Since Iran's groundwater hydrochemistry has not been well understood, we explored it using a rich, nationwide data sampled from 9,468 wells. Findings indicate the dominance of mix water type in 13 out of 30 of Iran's sub-basins, followed by NaCl water type in nine sub-basins. A distinctive relationship exists between Iran's geological-geomorphological features and hydrochemical facies/groundwater quality. This study's results can be used in the formulation of new strategies and policies for Iran's groundwater quality management in the future.