So I and a friend of mine had a fort a few months ago that was set on a seashore. The fortress was built below the sea between cavern layers 1 and 2. Since we initially thought that the first layer was much lower, A screw pump base was built in the shore and a water tunnel that lead to the fortress (an underground desalination station was planned). However, the water pipe happened to intersect with a passage between the two caves. Later the passage below was walled shut (but the upper cave was still connected) and the pump engaged.
While the pump did purify the seawater from salt, the water was contaminated immediately again from touching the natural cave walls. The pipeline filled as expected and the excess water started to spill into the cave.
Then something unexpected happened: The whole pipeline was suddenly devoid of salt! I can't think of any better explanation but that when a body of saltwater intersects with the fresh water from an underground lake (for the record: this was sourced water since the lake was on the edge of the map), all the water is considered fresh.
When I later shut down the pumps (also causing the water level in the cavern drop, so the water inside the pipeline was disconnected from any source) the water remained drinkable even though it was connected to natural cave walls in the surface level.
Thesis:
I) A static body water of will, when connected to a sourced one, change it's saltiness flag according to the water source.
II) Natural walls in the z-levels that accomodate cave systems do not cause water to become salty, even in an ocean biome.
III) A body of water will retain it's saltiness status when disconnected from a water supply.
Suggested research:
- If water is pumped from an underground lake and introduced to sourced salt water, will it become salt water?
- If unsourced salt water and unsourced fresh water are connected, will the resulting pool be salty?
- If an ocean and an underground lake are combined, will the lake become salty or the sea saltless?
edit: part of the post was missing