We're set in a fantastic medieval world. The underground water sources have some special properties because they are exposed to something. How would these sources be mixed with the substance if they aren't all connected through water networks? It could be related to minerals deep in earth of something like that?
After posting this question, it's been pointed to me that my characters wouldn't survive such a scenario without use of magic. Bear in mind this isn't the same character as my other post, as this would have created incoherent memories. I just like to set things up with characters :p
"The cursed brought us to our knees. Lacking energy, helpless, we couldn't hope to survive any longer. Some were starting to speak strangely, words I dare not repeat to you. The great fireball seemed to give us an energy that was taken from us slowly.
Plants started dying, but not all of them. Older hunters said these plants were the toughest, and were usually hard to pull of the ground due to their long tentacles into the ground. Animals also didn't seem to be the target of this curse, as they were as energetic as before. Though the less cold resistant animals usually didn't last long, animals that had a big fur were resisting as if nothing was happening. Why was this curse targeting us specifically? No one could give us an answer..
One day, as I was tracking an harmed animal, the steps led me to a cave. It wasn't usual for us to go into small caves since they could hold great danger and were too small to be potential shelters. Today, I thank the gods I decided to follow this animal as everything became crystal clear. Inside the cave was a pool of strange waters. It looked as if it were tainted by the fire. But we knew alchemy didn't allow water and flame to mix one another, so this was impossible. From the tracks, I could say lots of animals came here to drink from the pool. Considering that there were no corpses around, I decided to taste this water myself, since the animal I was looking for wasn't here anymore and that I was very thirsty.
Suddenly, I felt a new energy running in my veins. Very similar to the one the great fireball was giving us. I started feeling dizzy from this warm feeling. But this weird feeling didn't matter, I felt warm, energized. I had just found what kept the animals alive, this source was the key to our survival and everyone needed to know about it!"
There it is! My key to survival (which oddly looks like Rhum when described that way). Using unknown means, some underground water sources have been magically enhanced to replicate the effects of the sun. Animals found these sources well before humans did, because they didn't have other means to drink (considering everything is frozen/cold and they can't melt the snow like humans would). Some plants also survived because of their long roots, which meant the earth they took their minerals from (I'm no biologist) was "impregnated" with the magical water.
I think this solves the problem of animals, plants and humans to survive.
Let's assume this "magic" comes from another substance that is mixed into water. How could the substance be propagated only in the water sources if the sources aren't connected by some water networks?
At first I didn't want magic to intervene in my environment to the exception of the "curse". Though after reflection it's been shown I needed magic to make my characters survive. I don't want to solve every problems I have with "Because magic".