FAULT CONTROLLED FLUID FLOW AT EL SALVADOR VOLCANOES: EVIDENCE FROM DIFFUSE SOIL DEGASSING

During the last eight years several surveys of diffuse soil gases have been carried out at several volcanoes and hydrothermal systems in El Salvador (Ahuachapan Geothermal Field, Santa Ana-Coatepeque-Izalco volcanic complex, San Salvador volcano, Ilopango caldera, San Vicente volcano, Berlin Geothermal Field and Tecapa volcano, Chinameca Geothermal Field, and San Miguel volcano). These studies include the determination of soil radon concentrations using a radon monitor (Pylon AB5), the accumulation chamber method for the determination of CO2 efflux from the ground, and determination of the concentration of other gases (e.g. He, O2, CH4, N2, Ar, SO2) using gas chromatography or a quadrupole mass spectrometer, Two type of studies have been done: regional studies with grid spacing ranging between 200 and 500 m, and more detailed studies at and around areas that are discharging heat and gases advectively in the form of fumaroles, hot springs, or steaming ground. In this last type of study, distance between sampling points has been only a few meters. The regional studies show correlations between regions of high release of gases such as carbon dioxide and radon and the occurrence of permeable faults in the area. The zones of advective discharge of fluids (fumaroles, hot springs, steaming ground) occur usually at interceptions between two faults or the interception of a fault with a contact between two different lithologies. The diffuse soil degassing at these smaller areas show correlation with the fault traces and the spatial distribution of isoconcentration curves for the soil gases or CO2 efflux displays an orientation related to the fault that is likely transporting the fluid. In some volcanoes, the regional studies show that if the second degree regression surface is statistically significant, this surface is centered at the area where the deep source of gases is likely occurring (e.g. carbon dioxide fluxes at San Salvador volcano). In some other cases, the second degree regression is not statistically significant but it is not centered at the possible source either. These studies show that diffuse soil gases can be important indicators of vertical permeability and can be used to determine permeable faults in volcanic systems.