HR: 14:40h
AN: V13C-04    [Abstracts]
TI: Calibration of a Groundwater Model of Masaya Volcano, Nicaragua
AU: * Sanford, W E
EM: wsanford@usgs.gov
AF: U. S. Geological Survey, Mail Stop 431, Reston, VA 20192 United States
AU: MacNeil, R E
EM: rrmacneil@tampabay.rr.com
AF: Department of Geology, University of South Florida, Tampa, FL 33620 United States
AU: Connor, C B
EM: cconnor@chuma1.cas.usf.edu
AF: Department of Geology, University of South Florida, Tampa, FL 33620 United States
AB: Masaya Volcano consists of an active, 400-m-high, 6-km², composite cone within a large (50-km²) basaltic caldera, and has a history of large phreatomagmatic eruptions. In order to better understand the hydrologic processes in this system, a groundwater model has been developed of the caldera using the USGS model MODFLOW. Transient electromagnetic (TEM) soundings were used to map the water table within the caldera. The water level of Lake Masaya, which occupies the lower one-fifth of the caldera, was used as a calibration point for the soundings. The TEM soundings revealed a water table mound beneath the cone, but not within the more permeable part of the caldera surrounding it. The differences between our estimated water levels inside the caldera and known regional water levels outside strongly suggest that the caldera walls are acting as hydrologic barriers, effectively isolating the groundwater-flow system within the caldera. A total of 29 estimated water levels and two ground-water-flux measurements were used to calibrate the model. The flux measurements were (1) a net flux into Lake Masaya of 1.2 m/yr, calculated from an estimate of lake evaporation and a transient lake-level record during the dry season, and (2) a net steam emission flux of 400 kg/sec from the active vent in Santiago crater. The lake and the steam vents are the only substantial discharges of groundwater within the caldera, each accounting for about half of the annual recharge. The steam discharge is substantially larger than other similar volcanoes in the world, suggesting its origin may be dominantly meteoric. The model calibration revealed that a deep, highly permeable layer must feed the active vent in order for the steam emissions to be maintained at their current levels. Quantifying this type of groundwater-vent interaction could be important to the understanding and prediction of future phreatomagmatic eruptions.
DE: 0684 Transient and time domain
DE: 1829 Groundwater hydrology
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly