HR: 14:00h
AN: V13C-02    [Abstracts]
TI: Volcanic Aquifers Feed Large Springs on the Oregon High Cascades Mafic Platform
AU: * Jefferson, A
EM: jeffersa@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AU: Rose, T P
EM: rose23@llnl.gov
AF: Lawrence Livermore National Laboratory, Analytical and Nuclear Chemistry Division, L-231, Livermore, CA 94550 United States
AU: Grant, G E
EM: gordon.grant@orst.edu
AF: USDA Forest Service, Pacific Northwest Research Station, 3200 SW Jefferson Way, Corvallis, OR 97331 United States
AB: 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.
DE: 1829 Groundwater hydrology
DE: 1860 Runoff and streamflow
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly