Groundwater in California Cascade Aquifers: First-Magnitude Springs and Regional-Scale Storage and Flow
Northeastern California Cascade region contains 22,000 km2 of hard volcanic rock aquifer consisting mostly of surficial Neogene and Quaternary age basalts. Based on stable isotope evidence, volcanic centers of Mt. Shasta, Medicine Lake Volcano, and Lassen Peak are focal points of high elevation groundwater recharge that flows 20-50 km in these aquifers before emerging as first-magnitude spring flows. In total, these spring discharges are estimate at ~85 m3/s, forming the upper Sacramento River which supplies an annual critical base flow to the 5.7 million m3 Shasta Reservoir storage, provides a source of the >2000 gigawatt-hours of hydroelectric power, and supports unique spawning habitat for thermal- and nutrient- dependent native and endangered aquatic life. This volcanic aquifer area has never been comprehensively investigated and is currently under-recognized in California State water management. Recent baseline monitoring of discharge in the western portion of the Fall River Springs (34 m3/s) shows strong correlated response to regional precipitation recorded 50 km up-gradient in the recharge area, suggesting preferential fracture-driven flow paths traversing many kilometers. Distinct flow paths are further indicated in measured temperatures of individual discharge points <1 km apart distinguished by 9.0°C versus 12.5°C and electrical conductivities 105 mS versus 145 mS that remain constant throughout the year. The δD and δ18O further shows either a consistent, distinct recharge source mixture or an evaporation influenced snow-melt process driving differences in isotopic compositions between different discharge points. Higher temperature and electrical conductivity also is associated with magmatic helium, although corrected 3H-3He ages suggest groundwater storage times of <30 years. Accordingly, changes in aerial groundwater storage is captured by inter-annual differences in late summer base flow discharge in downstream rivers and records intra- and inter-decadal climate cycling. Furthermore, this spring-fed base flow during extended droughts showed 20-50% decrease in volumetric discharge.