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San Miguel volcano in eastern El Salvador is one of the most active volcanoes in Central America. During the last 250 yr, it has erupted at least 28 times. The city of San Miguel (more than 300,000 inhabitants) is located 10 km from the summit. An investigation of the concentration of diffuse soil gases—radon, carbon dioxide, and mercury—and soil fluxes of carbon dioxide was done in December 1999–January 2000. Radon (222Rn) concentrations ranged from 2 to 833 pCi/L (picocuries per liter) with an average of 110 pCi/L. Thoron (220Rn) concentrations ranged from 20 to 2178 pCi/L with an average of 356 pCi/L. These are similar to concentrations measured at other erupting volcanoes of the world.

Carbon dioxide concentrations and fluxes at San Miguel are low, with fluxes ranging from less than 0.1 to 5.0 g m−2 d−1, with an average of 1.5 g m−2 d−1. These fluxes are within the background levels of San Salvador volcano and the Santa Ana–Izalco–Coatepeque volcanic complex, and also compare with values found at Ilopango Caldera in El Salvador. Mercury soil gas concentrations were also low, with values ranging from zero to 56 µg m−3, with an average of 2 µg m−3. Carbon isotopic compositions indicate that the soil carbon dioxide is predominantly biogenic. The concentrations of the investigated gases as well as the flux of carbon dioxide are generally lower at the higher elevations of the volcanic edifice and higher at the lower elevations, close to NNW-trending faults, and to contacts between the different rock units. The low fluxes of carbon dioxide throughout the soils of San Miguel volcano are possibly due to low permeability of the volcanic cover, which is thicker at the higher elevations, and to ready degassing through the open volcanic pipe. This low flux is also consistent with the reported small but frequent historical eruptions of this volcano and its low SO2 fluxes.

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