Sounds like it would be cheaper to build desalination plants on the coast, and pipe the water in. Iran certainly has the technology and brainpower to do that.
I suggest you read the article it talks about the viability of that very point
No it has not and due to sanctions of USA, they struggle getting the equipement they need.
Too busy building rockets and drones
They share the same gas field with Qatar, who does all their desalination with all the excess gas production they can’t sell.
Qatar has no surface freshwater or groundwater. So all of their water is desalinated. It’s often still quite salty to the taste though - the last few ppms would be an exorbitant cost to remove.
However, Qatar has 3 million people. Iran has 92 million people - 9 million in Tehran alone. So their half of that gas field in the Gulf contributes far less energy per capita.
And even if the energy is free (unlimited natural gas, fusion, magic, whatever) desalination is still fairly expensive. I think only about 50% of the cost is energy, the other half is CapEx, operations, and replacing the membranes as they get used up.