pure
洌 centers on the idea of water that is clear and pure, free from impurity or cloudiness.
洌 combines the water radical 氵 with 列, which likely contributes the sound. The character is associated with pure, clear water.
The water radical 氵 beside 列 suggests water arranged in neat rows, like a clear, pure stream flowing in an orderly line.
For レツ, imagine a pure, clear stream that lets you see every pebble: the water is so clear it's like a lens, and 'lens' cues レツ.