Costa Rica is characterized by a magmatic arc associated with high enthalpy 
geothermal systems. The mean geothermal potential to produce electricity in 
Costa Rica is on the order of 1000 MWe, but only 170 MWe are produced at 
present with the installed capacity. The estimated potential is based on 
temperature measurements in deep boreholes drilled in oil exploration, on 
surface gravimetric studies, and on estimated hydrogeological features of 
aquifers. The main geothermal potential of the country, characterized by 
geothermal manifestations, is estimated to be provided mainly by the 
Guanacaste area (Miravalles and Rincón de la Vieja areas) and Central volcanic 
ranges. It is expected that non-volcanic zones with surface manifestations 
have a medium potential to produce electricity, but the geothermal potential of 
these regions is not well known. At present, all exploitation activity is 
concentrated on the Miravalles geothermal field, mostly due to limits set by the 
existence of environmental parks and habitat of wild life and due to the limited 
current economic potential to finance exploitation at larger scale. Normally, 
boreholes are used for the identification of the geological structure and for 
testing the aquifer performance in terms of fluid production. Seismic and 
magnetotelluric investigations were made to delineate the main areas of fluid 
containment. These data were incorporated in hydrogeological modelling 
performed with sophisticated software (e.g. FFFLOW, TOUGH) to simulate 
different geological scenarios in specific zones and their impact on fluid and 
electricity production. 

Temperatures at Miravalles geothermal field and in the non-exploited area of 
Las Pailas (northwestern slope of Rincón de la Vieja volcano) range between 
230 ˚C and 250 ˚C. In the areas of Arenal and Poás volcanoes, some springs 
identified along the slopes have temperatures of about 60 ˚C or lower. Hydro-
geochemical data from Rincón de la Vieja, Miravalles, Arenal and Poás volcanic 
zones and from the southeastern part of the central valley (a non-volcanic 
zone), already published in scientific articles or available at Instituto 
Costarricense de Electricidad (ICE) allow a basic characterization of geothermal 
fluids. The majority of water samples from the Miravalles and Rincón de la Vieja 
volcanic areas is neutral in pH and of bicarbonate- sodium/potasium or mixed 
type. In the Arenal and Poás areas, the waters are neutral in pH and of 
Bicarbonate/sulfate-calcium character. The samples in the southeastern part of 
the central valley are of chloride/sulfate-sodium type. Several hot springs in the 
volcanic and non-volcanic zones have a northwestern orientation, associated 
with the main geological structures on the pacific slope of the volcanic chain. 

Further research is needed to explore the Costa Rica resources in more detail. 
Open questions pertaining to reservoir temperatures at depth in the 
Guanacaste and Central volcanic range and the southeastern part of the 
central valley of Costa Rica could be tackled by the use of hydrochemical data 
from hot springs, wells and small streams. The data allow to make use of the 
Na-K-(Mg)1/2 and SiO2 chemical geothermometer.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy