'Gage' in the work style book. It's definitely a USGS thing, and I don't know how much farther it spreads.

I think 'gage' is not as hard a sell as 'ground water', which USGS spells as 2 words, same as surface water. (And hyphenates both when used as adjectives, of course.) Most of the rest of the English-speaking world uses 'groundwater'.