In regions of active volcanism, the presence of magmatic carbon dioxide (CO2) in regional hydrologic systems provides a radiocarbon-depleted tracer for delineating ground-water transport and mixing processes and provides a means of assessing regional magmatic carbon fluxes, Variations in the stable carbon isotopic composition (delta(13)C) and carbon-14 values of springs and surface waters from the southern Cascade Range show consistent patterns of carbon isotopic mixing between magmatic, biogenic, and atmospheric CO2 reservoirs. Radiocarbon measurements of waters from the Lassen region in northern California were used to construct a ground-water carbon-14 contour map, revealing principal subsurface flow paths and a broad region of diffuse magmatic CO2 flux.