The Ría de Vigo catchment is situated in the largest radon-prone area of the Iberian Peninsula. High local indoor radon (222Rn) levels are the preeminent source of radiation exposure, with negative effects on health. Nevertheless, information on radon levels of natural waters and the potential human exposure risks associated with their domestic use is very sparse. To elucidate the environmental factors increasing human exposure risk to radon during domestic water use, we undertook a survey of local water sources, including springs, rivers, wells, and boreholes, over different temporal scales. Continental waters were highly enriched in222Rn: activities ranged from 1.2 to 20.2 Bq L−1in rivers and levels one to two orders of magnitude higher were found in groundwaters (from 8.0 to 2737 Bq L−1; median 121.1 Bq L−1). The geology and hydrogeology of local crystalline aquifers support one order of magnitude higher222Rn activities in groundwater stored in deeper fractured rock compared to that contained within the highly weathered regolith at the surface. During the mean dry season,222Rn activities nearly doubled in most sampled waters in comparison to the wet period (from 94.9 during the dry season to 187.3 Bq L−1during wet period;n= 37). Seasonal water use and recharge cycles and thermal convection are postulated to explain this variation in radon activities. The high222Rn activities cause the total effective dose of radiation received from domestic use of untreated groundwaters to exceed the recommended 0.1 mSv y−1. Since more than 70% of this dose comes from indoor water degassing and subsequent222Rn inhalation, preventative health policy in the form of222Rn remediation and mitigation measures should be implemented prior to pumping untreated groundwater into dwellings, particularly during the dry period.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11356-023-27305-6.