These two words are difficult to render in English, in this context. The term "oligometallic" is used some in geology, mostly in geochemistry. I'm not familiar with its use in English in the description of mineral waters, but that's certainly not my area of specialization. Etymologically, it means "not many minerals" or "few minerals."
Spanish "acratópega" is a very odd word. I found almost no references to it in Spanish, & not much in the way of English equivalents. I did find a reference to "acratopège" in French, in French Wiktionary; Sense 2 is given as "(En particulier) Qui a, en parlant d’une eau, une faible teneur en sels minéraux ou une teneur moindre que celle d’une eau minérale." Roughly translated, "Which has, speaking of a water, a low mineral content, or a content less than a mineral water."