Volcano Hydrology I
Presiding: S Hurwitz, U.S. Geological Survey; J Major, U.S. Geological Survey Cascades Volcano Observatory
V13C-01 INVITED 13:35h
The Ultimate Hydrologic Sponge: Hydrology and Dynamics of a Young Volcanic Arc in a Mediterranean Climate
Young basaltic landscapes in wet geographic settings can store prodigious quantities of water. Factors contributing to their behavior as vast hydrologic sponges include extremely high porosities and permeabilities due to large interstitial spaces, cracks, cavities, and tubes, along with the intrinsically high permeability of basalt. In addition, relatively flat hydraulic gradients result in long residence times and slow release of water. Volumes of water stored as groundwater in even relatively small areas can be on the same order as large continental lakes or ice sheets or total global volumes of streamflow. This groundwater hydrology dominates the flow, stream temperature, sediment transport, and landscape evolution of young volcanic landscapes. For example, our research has revealed that streamflow, sediment transport, and temperature regimes in the Cascade Mountains of Oregon vary dramatically between the geologically distinct Western and High Cascade regions. A key control on streamflow response between these two regions is the partitioning of water input between a fast-draining shallow subsurface flow network (Western Cascades) versus a slow-draining deeper groundwater system (High Cascades). These differences result from the extremely high contrasts in rock permeability and porosity and drainage density between landscapes dominated by old versus young volcanic rocks. Along with controlling streamflow regimes, such geologically-based differences in groundwater storage capacity can significantly alter streamflow response to climatic warming. In particular, we expect that for the young volcanic terrains comprising the High Cascade Range of Oregon and Northern California, ground water storage is of sufficient magnitude to buffer potential changes in snowpack volume, hence summer streamflow, due to changing climate. Older volcanic and granitic landscapes in the Oregon Western Cascades and California Sierras, in contrast, will be much more sensitive to diminished snowpacks and summer streamflow changes. Even within the Sierras, local variations in bedrock geology and associated differences in volume and seasonal fluxes of subsurface water will likely result in significant spatial variability in sensitivity to climate forcing. Taken together, these results imply that current models linking climate and streamflow changes need to account for differences in groundwater storage as a first-order control in volcanic terrain.
http://www.fsl.orst.edu/wpg
V13C-02 14:00h
Volcanic Aquifers Feed Large Springs on the Oregon High Cascades Mafic Platform
Voluminous cold springs emanate from Quaternary basalts and basaltic andesites in the Oregon High Cascades, providing ~55% of late summer discharge to the Willamette River at Portland. Six springs on the west side of the Cascades have been continuously monitored for discharge and temperature since July 2003, and periodically sampled for Δ18O, ΔD, tritium and dissolved noble gases. Spring-fed streams are nearly constant in temperature and stable isotopes, with peak flow rates less than three times the baseflow values. Nearby streams that are not fed by springs show larger fluctuations in temperature (>10 C) and stable isotope values, with peak flows that are two orders of magnitude greater than baseflow. Oxygen isotope data reveal that mean recharge elevations for the springs are coincident with extensive young lavas which form the High Cascades mafic platform. Lava flows from shield volcanoes and cinder cones appear to be more important for water storage than composite volcanoes, because of larger surface areas and high permeabilities. Anomalously high unit discharges suggest that topographically-defined watersheds may not correspond to aquifer boundaries, but 3-He/4-He ratios in most of the springs are close to atmospheric, implying shallow flowpaths. Aquifer boundaries may correlate to paleotopography obscured by successive lava flows or glaciation. Tritium activities in spring waters (3.6 to 4.7 TU) are similar to that of recent precipitation, suggesting aquifer residence times of ~ 7-14 years. Complex relationships between lava flow geometries and groundwater flow patterns require a combination of hydrologic and volcanologic insights to understand the spring systems of young mafic terrains.
V13C-03 INVITED 14:15h
Geophysical investigations of volcano hydrogeology at Masaya Caldera, Nicaragua
The distribution and flow of groundwater is poorly known for the vast majority of active volcanoes, and hydrologic properties of active volcanoes are rarely characterized. This situation persists because such volcanic systems are not typically developed for their groundwater resources. Given that theses systems need to be characterized in the absence of sufficient well data, characterization of the groundwater system on active volcanoes is practical through the application of transient electromagnetic (TEM) soundings and innovative inversion techniques. Here, we employ this approach to constrain depth to the groundwater table at Masaya Caldera, Nicaragua, one of the largest active basaltic calderas on Earth. The active Santiago Crater within the caldera is the site of persistent volcano degassing, and the scene of occasional phreatic explosions, the most recent of which occurred in June, 2004. We present first estimates of depth to the groundwater table within the caldera, based on interpretation of 30 soundings. Uncertainty in depth estimates is constrained by using alternative one-dimensional inversion methods. Our results suggest that Masaya caldera is hydrologically isolated from the surrounding groundwater flow system by its caldera-bounding faults. Thus, these geophysical data potentially will provide a baseline for forecasting the response of this isolated groundwater system to changes in magmatic activity. In addition, characterization of the depth to groundwater provides context for interpretation of VLF anomalies, which indicate that a shallow degassing magma body (400 m diameter; 250 m depth) is located in the shallow subsurface beneath Santiago Crater, and interpretation diffuse degassing from the flanks of Masaya volcano, which produces elevated carbon dioxide flux in thermal areas, and related spontaneous potential anomalies.
V13C-04 14:40h
Calibration of a Groundwater Model of Masaya Volcano, Nicaragua
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.