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[Loud Whispers]

I find it more hilarious that rock salt reservoirs don't  contaminate water

[astaldaran]

Sphalerite,

You've contributed to our understanding...even if we are left confused...time to update the wiki probably--this seems pretty conclusive.

[Sphalerite]

Next experiment:  On the permanent contamination of a location by salt.

I built a small desalination plant by the ocean.  This one had a lever-controlled floodgate on the side which permitted the water in it to be dumped to a channel connected to the ocean.

Spoiler (click to show/hide)

As before, when filled with water with a pump, the cistern was usable as a water source, indicating that the water was not salty.

Spoiler (click to show/hide)

I opened the floodgate for a moment, allowing water to flow out into the channel.

Spoiler (click to show/hide)

I then shut the floodgate and refilled the cistern.  Checking the water source zone revealed that the water was now salty.

Spoiler (click to show/hide)

I then completely deconstructed the cistern, including the walls, floor, pump, door and floodgate.

Spoiler (click to show/hide)

In order to ensure that no trace of mud or salty water remained, I built a stone floor where the cistern was, then tore up the floor and threw away the stones.  When I was finished the location which had been occupied by the cistern had been physically scrubbed clean of all contamination.

Spoiler (click to show/hide)

I then rebuilt a new cistern and pump in the same location, and filled it with water.

Spoiler (click to show/hide)

I then checked the water zone on top of the cistern.

Spoiler (click to show/hide)

Still salty.

Conclusions:

Salt water contamination is essentially instantaneous, and can travel uphill and against the flow of water, so that a cistern that drains into the ocean will become salty.

Once a tile has become salty, it cannot be made unsalty, even if you remove all mud and other contamination and completely replace all structural materials.  It's similar to how once a tile has become light/aboveground, it can never become dark/underground again, even if you build a roof over it.

[Loud Whispers]

.....Resalination plants?

[HollowClown]

I injured one of the dwarves in the seaside embark, and created a minimalist hospital to treat him with.  Another dwarf went to the well and filled a bucket with salt-laced water to wash his wounds.

Am I the only one who finds this hilarious?

[Kamamura]

It also has important, far-reaching implication. Basically:

That, what has become salty, can never be unsalted again.

It also means that eventually, the whole world will become salty as a herring, incapable of supporting life. Saltiness is like entropy - always increasing.

Therefore, never salt anything voluntarily. Adapt to unsalted diet. Treat salt with utmost care, contain it, guard it, a single madman with a salt shaker can bring the ultimate salty Armageddon closer!

[Il Palazzo]

Kamamura-sensei, I challenge your teachings!
Can we not decrase salt-tropy by producing obsidian? The new tiles should be salt-free, no? Perhaps even when made from magma+salty water? Another mystery...

[LilGunmanX]

Have you tried washing out the area that you tore up with fresh water not from the cistern? Maybe from an outside source of naturally fresh water?

[Sphalerite]

Can we not decrase salt-tropy by producing obsidian? The new tiles should be salt-free, no? Perhaps even when made from magma+salty water? Another mystery...

An interesting question.  It will take longer to test, as I'll have to build a pump stack to raise magma to the surface.

Have you tried washing out the area that you tore up with fresh water not from the cistern? Maybe from an outside source of naturally fresh water?

If the tiles are salty, then any fresh water that enters them is going to become salty.  I don't expect that it would matter if the water is from another cistern or from a pump output.

[Nan]

I tried some tests for further debunking of desalination myths and to help confirm Sphalerite's results.

I channeled off a section of the beach (channels are a barrier to waves). Behind the channel, I channeled a cistern in the beach (conglomerate), I then pumped water into it.
Result: Fresh

Next I build a cistern - without a constructed floor - on the beach (again, walling off the waves themselves). Result: Fresh.

I also build a cistern with a constructed floor on the beach. In that case I didn't make any special effort to stop the waves, in fact I deliberately watched them wash all over the floor before finishing off the wall and pump. Result: Fresh.

And I build a cistern without a constructed floor in the waviest bloody spot I could find with the waves going right in the front. Result: Fresh.

The walled cisterns all remained fresh permanently (whether or not they had constructed floors).

However the cisterns channeled into the beach, went salty after a short time, even if they were fully sealed in by walls.

I also noticed something weird with pumping on the beach. Sometimes waves would emanate from the pumps even though the pump output had no possible way to escape (I suspect it is these "pump waves" or related breaches of physics which re-salinate fully-sealed dug out reservoirs on beaches).

Here's an image of my horribly messy testing setup:

Spoiler (click to show/hide)

What I particularly want to draw your attention to is the little reservoir on the far left; now I couldn't fill that one because it was under the influence of beach-physics, the beach seems to function as an aquifer which doesn't produce water, but does absorb pressurized water, so if you channel into the beach, water on top of the beach will drain into the channels. But the funny thing is, the water in the channel itself, is still fresh, that is a fresh water cistern. In other words, even contact with the beach-pseudo-aquifer - even in fact being stored in the beach-pseudo-aquifer - isn't sufficient to re-salinate water.

On the right side of the screen, the small walled-reservoirs on the beach are all perfectly fresh water, in spite of the fact that I filled them, removed the pumps, let them drain, let the waves wash through them, rebuilt the pumps and finally re-filled them. They remained fresh. Waves seem to have no ability at all to contaminate on their own z-level - they need to go down a channel (and "manifest" as real salt water) to do this.

So I concluded that water only re-salinates when it contacts genuinely existing salty water. I went out to test this further and confirm Sphalerite's result:

Spoiler (click to show/hide)

This is a perfectly fresh water cistern. Now my hypothesis was, when I opened the floodgate, the fresh water would flow out, contact salty water, and the saltiness, like electrical current, would propagate back into the reservoir. This turned up to be exactly what happened, when the fresh water stream hit the salt water, the whole reservoir turned salty. However, it wasn't sufficient for the fresh water stream to merely touch the waves, it was only when I dug out a channel bringing salt water nearer to the drain, that a "genuine water" connection could be made. It seems that waves by themselves are utterly harmless.
And I also repeat my earlier results, that I managed to empty small, 2x2 and 1x1 reservoirs and refill them without them being contanemated, even though the water drained into saltwater channels. I believe what must have happened there is that the fresh water quickly spread out to less than 3/7 depth before contacting the salt water. It is likely that two bodies of water need to be connected by at least 3/7 water before they count as the same body of water allowing saltiness to propagate.

So it seems that the only two critical things, is to avoid digging cisterns into the beach (this should be common sense), although building cisterns ON the beach is fine, and avoid letting the body of fresh water contact a body of salty water. Other than that, it seems you can dig or build your desalination cisterns wherever and however you like.

I also noticed some funny things with waves. Waves seem to propagate line-of-sight from the ocean. A straight horizontal mine shaft, going across the map and entering the ocean (at the highest water layer of the ocean), produced waves (and mist) on the floors of a chamber at the far end, 3 embark tiles away from the ocean in a different biome. Sadly though the mist didn't make the dwarves happy.

[Sutremaine]

An interesting question.  It will take longer to test, as I'll have to build a pump stack to raise magma to the surface.

Why not use DFliquids to spawn water and magma well above a pre-built container? If you create water directly on a tile it doesn't interact with the tile properly (it doesn't make it muddy), but any subsequent liquid movement is handled as though the water was put in the original tile using in-game means.

[ZzarkLinux]

Have you tried involving SOAP in the mix?

I had a ocean fortress recently where I had a reservoir working with no well.
It worked for a little bit, but then for some reason the dwarves stopped using it.

I noticed that there was the typical "blood smear" next to the reservoir, so I assume the soap somehow contaminated it.
Not sure though

[khearn]

Good science, Sphalerite!

[Sphalerite]

Excellent work, Nan.  It's good to have independent confirmation of my results.  I had been wondering if ocean waves could contaminate a freshwater cistern.

I've seen the beach pseudo-aquifer behavior as well.  It appears that near the ocean any tile of water that is at the same Z-level as the topmost level of water contained in the ocean can absorb an infinite amount of water from above.  This appears to be a hack to make the ocean act as an infinite sink of water, but it also makes any water on the map at ocean level act as an infinite sink, even if that water is completely isolated from the ocean.

[urick]

Is all the testing happening at the beach? I embarked on a salty river, we all died of thirst and I built desalinization cistern upon my reclamation, with success.