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[Flare]

Having pierced the aquifer and gotten my fort up and running, I decided to make a cistern of sorts so that the dwarves would have some sort of clean water when some of them inevitably become injured, and so I've run into my problem.

It would seem the wiki is mistaken on the subject of water desalination, or at least not complete as to all the mechanics at work when water becomes, or rather does not become, desalinated. I am aware of wells being able to desalinate water, but I'm also conducting another test to see if fish will appear in salt water underground ponds as well as fresh water underground ponds.

On the topic of the "fresh" water coming out of the screw pump from a salty source, it seems to be the case that it doesn't really matter if it touches natural ground and walls or not. I've built cisterns entirely out of normal building material, pumped water down into it, yet still the water is not desalinated. I've then built a small one, in the air, entirely out of blocks, and again the problem persists.

After delving into certain threads with posters therein trying to deal with said problem, I think I've hit upon the problem of what is the real cause of water becoming salted. It's not that the water is touching the ground, but rather whether it is salted or not depends on the z-level it is on. This seems to be evident in the pumping contraption that I've built to irrigate the surface. The water on the ground which the pump stack pumped through is desalinated, as is the water inside the pumpstack itself, even if it was touching natural walls and floors. Some constructed floors and walls in the pump stack were not made out of blocks either, which rules out the two hypothesis. What is important to note here, or in other words, what caused the water to desalinate is that the water was pumped upwards and not downwards.

One of the cisterns I've built is 15 z-levels deep. the water coming from the pump isn't in contact with any natural stone or wall, and yet it still somehow finds a way to be salt water. There was no dirt in the cistern to contaminate the water either. Blocks didn't seem like an issue then as some of the floors in the pump stack to the surface was made out of mined rock and the presence of fresh water on those tiles would indicate that this isn't a problem. In support of the theory that natural rock has nothing to do with it, the bath I dug into the earth near the entrance that had inadvertently filled during the mass irrigation process did become a water source when "i" was pressed even when the water had crawled along the rocky tiles and general wild life bushery.

I'm wondering whether or not anyone else has had this type of experience. I've read some posts about the topic but none of them addressed the fact of what the wiki purports to be the method of the desalination of water.

[Sphalerite]

On the topic of water salinity being based on Z-level - I have suspected this myself, and it might be partially the case.  I did however have a cistern on a seaside fortress recently which was built entirely above ground.  Initially, this cistern was filled with fresh water.  During experiments into desalination I without realizing it allowed water spilling from this cistern to come in contact with salt water several levels down, at which point the above-ground cistern became permanently salty.  Even when I completely deconstructed and rebuilt the cistern, water in that location would sill be salty.

So what I suspect is that each square on the map is flagged as to whether water in that square is salty or fresh.  Pumps desalinate water by transporting it from a salty square to a freshwater square.  Saltyness spreads from square to square if they're adjacent and both have water in them, and it doesn't seem to be possible to make a salty square back into freshwater.  Natural soil and stone rocks, at least in layers near the surface, are flagged as salty from the start, which is why making a cistern in natural stone layers turns out salty.

That's my theory, but more science needs to be done.

[Naryar]

One word: Well.