noun
Water drawn from a well, typically for household use or drinking.
この地域では、今でも井戸水を使っている家庭が多い。
In this area, many households still use well water.
井戸水は冷たく
The well water is cold and delicious.
地下水 is groundwater in general, often used in scientific or environmental contexts, whereas 井戸水 specifically refers to water obtained via a well.
Compound of 井戸 (well) and 水 (water).