deep fluid supply, characterising waters sampled in the different zones. In some cases a good correspondence between vertical soil movements and chemical changes in the waters has been observed. In particular, in pools and wells located around the Solfatara crater, interesting variations in some chemical parameters (pH, ammonia and/or SO42−/Cl− ratios) have been recorded. Variations can be observed in all of the sampling locations, but pools and wells located near the Solfatara crater are the best sites for detecting changes occurring in the deep hydrothermal system and possibly in the activity of the underlying magma chamber.