The hydrothermal systems associated with the restless high-threat volcanoes at Lassen and Long Valley, California, each release large amounts of arsenic (As) to surface waters – ~6 and ~8 metric tons/yr, respectively. The hydrothermal As output can increase during volcanic unrest, as illustrated by a two-fold increase during unrest at Lassen in 2014–15. During that period of unrest, increased As concentrations and fluxes were measured up to 75 km downstream from the Lassen source, in Mill Creek near the confluence with the Sacramento River. In eastern California, the Long Valley hydrothermal system feeds into the Los Angeles Aqueduct (LAA), and the Los Angeles Department of Water and Power (LADWP) actively manages the LAA system to remove hydrothermal As. In 1980, during a series of ~M_w6 earthquakes, the discharge of a particular group of hydrothermal vents in Long Valley increased approximately 7-fold, though the total increase in As flux to the LAA system at that time is unknown. The likely mechanism for increased hydrothermal discharge in each case is permeability enhancement due to strong ground motion. A review of the global literature on earthquake hydrology suggests a worst-case scenario of a roughly 10-fold increase in permeability, with commensurate increase in the hydrothermal As flux persisting for days to months. Here we evaluate the potential impact of such increases in hydrothermal As flux on the California water-supply system.