A wizards in a play-by-post game I was playing in have a large lake which was once part of an ocean, but now is being fed by freshwater streams and rivers (big island raising style of magic). However till the lake is purged of all the salt water is not not overly useful to the surrounding landscape.
This is a very situational spell to remove all the salt from a water source.
As the need is ultra-specialised I went a step further and used the T: Body-of-Water spell parameter from Realms of Power: Magic (which comes from the Atlantean Magic virtue), specifically for large bodies of water. There are almost no spells using Atlantean magic virtue.
Purify the Lake, Waterway, and Stream
Perdo Aquam 45, R: Touch, D: Momentary, T: Body-of-Water
This spell removes the saltiness from all water within the current body of water, and may affect very large bodies of water.
(Base 15, +1 Touch, +3 Body-of-Water, +2 size)
Creating the spell with a conventional Ars Magica from the core rules might be similar, however the T: Body of Water has some inbuilt aspects which make it more efficient. A trick to note is that the “Body of water” target can affect a larger mass of liquid, and to perform the same spell without it Target: Part is useful. This allows successive castings to eventually remove the salt from the water, but only in small amounts.
Purify the Salty Pool
Perdo Aquam 25, R: Touch, D: Momentary, T: Part
This spell removes the saltiness from all the targeted water, affecting the whole or part of an amount of salt water up to 560 cubic feet of water.
(Base 15, +1 Touch, +1 Part)
A small lake is hundreds of millions of litres, so would take tens of thousands of casting to do; however a magic item might be perfect for that. Plonk it into the water and watch it slowly desalinate the water. It could also be made in a variety of shapes such as an attachment to the bottom of a boat, or an anchor to be placed at the bottom of the lake bed.
Eventual Reduction of Saltiness – (PeAq38) This small silver fisherman’s charm is triggered each time the sun moves its diameter in the sky (every two minutes), and casts the Purify the Salty Pool spell on the water it is placed in. (Base PeAq25, with +10 for unlimited uses, +3 environmental trigger)
Find these spell and hundreds more for Ars Magica in my grimoire.