Influence of volcanic history on groundwater patterns on the west slope of the Oregon High Cascades

Spring systems on the west slope of the Oregon High Cascades exhibit complex relationships among modern topography, lava flow geometries, and groundwater flow patterns. Seven cold springs were continuously monitored for discharge and temperature in the 2004 water year, and they were periodically sampled for δ 18 O, δ D, tritium, and dissolved noble gases. Anomalously high unit discharges suggest that topographically defined watersheds may not correspond to aquifer boundaries, and oxygen isotope data reveal that mean recharge elevations for the springs are coincident with extensive Holocene lava fields. The 3 He/ 4 He ratios in most of the springs are close to atmospheric, implying shallow flow paths, and aquifer thicknesses are estimated to be 30–140 m. Estimates using 3 H/ 3 He data with exponential and gamma distributions yield mean transit times of ∼3–14 years. Recharge areas and flow paths are likely controlled by the geographic extent of lava flows, and some groundwater may cross the Cascade crest.