sorry to answer your questions:
Tell us about the water you are using on them. TAP left out for 24 hours just in case of chlorine in my water
Where does it come from? NO clue - city water
Is there any chance of sodium contamination? Possible.
That damage pattern looks like more than a fert burn.
Well - considering its the only thing I put in them, I would agree. The tips of the plants are also pointing down, so is that another sign you know of???