Tracing submarine hydrothermal inputs into a coastal bay in Baja California using radon

► We use radon to quantify hydrothermal fluid inputs into an arid coastal embayment. ► Radon did not correlate with temperature as heat fluxes were masked by solar heating. ► Estimated hydrothermal groundwater fluxes were much higher than ephemeral rivers. ► About 42% of the fluxes were fresh water and the remaining was recirculated seawater. ► Faults apparently control the entry of thermal groundwaters into the bay.