[JimRed]

To put it in context: if it was melted, it would be more fresh water than is available on all of Earth.

How can they tell that it is FRESH water? It could be loaded with any soluble substances.

[Yossarian]

How can they tell that it is FRESH water? It could be loaded with any soluble substances.

Perhaps by analyzing the spectrum of the light coming from it and filtered by it? While I haven't done it since college, spectroscopy is a very valuable tool.