1Water flowing beneath the surface (groundwater). Metaphorically, someone's underlying true feelings. (Reading in this entry: したみず.)
井戸を掘ると豊かな下水が見つかった。
When they dug the well they found abundant groundwater.
表向きは穏やかだが、彼の下水には不満がたまっている。
He seems calm on the surface, but his underlying feelings are full of resentment.
この地域では下水の位置が浅いので井戸に注意が必要だ。
In this area the groundwater is shallow, so care is needed when digging wells.
2The 'water' kanji radical when it appears at the bottom of a character (radical 85), as in 泰.
字の部首としての下水は泰などで確認できる。
You can see the bottom-form water radical in characters like 泰.
漢字学習では下水の位置と形に注意すると読みやすくなる。
When learning kanji, paying attention to the bottom water radical's position and form makes reading easier.